Atomic and Molecular Oxygen Kinetics Involved in Low Temperature Repetitively
Pulsed Nonequilibrium Plasmas
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy
in the Graduate School of The Ohio State University
By
Sherrie S. Bowman, M.S.
Graduate Program in Chemistry
The Ohio State University
2013
Dissertation Committee:
Dr. Walter R. Lempert, Advisor
Dr. Heather Allen
Dr. Anne McCoy
Dr. Frank DeLucia
Copyright by
Sherrie S. Bowman
2013
Abstract
This dissertation presents novel results in the study of nanosecond pulsed, nonequilibrium plasmas. Specifically, an in-depth experimental study of the role of atomic
oxygen on the kinetic mechanisms involved in three distinct discharge geometries was
conducted. First, a low temperature (~300 K) and low pressure (<100 Torr) pulsed
plasma in a plane-to-plane dielectric barrier discharge was studied using a high repetition
rate (40 kHz) high voltage pulsed discharge. Second, a higher temperature (~1000 K) and
low pressure (<100 Torr) pulsed plasma in a bare metal, spherical electrode geometry
was studied using a 60 Hz repetition rate high voltage pulsed discharge. Third, a high
temperature (~1200 K) and high pressure (~760 Torr) pulsed plasma in a pin-to-plane
geometry was studied using a 10 Hz repetition rate high voltage pulsed discharge.
Additionally, a study of the role of electronically excited molecular oxygen, a1g, on the
kinetics of a low temperature (~300 K) and low pressure ( <100 Torr) nonequilibrium
plasma in a plane-to-plane dielectric barrier discharge was conducted. Kinetic modeling
results were compared to all the experimental results.
UV ICCD camera imaging was used to confirm the stable and diffuse nature of
the plasma under all of the conditions that were studied. Current and voltage traces were
measured using commercially available probes to determine the energy coupled to the
ii
plasma. All of these results were used for modeling of experimental results. Two photon
Absorption Laser Induced Fluorescence (TALIF) measurements were used for
determining atomic oxygen concentration.. Calibration by comparison with xenon gas
gave absolute O atom concentration in a variety of gas mixtures and discharge
geometries. IR emission spectroscopy was used for electronically excited molecular
oxygen, a1g, measurements. Calibration by comparison with a blackbody source was
used for absolute scale results. The effect of a1g on ignition delay time was measured
spontaneous OH A→X(0,0) emission spectroscopy was used. Ignition delay was defined
as the onset of continuous OH emission between discharge pulses.
It was found that while, in general, the mechanism for atomic oxygen formation
and decay in each of the plasmas studied can be compared there are significant
differences in quantitative values in each case. Initial conditions, such as the coupled
energy and number density of electrons, play a strong role in determining how the
chemistry propagates in time. The role of a1g was found to be complicated by concurrent
NOx chemistry happening in the discharge and significantly higher concentrations would
be needed to differentiate these effects.
iii
This work is dedicated to the following:
James H. Robinson
For being my biggest fan.
Harrell H. Bowman
For teaching me to be myself.
Lorene B. Robinson
For teaching me what is really important.
iv
Acknowledgments
First and foremost, I have to thank God for every opportunity He has put in my
path. At every turn in the road, I have struggled to understand where I'm going and every
time He has given me more than I ever thought to ask for and so much more than I
deserve.
I have been very blessed to have a family who supports me in all of my wild
ideas, including becoming a scientist. My mom, especially, has listened to me through
every excitement and every frustration along the way and she deserves all sorts of credit
for putting up with me. I truly wouldn't have made it to this point without her. My dad,
more so than anyone else, has showed me that it doesn't matter what I do as long as I do it
to the best of my ability. I hope he realizes how much I try to emulate him in all of my
work and my approach to life.
Everyone thinks my sister, Stacie, and I are complete opposites, but she and I both
know that isn't true. She is one of the most creative and intelligent people on Earth and
she doesn't need a Ph.D. to prove it. I am so glad we spent 10+ years screaming and
yelling and challenging each other in every way imaginable and I hope to spend many
more years than that laughing and supporting each other through everything life throws at
us.
v
There are some things you can't learn in a classroom or laboratory, and my
brother, Zach, seems to have been born with all of that knowledge. Even from a young
age he was the most caring member of our family and that hasn't changed at all in the
past 12 years. He's opinionated and strong willed but is the type of good ol' boy you can
trust with your life. The world needs more men like Zach Bowman in it.
It takes a special kind of crazy to pursue a life of scientific research. It takes an
even crazier person to love one of those scientists. I am so thankful that Marc Pilkington
meets that description and came into my life when he did. As the eternal optimist, he is
all the encouragement I didn't know I needed. As the most laid back person on the planet,
he is the patience I never had. As stubborn as a brick wall, he is the counterpart I have
always wanted. He is my best friend and I am looking forward to all the adventures we
are going to have in the post-graduate school world. More.
There are so many more people who have shaped my life leading up to this point.
I wish I could name every single one of them. From my grandmother Mary Bowman to
my beautiful little nieces Aeris and Aisley Frick, Jared Frick, Julie Dawson, Barbie
Dobbins, Katie Cilwa, and everyone else, thank you for being a part of my life.
It may sound obvious, but Dr. Walter Lempert has been the most important
person in terms of getting me through graduate school. He has really been the ideal
advisor for a student like me and I am very grateful for all the help, guidance, opinions,
and information that he has shared with me over the years.
I am also grateful for the other teachers who have helped me along the way. Mr.
Lynn and Mr. Williams were the crazy science guys at my high school, and I wanted to
vi
be just like them. Dr. Darrell Crick taught me how to do research and listen to the
Grateful Dead, and both of those things have been critical over the years. Dr. Igor
Adamovich has been an unending source of information on a wide range of topics.
Finally, I have to thank all the members of NETL, both past and present, for
making coming in to work every day so enjoyable. Everyone should get to have a desk
beside Yvette Zuzeek and her drawer full of candy, or listen to John Bruzzese tirade
about everything from Democrats to Big Ten football. If I wasn't asking Naibo Jiang for
help at least once a week, I just wasn't working hard enough. We have had a lot of fun,
but we've also helped each other to be greater scientists and I can't imagine a better
environment to spend such important, busy, and awesome years. Thank you all!
vii
Vita
October 5, 1984 ..............................................Born - Clarksburg, WV
May, 2006 ......................................................B.S. Chemistry, Concord University
March, 2010 ...................................................M.S. Chemistry, The Ohio State University
2010 - present ................................................Graduate Research Associate, Department
of Chemistry, The Ohio State University
Publications
1.
Bowman, S., I.V. Adamovich, and W.R. Lempert. "Atomic Oxygen Kinetics of Fuel/Air
Mixtures in Repetitively Pulsed Low Temperature Nanosecond Discharges" 51st AIAA
Aerospace Sciences Meeting, 7-10 January, 2013, Dallas, TX.
2.
Lanier, S., S. Bowman, I.V. Adamovich, and W.R. Lempert, “Development of Pure
Rotational CARS Thermometry for Nanosecond Pulsed Oxygen/Argon Plasmas,” 51st
AIAA Aerospace Sciences Meeting, 7-10 January, 2013, Dallas, TX.
3.
Bowman, S., I.V. Adamovich, and W.R. Lempert. "Atomic Oxygen Measurements in
O2(a1Δg) Injected Nonequilibrium Plasmas by Two Photon Absorption Laser Induced
Fluorescence" AIAA 2012, 50th AIAA Aerospace Sciences Meeting, 8-11 January,
2012, Nashville, TN.
4.
Bowman, S., I.V. Adamovich, and W.R. Lempert. "Effect of Singlet Delta Oxygen on the
Kinetics of Low Temperature Repetitively Pulsed Nonequilibrium Plasmas" AIAA 2011,
49th AIAA Aerospace Sciences Meeting, 3-6 January, 2011, Orlando, FL.
5.
Zuzeek, Y., S. Bowman, I. Choi, I.V. Adamovich, and W.R. Lempert. "Pure Rotational
CARS Studies of Thermal Energy Release and Ignition in Nanosecond Repetitively
Pulsed Hydrogen-Air Plasmas" Proceedings of the Combustion Intstitute, Vol. 33, Issue
2, 2011, pp3225-3232.
viii
6.
Bowman, S., I. Choi, K. Takashima, I.V. Adamovich, and W.R. Lempert. "Kinetics of
Low-Temperature Hydrogen Oxidation and Ignition by Repetitively Pulsed
Nonequilibrium Plasmas" AIAA 2010-1590, 48th AIAA Aerospace Sciences Meeting, 47 January, 2010, Orlando, FL.
7.
Zuzeek, Y., S. Bowman, I. Choi, I.V. Adamovich, and W.R. Lempert. "Pure Rotational
CARS Measurements of Thermal Energy Release and Ignition in Nanosecond Pulse Burst
Air and Hydrogen-Air Plasmas" AIAA 2010-648, 48th AIAA Aerospace Sciences
Meeting, 4-7 January, 2010, Orlando, FL.
Fields of Study
Major Field: Chemistry
Areas of Interest: Lasers and Laser Diagnostics, Plasmas, Kinetics, Combustion
ix
Table of Contents
Abstract.............................................................................................................................. ii
Dedication ......................................................................................................................... iv
Acknowledgments ............................................................................................................. v
Vita .................................................................................................................................. viii
List of Tables .................................................................................................................. xiii
List of Figures................................................................................................................. xiv
Chapter 1: Introduction and Background..................................................................... 1
1.1 Introduction to Plasma Assisted Combustion…………………………………1
1.2 Introduction to Atomic Oxygen Kinetics in PAC……………………………..8
1.3 Introduction to Molecular Oxygen Research: Singlet Delta Oxygen……..…14
1.4 Discharge Geometries……………………………..…………………………21
1.5 Objectives………………………………..…………………………………..22
Chapter 2: Experimental Instrumentation………………………………….…………….25
2.1 Introduction…………………………………………………………………..25
2.2 Plasma Creation by High Voltage Pulser Units………………...……………26
2.3 Plasma Imaging and Coupled Energy Measurements……………………….29
2.4 Oxygen Atom TALIF Apparatus and Calibration…………………...………31
2.5 Characterization of Singlet Delta Oxygen…………………...………………40
2.6 Measurement of Ignition Delay Time……………………………………….43
Chapter 3: Nanosecond Pulse Discharge and Plasma Chemistry Model………….…….45
x
3.1 Introduction……………………………………………………………….….45
3.2 Plane-To-Plane Pulsed Discharge Model……………………...……….……46
3.3 Plane-To-Plane Plasma Chemistry Model…………………………….…….57
3.4 Bare Metal Electrode Discharge Model………………………………….…..59
Chapter 4: Plane-To-Plane Discharge Results……………………………………...……61
4.1 Introduction……………………………………………………………….….61
4.2 Characterization of the Plasma - Imaging……………………………………61
4.3 Characterization of the Plasma - Coupled Pulse Energy………….…………70
4.4 TALIF Results - Baseline Measurements……………………………….…..75
4.5 TALIF Results - Hydrogen Fuel……………………………………….…….81
4.6 TALIF Results - Ethylene Fuel………………………………..……………..87
Chapter 5: Singlet Delta Oxygen Results…………………………………………..……92
5.1 Introduction……………………………………………………….…………92
5.2 Experimental Considerations for SDO Measurements…………….……….94
5.3 SDO Yield Results…………………………………………..……………….97
5.4 Plasma Uniformity Measurements………………………………….……..103
5.5 O Atom Measurements and Kinetic Modeling……………………….……106
5.6 Summary and Conclusions for SDO Study……………………….……….119
Chapter 6: Pin-To-Pin Discharge Results……………………………………….……..121
6.1 Introduction………………………………………………….…………….121
6.2 Characterization of the Plasma - Coupled Pulse Energy………………….122
6.3 Characterization of the Plasma - Imaging……………………..……………126
xi
6.4 TALIF Results - Baseline Measurements…………………….……………133
6.5 TALIF Results - Hydrogen Fuel………………………………..…………..135
6.6 TALIF Results - Ethylene Fuel……………………………………..………136
Chapter 7: Pin-To-Plane Discharge Results………………………………...………….140
7.1 Introduction…………………………………………………………………140
7.2 Streamer Discharge Characterization Results………………………………143
7.3 TALIF Results - Air and Calibration……………………………………….147
7.4 TALIF Results - Methane Fuel…………………………………….……….152
7.5 TALIF Results - Ethylene Fuel……………………………………….…….154
7.6 TALIF Results - Propane Fuel………………………………………..…….156
7.7 Conclusions on Pin-to-Plane Study……………………………….……….158
Chapter 8: Summary and Conclusions…………………………………….……….…..160
8.1 Plane-To-Plane Discharge Conclusions…………………………………….160
8.2 Pin-To-Pin Discharge Conclusions……………………………………..….162
8.3 Pin-To-Plane Discharge Conclusions………………………………………162
8.4 Suggestions for Further Study………………………………………..…….163
Appendix A: Flow Controller Settings and Equivalence Ratio Calculations…………..165
References……………………………………………………………………….………171
xii
List of Tables
Table 2.1:
Summary of literature quenching coefficients relevant to this work.
Values used for this work are indicated by asterisks. Note that C2H4 value
is assumed equal to that of CH4………………………………………....38
Table 3.1:
Dominant radical species generation processes in the plasma………..…56
Table 3.2:
H2 and C2H4 fuel reactions used in the plasma chemistry model……..…57
Table 4.1:
Coupled pulse energy results from capacitive voltage probe and shunt
current probe measurement. Red values correspond to positive polarity
pulses, blue values correspond to negative polarity pulses………..…..71
Table 5.1:
SDO quenching reactions and room temperature rates for some common
species…………………….…………………………………..……….…99
Table 7.1:
Electrical pulse characteristics for different gas mixtures……………144
xiii
List of Figures
Chapter 1
Figure 1.1:
Ignition delay time as a function of the number of C atoms, n, in CnH2n+2
molecules. Closed points correspond to experiment, while open points are
calculations T=1430-1450K, P=0.4 to 0.5 atm……………………...……3
Figure 1.2:
Emission spectra of flame, taken 12 mm above a bluff body, as a function
of discharge power………………………………………………………...4
Figure 1.3:
Digital camera images of nanosecond repetitively pulsed glow and spark
discharges. Anode at top, cathode at bottom. The electrode gap is 4.5 mm.
The applied voltage is 5.5 kV for the glow discharge and 6 kV for the
spark discharge. Atmospheric pressure air preheated to 1000 K………….5
Figure 1.4:
Sequence of video stills taken over thirty minutes. The flame front moves
towards the burner exit (against the flow of air/fuel). In this case, methane
is the fuel being used………………………………………………………6
Figure 1.5:
Schematic diagram of proposed flame structure………………………….7
Figure 1.6:
Oxygen atom mole fraction as a function of time after a single highvoltage pulse in air and in ethylene-air at P=60 Torr. Dots and squares
correspond to experimental results while lines correspond to kinetic
modeling…………………………………………………………………..9
Measurements of O, N2(B), and N2(C) in air discharges as a function of
applied discharge voltage……………………………………….………..10
Figure 1.7:
Figure 1.8:
Schematic diagram of a diffusion flame…………………………………11
Figure 1.9:
Dependence of extinction strain rates as a function of fuel mole fractions
for various plasma repetition rates…………………………………….…12
Figure 1.10:
Extinction strain rates as a function of fuel mole fractions with and
without plasma presence (solid data points), in heated gas flows (open data
points), alongside computational results (lines)……………………….…13
xiv
Figure 1.11:
Relative reduction in induction time in stoichiometric H2:air with
preferential addition of various species……………………………….…14
Figure 1.12:
Induction time of ignition in a stoichiometric H2:air mixture at 0.5 atm.
The curves correspond to modeling results: (1) no SDO addition, (2) 1%
SDO addition with H2 quenching, (3) 1% SDO addition without H2
quenching……………………………………………………………...…16
Figure 1.13:
Maximum SDO yield as a function of E/N. (1) O2:Ar = 1:1, (2) O2:Ar:H2
= 0.95:1:0.05, (3) O2:Ar:D2 = 0.95:1:0.05, (4) O2:Ar:CO = 0.9:1:0.1…17
Figure 1.14:
Variation of temperature as a function of time in a CH4/air mixture, =3,
T0=100K, P0=105Pa………………………………………………...……18
Figure 1.15:
Concentration of species as a function of time calculated in a mixture with
V0=1205m/s and P=50 Torr……………………………………….……19
Figure 1.16:
Experimental flame liftoff height as a function of SDO and O3
concentration. Plasma power is 80 W. The error bars correspond to
experimental uncertainties in both concentration and flame liftoff
height…………………………………………………………………..…20
Figure 1.17:
Arrhenius temperature dependence of SDO by H2 and C2H4…………21
Chapter 2
Figure 2.1:
Typical positive and negative polarity voltage pulse waveforms used to
sustain a repetitively pulsed discharge. 20% O2 in argon gas mixture, P=40
Torr, and a pulse repetition rate of =40kHz……………………………26
Figure 2.2:
Schematic diagram of pulser output in burst mode operation…………27
Figure 2.3:
Typical voltage and current waveforms used in discharge. Air, P=760
Torr………………………………………………….……………………28
Figure 2.4:
Schematic diagram of custom designed shunt current and capacitive
voltage probe……………………………………………………….…….31
Figure 2.5:
Simplified TALIF energy level diagram for oxygen (left) and xenon
(right)………………………………………………………………..……32
Figure 2.6:
Energy level diagram showing all the possible transitions for an atom
excited to the n* state……………………………………………………33
xv
Figure 2.7:
Schematic diagram of the TALIF setup……………………………..…..39
Figure 2.8:
Schematic diagram of the calibration of SDO yield measurements……41
Figure 2.9:
Time-resolved OH emission…………………………………………..…44
Chapter 3
Figure 3.1:
Schematic of discharge geometry……………………………….………49
Figure 3.2:
Time dependent electric field in the plasma region as a function of
time………………………………………………………………….……50
Figure 3.3:
Time dependent electric field in the plasma region as a function of
location across the electrode gap…………………………………...……51
Figure 3.4:
Discharge model predictions for applied electric field, stored/coupled
energy, and field in the plasma………………………………………….53
Figure 3.5:
Current/Voltage waveforms in 20% O2/argon mixture at P=40 Torr. 100th
pulse in a burst of 451. Coupled pulse energy = 0.7mJ/pulse……..……54
Figure 3.6:
Fraction of energy transferred by electrons in air to (1) vibrations of O2,
(2) rotations of O2 and N2, (3) elastic losses, (4) vibrations of N2, (5)
electronic excitations of N2, (6) electronic excitations of O2, (7) ionization
of O2 and N2…………………………………………………………...…55
Figure 3.7:
Typical current (black) and voltage (red) results in 40 Torr of air in the
spherical electrode discharge cell. The coupled pulse energy is shown in
blue……………………………………………………………………….60
Chapter 4
Figure 4.1:
Optical access plasma flow channel with copper plate electrodes and gas
lines indicated……………………………………………………………62
Figure 4.2:
Series of ICCD camera images taken through the side of the discharge cell
of individual discharge pulses in Ar/O2, P=40 Torr and a pulse repetition
rate of 40 kHz. The camera gate is 5s…………………………….……63
xvi
Figure 4.3:
Series of ICCD camera images of individual discharge pulses in H2/O2/Ar
at =0.1, 0.5, and 1.0. P=40 Torr and v=40 kHz. The camera gate is
5s……………………………………………………………….…..……64
Figure 4.4:
Series of ICCD camera images of individual discharge pulses in
C2H4/O2/Ar at =0.08, 0.43, and 0.83. P=40 Torr and v=40 kHz. The
camera gate is 5s………………………………………………..………64
Figure 4.5:
Spectra of emission lines resulting from argon pen lamp (black) and
plasma (red).……………………………………………………………..65
Figure 4.6:
Image taken through the end of the plane to plane cell, with illumination
from an argon pen lamp. The dashed lines show the area of the test
cell………………………………………………………………..……….66
Figure 4.7:
Series of ICCD camera images taken through the end of the plane to plane
discharge cell of individual discharge pulses in Ar/O2, P=40 Torr and a
pulse repetition rate of 40 kHz. The camera gate is 5s………………..67
Figure 4.8:
Series of ICCD camera images taken through the end of the plane to plane
discharge cell of individual discharge pulses in Ar/O2, P=40 Torr and a
pulse repetition rate of 40 kHz. The camera gate is 5s…………………68
Figure 4.9:
Series of ICCD camera images of individual discharge pulses in H2/O2/Ar
at =0.1, 0.5, and 1.0. P=40 Torr and v=40 kHz. The camera gate is
5s…………………………………………………………….…………..69
Figure 4.10:
Series of ICCD camera images of individual discharge pulses in
C2H4/O2/Ar at =0.08, 0.43, and 0.83. P=40 Torr and v=40 kHz. The
camera gate is 5s………………………………………………………..70
Figure 4.11:
(top) Experimental applied field (red) and stored/coupled energy curves
(black). Discharge model predicted electric field (blue). (bottom)
Discharge model predictions for electric field (black), electron number
density (solid blue), coupled energy (red), and O atom concentration
(dotted blue)…………………………………………………….………..72
Figure 4.12:
Plasma discharge modeling predictions for atomic oxygen concentration
and temperature in Ar/O2………………………………….…………….74
Figure 4.13:
Plasma discharge modeling predictions for atomic oxygen concentration
and temperature in air……………………………………………………75
xvii
Figure 4.14:
Two photon excitation line profiles for atomic oxygen………………….76
Figure 4.15:
Typical TALIF excitation spectrum…………………………….………..77
Figure 4.16:
Atomic oxygen number density as a function of time after a 21 pulse burst
at P=40 Torr in air…………………………………………….………….78
Figure 4.17:
Atomic oxygen number density as a function of time after a 21 pulse burst
at P=40 Torr in 20% O2 in argon……………………………....…………79
Figure 4.18:
Effect of flow velocity on atomic oxygen number density in 20% O2 in
argon at a pressure of 40 Torr. Red points correspond to a flow velocity of
0.5 m/s, while black points correspond to a flow velocity of 1.0 m/s…...81
Figure 4.19:
Experimental and predicted atomic oxygen number density in Ar/O2/H2
mixtures………………………………………………………….………..82
Figure 4.20:
Experimental CARS results and plasma chemistry modeling predictions
(Popov's mechanism) for temperature under the same conditions as Figure
4.19………………………………………………………………..………84
Figure 4.21:
Experimental and predicted (Popov's mechanism) atomic oxygen number
density in Ar/O2/H2 mixtures……………………………………….……85
Figure 4.22:
Experimental and predicted (Konnov's mechanism) results for temperature
rise in Ar/O2/H2 mixtures………………………………………………...86
Figure 4.23:
Experimental and predicted (GRI 3.0 mechanism) atomic oxygen number
density in Ar/O2/C2H4 mixtures…………………………………….……88
Figure 4.24:
Experimental and predicted (Wang/USC mechanism) atomic oxygen
number density in Ar/O2/C2H4 mixtures………………………….……..88
Figure 4.25:
Experimental and predicted (GRI Mech 3.0 mechanism) results for
temperature rise in Ar/O2/C2H4 mixtures………………………………..90
Figure 4.26:
Experimental and predicted (Wang/USC mechanism) results for
temperature rise in Ar/O2/C2H4 mixtures………………………………...91
Chapter 5
Figure 5.1:
Plot of reactions involving O2 with H and H2 both with and without SDO
enhancement……………………………………………….……………..94
xviii
Figure 5.2:
Schematic diagram of the experimental apparatus. Electrodes are located
above/below the plane of the paper on the TALIF Discharge Cell and
wrapped around RF Discharge Cell…………………………….………..95
Figure 5.3:
Typical voltage waveform during a burst of nanosecond pulses in air at 40
Torr and a pulse repetition rate of 40 kHz, for grounded and floating
discharge operation……………………………………………..………..96
Figure 5.4:
Effect of flow rate on signal from SDO molecules. 20% O2 in helium, 40
Torr. Flow rate is as calculated in the RF discharge……………………..98
Figure 5.5:
Typical SDO emission spectra. (a) Comparison of signal levels upstream
(in the RF discharge) and downstream (in the flow channel cell) without
any NO2 titrant added to the system. (b) Same comparison with 0.1% NO2
by volume added. 20% O2 in helium, P=60 Torr………………..……..101
Figure 5.6:
Typical SDO emission spectra. (a) Comparison of signal levels upstream
(in the RF discharge) and downstream (in the flow channel cell) without
any NO2 titrant added to the system. (b) Same comparison with 0.1% NO2
by volume added. 20% O2 in argon, P=60 Torr…………….………….101
Figure 5.7:
Plot of raw SDO signal intensity against percent of NO2 titrant. Signal
levels off above ~0.06% NO2 by volume………………………………102
Figure 5.8:
Percent yield of SDO as a function of pressure in both the RF discharge
(green squares) and downstream in the flow channel. These measurements
were taken in 20% O2 in argon…………………………………...……..103
Figure 5.9:
Series of broadband ICCD images of =0.75 C2H4 in Ar/O2 mixture at 65
Torr. The camera gate is 2 microseconds for images of individual pulses,
and 20 microseconds for images taken between pulses………..………104
Figure 5.10:
Series of broadband ICCD camera images at =0.75 C2H4 in Ar/O2
mixture at 65 Torr, obtained at differing times after the final pulse of a 25
millisecond, 50 kHz burst. Camera gate is 100 microseconds……...….106
Figure 5.11:
O atom number density as a function of time after a 21 pulse burst in a
20% oxygen in argon oxidizer mixture, compared with kinetic modeling
results. P=40 Torr, discharge pulse repetition rate is 40 kHz……….…108
Figure 5.12:
Experimental and predicted atomic oxygen number density in Ar/O2/H2
mixtures. (Same as Figure 4.16)……………………………………..…109
xix
Figure 5.13:
Species concentrations and temperature vs. number of pulses in the
discharge burst, predicted by the kinetic model in a H2/Ar/O2 mixture at
=0.5, at the conditions of Figure 5.14. Data points show experimental O
atom number density……………………………………..……………..110
Figure 5.14:
Kinetic modeling calculations illustrating the effect of NO2 titration on
composition of 20% Ar/O2, 40 Torr oxidizer mixture after the RF
discharge. Initial SDO mole fraction 0.014 (7% yield), initial O atom mole
fraction 6x10-4 (0.06%). Top, no NO2 titration; bottom, with NO2 titration
(NO2 mole fraction same as O atom mole fraction)…………..………..112
Figure 5.15:
O atom number density as a function of number of pulses in the discharge
burst. Experimental results…………………………………………..…114
Figure 5.16:
O atom number density as a function of number of pulses in the discharge
burst. Kinetic modeling results……………………………...………….115
Figure 5.17:
Experimental O atom number density as a function of number of pulses in
the discharge burst. H2/Ar/O2 mixture at =0.5, 40 Torr, 40 kHz……..116
Figure 5.18:
Predicted O atom number density as a function of number of pulses in the
discharge burst, at the conditions of Figure 5.19. Left: baseline model.
Right: illustration of effect of SDO quenching by HO2 and H
atoms…………………………………………………..………………..118
Chapter 6
Figure 6.1:
Schematic diagram with spherical electrode geometry……………..….123
Figure 6.2:
Current and voltage waveforms in air at 40 Torr……………….………124
Figure 6.3:
Voltage trace used as input in kinetic model………………….………..125
Figure 6.4:
Experimental (gray) and calculated (red) current traces for 40 Torr of air
in the spherical electrode geometry………………………….…………126
Figure 6.5:
ICCD camera image of the electrode region in 40 Torr of air. No discharge
present……………………………………………………..……………127
Figure 6.6:
ICCD camera images taken in 40 Torr of air. The main pulse (left)
couples significantly more energy into the plasma than the pre-pulse
(right)………………………………………………………...………….128
xx
Figure 6.7:
ICCD camera images of individual discharge pulses in air with H2 fuel,
=0.07. P=40 Torr. The camera gate is <100ns………………….……..129
Figure 6.8:
ICCD camera images of individual discharge pulses in air with H2 fuel,
=0.22. P=40 Torr. The camera gate is <100ns………………..……….129
Figure 6.9:
ICCD camera images of individual discharge pulses in air with H2 fuel,
=0.43. P=40 Torr. The camera gate is <100ns…………..…………….130
Figure 6.10:
ICCD camera images of individual discharge pulses in air with C2H4 fuel,
=0.19. P=40 Torr. The camera gate is <100ns……………………..….131
Figure 6.11:
ICCD camera images of individual discharge pulses in air with C2H4 fuel,
=0.48. P=40 Torr. The camera gate is <100ns………………..……….131
Figure 6.12:
ICCD camera images of individual discharge pulses in air with C2H4 fuel,
=0.87. P=40 Torr. The camera gate is <100ns………………..……….132
Figure 6.13:
ICCD camera images of individual discharge pulses in air with C2H4 fuel,
=0.1.74. P=40 Torr. The camera gate is <100ns……………..………..132
Figure 6.14:
Atomic oxygen number density as a function of time after a single
discharge pulse at 40 Torr in air………………………………...………134
Figure 6.15:
Experimental atomic oxygen number density in air and air/H2 mixtures at
P=40 Torr……………………………………………………....………..135
Figure 6.16:
Experimental atomic oxygen number density in air and air/C2H4 mixtures
at P=40 Torr……………………………………………....……………..137
Figure 6.17:
Intensity of TALIF signal in 500 microseconds after the discharge pulse in
air (black), and =1.74 C2H4 (red), as well as without any discharge
present (blue)………………………………………………..………….139
Figure 6.18:
Intensity of TALIF signal in 500 microseconds after the discharge pulse in
=1.74 C2H4 (red), as well as without any discharge present (blue).….139
Chapter 7
Figure 7.1:
Photograph of the pin-to-plane electrode geometry and three axis
translation stage………………………………………………..……….141
xxi
Figure 7.2:
Typical voltages and current traces for the streamer discharge, taken in air
at atmospheric pressure…………………………………………………143
Figure 7.3:
Typical image of the streamer discharge taken in air. Distance between the
anode and cathode is 8mm. TALIF measurements were conducted a few
millimeters below the anode tip…………………………………..…….145
Figure 7.4:
Rotational temperature overlaid with vibrational v=1 signal in air showing
required overlap between population and temperature extraction….….147
Figure 7.5:
TALIF results of O atom number density in pure air at atmospheric
pressure……………………………………………..…………………..148
Figure 7.6:
Calculated decay due to diffusion shown with measured atomic oxygen
concentration in pure air (experimental data from Figure 7.5)…………151
Figure 7.7:
TALIF measurements of O atom number density in CH4/air mixture.…153
Figure 7.8:
TALIF Results of O atom concentration in C2H4/air……………...…….155
Figure 7.9:
TALIF results of C3H8/air mixtures………………………………..…...157
xxii
Chapter 1
Introduction and Background
1.1 Introduction to Plasma Assisted Combustion (PAC)
In recent years, there has been significant progress in the utilization of Plasma
Assisted Combustion (PAC) for a wide variety of applications. Some of these
applications, such as increasing the efficiency of engines and gas turbines, have led to the
development of new discharge and ignition techniques. The use of non-equilibrium
plasmas differs from other techniques, such as pilot flames and bluff bodies, in that they
don't rely, primarily, on thermal energy transfer. Conventionally, sparks or arc discharges
have been used, but these are known to be ineffective at high flow velocities or low
pressure (due to the small volume of the spark or arc). Non-equilibrium discharges,
however, can be created by a dc, rf, or microwave discharge and are not subject to the
same volume constraints as the conventional methods. A detailed review of these
techniques is given by Starikovskaya [1].
In particular, utilization of a nanosecond-pulsed, high peak voltage (~20kV), high
repetition rate (1-100 kHz) discharge has been shown to have a significant effect on the
1
augmentation of combustion phenomena, and this is the technique that has been studied
in this work. This type of non-equilibrium plasma discharge is characterized by
inherently high reduced electric fields, E/n, of up to several hundred Townsend (1 Td =
10-17 V-cm2). At these high reduced electric field strengths, a significant fraction of the
total discharge energy goes into molecular dissociation and population of electronically
excited states. From a fundamental kinetics perspective, however, the mechanisms that
dominate in these discharges are not well understood. The addition of fuels (for
combustion studies) further complicates the kinetic pathways due to the wide range of
rates for electron impact and chemical branching reactions that become available.
Several experimental and computational studies have been conducted with the
goal of understanding the plasma processes involved in combustion. Specifically, studies
have looked at the role of plasma generated species on important combustion
characteristics; flame stability, ignition delay, flame speed, etc. Using nanosecond pulsed
discharges, Kosarev et al [2] recently showed a decrease in ignition delay time by more
than an order of magnitude in hydrocarbon and oxygen mixtures.
2
Figure 1.1: Ignition delay time as a function of the number of C atoms, n, in CnH2n+2
molecules. Closed points correspond to experiment, while open points are calculations
T=1430-1450K, P=0.4 to 0.5 atm. Taken from [2].
In Figure 1.1, these results are shown for both the case where no plasma was utilized
(autoignition) and when the plasma was present (ignition by discharge) as a function of
number of C atoms present in the fuel molecules, CnH2n+2 in a stoichiometric mixture of
fuel and O2 with argon diluent (90%). Through numerical modeling of the system (open
black points), they were able to attribute this reduction in ignition delay time to an excess
of atomic oxygen produced by electron impact dissociation of O2 during the active phase
of the plasma discharge [2].
In addition, Pilla et al [3] were able to stabilize flames in lean conditions using a
nanosecond pulsed discharge with only 0.3% of the maximum power of the flame. Figure
1.2 shows emission spectra from this study as a function of plasma discharge power. OH
3
and CH radicals are direct indicators of combustion. Electronically excited N2 reacts with
hydrocarbons to form radicals which enhance the decomposition of C3H8 which leads to
enhanced combustion. All of these species are shown to be enhanced by the presence of a
plasma. The flame power in each case is 10 kW, so the total plasma power remains very
low in comparison (less than 1%).
Figure 1.2: Emission spectra of flame, taken 12 mm above a bluff body, as a function
of discharge power. Taken from [3].
Another study that looked into the characteristics of plasma development was
conducted by Pai, et al [4] who, using a nanosecond pulsed discharge, were able to
produce glow discharge characteristics (cathode directed streamer followed by potential
redistribution) at temperatures an order of magnitude lower than previously reported. A
glow discharge differs from a spark discharge in that it is very diffuse as opposed to
4
filamentary and this is desirable for some applications (i.e. Transversely Excited
Atmospheric pressure, TEA, lasers). Figure 1.3 shows digital camera images of glow and
spark discharges each created by nanosecond repetitive pulses in atmospheric pressure air
preheated to approximately 1000 K.
Figure 1.3: Digital camera images of nanosecond repetitively pulsed glow and spark
discharges. Anode at top, cathode at bottom. The electrode gap is 4.5 mm. The applied
voltage is 5.5 kV for the glow discharge and 6 kV for the spark discharge. Atmospheric
pressure air preheated to 1000 K. Taken from [4].
In a similar vein, Zaidi, et al. [5] reported the use of microwave energy addition as a
mechanism of flame speed enhancement. The intensity of the microwave energy used
was kept below the threshold for breakdown to occur, and even with that constraint they
found that the flame speed could be enhanced by up to 68%. Theoretical modeling of the
5
experiments led to the conclusion that this enhancement is due to an increase in
temperature in the thin flame front. The temperature rise is, most likely, due to reactions
amongst ions which produce electrons and ions. Figure 1.4 shows a series of video stills
taken over 30 minutes. In each frame, the microwave power is increased and the flame
front moves against the gas flow, towards the burner exit (bottom of the image).
Figure 1.4: Sequence of video stills taken over thirty minutes. The flame front moves
towards the burner exit (against the flow of air/fuel). In this case, methane is the fuel
being used. Taken from [5].
Not only has work been done in understanding the effects of these plasmas, but a
body of literature exists on the topic of the chemistry involved in the plasmas. There are
two primary mechanisms by which nanosecond pulsed plasmas are thought to manipulate
combustion kinetics. The first mechanism, Reforming, refers to the creation of additional
fuel species due to fragmentation of the parent fuel, in this case by electron impact
processes, and subsequent recombination.
Reforming products, such as H2, C2H2,
H2CO+, and CO, may be more reactive at low temperatures than the parent fuels which
leads to plasma enhancement. In general, studies looking into this mechanism are done
in the absence of oxygen and/or in lean fuel/oxidizer mixtures. Figure 1.5 shows a
6
schematic diagram of flame structure, proposed by Kim et al [6]. In this work, the parent
fuel is CH4, which breaks apart to produce radicals in the discharge along Stream A (CH4
+ e- → CH3 + H + e-). These radicals rapidly form intermediate species that move
outwards along the yellow arrows. These intermediate species form a barrier between the
inner flame and the outer flame and Stream B (no flame). This barrier is what causes both
flame stabilization and eventual ignition of Stream B by the intermediate species.
Figure 1.5: Schematic diagram of proposed flame structure [6].
7
The second primary mechanism, Enhancement, is based on the production of
transient radical (such as H, OH, O), and/or excited state species, created (either directly
or indirectly) by electron impact, which has the effect of accelerating low temperature
chemical oxidation. This is the mechanism that has been probed in the current work.
1.2 Introduction to Atomic Oxygen Kinetics in PAC
There have been several studies that show PAC can reduce ignition delay times,
lower ignition temperature [7,8,9], and increase flame propagation speed [5]. However,
there is little experimental data available that probes the key species and kinetics
involved in these enhancement processes. In order to determine the effect of individual
species or pathways, it is necessary to decouple effects from not only competing
reactions, but diffusion and mixing, thermal processes, and hydrodynamics [10].
In 2009, Uddi et al [11] matched experimental results with kinetic modeling to
show that in a nanosecond pulsed discharge approximately half of the total energy
deposited, in this case meV/molecule, resulted in O2 dissociation. The two dominant
mechanisms for atomic oxygen production were found to be electron impact and
collisions with electronically excited nitrogen molecules.
O2 + e → O + O + e
(Eq. 1.1)
N2(A3) + O2 → N2(X1) + O + O
(Eq. 1.2)
As can be seen in Figure 1.6, this led to significant atomic oxygen mole fractions, on the
order of 5.0 x 10-5, in air at P=60 Torr. Equally important, the time scale of atomic
8
oxygen production is on the order of 10s after the pulse, making this one of the first
species created by the discharge that likely plays a key role in later combustion kinetics.
Figure 1.6: Oxygen atom mole fraction as a function of time after a single high-voltage
pulse in air and in ethylene-air at P=60 Torr. Dots and squares correspond to
experimental results while lines correspond to kinetic modeling.
In a separate study done by Stancu et al [12], similar conclusions were reached. In
their study, they used a nanosecond repetitively pulsed discharge in a pin-to-pin
configuration in an atmospheric pressure plasma of air, preheated to 1000K. Atomic
oxygen formation was found to occur primarily through dissociative quenching of O2 by
N2(B) and N2(C) with N2(A) being less important. In order to further understand the
pathways involved in this mechanism, they later studied the absolute concentrations of all
four species, N2(A,B,C) and O, through various laser diagnostic techniques [13].
Formation of O atoms coincided with loss of N2(B) and N2(C), and the production of all
9
three species were found to be related to the applied discharge voltage (as shown in
Figure 1.7).
Figure 1.7: Measurements of O, N2(B), and N2(C) in air discharges as a function of
applied discharge voltage [13].
Sun et al [14] have studied the effects of atomic oxygen production on the
extinction limits of methane diffusion flames at low pressure conditions. The authors
used what is known as a counterflow diffusion flame. This is an essentially one
dimensional geometry where two gas flows (oxidizer and fuel) are introduced opposite
one another as in Figure 1.8. The two gas flows produce a stagnation plane (shown in the
center of Figure 1.8) which allows for diffusion between them. This diffusion leads to the
ignitable flame. Using Two Photon Absorption Laser Induced Fluorescence (TALIF) for
the measurement of O atoms, they were able to determine that both the atomic oxygen
concentration and plasma streamer temperature had a large effect on extinction limits.
10
Figure 1.8: Schematic diagram of a diffusion flame.
Through numerical modeling of this system, they were able to determine the specific
temperature and concentration range where significant kinetic enhancement is possible
[14]. Figure 1.9 shows the results of their extinction strain rate measurements as a
function of fuel mole fraction. Extinction strain rate is calculated, based on the gas flow
velocities of fuel and oxygen, Uf and UO, as well as the species densities, pf and pO, and
the separation between burner nozzles, L [14].
11
Figure 1.9 shows that in the presence of a plasma discharge, the extinction strain rate
goes up, compared to the no plasma case. At higher repetition rates, this difference is
even more apparent.
Figure 1.9: Dependence of extinction strain rates as a function of fuel mole fractions
for various plasma repetition rates. Taken from [14].
These same authors later expanded upon this study to look at the kinetics of CH4
oxidation at low temperatures. Through a combination of TALIF, Fourier Transform
Infrared Spectroscopy (FTIR), and Gas Chromatography (GC) they were able to
characterize the products coming from the CH4/O2 plasma. These results, when used to
validate a kinetic model, give critical insight into understanding the formation of atomic
oxygen and its role in plasma enhancement of oxidation. Specifically, atomic oxygen was
12
produced mainly through electron impact and collisional dissociation with electronically
excited molecules with O2, as described previously [15]. Figure 1.10 shows extinction
strain rate data from Figure 1.9, as well as additional data points and computational
results. It can be seen that the experimental and computational results are in good
agreement.
Figure 1.10: Extinction strain rates as a function of fuel mole fractions with and
without plasma presence (solid data points), in heated gas flows (open data points),
alongside computational results (lines). Taken from [15].
Lastly, Popov [16] studied the effects of non-equilibrium excitation on ignition of
hydrogen/oxygen mixtures. In a meticulous computational study of the kinetics involved
between these species, it was found that atomic oxygen, more than any other major
combustion species, has a large effect on the occurrence of ignition. Figure 1.11
summarizes the results in regards to induction time, i.e. the time between when fuel is
13
added and when combustion begins to occur. When atomic oxygen is preferentially
added to the system, this induction time is reduced by more than 70%.
Figure 1.11: Relative reduction in induction time in stoichiometric H2:air with
preferential addition of various species. Taken from [16].
The author went on to systematically determine the reactions and kinetic rates that
dominate in H2:O2 mixtures. These 44 reactions (22 forward, 22 reverse) have been used
extensively in the modeling of the chemistry in the present work and will be summarized
in more detail in a later chapter.
1.3 Introduction to Molecular Oxygen Research: Singlet Delta Oxygen (SDO)
Singlet delta oxygen (SDO), a1g, is of particular interest in PAC systems because
it has a very long radiative lifetime, more than 4500 seconds. Due to it having a low
14
energy state, 0.98eV, it is present in almost all oxygen containing plasmas [17]. In
addition, SDO is spin forbidden from transitioning to the ground state which makes it
likely to be available for oxidation reactions [17].
In the Popov paper discussed above [16], the author went on to study the effect
that SDO has on ignition and flow velocity. Figure 1.12 shows experimental (from [18])
and computational results of induction time in an H2:air mixture as a function of initial
temperature of the gas flow. The three cases that are listed are as follows; (1) is the model
without any SDO reaction rates, (2) is with 1% of the O2 converted to SDO with
quenching of the SDO by H2 being considered, and (3) is with 1% of the O2 converted to
SDO, but without any quenching of the SDO by H2 being considered. It can be seen that
at high temperatures (left side of the graph), curves (2) and (3) are very similar, implying
that quenching due to H2 is insignificant. At lower temperatures, this is not the case and
taking quenching into account allows the model to fit the experimental data much more
closely.
15
Figure 1.12: Induction time of ignition in a stoichiometric H2:air mixture at 0.5
atm. Data points correspond to experimental data from [18]. The curves correspond to
modeling results: (1) no SDO addition, (2) 1% SDO addition with H2 quenching, (3) 1%
SDO addition without H2 quenching.
Ionin et al. recently presented an in-depth topical review, detailing both
experimental and theoretical studies on SDO and its application for electric chemical
oxygen-iodine lasers (COILs). The active component of a COIL laser is atomic iodine
emitting photons from changing electronic states. Prior to this, SDO molecules undergo
an energy transfer process with ground state iodine to produce the necessary excited state
iodine (2P1/2) [19].
O2(a1g) + I(2P3/2) → O2(1) + I*(2P1/2)
I*(2P1/2) → I(2P3/2) + h

The authors were able to computationally determine maximum SDO yield values for a
variety of conditions that can be experimentally probed in their system. Figure 1.13
16
shows the results of these calculations, with each of the four curves corresponding to
different gas mixtures. It is important to note that in all of these conditions, there is no N2
present. The reasons for this will be discussed in further detail in Chapter 5.
Figure 1.13: Maximum SDO yield as a function of E/N. (1) O2:Ar = 1:1, (2) O2:Ar:H2 =
0.95:1:0.05, (3) O2:Ar:D2 = 0.95:1:0.05, (4) O2:Ar:CO = 0.9:1:0.1. Taken from [19].
Computationally, there have been a plethora of studies on SDO effects on
combustion systems. Of particular relevance to this work, Starik et al. showed that SDO
could potentially be used to reduce ignition delay time and temperature in a supersonic
H2/air flow [20,21]. In their study, they focused on SDO molecules that were produced
by either laser radiation or an electric discharge. Figure 1.14 shows some key results from
their study. Each curve corresponds to a different initial SDO mole fraction: (1) 0, (2)
0.01, (3) 0.03, (4) 0.05, and (5) 0.10. As the initial concentration of SDO molecules
increases, the temperature raises more rapidly which may lead to a quicker onset of
ignition.
17
Figure 1.14: Variation of temperature as a function of time in a CH4/air mixture, =3,
T0=100K, P0=105Pa. Taken from [20].
Experimentally, there is very limited literature on the effect of O2(a1g) on
combustion systems. In 2007, Skrebkov et al. measured electronically excited OH
radicals behind a shockwave in an H2/O2 system. They accurately modeled these results
using a mechanism that included O2(a1g) kinetics. It was found that OH radicals may be
a good indicator of enhancement due to O2(a1g) because of a notable increase in radicals
that were formed [22]. Figure 1.15 summarizes their computational results for a variety
of species. SDO is indicated as O2*. It can be seen that the increase in SDO corresponds
to an increase in OH, as well as OH*.
18
Figure 1.15: Concentration of species as a function of time calculated in a mixture
with V0=1205m/s and P=50 Torr.
In addition to the Skrebkov study, Ombrello et al [10,23] isolated the kinetic effects
resulting from SDO and O3 from other species produced in the plasma. They found that
even at low concentrations, on the order of parts per million, each of these species can
significantly enhance a variety of flame parameters, including flame liftoff height.
Flames become lifted from the burner when the gas flow velocity is faster than the flame
velocity, and this is a major issue with regards to flame stability. Figure 1.16 shows the
effect that SDO and O3 concentration can have on flame liftoff height. With less than
6ppm of SDO, the liftoff height can be increased by ~7mm [23]. In this particular study,
the fuel and oxidizer are mixed in such a way as to have a very short residence time prior
19
to entering the flame region. This helps to enhance separation of kinetic effects
(reactions) and gas flow effects.
Figure 1.16: Experimental flame liftoff height as a function of SDO and O3
concentration. Plasma power is 80 W. The error bars correspond to experimental
uncertainties in both concentration and flame liftoff height. Taken from [23].
Kinetic modeling of this system exposed the need for further understanding of
collisional quenching rates of excited species with hydrocarbons [23]. In fact, the authors
point out that verified quenching rates for SDO with hydrocarbon species at intermediate
temperatures are completely absent from the literature. Figure 1.17 shows reaction rate
modeling data for SDO quenching according to a variety of sources. First, hydrogen
quenching has been reported by two sources; Borrell et al [24] and Popov [16]. Both of
these sources predict a temperature dependence on the quenching rate that is
20
approximately Arrhenius. For hydrocarbon quenching, however, Ombrello et al
calculated Arrhenius temperature dependences based on room temperature quenching
values. The "Quenching A" curves correspond to estimated activation energies of
15kJ/mole and 30kJ/mole, while the "Quenching B" curve corresponds to an estimated
activation energy of 48.6kJ/mole. The details of why each of these activation energies
were chosen is given in [23]. It can be seen, though, that there is quite a bit of disparity in
these calculations and experimental verification is needed.
Figure 1.17: Arrhenius temperature dependence of SDO by H2 and C2H4. Taken from
[23].
1.4 Discharge Geometries
As can be seen from the previous sections, a large body of literature exists in the
field of plasma assisted combustion when it comes to nanosecond pulsed discharges. A
21
wide variety of plasma and flame conditions are studied regularly, and each contains
distinct and complex chemistry to be considered. Prior to being able to study the
chemistry that occurs, understanding of the discharge itself is vital. A wide variety of
discharge conditions and geometries are available for study. The nanosecond pulsed
discharges that are utilized can vary in voltage and current characteristics, duration of
pulse, duty cycle, shape of pulse, and several other parameters. In addition to the
nanosecond pulsed discharge being variable, the geometry of the plasma is variable and
this changes the conditions of the system being studied as well. The geometry of the
plasma is defined, in large part, by the shape and size of the electrodes that are used. In
this dissertation, three geometries are studied; plane-to-plane, pin-to-plane, and spherical.
The differences between each of these, and the characteristics they correspond to will be
discussed in more detail in later chapters.
1.5 Objectives
The goal of this work is to develop a better understanding of the non-equilibrium
kinetics involved in plasma assisted combustion and ignition. Those pathways that are
directly involved in atomic and molecular oxygen kinetics are of particular interest due to
the fact that oxygen is one of the first species created in the discharge and initiates chain
branching and combustion mechanisms. The body of literature on temperature dependent
atomic oxygen kinetics in the low temperature regime (< 1000 K), prior to this work, was
significantly lacking in experimental data. The primary focus of this study is to develop
an extensive set of experimental results of atomic oxygen concentration in low
22
temperature, nonequilibrium plasmas. These results can be useful in the PAC and
combustion communities in leading to understanding of the dominant mechanisms in the
low temperature regime. The secondary focus of this study is to compare experimental
results with existing kinetic models for the purpose of illuminating the specific reactions
and their rates that dominate in these systems. Specifically,
Chapter 2 presents a detailed description of the laser diagnostics involved in
measuring time-dependent O atom number density using Two Photon Absorption
Laser Induced Fluorescence (TALIF) along with a description of the high voltage
pulse generators used in this study. Additionally, a description of SDO
characterization by emission spectroscopy, Ignition Delay Time measurements by
OH emission, and plasma imaging using an ICCD camera are also given.
Chapter 3 gives a description of the plasma discharge and chemistry kinetic
models used in to model the data in this study.
Chapter 4 presents atomic oxygen Two Photon Absorption Laser Induced
Fluorescence (TALIF) measurements in a plane-to-plane discharge geometry in
Ar/O2 gas mixture with H2 and C2H4 fuels.
Chapter 5 presents the results of emission spectroscopy measurements for SDO
number density with both helium and argon buffer gas, as well as a description of
the effect of NO2 titrant on the SDO concentration. In addition, atomic oxygen
number density results from TALIF measurements in various gas mixtures both
with and without SDO addition to the system, respectively.
23
Chapter 6 presents atomic oxygen TALIF measurements in a pin-to-pin electrode
geometry in air with C2H4, and H2 fuels.
Chapter 7 presents the results of atomic oxygen TALIF measurements in a pin-toplane streamer discharge at atmospheric pressure in Air/CH4, Air/C2H4, and
Air/C3H8 gas mixtures.
Chapter 8 summarizes and gives conclusions for all the results presented in
chapters 3 through 7. Additionally, suggestions for future work are suggested.
24
Chapter 2
Experimental Instrumentation
2.1 Introduction
In order to understand atomic and molecular oxygen kinetics involved in PAC, it
must be possible to experimentally determine both the dynamics of these species as well
the characteristics of the plasma involved. The latter is done by taking ICCD images of
the plasma to ensure that it has a volumetric and diffuse nature. Current and voltage
measurements by the use of probes lead to quantitative coupled energy to the plasma per
pulse. To understand atomic oxygen, Two photon Absorption Laser Induced
Fluorescence (TALIF) measurements give spatially and temporally resolved absolute
number density. Molecular oxygen in its lowest lying electronic energy level (singlet
delta oxygen, SDO) can be created using an RF side discharge and injected into the main
gas flow. The number density of this species can be measured by IR emission
spectroscopy and its effect on ignition delay time then probed by OH emission
spectroscopy. A description of each of these techniques is given below, while details
particular to each of the electrode geometries used in this study (plane-to-plane, spherical,
and pin-to-plane) will be discussed in Chapters 4, 6, and 7, respectively.
25
2.2 Plasma Creation by High Voltage Pulser Units
The nanosecond pulsed high voltage discharges used in this study were created
using custom designed high voltage nanosecond pulse generators [25,26]. For the
measurements conducted in the plane-to-plane and spherical electrode configurations
(described in Chapters 4 and 5), the pulse generator was developed by Takashima et al at
The Ohio State University [25]. It produced pulses with a peak voltage of ~20kV per
pulse, and a pulse duration of ~70 nanoseconds FWHM. These high voltage pulses were
created using a magnetic pulse compression method. The pulse output voltage is varied
by adjusting the input DC voltage (up to 800 V, depending on conditions in the discharge
cell).
Figure 2.1 shows typical positive and negative pulse outputs that can be
approximated as Gaussians.
Figure 2.1: Typical positive and negative polarity voltage pulse waveforms used to
sustain a repetitively pulsed discharge. 20% O2 in argon gas mixture, P=40 Torr, and a
pulse repetition rate of =40kHz.
26
For all of the measurements taken in the plane-to-plane electrode configuration,
the pulse generator was used in burst mode operation and a repetition rate of 40 kHz,
with alternating pulse polarity. Each "burst" of pulses could be triggered a single time or
repeated at 10Hz to match the laser diagnostics systems (shown schematically in Figure
2.2). This was done for two reasons. First, so that atomic oxygen measurements could be
taken both as a function of time after the pulse and burst size. Second, because the planeto-plane electrode cell has a quartz dielectric layer between the electrodes and the gas
flow, there was significantly less energy coupled to the plasma per pulse, compared to
plasmas formed without such a dielectric barrier. Utilizing burst mode operation was a
way to partially nullify that limitation.
Figure 2.2: Schematic diagram of pulser output in burst mode operation.
Unlike in the plane-to-plane electrode configuration, the spherical electrodes had
no dielectric barrier between them and the gas flow. For this reason, it was possible to
take measurements in single pulse operation. This allowed for a closer analysis of the
27
characteristics of each pulse. Measurements were taken with a pulser repetition rate of 60
Hz which was found to give greater stability of the plasma, while the laser repetition rate
was kept at 10 Hz.
The high voltage pulse generator used in the pin-to-plane electrode configuration,
to be described in Chapter 7, was different than that described above. This pulse
generator was nearly identical to that developed by Singleton et al at the University of
Southern California [26]. Changes were made and described by Pendleton et al [28] for
the purpose of timing precision and repetition rate control [27].
As in the spherical electrode geometry, there was no dielectric barrier between the
electrodes and the gas flow. The pulse generator was operated in single pulse mode at 10
Hz, to match the laser repetition rate. Figure 2.3 shows a typical Gaussian-like voltage
waveform with a variable peak voltage of 10-60 kV and a pulse duration of ~20
nanoseconds FWHM.
30
Voltage
Current
15
10
10
5
Current (A)
Voltage (kV)
20
0
0
0
10
20
30
Time (ns)
40
50
60
Figure 2.3: Typical voltage and current waveforms used in discharge. Air, P=760 Torr.
28
2.3 Plasma Imaging and Coupled Energy Measurements
Before conducting any quantitative measurements on kinetics in low-temperature
plasmas, it is necessary to confirm that the discharge is volumetric, diffuse, and stable
throughout the entire burst of pulses. If it is not, arc filaments (or "hot spots") can form
and lead to thermal ignition which skews the results. Qualitative measurements of the
plasma's uniformity were conducted using a Princeton Instruments PIMAX-ICCD camera
with UV lens (UV-Nikkor 105 mm f/4.5, Nikon). Triggering of the camera was done by
the same Stanford Research Systems delay generator used to trigger the high voltage
pulse generator, ensuring synchronization of the measurements.
In order to look at the plasma uniformity during a pulse, a camera intensifier gate
of 5s with a gain of 150 was wrapped around a single pulse (within the burst if
applicable). In air/fuel mixtures, the emission in these images is mainly comprised of N2
second positive emission (N2 C3u → N2 B 3g) and, to a lesser extent, OH A→X (0,0)
emission. In argon or helium buffer gas with oxygen and fuel mixtures, however, there is
no N2 in the system, causing the images, dominated by Ar* emission, to be much fainter
(see Chapter 4).
In addition to ICCD images, it was important to know how much energy was
being coupled into the plasma. In principle, this is a straightforward calculation resulting
from the measurement of the current and voltage applied to the system, shown in
Equation 2.1.
29
For the pin-to-plane electrode geometry, a Northstar PVM-5 voltage probe and
Pearson 6223 current probe were used. For the spherical electrode geometry,
measurements were conducted using a Tektronix P6015 voltage probe and Pearson 2877
current probe.
This method was attempted for the plane-to-plane electrode geometry, but due to
the large amount of reflected signal (from the dielectric barrier) compared to the small
amount of coupled energy, these measurements were not found to be accurate. Instead,
current and voltage measurements were conducted using a custom designed capacitive
voltage probe and shunt current probe, described in detail by Takashima et al [29]. The
capacitive voltage probe setup functions by attaching probes to each electrode and
subtracting the two waveforms so as to leave just the voltage waveform coupled to the
load (i.e. the gas mixture). The inductive current probe setup functions similarly in that
two waveforms are collected and subtracted to give the current coupled to the load. The
two measurements are separated via transformers. While this is an inherently intrusive
technique, it allows for significantly more accurate measurements of current and voltage
in the plane-to-plane electrode configuration. A schematic diagram of this apparatus is
shown in Figure 2.4.
30
Figure 2.4: Schematic diagram of custom designed shunt current and capacitive voltage
probe. Taken from [29].
2.4 Oxygen Atom TALIF Apparatus and Calibration
Atomic oxygen concentration was measured by Two photon Absorption Laser
Induced Fluorescence (TALIF). This is a frequently used technique for both
concentration and temperature measurements based on the simultaneous absorption of
two photons from a lower state (usually the ground state) to a higher excited state. The
single photon fluorescence down to an intermediate state can then be detected. The main
disadvantage of TALIF is that since it is a two photon process, the signal is much weaker
than in single photon Laser Induced Fluorescence, LIF. However, because this is a
second order process, it is possible to probe atoms that absorb via single photon in the
vacuum UV range of the electromagnetic spectrum. This region is particularly difficult to
probe because of the large amount of molecular absorption from air. The details of
31
TALIF have been described in detail previously by Niemi et al [30,31]. Figure 2.5 shows
an energy level diagram for the two photon absorption and fluorescence for both atomic
oxygen and xenon, which is used as a calibration gas due to the similar wavelengths. The
transition between the 2p3P ground state and the 3p3P excited state is two photon allowed
with a wavelength of 225.7nm. Fluorescence by a single photon process to the 3s3S state
is then measured.
Figure 2.5: Simplified TALIF energy level diagram for oxygen (left) and xenon (right).
There are a few inherent assumptions that need to be made to use this technique.
First, it must be assumed that a negligible fraction of O atoms are being ionized by a third
225.7nm photon [32]. In order to determine this, analysis of the rate equations for the two
and three photon processes must be analyzed. Figure 2.6 shows an energy level diagram
with all of the possible transitions labeled.
32
Figure 2.6: Energy level diagram showing all the possible transitions for an atom excited
to the n* state.
The following three equations describe the rates for ionized atoms, n+, excited
atoms in the 3p 3P state, n*, and ground state atoms in the 2p 3P state, n0. The two photon
absorption cross section from the ground state to the excited 3p 3P state is (2) (which is
the calculated cross section from Bamford et al multiplied by the laser intensity [32]), the
cross section for ionization from the 3p 3P state is . The laser intensity is given by I, the
rate constant for quenching from the 3p 3P state is given by kq and the number density of
quenching species is given by Q. The spontaneous emission decay rate is 1/ [33].
33
(Eq. 2.2)
(Eq. 2.3)
(Eq. 2.4)
Using the steady state approximation, an equation for the concentration of n* can
be found, as shown in Equation 2.5. The four processes responsible for n* loss are in the
denominator. In order for ionization to be considered negligible, the combined quenching
and spontaneous emission rates must dominate. This can be experimentally verified by
measuring the TALIF signal as a function of laser intensity. When quenching and
spontaneous emission dominate, the signal increases quadratically with laser intensity.
Conversely, when ionization dominates, the signal becomes linear with laser intensity.
(Eq. 2.5)
It should be noted that the TALIF signal, STALIF, is proportional to [n*]SS ∙a21.
This leads to Equation 2.6, which is the full equation for calculating number density of O
atoms from the TALIF measurement. The first two terms are constants that quantify the
transmission characteristics of the collection optics, , and the quantum yield of the
detector (photomultiplier tube), V is the collection volume. The correction factor for
loss from the neutral density filter used during the calibration (described later) is given in
gND. The fluorescence quantum yield is a21, (2) is the two-photon absorption cross
section, g() is the line shape function, G(2) is a photon statistical factor [30], F(T) is the
Boltzmann factor for the lower level of the two photon absorption, NO is the ground
34
electronic state number density, and I0(T) is the time-dependent laser intensity at the
measurement location.
(Eq. 2.6)
While all of these terms can, in principle, be calculated or accounted for in the
experiment, obtaining quantitative results via these calculations is extremely difficult. For
example, the photon statistical factor (G(2)) and transmission characteristics of the
detection optics (would need to be determined precisely. As an alternative, the atomic
oxygen signal can be put on an absolute scale through comparison with signal from
xenon gas. As can be seen in Figure 2.5, the two photon absorption for xenon occurs at a
wavelength of 224.31nm (as opposed to 225.7nm for atomic oxygen) and the subsequent
fluorescence is then at 834.9nm (as opposed to 844.6nm for atomic oxygen). Because of
these similar wavelengths, the experimental setup can be kept the same between
measurements, causing several of the constant terms in Equation 2.6 to be the same for
the analogous SXe equation. Taking the ratio of these two, cancelling out the constant
terms, and rearranging to solve for No gives Equation 2.7.
(Eq. 2.7)
SO and SXe are the spectrally integrated signal levels for atomic oxygen and xenon,
respectively. NXe is the number density of xenon. In the experiments presented here, the
voltage applied to the photomultiplier tube is kept constant (so the gain, and therefore
35
quantum efficiency, is constant). Since xenon signal levels were inherently much higher,
by two to four orders of magnitude, a neutral density filter was used to attenuate the
signal in order that the photomultiplier tube gain (HV) could be maintained constant. It is
because of this that the gND term exists in the above equation.
The fluorescence quantum yield, a21, is actually a branching ratio, according to
Equation 2.8 of the quenching and spontaneous emission processes. In Equation 2.8, A21
is the Einstein coefficient for spontaneous emission (see the Figure 2.6) for the transition
being observed, A is the Einstein coefficient for all spontaneous emission transitions that
are selection rule allowed (in this case, A21=A), and Q is the sum of quenching
contributions from each major species.
(Eq. 2.8)
The sum of quenching terms, Q, is a product of number density for each quenching
species and their quenching coefficient, as shown in Equation 2.9.
(Eq. 2.9)
In the present work, the primary quenching species considered were N2 (or Ar, depending
on the gas being used), O2, and either H2, CH4, C2H4, or C3H8 (depending on which fuel
was being studied). In addition to these species, H2O should, in principle, be considered
as the quenching cross section is significantly larger than that of the other major species,
on the order of ~5-10 times. However, for the conditions of the experiments performed in
36
this work, H2O production was predicted to be quite low and so has not been included in
the calculation of quantum yield used to infer atomic oxygen number densities.
Table 2.1 shows quenching coefficients available in literature for all of the species
discussed above as well as xenon. It can be seen that there is significant variance,
particularly for O2 and argon. The values assumed for the results presented throughout
this dissertation are indicated by an asterisk.
37
Species Quenching Rate, kq (10-10cm3/s)
O atoms
Ar
Reference
Used?
0.21
Bittner [34]
0.14
Niemi [30]
0.25
Niemi [31]
9.3
Niemi [31]
6.3
Bittner [34]
8.6
Bamford [32]
9.4
Niemi [30]
*
10.9
Niemi [30]
*
6.5
Bittner [34]
CH4/C2H4
5.5
Bittner [34]
*
H2O
49
Bittner [34]
*
25
Gasnot [35]
5.9
Niemi [31]
O2
H2
N2
4.3
*
Bittner [34]
-10
3
Species Quenching Rate, kq (10 cm /s)
Xenon
Xe
*
Reference
Used?
3.6
Niemi [30]
*
4.2
Alekseev [36]
4.3
Bruce [37]
Table 2.1: Summary of literature quenching coefficients relevant to this work. Values
used for this work are indicated by asterisks. Note that C2H4 value is assumed equal to
that of CH4.
The experimental apparatus used for TALIF measurements is shown
schematically in Figure 2.7. The second harmonic output (532nm) of an injection-seeded,
Q-switched Nd:YAG laser (Continuum Surelite III or Precision 8010) is used to pump a
38
tunable dye laser (Continuum ND6000). The output of the dye laser (~619nm) is mixed
with the third harmonic output (355nm) of the Nd:YAG laser in a Type 1 BBO crystal,
generating the necessary 226nm UV beam. An autotracker device (InRad) is used to
control the BBO phase matching angle as the dye laser is scanned over the two photon
absorption transition. The UV beam energy is separated from residual 355 and 619nm
light by a series of four turning prisms.
Figure 2.7: Schematic diagram of the TALIF setup.
The energy in the UV beam is kept below 500J per pulse and is focused into the
plasma using a 300 millimeter focal length, plano-convex lens. The single photon
fluorescence signal (844nm, see Figure 2.5) is then 1:1 imaged (f/2) onto a standard
39
photomultiplier tube (PMT) after passing through an 80nm bandpass filter centered at
850nm. An important detail to note is that while the collection optics are located at a
ninety degree angle from the center of the plasma discharge region, the 226nm beam is
focused a few millimeters past that point. This is done to ensure that saturation is not
occurring. The PMT signal goes through 25x pre-amplification process. A photodiode is
used to detect diffuse UV light during the experiment as a way of providing a
normalization channel that can be continuously monitored. The timing of the experiments
is controlled using a delay generator from Stanford Research Systems that triggers the
pulser, laser, and oscilloscope in order to have consistent, variable signal delay times with
respect to discharge initiation.
2.5 Characterization of Singlet Delta Oxygen
As will be discussed in Chapter 5, an additional study on the potential role of
electronically excited oxygen molecules in plasma assisted combustion was conducted.
The lowest lying excited state of O2 is the singlet delta oxygen, a1g or SDO, state. The
study of this species consisted of three main components: creation of SDO and
measurement of its' concentration, measurement of the effect of SDO on ignition delay
time, and measurement of the effect of SDO on atomic oxygen kinetics.
In order to create the SDO molecules, a capacitively coupled radio frequency
(RF) discharge was used. The power supply for this discharge was produced by ENI,
while the manual impedance matching apparatus was produced by MFJ Enterprises. The
SDO molecules were then measured by IR emission spectroscopy, using an Optical
40
Multichannel Analyzer (OMA). The OMA had a 0.5 meter IR spectrometer, with 600
lines per millimeter grating blazed at 1m. Roper Scientific makes the liquid N2 cooled,
1-D array, 1024 pixel, InGaAs PDA camera. The emission signal was collected using a
one meter long optical fiber (Thor Labs) with a 1.3 inch diameter collimator that could be
positioned at various locations along the flow. The same OMA system has been used
previously to measure SDO yield in a DOIL laser discharge [38].
Figure 2.8: Schematic diagram of the calibration of SDO yield measurements.
Calibration of the SDO yield from relative to absolute values is done using a
blackbody source from Infrared Systems (IR-564) set to a temperature of 800 K. In this
calibration, the SDO signal and blackbody signal are both collected in a given
wavelength range and can be used to find the SDO number density. Calibration by
blackbody radiation is experimentally useful since it is possible to ensure the collection
solid angle for both signals is the same, essentially causing this term to fall out of all the
resulting equations. Consider diffuse radiation from a volume that is incident on a
41
detector of size D (centered) and at a distance, d, as represented in Figure 2.8. The
fraction of radiation that is collected can be calculated.
(Eq. 2.9)
The total number of photons released from SDO is a function of the number density,
NSDO, the volume, dV, and the Einstein coefficient, A12 [24]. Combining these terms over
a specific wavelength range, , with the collection time, tSDO, and the photon to
intensity counts conversion efficiency term, C, gives an equation for intensity counts
where all the terms are known except for NSDO and C.
(Eq. 2.10)
The photon to intensity counts conversion efficiency term can be found through
calibration with the blackbody source. The emission intensity of the blackbody spectra
can be calculated using the wavelength range and known temperature, I(,T). The
fraction of collected emission, FBB, is found through an analogous calculation to the
fraction of SDO.
(Eq. 2.11)
Combining these terms with the collection time, tBB, and area, dA, as well as some
constants gives an equation where only the photons to intensity counts conversion
efficiency term is unknown.
42
(Eq. 2.12)
This method of calibration works well for SDO emission spectroscopy
measurements as long as the volume of SDO is small, the black body aperture is small,
and the detector distance is significantly larger than the detector diameter. This is due to a
breakdown in the assumptions used to develop the integrals in Equations 2.9 and 2.11.
Basically, instead of treating it as a point source, it would become necessary to formulate
a way to integrate over a point source at any arbitrary location and then integrate over the
entire collection volume/area. In addition, this would cause the radiation angle and
intensity to vary with location, so the photon to counts conversion term, C, would need to
be moved inside of the integral. Because of these complications, the source volume for
SDO and area for the blackbody source are kept significantly smaller than the detector
diameter in these experiments.
2.6 Measurement of Ignition Delay Time
To test for ignition in the flow channel, time resolved UV emission was measured
by observation of the spontaneous OH A → X transition during a single burst of pulses
using a PMT and UV filter (310nm +/- 2nm) for detection. Response time of these
emission diagnostics was approximately 10s and was controlled using a variable
terminator resistor set to 50k. Figure 2.9 shows a typical emission trace where ignition
was observed. As can be seen, starting around 10ms into the burst, OH emission between
the pulses no longer decays to zero and this leads to a "footprint" in the horizontal axis.
43
Ignition delay time was determined as the time when detectable OH emission was
observed in the "footprint."
Figure 2.9: Time-resolved OH emission.
44
Chapter 3
Nanosecond Pulse Discharge and Plasma Chemistry Model
3.1 Introduction
In order to gain insight into the kinetic mechanism of plasma and chemical
fuel oxidation, modeling of the nonequilibrium plasma and resulting chemistry was
conducted. These models, based on those previously developed at The Ohio State
University [39], can essentially be broken down into two basic steps. First, a
nonequilibrium pulsed discharge model which describes the plasma kinetics during the
pulse and at very short time scales (~1-100nanoseconds) was used. Second, the post
discharge pulse regime, dominated by fuel/oxidizer kinetics, were modeled at longer time
scales (up to ~10 milliseconds) using nonequilibrium air plasma chemistry [40],
expanded to include hydrocarbons and hydrogen dissociation processes in the plasma [16,
41, 42]. In both time regimes listed above, the plasma-chemical reactions were solved
numerically as coupled Ordinary Differential Equations (ODEs). In the air/fuel and
Ar/O2/fuel mixtures studied in Chapters 4 and 5, this resulted in approximately 112
species and 201 reactions to be solved for H2 fuel and 180 species and 1361 reactions to
be solved for C2H4 fuel [43,44].
45
3.2 Plane-To-Plane Pulsed Discharge Model
The pulsed discharge model was used for determining the direct effect of the high
voltage discharge on the gas flow during the pulse and shortly after, up to approximately
100 nanoseconds. During this time, electron impact processes such as dissociation of
molecules, loading of electronically excited states, loading of vibrationally excited states,
and ionization are the dominant mechanisms. The key properties that determine how
these processes develop are the number density of electrons produced in the discharge, ne,
and the electron energy distribution function, EEDF. The approach developed and used
by Adamovich et al [39] used in this study is very similar to that of Nikandrov et al [45]
and Tsendin et al [46].
In general, understanding the effect of the discharge on the gas comes down to
understanding the reduced electric field, E/n, and how it affects the electron temperature,
Te. As the electric field (E) is increased, the acceleration of the electrons is increased and
when E is decreased, the opposite occurs. Increasing the number density (N) decreases
the mean free path between collisions. Electron energy scales as the ratio, E/N, which is a
measure of the kinetic energy acquired by free electrons due to the field, in the time
interval between collisions.
Quantitatively, calculation of the electron energy distribution function (EEDF) is
done by numerically solving the Boltzmann equation. A general formulation of this
equation is shown below where c is the vector electron velocity,
operator with respect to positional space,
is the gradient
is the gradient operator with respect to
velocity space, E is the electric field, e and m are the charge and mass of the electrons,
46
and B is the magnetic field. The electron velocity distribution function (which contains
both isotropic and electric field perturbed components) is given by f. Simplifying this
equation with respect to space and expressing in terms of energy gives f as the EEDF
[47].
(Eq. 3.1)
The plasma region is joined to the walls of the test cell by a sheath layer. A set of
continuity equations is used to describe the pulse discharge between the electrodes. A full
kinetic model of this sort is extremely computationally intensive, so the Ohio State model
uses a hydrodynamic, or drift-diffusion, approximation described by Macheret et al [48]
to develop a set of equations that can be solved analytically. In the following equations,
n+ and ne are the ion and electron number densities, vi is the ionization frequency equal to
e|E| where  is the Townsend ionization coefficient,  is the electron-ion
recombination rate coefficient, and D+ and De are the diffusion coefficients, equal to
+kbTi/e and ekbTe/e, respectively. The electron and ion mobilities are e and +, which
characterizes the velocity that the electrons or ions are pulled due to the electric field. The
electric field and potential are given by E and
, respectively. The constants e,
, and kb
are the charge on an electron, the permittivity in free space, and Boltzmann's constant,
respectively. Finally, + and e are the drift-diffusion fluxes which are each a combined
term consisting of the classical diffusion gradient of the charged particles and the drift
velocity due to the electric field.
47
(Eq. 3.2)
(Eq. 3.3)
(Eq. 3.4)
Equations 3.2 and 3.3 describe how the ion and electron number densities change
in both time and space due to diffusion, collisions, and recombination. These equations
are setup so that a positive value for drift fluxes (+ and e) results in ions or electrons
drifting out of the system. Equation 3.4 is Poisson's Equation, where the electric field is
the change in potential with respect to space. Note that when n+ = ne (locally neutral) the
field gradient is zero. In the cathode "sheath" region, which is modeled as 1D and is
discussed below (see Figure 3.1), n+ > ne leading to a large gradient (drop) in the electric
field.
The input parameters of this model are those terms used to solve the above
equations under the conditions being studied experimentally. This includes the current
and voltage which are used to determine, ultimately, the EEDF and number density of
electrons via the coupled power and effective E/n. Two methods were used for
determining the input parameters (discussed below): discharge modeling or experimental
measurement of the input parameters.
A diagram of the discharge parameters assumed by the model for determining the
input parameters is shown in Figure 3.1. In the plane-to-plane geometry, the electric field
is oriented perpendicular to the gas flow. The plasma region makes up most of the area
between the dielectric barriers. The cathode sheath is a thin, positively charged layer that
48
connects the "neutral" plasma with the dielectric barrier surface along the cathode. A
similar layer does not form along the anode wall because electrons have a much higher
temperature and smaller mass than ions, allowing for greater mobility through the
plasma.
Figure 3.1: Schematic of discharge geometry
A dielectric barrier was used between the electrodes and the gas flow (shown
schematically in Figure 3.1). This was done to make the discharge more uniform over a
large area (1.4 cm wide by 6.5 cm long and 1.0 cm deep), though this makes modeling of
the plasma region more difficult. Basically, the dielectric barriers cause a uniform charge
to build up as in a capacitor, which acts to create a uniform electric field. Figure 3.2
shows a typical model result for the evolution of the electric field (kV/cm) with respect to
time in the bulk plasma region. The blue curve corresponds to the applied voltage that is
49
assumed to follow a Gaussian shape in time, with a HWHM of ~12 nanoseconds, where
for modeling purposes, the maximum in applied voltage is assumed to occur, arbitrarily,
at t=100 nanoseconds. The red and black curves are numerical and analytical results,
respectively, for the electric field in the plasma which can be seen to drop off steeply
around 88 nanoseconds. This "breakdown" occurs due to charge accumulation in the
sheath layer and corresponds to the onset of plasma in the discharge region.
Figure 3.2: Time dependent electric field in the plasma region as a function of time.
Figure 3.3 shows a typical numerical prediction of the time dependent electric
field in kV/cm as a distribution across the electrode gap for a test case of N2 at 60 Torr.
The cathode corresponds to x = 0, and the anode corresponds to x = 1.0 cm. The sheath
50
layer is approximately 0.1 cm wide, starting from the cathode, and it can be seen that it is
in this region that the electric field exhibits a large gradient.
Figure 3.3: Time dependent electric field in the plasma region as a function of location
across the electrode gap.
Each red curve in Figure 3.3 corresponds to a different time step from Figure 3.2.
It can be seen that before breakdown occurs (87 nanoseconds), the electric field is
constant in both the sheath and plasma regions. Once breakdown occurs, the electric field
quickly becomes perturbed causing a sharp voltage drop near the cathode before
maintaining a constant, though lower, value across the plasma region (88, 89, 90, and 92
nanoseconds).
51
The behavior of the voltage breakdown between the dielectric barriers can be
thought of in terms of a parallel plate capacitor where the coupled energy, Q, is
proportional to the capacitance, C, and breakdown voltage, Vb. Capacitance is a function
of both the area of the dielectric surfaces, A, and the sheath layer size, L.
(Eq. 3.5)
(Eq. 3.6)
The method for determining the input parameters for the discharge model is to
experimentally measure the current and voltage. The integrated product of these terms
gives the energy coupled to the plasma according to Equation 3.7. The coupled energy is
a key a parameter for solving the Boltzmann equation for EEDF, as discussed above.
(Eq. 3.7)
Figure 3.4 shows experimentally measured values for the applied field (kV/cm,
red dotted curve) and the stored/coupled energy (mJ, black dotted curve). The solid
curves are smooth fits used by the model. It can be seen that in the dielectric barrier
discharge, under these conditions, only a small fraction of the total energy initially stored
in the dielectric "capacitor" is coupled to the plasma (~0.12 mJ/pulse). The remaining is
reflected back to the power supply. The electric field (kV/cm) is shown in blue at the
bottom of the figure. Initially, the electric field follows the same rise as the applied field.
This corresponds to the time before breakdown occurs. After this, however, charge
separation causes the electric field to drop nearly to zero.
52
Figure 3.4: Discharge model predictions for applied electric field, stored/coupled energy,
and field in the plasma.
Figure 3.5 shows the field in the plasma, again, as the black curve. It is this
electric field waveform that is input into the plasma chemistry model described in the
next section. In addition, the solid blue curve corresponds to the electron number density
in the plasma discharge. The solid red curve corresponds to the coupled energy in the
plasma (not to be confused with the black curve in Figure 3.4 which is the coupled
energy as well as the energy stored in the dielectric barrier). Finally, the blue dotted curve
in Figure 3.5 corresponds to the O atoms that are produced during the discharge. A
complete description of these parameters is given in Chapter 4, alongside the
experimental results.
53
Figure 3.5: Current/Voltage waveforms in 20% O2/argon mixture at P=40 Torr. 100th
pulse in a burst of 451. Coupled pulse energy = 0.7mJ/pulse.
Finally, the EEDF is used to calculate rates for the chemical reactions that occur
during and very shortly after the discharge pulse (~1-100 nanosecond time scale). At this
time scale, the main reactions are electron impact with major species to produce
dissociation, vibrational excitation, electronic excitation, and in some cases ionization, as
demonstrated in Table 3.1.
54
Figure 3.6: Fraction of energy transferred by electrons in air to (1) vibrations of O2, (2)
rotations of O2 and N2, (3) elastic losses, (4) vibrations of N2, (5) electronic excitations of
N2, (6) electronic excitations of O2, (7) ionization of O2 and N2 [56].
As can be seen in Figure 3.6, there is a strong dependence on E/N in the
determination of how much electron energy is transferred to each of these modes. A
typical value of E/N for the work to be presented in this thesis is E/N ≈ 10-15 V cm-2 =
100 Td. For the plasmas being studied here, ionization processes were found to be
negligible. The rate of electron impact processes depend upon the electron energy
dependent impact cross sections. These cross sections were taken from Itikawa et al
[54,55] and the Boltzmann solver package known as BOLSIG [52]. These reactions give
the state of the gas mixture formed by the plasma which can then be used as an input
parameter for the plasma chemistry model that is used at longer time scales.
55
Process
Rate, cm3/s
A1 N2 + e- → N2(A3, B3, C3, a'1) + e1
4
4
1
A2
N2 + e → N( S) + N( S) + e

3
3 1
A3
O2 + e → O( P) O( P, D) + e

A4
Ar + e- → Ar* + e


A5
3.0
x
10-10
N2(C ) + O2 → N2(B ) + O2
A6
2.8 x 10-11
N2(a'1) + O2 → N2(B3) + O2
A7
3.0 x 10-10
N2(B3) + O2 → N2(A3) + O2
A8
2.5 x 10-12
N2(A3) + O2 → N2 + O + O
A9
Ar* + Ar → Ar + Ar
2.3 x 10-15
A10
Ar* + Ar + Ar → Ar + Ar + Ar
1.4 x 10-32
A11
Ar* + N2 → Ar + N + N
3.6 x 10-11
A12
Ar* + O2 → Ar + O + O
2.1 x 10-10
H1
H2 + e- → H + H e2
H2
2.6 x 10-11
N2(a'1) + H2 → N2 + H + H
H3
2.5 x 10-11
N2(B3) + H2 → N2(A3) + H2
H4
4.4 x 10-10 exp(-3170/T)
N2(A3) + H2 → N2 + H + H
H5
O(1D) + H2 → H + OH
1.1 x 10-10
H6
Ar* + H2 → Ar + H + H
6.6 x 10-11
M1
CH4 + e- → CH3 + H + e
-10
3
M2
1.2 x 10 exp(-3500/T)
N2(A ) + CH4 → N2 + CH3 + H
3
M3
3.0 x 10-10
N2(B ) + CH4 → N2 + CH3 + H
M4
5.0 x 10-10
N2(C3) + CH4 → N2 + CH3 + H
M5
3.0 x 10-10
N2(a'1) + CH4 → N2 + CH3 + H
M6
Ar* + CH4 → Ar + CH2 + H + H
2.2 x 10-10
M7
Ar* + CH4 → Ar + CH2 + H2
1.1 x 10-10
M8
Ar* + C2H2 → Ar + C2H + H
5.6 x 10-10
E1
C2H4 + e- → products3 + e
3
E2
9.7 x 10-11
N2(A ) + C2H4 → N2 + C2H3 + H
E3
3.0 x 10-10
N2(B3) + C2H4 → N2 + C2H3 + H
E4
3.0 x 10-10
N2(C3) + C2H4 → N2 + C2H3 + H
E5
4.0 x 10-10
N2(a'1) + C2H4 → N2 + C2H3 + H
E6
Ar* + C2H4 → Ar + C2H3 + H
5.4 x 10-10
Table 3.1: Dominant radical species generation processes in the plasma
[41,42,43,44,51,53].
56
3.3 Plane-To-Plane Plasma Chemistry Model
At longer time scales, modeling of the fuel/oxidizer chemistry becomes important.
This was done using a plasma kinetics model similar to that published previously
[11,41,49,50,51]. This kinetic model uses the results from the pulsed discharge model as
inputs to predict subsequent air or argon chemistry [40], as well as the hydrogen or
hydrocarbon chemistry in the plasma [16,41,42]. As a representative example, a few key
H2 and C2H4 reactions used in the model are given in Table 3.2.
Rate, cm3/s
Process
H + O2 + M → HO2 + M
O + HO2 → OH + O2
OH + H2 → H + H2O
H + O2 → O + OH
HO2 + HO2 → H2O2 + O2
OH + HO2 → H2O + O2
5.8 x 10-30
3.0 x 10-11
1.7 x 10-16
1.62 x 10-10
3.1 x 10-12
4.8 x 10-11
O + C2H4 → CH3 + HCO
O + C2H4 → H + CH2CHO
4.9 x 10-13
2.6 x 10-13
Table 3.2: H2 and C2H4 fuel reactions used in the plasma chemistry model
For hydrogen fuel, the complete chemical mechanism was developed by Popov
and contains 22 reactions involving H, O, OH, H2, O2, H2O, HO2, and H2O2 [16]. For
C2H4 fuel, a mechanism developed by the Gas Research Institute (GRI Mech 3.0) was
used [52]. It should be noted that both of these mechanisms were developed and validated
for temperatures significantly higher than those found in the experiments presented in this
study. This is a potential source of error that will be discussed in Chapter 4 alongside the
atomic oxygen results.
57
Due to the temperature dependence of many of the reactions listed in Table 3.2,
the model is capable of calculating temperature along the plasma centerline. In the latter
case, heat transfer to the walls is the dominant energy loss mechanism, according to
Equation 3.8. In this equation,
is the spatially averaged
temperature (not to be confused with T, the time-dependent temperature), qpulse is the
coupled pulse energy per molecule,  is the pulse repetition rate in a burst, hi are
enthalpies of chemical and excited species, dni/dt are the rates of species concentration
change in chemical reactions,
is the thermal diffusivity,  is the thermal
conductivity, Tw = 300 K is the wall temperature, L is the characteristic spatial scale (the
distance between the high voltage electrode and the grounded test section wall), and L/
is the spatial scale for conduction heat transfer. In the model, the spatial dependence of
temperature across the plasma is approximated by a cosine function.
(Eq. 3.8)
The results from the plane-to-plane plasma chemistry model were validated by
comparison with atomic oxygen concentration measurements (discussed in Chapter 4). In
addition, comparison with other experimental measurements is ongoing. As an example,
pure rotational Coherent Anti-Stokes Raman Scattering, or RCARS, spectroscopy is
being utilized for time resolved temperature measurements in the same repetitively
pulsed nanosecond discharge in air, Ar/O2, and O2/fuel mixtures.
58
3.4 Bare Metal Electrode Discharge Model
In Chapter 6, a study of a plasma discharge using spherical bare metal electrodes
will be discussed. Modeling of this system was conducted in a very similar way to that of
the dielectric barrier discharge discussed at length in the above sections. The main
difference between the two discharges is the lack of a dielectric barrier, which allowed
for direct measurement of the current and voltage (leading to a value for the coupled
pulse energy) as shown in Figure 3.7. Note that the coupled pulse energy, ~8mJ/pulse for
the case shown in Figure 3.7, is much higher than for that typical of the dielectric barrier
discharge case. In addition, as will be discussed in Chapter 6, the plasma volume for the
spherical electrode geometry is much smaller than for the plane-to-plane geometry. As
such, the specific pulse energy (or energy coupled per gas molecule) is many orders of
magnitude higher for this discharge.
59
Figure 3.7: Typical current (black) and voltage (red) results in 40 Torr of air in the
spherical electrode discharge cell. The coupled pulse energy is shown in blue.
60
Chapter 4
Plane-To-Plane Discharge Results
4.1 Introduction
Two photon Absorption Laser Induced Fluorescence (TALIF) studies have been
done to determine absolute atomic oxygen concentrations in nanosecond repetitive pulse
discharges in burst mode operation, at a 40 kHz repetition rate and low pressure, 40 Torr.
These measurements were conducted in a plane to plane electrode configuration in a
dielectric barrier cell. In order to decouple the effects of NOx chemistry from that of
oxidation, gas mixtures of argon buffer gas with 20% O2 and H2 or C2H4 fuel were
studied at a series of equivalence ratios and burst sizes. The results were compared to
predictions from the plasma kinetic model described in Chapter 3. Xenon calibration was
performed for each set of measurements.
4.2 Characterization of the Plasma - Imaging
A single piece of rectangular cross section quartz is used as the flow channel for
the measurements described in more detail below. It has dimensions of 220 mm long by
22 mm wide and 10 mm tall with walls that are 1.75 mm thick. There are flanges at either
end of the channel for connection of the gas inlet and outlet lines as shown in Figure 4.1.
61
The gas outlet line is connected by a tee shape connection to a gas pressure sensor. There
are two copper plate electrodes located on the top and bottom outer surfaces of the quartz
channel. These electrodes are encased in dielectric plates made of acrylic plastic. The
electrodes are 14 mm wide and 65 mm long, with curved edges to help facilitate a diffuse
plasma and field uniformity. Unless otherwise noted, the flow velocity of the gas is
~1m/s which corresponds to a residence time in the discharge region of 0.08 seconds. The
flow rate is controlled, for each gas, using an individual MKS mass flow controller.
Figure 4.1: Optical access plasma flow channel with copper plate electrodes and gas lines
indicated.
Initial ICCD images (see Chapter 2 for a description of the experimental
procedure) were taken in 20% O2 in argon buffer gas as a method of determining the
structure of the plasma. The burst repetition rate was held constant at 40 kHz, and all of
the images correspond to a single burst of pulses 11ms in duration. The camera intensifier
gate was set at 5s and wrapped around a single ~70 nanosecond pulse within the burst.
The gain was set to 150. It should be noted that while the images correspond to specific
62
pulses within the burst, they come from different burst events. Figure 4.2 shows the
results of these ICCD images taken at 40 Torr, which corresponds to the pressure used in
the TALIF measurements discussed below. The electrodes were located above and below
the plasma in all images. It was found that while the first pulse in the burst shows some
filamentary structure, this disappears rapidly and the plasma appears volumetric and
diffuse at all other pulses within the burst.
Figure 4.2: Series of ICCD camera images taken through the side of the discharge cell of
individual discharge pulses in Ar/O2, P=40 Torr and a pulse repetition rate of 40 kHz.
In addition to ICCD imaging of Ar/O2, the same measurements were taken in the
presence of fuels. Figure 4.3 shows images taken in three equivalence ratios of H 2 fuel in
the same 20% O2 in argon mixture at 40 Torr. As in the pure Ar/O2 mixture, the plasma
appears very diffuse and uniform throughout the duration of the burst.
63
Figure 4.3: Series of ICCD camera images of individual discharge pulses in H2/O2/Ar at
=0.1, 0.5, and 1.0. P=40 Torr and v=40 kHz. The camera gate is 5s.
Figure 4.4 shows parallel results in C2H4 fuel mixtures in Ar/O2. All of the
conditions are the same as in Figures 4.2 and 4.3.
Figure 4.4: Series of ICCD camera images of individual discharge pulses in C2H4/O2/Ar
at =0.08, 0.43, and 0.83. P=40 Torr and v=40 kHz. The camera gate is 5s.
64
While the images taken with C2H4 do not appear to show any filamentary
structure, there was very little emission present making characterization difficult.
Emission was mostly due to a combination of argon and O2 emission lines, as shown in
Figure 4.5. In these spectra, the black lines are argon pen lamp emission. The red lines
are plasma emission in 40 Torr of 20% O2 in argon (without fuel). Peaks from the plasma
not matching argon lines were found to correspond to O2 emission lines from the c1u- →
X3g- transition
Figure 4.5: Spectra of emission lines resulting from argon pen lamp (black) and plasma
(red).
65
In addition to imaging the plasma through the side of the test cell, ICCD images
were taken through the end of the test cell. This allowed for confirmation of the structure
in both dimensions. When measuring through the end of the cell, there were significantly
more reflections, and so the area of the plasma had to be determined by taking an image
without the plasma present and the sides of the cell illuminated by an argon pen lamp as
shown in Figure 4.6. The dashed line corresponds to the area of the plane to plane test
cell.
Figure 4.6: Image taken through the end of the plane to plane cell, with illumination from
an argon pen lamp. The dashed lines show the area of the test cell.
Figure 4.7 shows the ICCD images taken in 20% O2 in argon during the first, one
hundredth, and two hundredth pulses within the burst. Unlike in the images taken through
the side of the discharge (Figure 4.2), there is significant filamentary structure seen in this
dimension.
66
Figure 4.7: Series of ICCD camera images taken through the end of the plane to plane
discharge cell of individual discharge pulses in Ar/O2, P=40 Torr and a pulse repetition
rate of 40 kHz. The camera gate is 5s.
In order to determine if the filamentary structure was a result of defect in the
quartz test cell or electrodes (as opposed to random plasma instability), several images
were taken around the one hundredth pulse in the burst. As can be seen in Figure 4.8, the
location and intensity of the plasmas change randomly on a shot-to-shot basis. While
non-ideal, these instabilities will (at least partially) at least somewhat average out during
67
the course of a single pulse burst TALIF experiment. Each measurement consists of an
average of a large number of bursts (1000 - 4000, depending on the collection time),
providing further averaging.
Figure 4.8: Series of ICCD camera images taken through the end of the plane to plane
discharge cell of individual discharge pulses in Ar/O2, P=40 Torr and a pulse repetition
rate of 40 kHz. The camera gate is 5s.
Additionally, ICCD images were taken through the end of the plane-to-plane
discharge cell for a variety of fuel equivalence ratios. Figure 4.9 shows the results in
three equivalence ratios of H2 fuel: 0.1, 0.5, and 1.0. It can be seen that in all three cases,
a coarse filamentary structure persists throughout the duration of the burst. Also, as the
68
amount of fuel was increased the total emission decreased as a result of quenching of
argon emission.
Figure 4.9: Series of ICCD camera images of individual discharge pulses in H2/O2/Ar at
=0.1, 0.5, and 1.0. P=40 Torr and v=40 kHz. The camera gate is 5s.
Figure 4.10 shows parallel results in C2H4 fuel. In this case, the filamentation
appears to be less than in the previous cases; however the emission is faint, making this
hard to determine, particularly at =0.83.
69
Figure 4.10: Series of ICCD camera images of individual discharge pulses in C2H4/O2/Ar
at =0.08, 0.43, and 0.83. P=40 Torr and v=40 kHz. The camera gate is 5s.
4.3 Characterization of the plasma - Coupled Pulse Energy
Along with ICCD imaging of the plasma, current and voltage measurements were
conducted in order to determine the amount of energy coupled to the plasma during the
burst, a result which is used in the plasma chemistry model (see Chapter 3). Generally,
this can be done using voltage and current probes and monitoring the signal on an
oscilloscope. In principle, the coupled pulse energy can then be determined from
integration of the I(t)V(t) product. In practice, as discussed in Chapter 3, the result was
found to be very dependent upon phase shift between the current and voltage traces
which is difficult to accurately determine.
70
Due to the difficulty with the standard voltage/current probe technique, a second
experimental method was utilized to determine the coupled pulse energy. A capacitive
voltage probe and shunt current probe, developed by Takashima et al [57], was used
under a variety of conditions in both Ar/O2 and air. The results of these measurements are
summarized in Table 4.1. For all the conditions that were measured, the coupled pulse
energy was found to be even lower than the nanosecond discharge model predictions,
~0.08 mJ/pulse in Ar/O2 and 0.25 for air.
Table 4.1: Coupled pulse energy results from capacitive voltage probe and shunt current
probe measurement. Red values correspond to positive polarity pulses, blue values
correspond to negative polarity pulses.
Figure 4.11, on the top, shows the results of a single pulse. The red and black
dotted curves are experimentally determined values for the applied field (kV/cm, red) and
the combined coupled/stored energy (mJ, black). The solid curves of the same colors are
smooth fits to the experimental data. The energy coupled to the plasma is the offset of the
black curves at long times and is found to be ~0.12 mJ in this case. The electric field in
the plasma is shown in blue.
71
Figure 4.11: (top) Experimental applied field (red) and stored/coupled energy curves
(black). Discharge model predicted electric field (blue). (bottom) Discharge model
predictions for electric field (black), electron number density (solid blue), coupled energy
(red), and O atom concentration (dotted blue).
72
The bottom portion of Figure 4.11 shows the electric field in the plasma (same as the blue
curve in the top portion of the Figure), as well as the number density (solid blue curve),
coupled energy (red solid curve), and predicted atomic oxygen concentration (dotted blue
curve). The electric field waveform, number density of electrons, and coupled energy are
all used as inputs in the plasma chemistry model that is compared with the experimental
O atom concentration results.
Since the focus of this work is on the formation and decay of atomic oxygen, a
closer analysis of the model predictions for this species is necessary. The solid blue curve
in Figure 4.12 is the prediction from the plasma discharge model for atomic oxygen
concentration as a function of time during the burst in 20% O2 in argon (40 Torr). Each
"step" in the curve corresponds to a different pulse in a burst of 21 pulses at a 40 kHz
repetition rate (conditions studied experimentally in the next section, and shown as black
dots on the right of Figure 4.12). The dotted blue curve corresponds to model predictions
for temperature. While not immediately obvious from looking at the curve, the rise in
atomic oxygen concentration at each step is incredibly rapid. This implies that the
dominant mechanism for atomic oxygen formation is electron impact of O2 (Equations
4.1 and 4.2) and not metastable argon dissociating O2 (Equations 4.3 and 4.4).
O2 + e- → O + O
(Eq. 4.1)
→ O + O(1D)
(Eq. 4.2)
Ar + e- → Ar* + e-
(Eq. 4.3)
Ar* + O2 → Ar + O + O
(Eq. 4.4)
73
Figure 4.12: Plasma discharge modeling predictions for atomic oxygen concentration and
temperature in Ar/O2.
As a comparison, Figure 4.13 shows the plasma discharge model prediction for
atomic oxygen concentration as a function of time during the burst in air (40 Torr, 40
kHz repetition rate). While being qualitatively similar, the time scales for O atom
production resulting from each pulse are significantly longer. This implies that the
dominant O atom formation mechanism in air is not electron impact, but dissociation of
O2 from metastable N2 species.
N2 + e- → N2* + e-
(Eq. 4.5)
N2* + O2 → N2 + O + O
(Eq. 4.6)
74
Figure 4.13: Plasma discharge modeling predictions for atomic oxygen concentration and
temperature in air.
4.4 TALIF Results - Baseline Measurements
As described in Chapter 2, Two photon Absorption Laser Induced Fluorescence
(TALIF) was used to determine atomic oxygen concentration in a variety of fuel/oxidizer
mixtures. A dye laser was scanned across the 2p 3P → 3p 3P two photon absorption
transition while the single photon allowed 3s 3S → 3p 3P fluorescence signal was
collected. Figure 4.14 (taken from [30]) shows the relative signal intensities resulting
from each possible transition. Each spectrum in Figure 4.14 results from a different J
level in the ground state. The splitting between J levels in the excited state is too small to
completely resolve the peaks, however, and so a convolution of their overlap is shown.
75
Figure 4.14: Two photon excitation line profiles for atomic oxygen. Taken from [30].
Figure 4.15 shows a typical fluorescence excitation spectrum, where the structure
is due to the triplet splitting of the excited (3p 3P) state.
76
Figure 4.15: Typical TALIF excitation spectrum.
While the ground state (2p 3P) is also a triplet, the spacing between those levels is
sufficient that only the lowest level (J=2) was probed during the measurement. It should
be noted that for inference of atomic oxygen number density the Boltzmann fraction of
this lower level of the triplet is assumed to correspond to the ambient room temperature
conditions (Boltzmann fraction equal to 0.74). This assumption was justified by the low
coupled pulse energy as well as by the low predicted temperature values shown in the
following sections.
While the focus of this work was on the kinetic study of fuel mixtures in 20% O2
in argon, an initial, baseline, measurement was performed in air at a pressure of 40 Torr.
Figure 4.16 shows the temporal evolution of atomic oxygen as a function of time after a
burst of 21 pulses at a 40 kHz repetition rate. The 21 pulse burst was chosen so as to
reduce potential effects from temperature rise while still exhibiting a strong signal. It can
77
be seen that the agreement, both in absolute atomic oxygen magnitude and temporal
decay, was quantitative. This indicates that both the energy coupling, and the kinetics (to
be described in more detail below), were being captured accurately by the kinetic model.
It also demonstrates that the course filamentary structure observed in the spanwise
dimension does not preclude quantitative measurement, presumably due to spatial and
temporal averaging.
Figure 4.16: Atomic oxygen number density as a function of time after a 21 pulse burst at
P=40 Torr in air.
Figure 4.17, similar to Figure 4.16, shows the results of TALIF measurements
where the atomic oxygen decay was again determined as a function of time after a 21
pulse, 40 kHz burst from the pulse generator. In this case, however, the mixture was 20%
78
O2 in argon at a pressure of 40 Torr. As in Figure 4.16, the kinetic modeling results were
found to agree well with the data. In addition, it can be seen that the predicted
temperature rise was only 1 Kelvin. It should be noted that even though the plasma
images (shown in section 4.3) were found to be filamentary in the span-wise dimension,
this effect seems to be averaging itself out and not affecting the O atom concentration
results.
Figure 4.17: Atomic oxygen number density as a function of time after a 21 pulse burst at
P=40 Torr in 20% O2 in argon.
Under these near room temperature conditions, the dominant mechanisms for
atomic oxygen decay, in both air and argon mixtures are three body recombination to
form ozone, and two body reaction of atomic oxygen with ozone to form molecular
79
oxygen. Analysis shows that under the conditions of this measurement, both of these
processes are important, a result which provides confidence in the absolute inferred
concentrations. To illuminate this point, the rate coefficient for Equation 4.7 (from
Kossyi et al [40]) for Ar as a third body collision partner (at T= 300 K) is 4.4 x 10-34
cm6/s. This corresponds to a half life, , of approximately 7.9 milliseconds
(=1/k[Ar][O2]). As can be seen in Figure 4.17, this lifetime agrees well with both the
experimental results and modeling predictions.
O + O2 + M → O3 + M
(Eq. 4.7)
O + O3 → O2 + O2
(Eq. 4.8)
As a third baseline measurement, atomic oxygen was measured as a function of
burst size at two nominal flow velocities: 1.0 m/s and 0.5 m/s. As in the previous
measurement, these were taken in 20% O2 in argon at a pressure of 40 Torr. Figure 4.18
shows that the two velocities essentially identical results, confirming that the gas refresh
rate was sufficiently high that the gas mixtures experienced only a single discharge burst
prior to being probed by the laser. For all further measurements in the plane to plane
discharge configuration, the flow velocity was kept at 1.0 m/s.
80
Figure 4.18: Effect of flow velocity on atomic oxygen number density in 20% O2 in
argon at a pressure of 40 Torr. Red points correspond to a flow velocity of 0.5 m/s, while
black points correspond to a flow velocity of 1.0 m/s.
4.5 TALIF Results - Hydrogen Fuel
A series of atomic oxygen measurements were conducted in 20% O2 in argon gas
mixture with the addition of H2 fuel at three equivalence ratios: 0.1, 0.5, and 1.0. In this
case, measurements were performed as a function of number of pulses in the burst, again
at 40 Torr total pressure and a pulse repetition rate of 40 kHz. Figure 4.19 shows
experimental data, along with kinetic modeling predictions (using the plasma chemistry
reaction mechanism by Popov [16], plotted as atomic oxygen number density as a
function of number of pulses/burst in the range of 10-450.
81
Figure 4.19: Experimental and predicted (Popov’s mechanism) atomic oxygen number
density in Ar/O2/H2 mixtures.
Focusing first on the experimental Ar/O2 data it can clearly be seen that the
atomic oxygen number density rises rapidly with increasing number of pulses in the
burst, reaching an approximately constant plateau of ~2.0 x 1015 cm-3 after ~50 pulses.
Other than a slightly more rapid experimentally observed rise time, the overall agreement
between the model prediction and the experimental data is quantitative.
In the H2/O2 mixtures, the experimental atomic oxygen concentration again
rapidly reaches a steady state plateau, but in all cases the steady state concentration was
significantly lower than the pure Ar/O2 case, without fuel. This indicates an accelerated
net rate of atomic oxygen loss processes. In previous studies of OH creation and loss
[49], in the temperature range of ~300 - 400 K, the oxidation kinetics of H2/air plasmas
82
were found to be dominated by a three step sequence which clearly results in a net loss of
atomic oxygen at a rate faster than that corresponding to reactions 4.7 and 4.8 alone.
H + O2 + M → HO2 + M
(4.9)
O + HO2 → OH + O2
(4.10)
OH + H2 → H + H2O
(4.11)
While it can be seen from Figure 4.19 that the plasma kinetic model captured the
general trend in experimental data, the agreement was not quite quantitative. In
particular, while the ratio of the atomic oxygen steady-state concentrations for the three
equivalence ratios studied was predicted quite well, the ratio between the fuel containing
mixtures and the Ar/O2 mixture was over predicted by approximately a factor of two. It
should be noted that some of this discrepancy could be due to uncertainty in the
quenching rate of atomic oxygen by hydrogen (See Chapter 2). In particular, as shown in
Table 2.1, there is a discrepancy of a factor of ~60% between the rate coefficient reported
in [30], assumed in this work, and that of [34] would result in an increase of the inferred
atomic oxygen number densities of ~30% and 10% for =1.0 and =0.5, respectively.
The result for =0.1 would be negligible. Other possible causes for this discrepancy
include inaccuracy in the coupled pulse energy for the fuel containing mixtures relative to
the Ar/O2 mixture and an underestimation in the net low temperature rate of atomic
oxygen loss due to processes such as 4.9, 4.10, and 4.11 listed above.
In addition to atomic oxygen concentration, temperature measurements have been
conducted via the pure rotational Coherent Anti-Stokes Raman Scattering (CARS)
technique. Figure 4.20 compares these results with those of the plasma chemistry model
83
(using Popov's mechanism) under the same conditions as the TALIF results. While the
model predicts slightly higher temperatures in the fuel mixtures than were measured, the
agreement is well within 20 K.
Figure 4.20: Experimental CARS results and plasma chemistry modeling predictions
(Popov's mechanism) for temperature under the same conditions as Figure 4.19.
By comparison, Figures 4.21 and 4.22 show the same experimental results with
plasma chemistry modeling results using the Konnov mechanism [58]. There is
significantly less agreement between the modeling and experimental results.
84
The difference between these two mechanisms is in the rate coefficient for the reaction of
O atoms with OH radicals to form H atoms and O2 molecules. Konnov uses a
significantly faster mechanism (by a factor of four) which leads to an over prediction of
O atom loss.
O + OH → H + O2
k = 1.2 x 10-11 cm3/s (Popov)
(Eq. 4.12)
O + OH → H + O2
k = 5.3 x 10-11 cm3/s (Konnov)
(Eq. 4.13)
Figure 4.21: Experimental and predicted (Konnov's mechanism) atomic oxygen number
density in Ar/O2/H2 mixtures.
85
Figure 4.22: Experimental and predicted (Konnov's mechanism) results for temperature
rise in Ar/O2/H2 mixtures.
The Popov mechanism uses the rate of O + OH → H + O2 that is only accurate at
high temperatures (above 1000K, according to the NIST chemical kinetics database).
Because of this, it underestimates the NIST recommended rate (for T=300K) of k = 3.5 x
10-11 cm3/s by a factor of three, which results in an over prediction of O atoms as shown
in Figure 4.19. Conversely, the Konnov mechanism overestimates the rate by
approximately 50% and thus under predicts the O atom concentration as shown in Figure
4.22. Substituting the NIST recommended rate into these mechanisms gives better
agreement between experiment and modeling, as shown in Figure 4.23.
86
Figure 4.23: Experimental and predicted (NIST adjusted mechanism) atomic oxygen
number density in Ar/O2/H2 mixtures.
4.6 TALIF Results - Ethylene Fuel
Additional TALIF measurements were also conducted in Ar/O2 mixtures
containing C2H4 fuel mixtures, the results of which are shown in Figures 4.23 (compared
with the GRI 3.0 mechanism [52]) and 4.24 (compared with the Wang/USC mechanism
[59]), along with the Ar/O2 data from the previous section. When considering this data, it
should be noted that there are no published values for the quenching rate of atomic
oxygen by C2H4. In this work, a C2H4 quenching rate equal to that of CH4 in [34] was
used.
87
Figure 4.23: Experimental and predicted (GRI 3.0 mechanism) atomic oxygen number
density in Ar/O2/C2H4 mixtures.
Figure 4.24: Experimental and predicted (Wang/USC mechanism) atomic oxygen number
density in Ar/O2/C2H4 mixtures.
88
It can be seen that, as for the H2 fuel cases, the atomic oxygen number density
rises rapidly with increasing number of pulses in the burst, reaching a steady-state plateau
after ~50-100 pulses, the value of which is a strong function of fuel equivalence ratio.
Comparing the behavior of C2H4 and H2 fuels, the atomic oxygen steady-state number
density decreases more rapidly for C2H4 than for H2. This results in a number density at
=0.83, ~6 x 1013 cm-3, which is lower by more than an order of magnitude than the
steady-state value of ~1 x 1014 cm-3 for H2 at =1.0. Such behavior was expected due to
the known rapid rate of low temperature reaction of atomic oxygen with C 2H4. Two
processes, shown below, have rate coefficients at room temperature of k=4.9 x 10-13 cm3/s
and k=2.6 x 10-13 cm3/s, respectively.
O + C2H4 → CH3 + HCO
(4.6)
O + C2H4 → H + CH2CHO
(4.7)
As in the previous section on H2 fuel, comparison of the experimental data with
modeling predictions showed qualitative agreement to within an average factor of ~50%,
but there were some differences that can be discerned. First, both mechanisms that were
studied (the GRI Mech 3.0 and the Wang/USC mechanisms) predicted very similar
atomic oxygen concentrations. There were no major differences between the two.
Second, contrary to the result for H2 fuel, both modes generally predicts steady-state
atomic oxygen number densities which are greater than the experimental values, although
the agreement improves with increasing equivalence ratio. Finally, while the
experimental data initially increased with increasing number of pulses in the burst, the
modeling predictions exhibit the opposite behavior, dropping with increasing number of
89
pulses. Finally, the experimental atomic oxygen number densities decreased more rapidly
with increasing equivalence ratio than was predicted by the model.
There are multiple potential causes for the discrepancy between experimental data
and modeling predictions. Potentially most importantly, kinetics are strongly dependent
on temperature, which is in turn strongly related to the coupled pulse energy and effective
E/N. Experimental verification of the predicted temperature could alleviate this issue.
Figures 4.25 and 4.26 show modeling predictions of temperature and indicate that the rise
is significantly greater than in the H2 fuel containing mixtures.
Figure 4.25: Experimental and predicted (GRI Mech 3.0 mechanism) results for
temperature rise in Ar/O2/C2H4 mixtures.
90
Figure 4.26: Experimental and predicted (Wang/USC mechanism) results for temperature
rise in Ar/O2/C2H4 mixtures.
91
Chapter 5
Singlet Delta Oxygen Results
5.1 Introduction
A study of the role singlet delta oxygen, O2(a1g) or SDO, plays on low
temperature, repetitively pulsed nanosecond nonequilibrium plasmas has been conducted.
In particular, SDO was created in an RF discharge in O2/Ar/H2 gas mixtures prior to
excitation with a pulsed nanosecond discharge. The plane-to-plane electrode geometry
and pulse generator identical to those discussed in Chapter 4 were also used in this study.
The yield of SDO was measured quantitatively by calibrated IR emission spectroscopy in
both the immediate RF afterglow and in the nanosecond discharge cell. Atomic oxygen
measurements were performed by Two photon Absorption Laser Induced Fluorescence
(TALIF), as a function of both time and number of pulses in a 40 kHz burst with, and
without, the addition of SDO. While kinetic modeling indicated that a measurable SDO
effect on O atom number density should be observable, no effect that can be
unambiguously traced to SDO was detected. One possible explanation, discussed in
detail, is that it is difficult to experimentally isolate the effect of SDO from that of NO
and/or NO2 present in the gas mixture.
92
SDO has an excitation energy of ~1eV, which may be significant enough to
overcome the activation energy for key chemical reactions involved in plasma
combustion. In particular, replacing a percentage of ground state O2 with SDO has the
possibility of enhancing key chain branching reactions such as the two shown below
(discussed in more detail in Chapter 4) [60].
O2(a1g) + H → OH + O
k1 = 1.8 x 10-10 exp[-3188/T]
k2 = 1.62 x 10-10 exp[-7470/T]
O2 + H → OH + O
O2(a1g) + H2 → H + HO2
k3 = 3.5 x 10-11 exp[-18216/T]
k4 = 3.0 x 10-11 exp[-24080/T]
O2+ H2 → H + HO2
Figure 5.1 shows these reactions graphically as a function of temperature. It can be seen
that in both reactions, introducing SDO into the system enhances the rate coefficient (by
more than an order of magnitude in the low temperature regime being studied here). The
remainder of this chapter describes the experimental and computational attempt to isolate
the effect of SDO on the plasma from all other species.
93
10
Rate (cm3/molecule/sec)
10
10
10
10
10
10
10
-10
-15
-20
-25
-30
-35
-40
SDO+H->OH+O
SDO+H2->H+H02
O2+H->OH+O
-45
O2+H2->H+HO2
10
-50
300
400
500
600
700
800
900
1000
Temperature (K)
Figure 5.1: Plot of reactions involving O2 with H and H2 both with and without SDO
enhancement.
5.2 Experimental Considerations for SDO Measurements
The experimental apparatus used in this study is shown in Figure 5.1 The TALIF
Discharge Cell is the same as that described in Chapter 2 and this is where a majority of
measurements were conducted. The SDO is initially created in the RF Discharge Cell.
The RF cell is a cylindrically shaped glass tube with an outer diameter of 0.25 inches and
a wall thickness of 0.02 inches. Three synthetic air mixtures have been used in these
experiments: 20% O2 in helium, 50% O2 in helium, and 20% O2 in argon. Along with
these, two fuels (H2 and C2H4) have been studied.
94
O2-Ar
Figure 5.2: Schematic diagram of the experimental apparatus. Electrodes are located
above/below the plane of the paper on the TALIF Discharge Cell and wrapped around RF
Discharge Cell.
Along with the power supply described in the previous chapter, repetitively
pulsed plasmas were generated using two different power supplies identical to those used
in our previous work [41,60]. These pulse generators are manufactured by Chemical
Physics Technologies (CPT) and produce ~25 nanosecond pulses with a ~20kV peak
voltage. The power supplies generate negative polarity pulses (as opposed to the
alternating pulses in the pulse generator described in previous chapters), while the
terminal connected to the positive electrode can grounded or left floating. Figure 5.3
shows a typical single pulse voltage waveform during a burst of nanosecond pulses in air
95
at 40 Torr and a pulse repetition rate of 40 kHz (pulse #100 in the burst), for grounded
and floating discharge operation. For all the measurements in this study, the pulser was
operated in floating mode. This reduced the pulse duration slightly (from approximately
35 nanoseconds to 25 nanoseconds) while also significantly reducing voltage oscillations
after the pulse. Floating mode has only a weak effect on the peak voltage. It has
previously been found that leaving the positive terminal floating considerably improves
the discharge stability [61,62,65]. The pulser is operated in repetitive burst mode,
generating sequences of 2000 to 2500 pulses at a repetition rate of 40 kHz to 50 kHz. To
consistently produce breakdown in the nanosecond discharge cell on the first pulse, the
section was irradiated by a deuterium UV lamp (Resonance Ltd.) through the side wall,
providing pre-ionization of the discharge volume.
Voltage, kV
20
10
0
-10
Air, P=40 torr
grounded
-20
floating
0
50
100
150
200
250
Time, nsec
Figure 5.3: Typical voltage waveform during a burst of nanosecond pulses in air at 40
Torr and a pulse repetition rate of 40 kHz, for grounded and floating discharge operation.
96
A capacitively coupled radio frequency (RF) discharge is used to generate SDO
molecules. The power supply for this discharge is produced by ENI, while the manual
impedance matching apparatus is produced by MFJ Enterprises.
5.3 SDO Yield Results
In the present experiments, the RF discharge is operated at a power of
approximately 200 Watts for duration of up to 90 seconds, with no sign of instability or
arc filament formation. EMI noise has not been found to interfere with other equipment
involved in the experiments (infrared camera, OMA spectrometer, pulser, or pressure
gauges). In order to ensure that there is not significant heating from the RF discharge,
temperature measurements have been taken using a metal thermocouple downstream of
the plasma. A temperature increase of less than 7K has been observed and is not
dependent on gas mixture, electrode gap, or RF power being put into the system.
97
Figure 5.4: Effect of flow rate on signal from SDO molecules. 20% O2 in helium, 40
Torr. Flow rate is as calculated in the RF discharge.
Initially, the IR camera and OMA spectrometer are used to measure SDO yield as
a function of flow rate in the 20% O2 in helium gas mixture. Figure 5.4 shows the result
of this measurement at a pressure of 40 Torr. Increasing the flow rate linearly increases
the intensity of the SDO signal and so for the rest of the measurements, the flow rate is
set to 7 m/s in the RF discharge cell. This corresponds to a flow rate of 1 m/s in the
nanosecond pulsed cell, which is the same as reported in other chapters.
While SDO has a radiative lifetime of more than one hour [17], the actual lifetime
can be much shorter due to collisional quenching and, to a smaller extent, reactions with
other species in the system. The quenching rate constants for some relevant species with
SDO are summarized in Table 5.1. It can be seen that quenching occurs primarily through
collisions with the walls and so it becomes important to minimize the distance the gas has
98
to flow between the RF cell and the nanosecond pulsed discharge. This distance is
approximately 0.6 meters.
Reaction
Rate Constant (cm3/s) Reference
1.0 x 10-20
[63]
O2(a1g) + Ar → O2 + Ar
-18
1
2.2 x 10
[63]
O2(a g) + O2 → O2 + O2
-16
1
2 x 10
[63]
O2(a g) + O → O2 + O
-15
1
3.8 x 10
[63]
O2(a g) + O3 → 2O2 + O
-18
5 x 10
[17]
O2(a1g) + NO2 → O2 + NO2
-17
1
3.5 x 10
[17]
O2(a g) + NO → O2 + NO
-18
1
4.5 x 10
[16]
O2(a g) + H2 → O2 + H2
-14
1
1.1 x 10
[16]
O2(a g) + H → O2 + H
-11
1
2.0 x 10
[16]
O2(a g) + HO2 → O2 + HO2
-5
1
~2 x 10 (quartz)
[64]
O2(a g) + wall → O2
Table 5.1: SDO quenching reactions and room temperature rates for some common
species.
In spite of the minimal distance between test cells, most of the SDO molecules
were still quenched before reaching the nanosecond discharge cell. NO2 titration shortly
after the formation of SDO is used to minimize the presence of ozone and atomic oxygen,
both of which are produced in the RF discharge and are known to quench SDO. This
process relies on the cyclical reactions of NO2 and NO with atomic oxygen and O3. It is
important to note that the reaction of NO with O3 is over three orders of magnitude faster
than the reaction of NO with SDO [23].
NO2 + O → NO + O2
(Eq. 5.1)
NO + O3 → NO2 + O2
(Eq. 5.2)
Figures 5.5 and 5.6 show typical SDO signal intensities in 50% O2 in helium and
20% O2 in argon, respectively. In each, the signal intensities located in the RF discharge
99
cell (red curves) are compared to signal intensities downstream, in the nanosecond pulsed
discharge cell (blue curves). It can clearly be seen that the addition of NO 2 to the system
significant increases the SDO yield delivered to the nanosecond discharge cell. The
baseline shift in Figure 5.5(b) is due to the afterglow of the NO2 from the RF discharge.
As can be seen from Table 5.1, NO2 titration both eliminates ozone, a rapid
quencher of SDO, and prevents its later formation through three body recombination O +
O2 + M → O3 + M. It can be seen that without the addition of NO2 titrant, the SDO
concentration falls to almost below the detection limit before reaching the nanosecond
pulsed discharge cell. Upon addition of NO2, however, a much larger SDO emission
signal is observed.
100
(a.)
(b.)
Figure 5.5: Typical SDO emission spectra. (a) Comparison of signal levels upstream (in
the RF discharge) and downstream (in the flow channel cell) without any NO2 titrant
added to the system. (b) Same comparison with 0.1% NO2 by volume added. 20% O2 in
helium, P=60 Torr.
(a.)
(b.)
Figure 5.6: Typical SDO emission spectra. (a) Comparison of signal levels upstream (in
the RF discharge) and downstream (in the flow channel cell) without any NO2 titrant
added to the system. (b) Same comparison with 0.1% NO2 by volume added. 20% O2 in
argon, P=60 Torr.
101
Figure 5.7 plots relative SDO yield in the nanosecond pulsed discharge cell
(arbitrary units) vs. NO2 percentage in the flow. Adding approximately 0.6% NO2 (by
volume) results in SDO yield leveling off. Due to the extremely rapid rate of process 5.1,
this value corresponds to a titrant concentration which is approximately equal to that of
atomic oxygen created by the RF discharge.
Figure 5.7: Plot of raw SDO signal intensity against percent of NO2 titrant. Signal reaches
a plateau above ~0.06% NO2 by volume.
More quantitatively, Figure 5.8 shows SDO yield just downstream of the RF
discharge (green curve) and in the nanosecond discharge cell for 0%, 0.06%, and 0.1%
NO2 added, over a range of pressures in the range of 40 to 100 Torr. While a significant
loss of SDO occurs during the transport to the nanosecond discharge cell, yield of up to
2.5% is obtained at the 40 Torr pressure, for which TALIF measurements were
performed. All present TALIF data with SDO injection is performed with 0.06% NO2
102
titrant added. As will be discussed later, this implies that essentially all added NO2 is
converted to NO prior to injection in the nanosecond pulsed discharge cell. The
ramifications of this on the plasma kinetics will also be discussed.
Figure 5.8: Percent yield of SDO as a function of pressure in both the RF discharge
(green squares) and downstream in the flow channel. These measurements were taken in
20% O2 in argon.
5.4 Plasma Uniformity Measurements
In order to confirm that the nanosecond pulsed discharge remains diffuse and
spatially uniform during the entire pulse burst sequence, a set of UV ICCD camera
images have been obtained. Specifically, images taken in the same =0.75, 20% O2 in
argon with C2H4 mixture as discussed above is of particular interest. As a representative
example, Figure 5.9 shows a series of images taken at a pressure of 65 Torr and a pulse
103
repetition rate of 40 kHz. Each image corresponds to a single burst of pulses 50
milliseconds long, with pulse numbers in the burst ranging from #1 to #1450. The top
nine images have been taken with a 2 microsecond intensifier gate, timed to include a
single, 25 nanosecond discharge pulse. The bottom three images have been taken with a
20 microsecond intensifier gate and timed to collect emission from between pulses within
a burst. It is important to note that individual images are from different bursts and do not,
therefore, illustrate discharge development of a single burst.
Figure 5.9: Series of broadband ICCD images of =0.75 C2H4 in Ar/O2 mixture at 65
Torr. The camera gate is 2 microseconds for images of individual pulses, and 20
microseconds for images taken between pulses.
Some filamentary structure is seen to develop in the early pulses of the burst, but
is consistently contained in the central position of the plasma. This result is similar to
previous nanosecond pulsed plasma images taken in C2H4/air mixtures in which much
104
larger scale filamentation was observed to develop after approximately 50 to 100 pulses,
depending on the equivalence ratio [53]. Filamentation in both these and the current
plasmas is most likely due to the onset of ionization and heating instability [56]. C2H4 has
a particularly low ionization potential (10.5eV) compared to H2 fuel (15.4eV). Another
factor is the differences in thermal conductivity of N2, argon, and helium. In the images
presented here, filamentary structure disappeared before any evidence of ignition
occurred.
In previous work from this laboratory [65], UV ICCD images were obtained in
fuel/air mixtures. In these cases, all images were considerably brighter due to emission
from the N2 second positive band. In the current fuel/synthetic air mixtures, this emission
is not present and so the image brightness is due to emission from the OH A→X(0,0)
band. In order to confirm that the faint ignition between pulses is due to ignition, a series
of ICCD images have been obtained with a much wider intensifier gate (100
microseconds) after the final pulse in a burst as shown in Figure 5.10.
105
Figure 5.10: Series of broadband ICCD camera images at =0.75 C2H4 in Ar/O2 mixture
at 65 Torr, obtained at differing times after the final pulse of a 25 millisecond, 50 kHz
burst. Camera gate is 100 microseconds.
5.5 O Atom Measurements and Kinetic Modeling
Prior to studying the effect of SDO on atomic oxygen concentration, three
baseline measurements need to be performed. First, TALIF measurements were
performed with the RF discharge turned on and 0.06% NO2 titrant added to a 20% O2 in
argon mixture at 40 Torr. When the nanosecond pulsed discharge was not operated, it
was confirmed that no atomic oxygen, to within the detection limit of the TALIF
apparatus, was detected in the pulsed nanosecond discharge cell. This confirms that the
concentration of atomic oxygen transported to the nanosecond discharge cell from the RF
discharge is completely negligible.
106
Second the measurement above was repeated with both the RF and nanosecond
discharges not operated. In this case, a very small amount of atomic oxygen was detected,
but it was orders of magnitude lower than the values which were measured with the
nanosecond discharge in operation. While the origin of this small atomic oxygen signal is
not entirely certain, it is likely due to photo-dissociation of NO2 by the 226nm TALIF
beam.
Finally, a series of TALIF measurements was performed in which the decay of the
atomic oxygen was determined as a function of time after a burst of discharge pulses. As
an example, Figure 5.11 shows the temporal decay of atomic oxygen generated in 20%
Ar/O2 oxidizer mixture at 40 Torr excited by a 21 pulse burst at a 40 kHz repetition rate,
along with kinetic model predictions.
O + O2 + M → O3 + M
(Eq. 5.5)
O + O3 → O2 + O2
(Eq. 5.6)
It can be seen that the predicted decay, which is dominated by three body
recombination to form ozone, as well as the two body reaction, is in good agreement with
experimental results.
107
Figure 5.11: O atom number density as a function of time after a 21 pulse burst in a 20%
oxygen in argon oxidizer mixture, compared with kinetic modeling results. P=40 Torr,
discharge pulse repetition rate is 40 kHz.
In the next series of measurements, O atom number density is measured in the
20% Ar/O2 oxidizer mixtures, as well as in the H2/Ar/O2 mixtures at =0.1 and =0.5,
after a burst of discharge pulses of variable duration, P=40 Torr and a repetition rate of
40 kHz. Figure 5.12 plots O atom number density as a function of the number of pulses in
the discharge burst, varied from 50 to 450, compared to modeling calculations. From
Figure 5.12, it can be seen that in all three cases the O atom number density nearly levels
off during the burst. In H2/Ar/O2 mixtures, the quasi-steady state O atom concentration
during the burst is much lower than the Ar/O2 mixture, by more than an order of
magnitude. The O atom number density reached during the quasi-steady state stage (after
~200 to 300 pulses), predicted by the kinetic model, is in fairly good agreement with the
108
experimental data, although the model over predicts O atom number density, particularly
at large burst sizes.
Figure 5.12: Experimental and predicted atomic oxygen number density in Ar/O2/H2
mixtures.
Kinetic analysis of dominant O atom decay processes in H2/Ar/O2 mixtures shows
that lower O atom concentrations achieved when hydrogen is added to the flow are
primarily due to three reactions [49].
H + O2 + M → HO2 + M
(Eq. 5.7)
O + HO2 → OH + O2
(Eq. 5.8)
OH + H2 → H + H2O
(Eq. 5.9)
The model predicts that in the presence of hydrogen, the number densities of O atoms and
OH radicals level off gradually, and start decreasing with the burst duration, while the
109
number density of H atoms keeps increasing (see Figure 5.13). This results in gradual
hydrogen oxidation and water vapor formation, although ignition in the discharge cell is
not achieved due to relatively low temperature, T< 600K.
Figure 5.13: Species concentrations and temperature vs. number of pulses in the
discharge burst, predicted by the kinetic model in a H2/Ar/O2 mixture at =0.5, at the
conditions of Figure 5.12. Data points show experimental O atom number density.
Figure 5.14 illustrates the effect of NO2 titration on composition of 20% Ar/O2
oxidizer mixture at 40 Torr downstream of the RF discharge, predicted by the kinetic
model. In these calculations, the initial composition of the mixtures includes SDO mole
fraction of 0.014 (based on 7% SDO yield measured in the RF discharge at these
conditions, see Figure 5.7) and O atom mole fraction of 6x10-4 (0.06% based on NO2
titration measurements, see Figure 5.7. The upper plot in Figure 5.14 shows results of
calculations conducted without adding NO2 to the mixture, and the lower plot shows the
110
results predicted with 0.06% of NO2 added (in this case, NO2 mole fraction is the same as
O atom mole fraction). It can be seen that in the absence of NO2, O atoms decay very
slowly (mainly by recombination with O2 to form ozone), and SDO number density
decreases by about an order of magnitude, mainly due to fairly rapid quenching by O
atoms and ozone (see Table 5.1). With NO2 added to the flow downstream of the RF
discharge, O atoms are removed by their reaction with NO2 very rapidly, within ~1
millisecond, while NO2 is almost completely converted to NO, and ozone accumulation
remains insignificant. Note that at these conditions the rate of O atom removal by NO, in
reaction with ozone, is more than two orders of magnitude lower than that by reaction of
O atoms with NO2. Due to much lower O and ozone concentrations, SDO decay rate is
reduced considerably, with about one third of the initial yield remaining in the flow after
~1 second. These results are qualitatively consistent with the SDO yield measurements in
the pulsed discharge cell, shown in Figures 5.5 and 5.6.
111
Figure 5.14: Kinetic modeling calculations illustrating the effect of NO2 titration on
composition of 20% Ar/O2, 40 Torr oxidizer mixture after the RF discharge. Initial SDO
mole fraction 0.014 (7% yield), initial O atom mole fraction 6x10-4 (0.06%). Top, no
NO2 titration; bottom, with NO2 titration (NO2 mole fraction same as O atom mole
fraction).
112
Figure 5.15 shows the effect of SDO produced in the RF discharge and NO2
titration on O atom generation and loss in the repetitively pulsed nanosecond discharge.
The experimental results in Figure 5.17 are shown for a 20% Ar/O2 oxidizer mixture, 40
Torr and a repetition rate of 40 kHz at different discharge burst durations, for the
following three cases, (a.) baseline case without NO2 added to the flow and without SDO
(i.e. with RF discharge off), (b.) with 0.06% by volume NO2 added to the flow, still
without SDO, and (c.) with 0.06% NO2 added to the flow and with SDO (i.e. RF
discharge turned on). It can be seen that injecting NO2 into the flow, without adding
SDO, considerably reduces O atom number density accumulated in the cell during first
~200 to 300 pulses in the burst, compared to the baseline case. On the other hand, adding
SDO on top of NO2 (i.e. turning the RF discharge on) nearly offsets the effect of adding
NO2 and significantly increases O atom number density rise observed during first ~150
pulses, to nearly baseline levels.
113
Figure 5.15: O atom number density as a function of number of pulses in the discharge
burst. Experimental results.
Comparison with kinetic modeling calculations, shown in Figure 5.18, provides
insight into the kinetics of O atoms in the pulsed discharge in these same three cases. In
the presence of NO2 (without SDO), O atoms generated in the discharge are rapidly
removed by titration, until NO2 is converted to NO. After this conversion has occurred,
further O atom removal (by reaction with NO) becomes much slower and O atom number
density gradually approaches the value reached in the baseline case.
114
Figure 5.16: O atom number density as a function of number of pulses in the discharge
burst. Kinetic modeling results.
With SDO generated in the flow by the RF discharge (along with O atoms), NO2 addition
results in a rapid, nearly complete, conversion to NO (see Figure 5.14), such that the
mixture entering the nanosecond discharge cell contains SDO and NO rather than SDO
and NO2. In this case, the rate of O atom removal in the pulsed discharge is reduced by
about two orders of magnitude, while contribution of electron impact dissociation of
SDO into O atom generation is almost negligible (since SDO energy is much smaller
compared to the threshold energy of dissociation cross section). Therefore, in this case,
the O atom number density predicted by the model is very close to the one reached in
baseline case, as shown in Figure 5.16.
115
Figure 5.17 shows the effect of SDO produced in the RF discharge and NO2
titration on O atom generation in the repetitively pulsed nanosecond discharge, this time
in a H2/Ar/O2 mixture at =0.5 and 40 Torr. Note that the RF discharge, which is located
upstream of NO2 and fuel injection points, always operates in 20% Ar/O2 mixture.
Figure 5.17: Experimental O atom number density as a function of number of pulses in
the discharge burst. H2/Ar/O2 mixture at =0.5, 40 Torr, 40 kHz.
The experimental results in Figure 5.19 are shown at a pulse repetition rate of 40 kHz at
different discharge burst durations, for two cases. The red squares correspond to 0.06%
NO2 added to the flow, without SDO (RF is turned off), and the blue triangles correspond
to 0.06% NO (instead of NO2) added to the flow, with SDO (RF is turned on). It can be
seen that, again, adding SDO to the flow on top of the NO2 (i.e. turning the RF discharge
116
on) offsets the effect of O atom removal by NO2 and significantly increases O atom
number density generated during the first ~50 pulses in the burst.
This behavior is analyzed in Figure 5.18. In the plot on the left, modeling
predictions for three different cases are shown: (a.) baseline case without NO2 added to
the flow and without SDO (RF turned off), (b.) with 0.06% NO2 added to the flow,
without SDO, and (c.) with 0.06% NO (not NO2) added to the flow, with SDO (RF
discharge turned on). Similar to the modeling results shown in Figure 5.16, NO2 (without
SDO) rapidly removes O atoms generated in the discharge by the titration until NO2 is
converted entirely to NO. After this, further O atom removal (by reaction with NO)
becomes much slower, and O atom number density approaches the value reached in the
baseline case after ~300 pulses. The critical effect of NO2 on O atom reduction is
illustrated further in case (c.), when NO2 is replaced by NO. As discussed above, with
SDO generated by the RF discharge (along with O atoms), NO2 in the flow rapidly
converts to NO upstream of the nanosecond discharge cell, as demonstrated in Figure
5.14, such that in this case the mixture entering the nanosecond discharge cell contains
SDO and NO, rather than SDO and NO2. The rate of O atom removal by NO in the
pulsed discharge is fairly insignificant, and in this case O atom number density follows
the baseline very closely. Finally, modeling calculations demonstrate that adding or
removing SDO from the mixture (while keeping NO mole fraction the same) does not
produce a detectable effect of O atom number density.
Comparison of the plot on the left (in Figure 5.18) with Figure 5.17 shows that the
model predictions are consistent with the experimental observations. This suggests that
117
the entire difference between the O atom number densities measured with RF discharge
on and off (i.e. with and without SDO added to the flow) may be due to the effect of
nitric oxides (NO and NO2) kinetics, rather than due to adding SDO.
Figure 5.18: Predicted O atom number density as a function of number of pulses in the
discharge burst, at the conditions of Figure 5.19. Left: baseline model. Right: illustration
of effect of SDO quenching by HO2 and H atoms.
The plot on the right of Figure 5.18 provides insight into SDO kinetics in the
nanosecond pulsed discharge in greater detail. It shows that turning off rapid SDO
quenching by HO2, generated primarily by the three body reaction of H and O2, results in
a modest increase of O atom number density, ~20-30% (see Table 5.1). Furthermore,
hydrogen atoms can interact in two ways with SDO. The reactive channel is shown below
as Equation 5.10.a and the non-reactive channel is shown as 5.10.b. If 100% probability
of the reactive channel (a.) and absence of non-reactive quenchingby H atoms is
assumed, there results in O atom number density an increase by a factor of 2.5.
118
O2(a1g) + H → OH + O
(5.10.a)
→ O2 + H
(5.10.b)
The difference becomes apparent after ~100 pulses in the burst, when the flow
temperature in the nanosecond discharge cell gradually increases to approximately 400 K
(see Figure 5.13), since the reaction (10) has a fairly high activation energy of Ea=2530
K. Finally, the analysis shows that at the present conditions, reactions of SDO with H2
have almost no effect on SDO kinetics, due to relatively low temperatures in the cell
(T<600 K) and very high activation energy, Ea=17900 K. Thus, kinetic modeling predicts
that rapid non-reactive quenching of SDO by HO2 and by H atoms generated in the
nanosecond pulse discharge results in significant reduction of SDO number density, such
that its effect on O atom concentration becomes very weak.
5.6 Summary and Conclusions for SDO Study
The production of SDO molecules in an RF discharge has been studied by in
various synthetic air mixtures, most extensively in 20% O2 in argon buffer gas. In order
to observe the SDO molecules downstream of the RF discharge (in the nanosecond
pulsed discharge cell), NO2 titration is used to react with O3 and O atom byproducts of
the discharge through two cyclical reactions.
NO2 + O → NO + O2
(1)
NO + O3 → NO2 + O2
(2)
In addition, Two Photon Absorption Laser Induced Fluorescence (TALIF)
measurements and kinetic modeling have added insight into the temporal evolution of
119
atomic oxygen in H2/Ar/O2 plasmas. Experimental data is obtained by application of a
variable number of nanosecond pulses, in the range of 10-450, at a 40 kHz repetition rate,
using a house built high voltage power supply and pulser unit. The pulse burst is
repeated at 10 Hz, matching the repetition rate of the laser system and also assuring that
each sample of gas experiences only a single burst.
Qualitative agreement is found between the experimental atomic oxygen number
density and plasma chemical kinetic modeling predictions in both Ar/O2 and H2/Ar/O2 at
equivalence ratios of 0.1 and 0.5. In addition, the rate of atomic oxygen decay in Ar/O2
after a 21 pulse burst is found to agree well with kinetic model predictions.
Additional experimental measurements and modeling predictions focus on the
effects of adding SDO, generated in the RF discharge. No effect can be unambiguously
traced to SDO has been detected. One possible explanation is that it is difficult to
experimentally isolate the effect of SDO from that of NOx species which are also present
in the gas mixture due to the requirement to titrate atomic oxygen also formed in the RF
discharge. SDO effect may well be reduced significantly due to non-reactive quenching
of SDO by HO2 and H atoms, although non-reactive quenching rates are known with
some uncertainty. Kinetic modeling calculations, incorporating SDO quenching rates
recommended in the literature [16], predict no detectable effect of adding SDO to the
flow on number density of O atoms generated in the nanosecond pulsed discharge.
120
Chapter 6
Pin-to-Pin Discharge Results
6.1 Introduction
A Two photon Absorption Laser Induced Fluorescence (TALIF) study was
conducted in order to determine the absolute atomic oxygen concentration in a
nanosecond pulsed discharge with much higher specific energy loading than achievable
in the plane-to-plane dielectric barrier geometry. Measurements were taken as a function
of time after a single pulse, at a 60 Hz repetition rate and low pressure (40 Torr) air and
air/fuel mixtures. The bare metal electrodes were spherical, with a 7.5 mm diameter. Two
fuels were studied, H2 and C2H4. The results in air were compared to predictions from a
plasma kinetic model described in Chapter 3. Xenon calibration was performed for each
set of measurements. In addition, characterization of the plasma in the spherical electrode
geometry was conducted by ICCD plasma imaging and current and voltage
measurements.
The motivation for studying pin-to-pin electrode geometries, as opposed to the
plane-to-plane dielectric barrier geometry discussed in Chapters 4 and 5, is primarily to
achieve higher coupled pulse energies. Since the electrodes are in contact with the gas
121
flow, there is no dielectric barrier to behave as a capacitor as was present in the previous
geometry. To illustrate this point, typical pulse energy values for the plane-to-plane
dielectric barrier discharge were on the order of 0.07 mJ/pulse. In the pin-to-pin electrode
geometry, typical pulse energy values were on the order of 8 mJ/pulse. It should be noted
that while the plasma region in the pin-to-pin electrode geometry was smaller than that of
the plane-to-plane geometry, care was taken to ensure that the plasma volume was
reasonably large for laser diagnostics to be conducted.
6.2 Characterization of the Plasma - Coupled Pulse Energy
A rectangular acrylic discharge cell was used as the flow channel for the
measurements described in this chapter. The electrodes had a 7.5 mm diameter and were
made of bare copper. For all the results reported here, the electrode gap was 1 cm. The
flow region inside the cell was cylindrical with a 1.5 inch diameter and 2 inch optical
windows on three sides for TALIF experiments. The gas inlet and outlet lines were
located on either end of the cell on the side where no optical window was present. As in
the previous experiments, the flow rate was controlled for each gas using an individual
MKS mass flow controller. Figure 6.1 shows a schematic diagram of the discharge cell,
taken from Burnette et al [66].
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Figure 6.1: Schematic diagram with spherical electrode geometry.
Current and voltage measurements were taken in order to determine the total
amount of energy coupled to the plasma during each discharge pulse. A current Pearson
8277 current probe and Tektronix P6015As has been described in Chapters 3 and 4, the
coupled pulse energy, Q, can be calculated from the integrated product of the current and
voltage traces, according to Equation 6.1.
(Eq. 6.1)
Calculation of the coupled pulse energy was complicated by the fact that the high
voltage pulser unit, when operated in this electrode geometry, outputs both a "pre-pulse"
and "main pulse" for each trigger event. Figure 6.2 shows the current measurement (black
123
curve), voltage measurement (red curve), and the resulting coupled pulse energy (blue
curve) taken in 40 Torr of air. It can be seen that a small amount of energy is coupled
during the pre-pulse, while most is coupled during the main pulse.
Figure 6.2: Current and voltage waveforms in air at 40 Torr.
For the purposes of modeling data taken in this electrode configuration, the
voltage from both the pre-pulse and main-pulse were used as inputs. Figure 6.3 shows the
voltage trace used in the model. The pre-pulse is the negative going pulse on the left. The
main pulse is the large positive pulse on the right. The third peak, on the far right, was
not found to make a difference in the coupled energy as there is a negligible amount of
current corresponding to it.
124
Figure 6.3: Voltage trace used as input in kinetic model.
While the pulsed discharge model used for modeling this discharge geometry
used the voltage as an input, it generated a current profile which could then be compared
to the measured current. This was used as a confirmation of the pulsed discharge model
characteristics. Figure 6.4 shows both the experimentally measured current trace (gray
curve) and the discharge model produced current trace (red curve). The two qualitatively
agree, with the difference in peak occurring around 140 ns being attributed to limitations
from the current probe bandwidth. The red curve was used in the calculation of coupled
pulse energy in the modeling results presented below.
125
Figure 6.4: Experimental (gray) and calculated (red) current traces for 40 Torr of air in
the spherical electrode geometry.
6.3 Characterization of the Plasma - Imaging
Along with current and voltage measurements, ICCD images of the plasma were
taken. Initial ICCD images (see Chapter 2 for a description of the experimental
procedure) were taken in air as a method of determining the structure of the plasma. The
pulse repetition rate was held constant at 60 Hz. The camera intensifier gate was set to be
less than 100 nanoseconds wide and wrapped around a single pulse. The gain was set to
150. Figure 6.5 shows a camera image of the electrode region without any discharge (no
plasma present). Figre 6.6 shows images of both the main pulse (left) and the pre-pulse
126
(right). It can be seen that the plasma was much more intense during the main pulse, as
was to be expected given the amount of energy coupling from each pulse.
Figure 6.5: ICCD camera image of the electrode region in 40 Torr of air. No discharge
present.
127
Figure 6.6: ICCD camera images taken in 40 Torr of air. The main pulse (left) couples
significantly more energy into the plasma than the pre-pulse (right).
In addition to ICCD images of air, the same measurements were taken in the
presence of fuel mixtures. Three equivalence ratios of H2 fuel were studied: 0.07, 0.22,
and 0.43. In all three cases, the plasma was found to be stable and diffuse with a
centerline diameter of approximately 3 mm. Figures 6.7, 6.8, and 6.9 show both positive
and negative polarity plasma images (see Chapter 2) for each of these equivalence ratios.
128
Figure 6.7: ICCD camera images of individual discharge pulses in air with H2 fuel,
=0.07. P=40 Torr. The camera gate is <100ns.
Figure 6.8: ICCD camera images of individual discharge pulses in air with H2 fuel,
=0.22. P=40 Torr. The camera gate is <100ns.
129
Figure 6.9: ICCD camera images of individual discharge pulses in air with H2 fuel,
=0.43. P=40 Torr. The camera gate is <100ns.
`
A series of ICCD camera images were also taken with in an air-C2H4 mixture at
four equivalence ratios: 0.19, 0.48, 0.87, and 1.74. Similar to the images taken with H2
fuel, the plasma appears to be stable and diffuse at all of the conditions that were studied.
As was observed in other fuel/oxidizer cases (see Chapter 4), the plasma emission
becomes fainter at higher equivalence ratios of C2H4. This is most likely due to
quenching from the fuel species. Figures 6.10, 6.11, 6.12, and 6.13 show the negative and
positive polarity images in each of these mixtures.
130
Figure 6.10: ICCD camera images of individual discharge pulses in air with C2H4 fuel,
=0.19. P=40 Torr. The camera gate is <100ns.
Figure 6.11: ICCD camera images of individual discharge pulses in air with C2H4 fuel,
=0.48. P=40 Torr. The camera gate is <100ns.
131
Figure 6.12: ICCD camera images of individual discharge pulses in air with C2H4 fuel,
=0.87. P=40 Torr. The camera gate is <100ns.
Figure 6.13: ICCD camera images of individual discharge pulses in air with C2H4 fuel,
=0.1.74. P=40 Torr. The camera gate is <100ns.
132
6.4 TALIF Results - Baseline Measurements
As described in Chapter 2, Two photon Absorption Laser Induced Fluorescence
(TALIF) was used to determine atomic oxygen concentration in a variety of fuel/air
mixtures. As an initial experiment, atomic oxygen decay as a function of time after the
discharge pulse in 40 Torr of air was measured. Figure 6.14 shows the experimental
results (black squares) alongside plasma chemistry modeling results (red line). It can be
seen that at short time scales (less than ~1 millisecond), the experimental data shows a
slight increase in concentration before decaying. The kinetic model does not show this
trend. It is possible that this curvature is a function of a calibration issue, drifting laser
energy or alignment during the experiment, and a closer analysis of the atomic oxygen
concentration in this time regime is necessary. At longer time scales, however, when the
atomic oxygen concentration was decaying rapidly, the agreement becomes extremely
good.
133
Figure 6.14: Atomic oxygen number density as a function of time after a single discharge
pulse at 40 Torr in air.
As in the plane-to-plane electrode regime (see Chapter 4), the atomic oxygen
decay was found to be dominated by three body recombination to form ozone, and two
body reaction of atomic oxygen with ozone to form molecular oxygen.
O + O2 + M → O3 + M
(Eq. 6.2)
O + O3 → O2 + O2
(Eq. 6.3)
The rise in atomic oxygen concentration that was observed in the experimental results
around 200 microseconds was not captured by the plasma chemistry model, but as was
mentioned previously, further analysis is required to determine if this is a real kinetic
effect.
134
6.5 TALIF Results - Hydrogen Fuel
A series of atomic oxygen TALIF measurements were conducted as a function of
time in mixtures of air with H2 fuel at three equivalence ratios: 0.07, 0.22, and 0.43. The
results for these three cases are shown in Figure 6.15 alongside the results from pure air.
The results in air show an approximate plateau of ~3.0 x 1016 cm-3 until 1 millisecond
after the discharge pulse, when the concentration decreases rapidly. In each of the H2 fuel
equivalence ratios, the same general trend was observed with the difference being the
maximum concentration that was reached being lower and the rapid decay beginning
sooner.
Figure 6.15: Experimental atomic oxygen number density in air and air/H2 mixtures at
P=40 Torr.
135
In the very lean case of =0.07 H2, the atomic oxygen concentration reaches a
plateau around 1.8 x 1016 cm-3 and begins to decay after only 200 microseconds. In
=0.22 case, the atomic oxygen concentration reaches 1.5 x 1016 cm-3 and begins to decay
after approximately 100 microseconds. Finally, in the =0.43 case, the atomic oxygen
concentration reaches 1.0 x 1016 cm-3 and begins to decay after approximately 80
microseconds.
6.6 TALIF Results - Ethylene Fuel
Another series of atomic oxygen TALIF measurements were taken in mixtures of
air with C2H4 fuel at a variety of equivalence ratios: 0.19, 0.48, 0.87, and 1.74. The
results from each of these measurements are shown in Figure 6.16 alongside the results
for pure air (same as in Figures 6.14 and 6.15).
In the three lean cases that were studied (f=0.19, 0.48, and 0.87), the atomic
oxygen decays rapidly without the observable plateau region. As the equivalence ratio
goes up, the maximum concentration of atomic oxygen goes down. This result was
expected because, as in the H2 fuel case, quenching of the atomic oxygen goes up
increased fuel in the system.
136
Figure 6.16: Experimental atomic oxygen number density in air and air/C2H4 mixtures at
P=40 Torr.
In the fuel rich case, =1.74, the atomic oxygen decayed rapidly for
approximately 50 microseconds and then reached a plateau that lasted until several
milliseconds after the discharge pulse. After ~5 milliseconds, the concentration decayed
further due, most likely, to diffusion out of the plasma region. A kinetic mechanism for
describing this plateau is unknown, and so measurements were taken with no pulser
present to ensure that this was not due to photo-dissociation from the laser beam. Figures
6.17 and 6.18 show the raw TALIF signal as the laser was scanned across the O atom
line. In Figure 6.17, the black curve corresponds to the signal in air at 500 microseconds
after the discharge pulse. The red curve is the signal in =1.74 C2H4 at 500 microseconds.
137
The blue curve is without any discharge present. The curves in Figure 6.18 are the same,
without the air curve present, to show the difference in the red and blue curves more
clearly. There is significantly more TALIF signal present in the =1.74 C2H4 than in the
no pulser case, indicating that the plateau region is not due to photo-dissociation.
138
Figure 6.17: Intensity of TALIF signal in 500 microseconds after the discharge pulse in
air (black), and =1.74 C2H4 (red), as well as without any discharge present (blue).
Figure 6.18: Intensity of TALIF signal in 500 microseconds after the discharge pulse in
1.74 C2H4 (red), as well as without any discharge present (blue).
139
Chapter 7
Pin-To-Plane Discharge Results
7.1 Introduction
A collaborative study between The Ohio State University, the University of
Southern California, and the Air Force Research Laboratory was conducted on the kinetic
mechanism of transient plasma ignition at atmospheric pressure. Dr. Scott Pendleton of
USC developed a high voltage pulsed discharge unit [26,67,68] that was brought to OSU
for one month in the summer of 2011 for the purposes of studying the discharge via Two
photon Absorption Laser Induced Fluorescence (TALIF) of O atoms and Coherent AntiStokes Raman Scattering (CARS) spectroscopy for temperature measurements within the
plasma.
The TALIF study was conducted in fuel/air mixtures in the afterglow of the
repetitively pulsed, nanosecond streamer discharge with the purpose of illuminating the
oxygen pathways in transient plasma ignition [69,70,71,72]. Different from those
described in Chapters 4 through 6, this discharge exists in a pin-to-plane electrode
configuration as opposed to plane-to-plane or spherical electrodes. It is higher pressure
140
and a smaller filament. Figure 7.1 shows a photograph of the experimental apparatus. The
arrows indicate the location of the pin (anode) and plane (cathode). The pin electrode
used in this configuration was a steel needle with a tip curvature radius of 75m and the
plane electrode was a sintered bronze surface of a McKenna flat flame burner (Halthius
and Associates). The pin electrode was the anode and the plane electrode was the cathode
for all the measurements being reported here. All measurements were recorded at a pulse
repetition rate of 10 Hz to match the laser repetition rate. It can be seen that the entire
apparatus is located on a three axis translation stage, which allowed for careful alignment
of the laser beam into the desired region of interest in the plasma.
Figure 7.1: Photograph of the pin-to-plane electrode geometry and three axis translation
stage.
141
This configuration was designed for measurements in atmospheric pressure
air/fuel mixtures, as opposed to the low pressure regime. Both the pin and plane
electrodes were exposed to the gas mixture and there was no dielectric barrier present.
Measurements were taken as a function of time after a single pulse in mixtures of CH 4,
C2H4, and C3H8 and air at a variety of equivalence ratios, all of which were outside the
ignition limits.
In a streamer discharge of this type, the plasma behavior is very non 1Dimensional, making modeling of the system difficult. The plasma not only expands as it
reaches from the anode (pin) to the cathode (plane), but moves spatially to different
points on the cathode. The small size of the plasma region causes both convection and
diffusion to be significant effects in understanding this type of plasma. For all of these
reasons, a comprehensive model of the system was not attempted.
The motivation for studying a plasma discharge of this type is because streamers
have been shown to be practical ignition sources and the field of transient plasma ignition
(TPI) has developed around improving engine performance in a variety of systems
(pulsed detonation engines, automobile engines). TPI has decreased ignition delay,
increased flame speed, and decreased the amount of fuel necessary to produce ignition as
compared to more conventional techniques (spark or arc gap) [26,69,70,71]. Up to this
point, however, understanding of the physical and chemical processes behind TPI has
been limited. This diagnostic study of the atomic oxygen concentration in a streamer
discharge has been conducted with the goal of building a body of knowledge on the
kinetic mechanism behind TPI.
142
7.2 Streamer Discharge Characterization Results
Before atomic oxygen TALIF measurements could be performed, a series of
discharge characterization measurements were conducted. First, voltage and current
traces were taken in each of the gas mixtures to ensure that any changes in electrical
impedance were accounted for. Figure 7.2 shows a typical voltage/current trace taken in
air. As can be seen, the pulse generator produced a Gaussian-like voltage waveform with
an amplitude of ~15kV and a duration of 20 nanoseconds FWHM.
30
Voltage
Current
15
10
10
5
Current (A)
Voltage (kV)
20
0
0
0
10
20
30
Time (ns)
40
50
60
Figure 7.2: Typical voltages and current traces for the streamer discharge, taken in air at
atmospheric pressure.
These measurements were taken both during the initial 50 pulses after the system had
been switched off for a prolonged amount of time and also 50 shots into an arbitrary
period after the system had been running continuously for a prolonged time. These "cold"
and "hot" results showed no difference, leading to the conclusion that there was no
143
discernable heating effect on impedance. In addition, current and voltage measurements
were taken in each of the gas mixtures studied. These were used to calculate the total
energy delivered into the plasma. Table 7.1 summarizes these results, which show that
the addition of fuel had a negligible effect on the total energy, peak voltage, and peak
current of the discharge.
Energy Delivered
Gas Mixture
(mJ)
Peak Voltage (kV) Peak Current (A)
Air
4.78±0.03
16.55±0.15
19.64±0.33
C2H4 Φ=0.25
4.79±0.02
16.53±0.19
19.96±0.33
C2H4 Φ=0.5
4.74±0.03
16.55±0.18
20.03±0.24
C2H4 Φ=2.4
4.54±0.02
16.66±0.20
20.19±0.33
CH4 Φ=0.3
4.68±0.03
16.54±0.17
19.89±0.37
CH4 Φ=0.6
4.64±0.02
16.61±0.19
19.84±0.34
CH4 Φ=1.2
4.62±0.02
16.56±0.18
19.83±0.34
C3H8 Φ=0.4
4.78±0.02
16.52±0.15
19.81±0.36
C3H8 Φ=0.8
4.65±0.03
16.56±0.16
20.01±0.38
C3H8 Φ=2.1
4.55±0.03
16.73±0.15
19.92±0.35
Table 7.1: Electrical pulse characteristics for different gas mixtures.
144
In addition to voltage/current traces, images of the discharge were taken in the
330-450 nm range. Emission in this range is primarily due to to N2 C3u → N2 B 3g
emission. These images give a good indication of the size and stability of the streamer
discharge. The most intense discharge, as well as the most stable, was found to be
directly below the high voltage anode tip. For this reason, atomic oxygen measurements
were taken as close to the anode as possible. A typical image (taken in air) is shown in
Figure 7.3 below.
Figure 7.3: Typical image of the streamer discharge taken in air. Distance between the
anode and cathode is 8mm. TALIF measurements were conducted a few millimeters
below the anode tip.
The TALIF laser beam was passed through the electrode gap directly beneath the
anode pin and is indicated with a square in Figure 7.3. Vertically, the location of the
145
beam was adjusted using the three-axis translation stage so that it could be as close to the
electrode pin as possible without producing any noticeable scattering. Horizontally, the
same thing was done by finding the most intense signal (center of plasma region). The
size of the region of interest (ROI) was initially determined using a 50m slit and
translating it both horizontally and vertically across the laser beam while the light passing
through the slit was monitored using a photodiode. This gave a beam size of 250m
(FWHM). During the TALIF measurements, a 250m slit was placed in the image plane
to limit the detection region to that of the area probed by the laser beam. This was an
important step in determining absolute O atom concentration because the size of the
region where xenon signal resulted from (during the calibration) was so much larger than
the plasma region. The 250m slit limited detection to a small, uniform area for both
measurements.
While the experimental apparatus was located at OSU, vibrational and rotational
CARS measurements were conducted to determine vibrational loading and rotational
temperature as a function of time after the discharge pulse. It was found that a significant
amount of vibrational loading of the v=1 state occurs, reaching a maximum concentration
approximately 20 to 100 microseconds after the discharge pulse. This unexpected result
is believed to be due to what is known as V-V transfer, where population in higher
vibrational levels, excited by electron impact during the discharge pulse population
"cascade" downward through sequential processes such as N2(v) + N2(w) ↔ N2(v+1) +
N2(w-1) where w > v. The rotational temperature was found to be approximately 1000 K
146
over the same time period [28]. Figure 7.4, taken from Pendleton et al [28], summarizes
these results in air. Additional measurements were taken in fuel/air mixtures.
Figure 7.4: Rotational temperature overlaid with vibrational v=1 signal in air showing
required overlap between population and temperature extraction [71].
7.3 Air Results and Calibration Discussion
Figure 7.5 shows the results of O atom TALIF measurements in air as a function
of time. Additional measurements at longer time scales were conducted (up to 5
milliseconds) but these showed no discernable atomic oxygen. For the sake of clarity,
uncertainty is shown via an error bar on only one data point, though this result can be
applied to all points in the figure. Uncertainty was calculated by the standard deviation of
the mean taken from ~3000 laser shots, with variations in laser intensity, TALIF signal,
and Xe calibration signal taken into account. It can be seen that the atomic oxygen values
147
were initially consistent with a plateau of approximately 4x1017cm-3. This corresponds to
a dissociation fraction of O2 equal to approximately 0.18 assuming atmospheric pressure
and rotational/translational temperature of ~1000 K, as determined in previous CARS
measurements on this system and under identical conditions (see Figure 7.4). After 40 to
50 microseconds, atomic oxygen decay began to be evident. After 200 microseconds the
concentration of atomic oxygen had plummeted and quickly fell below the detection
limits for the TALIF system. This qualitative trend agrees with results found previously
in Chapters 4 and 5 as well as in a TALIF study done by Uddi et al from 2009 [11].
Figure 7.5: TALIF results of O atom number density in pure air at atmospheric pressure.
148
The primarily loss mechanism for atomic oxygen in air under these conditions is
through three-body combination; O + O + M → O2 + M where is M is either N2 or O2
[40]. It should be noted that at the relatively high temperatures being studied here, ozone
production due to O + O2 + M → O3 + M is negligible due to the fast rate of the reverse
reaction. Using rate data from Kossyi et al [40], the rate coefficient, k, for N2 as a third
body collision partner (at T=1000 K) is ~5 x 10-34 cm6/s. This corresponds to a half life,
, of approximately 500 s which is an order of magnitude longer than the ~50 s
observed experimentally.
(Eq. 7.1)
There are multiple possibilities for the discrepancy between experimental and
calculated O atom decay rates. First, the size of the discharge, on the order of hundreds
of micrometers, must be taken into account. Since xenon signal comes from the entire
volume being seen by the PMT, this volume must be limited to the size of the plasma
discharge. This is done using a 250 micrometer slit at the entrance to the PMT. Second,
and perhaps more importantly, collisional quenching in air occurs at different rates in the
atmospheric pressures being studied here as compared to the sub 100 Torr regime
described in Chapters 4 through 6. Quenching terms were calculated for each of the
major species (N2, O2, CH4, C2H4, and C3H8) with pressure and temperature taken into
account.
(Eq. 7.2)
149
In Equation 7.1, Qi is the quenching rate contribution for a given species, i, ki is the rate
coefficient for that species, n0 is the Loschmidt number (2.45 x 1019 cm-3 at 300 K), Pi is
the pressure of the species, and Ti is the temperature.
Another complication for accurate measurement of atomic oxygen decay lies in
understanding the role of diffusion in this electrode geometry. The rate of decay of O
atoms out of the region of interest was estimated assuming that an initially uniform
cylindrical distribution of oxygen atoms (r < 0.125 m) diffuses into an infinite medium.
Diffusion of a trace amount of atomic oxygen in N2 was assumed and the binary diffusion
coefficient was derived using the temperature values reported in a thermometry study by
Pendleton et al [28] in the same discharge. Standard gas diffusion equations were used
and a complete description of this process is given by Carslaw et al [73] and Kee et al
[74]. This calculation shows a much reduced rate in the fall-off of atomic oxygen
concentration compared to that shown in Figure 7.5. Figure 7.6 shows the results of the
diffusion calculation overlaid onto the experimental data taken in air.
150
Figure 7.6: Calculated decay due to diffusion shown with measured atomic oxygen
concentration in pure air (experimental data from Figure 7.5).
The rate of reduction in initial concentration of O atoms by a factor of (1/e)2 is
predicted to be approximately 100 ms in the diffusion calculation, which is in reasonable
agreement with the data. In addition, it may be that due to the high specific energy
loading of the filament, the discharge induces bulk convection of the gas out of the region
of interest that causes the observed rapid decay in signal. Future measurements in this
electrode configuration should focus on multi-dimensional imaging of the flow (Schlieren
imaging, for example) to determine characterization of the effects from diffusion versus
chemical consumption.
151
7.4 Results in CH4/Air Discharges
Figure 7.7 shows atomic oxygen concentration results as a function of time after
the pulse in air and air/CH4 mixtures at atmospheric pressure. The equivalence ratios
studied correspond to those just outside of the limits for ignition (=0.6 and =1.2) and
an additional equivalence ratio further removed from ignition (=0.3). As described in
the previous section, error bars were calculated using the standard deviation of the means
for 3000 shots and is shown only on one point in each equivalence ratio for the sake of
clarity. These error bars were consistent, however, and can be applied to all points in the
sequence.
With the addition of even a small equivalence ratio of CH4, there was a rapid
increase in atomic oxygen consumption. While a full kinetic analysis was not within the
scope of this study, some insight can be drawn from consideration of important atomic
oxygen loss processes. It should be noted that the analysis is not meant to provide a
quantitative comparison with the data, but rather to illustrate that atomic oxygen kinetics
in the presence of fuel are predicted to be much faster than for air.
152
Figure 7.7: TALIF measurements of O atom number density in CH4/air mixtures.
The primary loss processes for O atoms in the presence of CH4 fuel are as follows
[59]. For methane dissociation fraction on the order of 1% the rate of atomic oxygen loss
due to reaction with methyl radical is comparable to that for reaction with methane.
O + CH4 → OH + CH3
k = 7 x 10-13 cm3/s
O + CH3 → H + CH2O
k = 8.4 x 10-11 cm3/s
O + CH3 → H + H2 + CO
k = 5.6 x 10-11 cm3/s
Considering only the methane reaction pathway, the initial CH4 number density
was approximately 4 x 1017 cm-3 at atmospheric pressure, =0.6, and T = 1000 K. The
predicted O atom half life is then ~2.2 s (Eq. 7.3). Assuming 1% CH4 dissociation to H
153
+ CH3 and ignoring any coupling between kinetic processes results in a total half life
from the three parallel processes of ~0.8 s.
(Eq. 7.3)
(Eq. 7.4)
The calculation of atomic oxygen half life is dependent upon the assumption that
the initial CH4 concentration is much larger than that of atomic oxygen. However, for
particularly lean cases, the initial CH4 concentration may actually be less than the initial
O concentration. With the fuel as the limiting reactant, this tends to limit the rate of decay
of O atoms. Under fuel rich conditions, where the initial CH4 concentration exceeds that
of O atoms, the observed rate of O atom decay is much faster due, presumably, to both
the absolute CH4 being larger and the fact that the CH4 concentration exceeds that of the
initially formed O atom concentration. It should be noted that these O atom consuming
reactions above are chain branching reactions and are ultimately responsible for the
runaway heat release of combustion.
7.5 Results in C2H4/Air Discharges
Figure 7.8 shows results in air and air/C2H4 mixtures at atmospheric pressure. As
in the previous case, measurements were taken just outside of the limits for ignition
(=0.5 and =2.4) and an additional equivalence ratio further removed from ignition
(=0.25). For both of the lean cases studied here, the peak atomic oxygen concentration
is very similar to that of pure air. Instead of a plateau and steep decay, though, these O
atoms begin decaying initially and do so slowly until the decay matches that of pure air.
154
In the rich case that was studied, =2.4, the atomic oxygen concentration dropped off so
rapidly that only a few data points were able to be collected.
Figure 7.8: TALIF Results of O atom concentration in C2H4/air.
A similar analysis to that conducted for the CH4 containing mixtures was
conducted for this case. There are two parallel processes that cause initial C2H4 loss. At
atmospheric pressure and a temperature of 1000 K, they are as follows:
O + C2H4 → CH3 + HCO
k = 6.0 x 10-10 cm3/s
O + C2H4 → CH2CO + H
k = 3.0 x 10-10 cm3/s
155
The corresponding atomic oxygen half life is very short, ~0.4 s. However, as was
discussed in the previous section, in lean cases the assumption that the C2H4
concentration is greater than that of the initial O atom concentration is no longer true.
This is a likely cause of the reduction in the rate of decay of O atoms compared to the
calculated value.
7.6 Results in C3H8/Air Discharges
Figure 7.9 shows the atomic oxygen results in C3H8/air mixtures. As before,
measurements were taken just outside of the limits for ignition (=0.8 and =2.4) and an
additional equivalence ratio further removed from ignition (=0.4). It can be seen that the
relative shape of the O atom decay is more similar to that of CH4 than that of C2H8 in that
the O atoms exist for a short time at a constant concentration and then decay rapidly.
156
Figure 7.9: TALIF results of C3H8/air mixtures.
There is one primary loss mechanism for O atoms with respect to C 3H8. The
formation of C3H7 radical occurs with two isomeric configurations, but for the purposes
of this analysis their rate coefficient can be combined into one total term.
k ~ 1011 cm-3
O + C3H8 → C3H7 + OH
At atmospheric pressure and a temperature of 1000 K, for an equivalence ratio of =0.8
the initial C3H8 concentration is 2.5 x 1017 cm-3 and the corresponding O atom half life is
0.3 s. For an equivalence ratio of =2.4, the initial C3H8 concentration is 7.1 x 1017 cm3 and the corresponding O atom half life is 0.1 s.
157
7.7 Conclusions
A comprehensive set of Two photon Absorption Laser Induced Fluorescence
(TALIF) measurements, describing the temporal evolution of atomic oxygen number
density in quasi-quiescent C2H4/air, CH4/air, and C3H4/air mixtures outside the limits of
combustion, has been conducted. These measurements were completed in a point to plane
nanosecond streamer discharge similar to that used in transient plasma ignition
experiments at atmospheric pressure.
It has been shown that absolute atomic oxygen concentrations are quite high in
the afterglow shortly after the discharge, approximately 5 x 1017 cm-3, with a dissociation
mole fraction of up to 0.18 in air which persists for a time scale of approximately 50
microseconds. After this, a rapid decay is observed due to a combination of three body
recombination (the principle atomic oxygen loss mechanism) and diffusion/convection.
In each of the fuel/air mixtures that have been studied, this monotonic decay is observed
while the absolute concentration of atomic oxygen decreased. Differences in O atom
behavior can be attributed to hydrocarbon chain branching reactions that vary for each of
the species studied.
Further measurements are necessary to determine spatial distributions of atomic
oxygen, as well as production of intermediate reactive species created by combustion
chain branching reactions in order to better understand the physics and chemistry behind
the plasma created by this pin-to-plane geometry. An example of this type of
measurement could be Laser Induced Fluorescence (LIF) measurements of OH radicals.
158
In addition, Schlieren imaging of the gas flow will help to separate chemical and physical
effects.
159
Chapter 8
Summary and Conclusions
8.1 Plane-To-Plane Discharge Conclusions
This dissertation has provided new data on plasma chemical oxidation in O2/H2
and O2/C2H4 nanosecond pulsed discharges in argon buffer gas in a plane-to-plane
dielectric barrier discharge. New experimental data for the temporal evolution of atomic
oxygen data has been obtained using Two-photon Absorption Laser Induced
Fluorescence (TALIF), as a function of both time after the discharge and number of
pulses in a high repetition rate (40 kHz) discharge burst. In each of the gas mixtures
studied, ICCD camera images of the plasma were taken in order verify that the plasma
was stable and diffuse over the entire volume. Current and voltage measurements, as well
as kinetic modeling of the discharge and resulting chemistry was conducted. A secondary
study of the role singlet delta oxygen, SDO, has on the plasma chemistry was also
conducted.
UV ICCD camera images showed that under the conditions studied in the atomic
oxygen TALIF experiments (40 Torr, 40 kHz repetition rate), the plasma is reasonably
diffuse and stable along the length of the discharge region. In the spanwise direction,
however, significant filamentation was observed. These filaments appeared random with
160
respect to space and time, making it possible that their influence averaged out during the
TALIF measurements. Additional images were taken in each of the gas mixtures that
were studied via TALIF, with comparable results in each case. As the concentration of
fuel (either H2 or C2H4) was increased, emission became fainter making imaging of the
plasma more difficult. This is primarily due to quenching of excited O2 molecules.
Current and voltage traces were collected in order to give a value for the coupled
pulse energy in the plasma. Two separate techniques were used as a way of verifying
their accuracy. Using commercially available voltage and current probes gave a coupled
pulse energy in the plasma of ~0.7 mJ/pulse. Using a house built system consisting of a
capacitive voltage probe and shunt current probe gave a coupled pulse energy of ~0.07
mJ/pulse. Modeling of the plasma discharge predicted the coupled pulse energy as being
~0.2 mJ/pulse.
Time-resolved, absolute atomic oxygen concentration was obtained in H2/O2/Ar
gas mixtures at equivalence ratios of =0.1, 0.5, and 1.0. Atomic oxygen concentration
was obtained in C2H4/O2/Ar gas mixtures at equivalence ratios of =0.07, 0.19, 0.42, and
0.84. All measurements were conducted at P=40 Torr and an initial temperature of T=300
K, excited by a pulse burst discharge (pulse repetition rate of 40 kHz and a burst
repetition rate of 10 Hz), for a number of pulses in the burst varying from 1 to 450. It was
clearly evident that the presence of even a small amount of fuel drastically reduces the
amount of quasi-steady state atomic oxygen in the system. Kinetic modeling of these
results gave reasonable agreement.
161
In addition, a study of the role of SDO on the plasma chemistry was conducted in
the same system, both experimentally and computationally. An RF discharge was used to
produce SDO. NO titrant was added to the gas mixture as a method of annihilating O and
O3 quenching species produced in the RF discharge. No effect that could be
unambiguously traced to SDO has been detected so far in the plasma kinetics.
8.2 Pin-To-Pin Discharge Conclusions
Similar to the study conducted in the plane-to-plane discharge, plasma chemical
oxidation was studied in H2 and C2H4 fuel mixtures with air. In this case, 7.5 mm
spherical electrodes were placed in direct contact with the gas mixture. Current and
voltage probe measurements confirmed that this geometry led to significantly higher
coupled pulse energy (8mJ/pulse as opposed to 0.07mJ/pulse) while maintaining a
reasonably large plasma volume.
Atomic oxygen TALIF measurements taken in this geometry show qualitatively
similar results to those in the plane-to-plane geometry. Measurements were taken as a
function of time after a single discharge pulse (60Hz while the laser repetition rate was
kept at 10 Hz). Atomic oxygen decay was found to be very rapid, potentially due to a
complex gas flow resulting from the presence of electrodes in the flow.
8.3 Pin-To-Plane Discharge Conclusions
A comprehensive study of the characteristics of a pin-to-plane nanosecond pulsed
streamer discharge, similar to that used for transient plasma ignition, was conducted.
162
Current and voltage was measured using commercially available probes and imaging of
the plasma was done using an ICCD camera. Atomic oxygen concentration was measured
using the TALIF diagnostic as a function of time after a single discharge pulse.
Measurements were conducted in CH4/air, C2H4/air, and C3H8/air gas mixtures over a
variety of equivalence ratios. All measurements were taken at atmospheric pressure
(~760 Torr) and at an initial temperature of 300 K and in a spatial region directly below
the high voltage anode (pin).
The atomic oxygen concentration in pure air was found to be ~5 x 1017 cm-3,
giving a dissociation mole fraction of ~18%. The rate of decay was ~10 times faster than
what can be reasonably attributed to the three body recombination reactions that are
thought to be the principle loss mechanisms in high temperature (~1000 K) filaments of
this type. Mass diffusion and/or convection are likely the cause of this discrepancy.
8.4 Suggestions for Further Study
In the plane-to-plane geometry, additional measurements to accurately determine
the temperature rise in the plasma as a function of burst size would greatly increase the
accuracy of the kinetic modeling predictions. Pure rotational Coherent Anti-Stokes
Raman Scattering Spectroscopy (CARS) measurements are currently being conducted
and are ideally suited to this task.
In the pin-to-pin geometry, kinetic modeling of the fuel/air mixtures will lead to a
greater understanding of the mechanism behind atomic oxygen generation and decay in
that system. Additionally, temperature measurements via CARS will potentially show a
163
steep temperature rise (compared to the plane-to-plane discharge geometry). Schlieren
imaging of the gas flow may also be useful in determining the gas flow pattern.
In the pin-to-plane geometry, kinetic modeling was not possible due to the
complex geometry of the system. Dedicated research into this area would lead to a firmer
understanding of both the chemistry and physical characteristics of streamer discharges.
Schlieren imaging, as in the pin-to-pin discharge, would be useful for determining the gas
flow pattern.
164
Appendix A
Flow Controller Settings and Equivalence Ratio Calculations
For all of the results presented in Chapters 4 through 6, mass flow controllers
from MKS Incorporated were used. Separate flow controllers were used for the oxidizer
and fuel lines and the values were set and monitored using a four channel readout
device/power supply (Model Number: 247D). There are three settings on the MKS mass
flow controller readout device that must be considered. First, the maximum flow value
must be set for the gas being used. This is done using the Scaling Control Factor (SCF)
on the back of the box. Second, the readout screen on the front of the box must be set to
the proper scale using the decimal adjustment on the back of the box. Finally, the
volumetric flow in standard cubic centimeters per minute (sccm) must be calculated and
set on the front of the device for each gas. The procedure for each of these settings is
given below.
1. Maximum Flow Controller Settings
Each flow controller is calibrated for a maximum flow rate (ranging from
approximately 100 sccm to 5000 sccm) for a specific gas (usually N2 or He). In order to
use a different gas than the one the flow controller is calibrated for, the Scaling Control
165
Factor (SCF) must be set. This term is the product of the gauge factor, G F, and the gas
correction factor ratio.
(Eq. A.1)
The gauge factor is a factory set value which scales the output signal to the
appropriate full scale range for the mass flow controller. In other words, the gauge factor
ensures that the digital panel on the front of the readout device scales properly. The gauge
factor is the same as the first digit of the calibrated flow for the controller. For example, a
mass flow controller calibrated to N2 for 5000 sccm will have a GF = 5, while a mass
flow controller calibrated to He for 100 sccm will have a G F = 1. The calibration
information for the flow controller can be found on the top or side of the device.
Gas Correction Factors (GCFs) can be found on the MKS website
(http://www.mksinst.com/docs/ur/MFCGasCorrection.aspx) for a large number of
frequently used gases. For a gas that is not on the website or a mixture of gases (i.e. air)
the GCF can be calculated using Equation A.2, where the subscripts denote each gas in
the mixture. The fraction of gas for each species is given by F, the molecular structure of
the gas by S (1.030 for diatomic molecules, 1.000 for monatomic molecules, 0.941 for
triatomic molecules, and 0.880 for polyatomic molecules with four or more atoms), the
density of the gas by d, and the specific heat by Cp.
(Eq. A.2)
The SCF term must be less than 1.999, so in cases where it comes out to be
greater than that it must be divided by a factor of ten. To give an example, for argon
166
flowing through a mass flow controller calibrated for 5000 sccm of N 2, the SCF is
calculated as follows:
(Eq. A.3)
Finally, the SCF is set for each mass flow controller via potentiometers on the
back of the readout device. Figure A.1, taken from the supplemental material to the
manual, shows a schematic of this potentiometer. The adjustment knob has both inner
and outer controls to set the factors corresponding to different decimal places.
Figure A.1: Scaling Control Potentiometer - Example Settings
2. Main Gas Flow Settings
Once the SCF is set for all of the gases being used, it is necessary to calculate the
desired flow rate, Q, for one of the gases in the system. For consistency, the oxidizer is
generally used. The first step is to decide the flow velocity (in m/s) that is necessary for
167
the experiment. This value is then converted to volumetric flow rate according to
Equation A.4, where V is the velocity, and r2 is the area of the flow channel. Equation
A.4 gives the volumetric flow rate for all of the gas in the channel.
(Eq. A.4)
Usually, Qox is calculated in units of cubic meters per second. This needs to be
converted to standard cubic centimeters per minute, sccm. Converting from experimental
conditions to standard conditions is straightforward, utilizing the ideal gas law, and
solving for standard volumetric flow, QSt. The subscript "Exp" represents experimental
conditions while the subscript "St" represents standard conditions (1 Atmosphere, 273
K).
(Eq. A.5)
Once the standard volumetric flow is calculated, it is important to verify that the
decimal place on the MKS readout device is in the correct position for the value you are
using. The rule is that the most significant (left most) digit must have a non-zero value
when operating the flow meter at its full range. For example, if the full range of the flow
meter is 2000 sccm (or 2 sLm) then the readout should be set to 0.00 (for three digits).
This will give flow rate in sLm. Operation at 1000 sccm would then be input as 1.00.
Operation at 400 sccm would be read as 0.40. As a second example, if the full range of
the flow meter is 700 sccm, then the readout should be set to 000. and the units become
sccm. In this case operation at 400 sccm would be read as 400. sccm.
Note that the
position of the decimal point is different for these two 400 sccm cases because the
maximum flow rate of the two meters is different. Also note that the maximum flow rate
168
includes the Scaling Control Factor. For example, a 1000 sccm flow meter, based on
calibration for nitrogen, becomes a 720 sccm flow meter if ethylene is the gas controlled
(GCF = 0.72)). Hence a “1000” sccm meter used for this gas has the decimal point set as
000. with readout in sccm units. Similarly a 1000 sccm flow meter, based on calibration
for nitrogen, becomes an ~700 sccm meter if the factory calibration was based on helium
(factor 1.45). The decimal point would again be set to 000.
3. Calculation of Equivalence Ratio
Equivalence ratio is defined as a ratio of ratios; the ratio of oxidizer to fuel under
stoichiometric conditions is compared to the ratio of oxidizer to fuel under experimental
conditions.
(Eq. A.6)
Note that  is dimensionless; there is no difference in value when calculating equivalence
ratiobased on a mass, or molar basis.
(Eq. A.7)
In order to calculate the equivalence ratio of a given system, it is necessary to
balance the chemical equation for combustion in order to get the molar ratio for
stoichiometric conditions. Note that the buffer gas that is present (i.e. N2, Ar, He, etc) is
on both sides of the reaction and is not a factor in the equation for equivalence ratio.
Combustion reactions for some commonly used fuels are shown below (with
stoichiometric fuel/oxidizer mole ratios of 2, 1/2, and 1/3, respectively).
169
N2 + O2 + 2H2 → N2 + 2H2O
(Eq. A.8)
Ar + 2O2 + CH4 → Ar + CO2 + 2H2O
(Eq. A.9)
He + 3O2 + C2H4 → He + 2CO2 + 2H2O
(Eq. A.10)
Substituting molar values for standard volumetric flow values and rearranging
Equation A.4 puts the equation in the form to be solved. If the gas being used is
premixed, the standard volumetric flow of the oxidizer will be the fraction of oxidizer in
the gas mixture (i.e. in premixed air, Qox is 21% of Qair).
(Eq. A.11)
At this point, the standard volumetric flow for both the oxidizer and fuel are
known and can be set in the MKS mass flow readout device.
170
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