22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma assisted low temperature combustion
Y. Ju, J.K. Lefkowitz, X. Yang, S.H. Won and W. Sun
Department of Mechanical and Aerospace Engineering, Princeton University, US-08544 Princeton, NJ, U.S.A.
Abstract: The recent progresses in plasma assisted low temperature combustion, in-situ
mid-IR diagnostics, and studies of low temperature plasma combustion kinetic mechanism
are discussed. The observations of plasma activated direction ignition to flame transition,
cool flames, and mild combustion are presented. The direct measurements of intermediate
species and the kinetic pathways of low temperature ethylene/methanol oxidation in a nonequilibrium plasma reactor and a photolysis reactor are reviewed. The validity of low
temperature plasma combustion kinetic mechanism is discussed. The recent development
high pressure kinetic mechanism for plasma assisted combustion and technical challenges
for future research are summarized.
2. Plasma Assisted Low Temperature Combustion and
Cool Flames
Plasma assisted low temperature combustion was
studied by using NRP discharge in CH 4 /He and O 2 /He
counterflow diffusion flame [5, 6]. The ignition and
flame extinction were measured by using both OH laser
induced fluorescence and the OH* emission intensity.
During the experiments, the strain rate (400 1/s) and the
discharge frequency (f = 24 kHz) were held constant,
while CH 4 and oxygen mole fractions (X F and X O ) were
varied. Fig. 2 compares the dependence of the peak OH*
intensity on the fuel mole fraction at two different oxygen
mole fractions, X 0 = 0.34 and 0.62. The temperature
distributions were also measured by using the Rayleigh
scattering method. As seen in Fig. 2, for X 0 = 0.34, with
the increase of fuel concentration ignition occurred at
X F = 0.26 with a sharp increase in the OH* intensity. On
the other hand, with the decrease of fuel concentration,
flame extinction occurred at X F = 0.20 with an abrupt
decrease in the OH* intensity. This hysteresis of OH*
intensity between ignition and extinction limits represents
the conventional ignition to extinction “S-curve”.
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Increasing energy per pulse
Increasing number of pulses
1. Introduction
Plasma provides a promising solution to control ignition
and flame stabilization of in engines and high speed
propulsion system [1-3]. As shown in Fig. 1 (top) [4],
non-equilibrium nanosecond repetitive plasma (NRP) can
accelerate ignition by varying the discharge pulses and
energy.
Four major plasma assisted combustion
enhancement pathways the thermal effect, the kinetic
effect via electronically and vibrationally excited
molecules and active radicals, the diffusion transport
enhancement via fuel decomposition by direct electron
impact dissociation, and the convective transport
enhancement due to plasma generated ionic wind,
hydrodynamic instability, and flow motion via Coulomb
and Lorentz force, are schematically shown in Fig. 1
(bottom). In this paper, we report the recent progress in
plasma assisted low temperature combustion, in-situ
diagnostics, and kinetic model development.
No Ignition
3 Pulses
3.2 mJ/pulse
1.9 mJ/pulse
0.8 mJ/pulse
20 Pulses
50 Pulses
MSD, 5.7 mJ/pulse, 3 pulses
Plasma discharge
O2+, N2+
Temperature
increase
Radicals
NO, O3
O, H, OH Int. species
N2*, N2(v)
O2 (a1Δg)
Thermal
Ionic wind
Instability
Ions/electrons
Fuel
fragments
Excited
species
Kinetic
H2 , CO
CH4
CH2O
Transport
Combustion Enhancement
Fig. 1. Top: Schlieren images of stoichiometric methaneair ignition 4 ms after initial discharge for both NRP and
MSD ignition systems. NRP pulse frequency = 40 kHz.
Gas flow = 10 m/s from left to right [4]. Bottom: Four
combustion enhancement pathways of plasma assisted
combustion [1].
However, when the oxygen mole fraction was increased
to X 0 = 0.62, the ignition and extinction limits merged,
resulting in a monotonic ignition curve without extinction
limit. The temperature measurements also demonstrated a
similar monotonic increase in the maximum temperatures.
This drastic change of ignition curve demonstrates that
plasma enhances ignition so fast that there is no extinction
limit. The main kinetic pathway for the observed ignition
enhancement is due to the low temperature atomic
O production by reactions such as,
1
e+O 2 =O+O(1D)+e
(R1)
e+He = He*+e
(R2)
*
He +O 2 =O+O+He
(R3)
which subsequently accelerate
reaction of
H+O 2 =OH+O
the
chain-branching
(R4)
O(1D)+CH 4 =CH 3 +OH (R5)
rate, and the peak cool flame temperature. Measurements
of the intermediate species, such as CH 2 O, acetaldehyde,
C 2 H 4 , and CH 4 showed that the current low temperature
kinetic model of n-heptane significantly over-predicted
the QOOH thermal decomposition reactions to form
olefins, resulting in substantial over-estimation of C 2 H 4
and CH 4 concentrations. The new experimental method
for cool diffusion flame formation also demonstrates that
plasma has a promising potential for low temperature
ignition control in engine and fuel reforming.
Therefore, plasma creates low temperature pathways to
enhance ignition of methane at low and intemediate
temperature.
OH number density (cm-3)
7x1015
15
6x10
χO2=34%
χO2=62%
(a) Cool diffusion flame
CH4
5x1015
4x1015
3x1015
2x1015
Smooth
Transition
Extinction
plasma
S-curve
1x1015
Ignition
0.05
0.10
0.15 0.20 0.25 0.30
Fuel mole fraction
0.35
Fig. 2. Change of the dependence of the maximum OH*
intensity on fuel mole fraction at two different oxygen
mole fractions (X 0 = 0.34 and X 0 = 0.62) with NRP
discharge frequency at f = 24 kHz and flow stretch rate of
a = 400 1/s.
However, unlike methane most transportation fuels
such as gasoline, diesel, and jet fuels are large
hydrocarbon fuels, which have strong low temperature
chemistry. In order to understand how plasma assisted
combustion affects low temperature ignition, we have
conducted experiments of plasma and ozone assisted
combustion in both diffusion and premixed counter flow
flames for n-heptane and dimethyl ether fuels.
By using adding ozone produced by a dielectric barrier
discharge in the n-heptane/oxygen diffusion flames [7], it
was discovered for the first time that ozone production by
plasma enabled a successful establishment of
self-sustaining cool flames (Fig. 3) at pressures and
timescales at which the conventional cool flames
normally do not exist. Similar cool flames were also
observed with direct NRP discharge in dimethyl-ether
diffusion flames [6]. The observed cool diffusion flame
formation by ozone and plasma clearly demonstrated that
non-equilibrium plasma can significantly enhance cool
flame chemistry. Moreover, the successful establishment
of cool diffusion flames provides a new platform to
validate cool flame chemistry. Comparisons between
experimental measurements of species concentrations and
numerical simulations showed that the model overpredicted the rate of n-heptane oxidation, the heat release
2
(b) Hot diffusion flame
Fig. 3. Direct photo of n-heptane/oxygen cool diffusion
flame (a) and the normal diffusion flame (b), observed at
the identical flow condition, fuel mole fraction of 0.07
and strain rate of 100 s-1.
3. In situ intermediate species diagnostics and low
temperature oxidation pathways
In situ measurements of plasma generated intermediate
species by mid-IR laser absorption spectroscopy of
C 2 H 4 /Ar and C 2 H 4 /O 2 /Ar mixtures activated by a
nanosecond repetitively pulsed plasma were also
conducted in a low temperature flow reactor (below
500 K) at a pressure of 60 Torr. A recently developed
kinetic mechanism (HP-Mech) for plasma activated C 2 H 4
oxidation was assembled to understand the fuel oxidation
pathways with plasma discharge at low temperature [8].
Fig. 4 (top) shows the comparisons between model
prediction and experiments. It is seen that although the
models (HP-Mech and USC-Mech II) predict the trends of
species evolutions, both models failed in predicting
quantitatively the H 2 O and CH 4 formation. The results
showed that the plasma combustion kinetics has a large
uncertainty for low temperature fuel oxidation. Fig. 4
(bottom) shows the measured and computed species time
histories of methanol oxidation by O(1D) in a photolysis
reactor [9]. The results also show that existing kinetic
mechanisms fails to predict the species concentrations.
The results suggest that there may be many missing
reaction pathways in plasma assisted low temperature fuel
oxidation.
4. Conclusion
New observations of plasma assisted low temperature
combustion and cool flames were reported. The results
show that plasma can enhance low temperature
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10000
[2]
1000
400
300
100
200
10
Temperature (K)
Mole Fraction (ppm)
500
[3]
[4]
100
C2H2, Exp.
CH4, Exp.
H2O, Exp.
T, Exp.
1
0
C2H2, HP Mech
CH4,HP Mech
H2O, HP Mech
T, HP Mech
0.005
C2H2, USC Mech II
CH4, USC Mech II
H2O, USC Mech II
T, USC Mech II
0
0.01
[5]
Time from first pulse (s)
[6]
[7]
[8]
Fig. 4. Top: Measurements and modeling of C 2 H 2 , CH 4 ,
H 2 O, and temperature after 150 pulses at 30 kHz
repetition rate for a mixture of 6.25/18.75/75
C 2 H 4 /O 2 /Ar. Bottom: Time-resolved mole fraction of
H 2 O in 266 nm laser photolysis of 0.112% CH 3 OH,
1.91% O 2 , and 596 ppm O 3 in AR mixture compared to
model simulations. ♦: Experimental measurement; ― :
simulation using the original model; ― : simulation with
modified rates.
[9]
A. Starikovskiy and N. Aleksandrov. “Plasma
assisted ignition and combustion”. Progr. Energy
Combustion Sci., 39, 61-110 (2013)
S.M. Starikovskaia. “Plasma assisted ignition and
combustion”. J. Phys. D: Appl. Phys., 39, R265R299 (2006)
J.K. Lefkowitz, M. Uddi, B.C. Windom, G. Lou and
Y. Ju. “In situ species diagnostics and kinetic study
of plasma activated ethylene dissociation and
oxidation in a low temperature flow reactor”. Proc.
Combustion Inst., 35, 3505-3512 (2015)
W. Sun, S.H. Won, T. Ombrello, C. Carter and Y.
Ju. “Direct ignition and S-curve transition by in situ
nano-second
pulsed
discharge
in
methane/oxygen/helium counterflow flame”. Proc.
Combustion Inst., 34, 847-855 (2013)
W. Sun, S.H. Won and Y. Ju. “In situ plasma
activated low temperature chemistry and the S-curve
transition in DME/oxygen/helium mixture”.
Combustion Flame, 161, 2054-2063 (2014)
S.H. Won, B. Jiang, P. Diévart, C.H. Sohn and Y.
Ju. “Self-sustaining n-heptane cool diffusion flames
activated by ozone”. Proc. Combustion Inst., 35,
881-888 (2015)
J.K. Lefkowitz, T. Ombrello, S.H. Won, C. Stevens,
J. Hoke, F. Schauer and Y. Ju. “Schlieren Imaging
and Pulsed Detonation Engine Testing of Ignition
by a Nanosecond Repetitively Pulsed Discharge”.
Combustion Flame, in press (2015)
X. Yang, J.K. Lefkowitz, B.E. Brumfield, Q. Chen,
G. Wysocki and Y. Ju. “Kinetics studies of
O3/O2/CH3OH/Ar mixtures in a photolysis flow
reactor”. in: 9th US Nat. Combustion Meeting.
(May 17-20; Cincinnati, Ohio) (2015)
combustion dramatically, and can lead to monotonic
ignition to flame transition without an extinction limit and
enable cool flame formation in a significantly reduced
timescale. Comparisons of time dependent measurements
of the formation of intermediate species of ethylene
oxidation in a non-equilibrium plasma discharge and
methanol oxidation by O(1D) with numerical modeling
showed that existing models fail to quantitatively the
intermediate species formation in low temperature plasma
oxidation. The results suggest that there are missing
reaction pathways in low temperature plasma assisted
combustion which need to be addressed in future study.
5. References
[1] Y. Ju and W. Sun. “Plasma Assisted Combustion:
Dynamics and Chemistry”.
Progr. Energy
Combustion
Sci.,
48,
June,
21-83;
doi:10.1016/j.pecs.2014.12.002 (2015)
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