Ignition of N2/O2/C3H8 mixtures by a single nanosecond pulsed
discharge at atmospheric pressure
Sabrina Bentaleb, Pierre Tardiveau, François Jorand, Pascal Jeanney, Lionel Magne, Stéphane
Pasquiers
Laboratoire de Physique des Gaz et des Plasmas, Université Paris-Sud, Orsay, France
Abstract: A single nanosecond pulsed corona discharge applied at atmospheric pressure in airpropane mixtures is investigated. In pure dry air, the discharge presents a diffuse regime which
becomes more and more filamentary when propane is added. In this filamentary regime, the ignition of
propane-air mixtures and the propagation of a self-sustained flame can be realized with a single
nanosecond range pulse. In this paper, we also propose to present time and space resolved Planar
Laser Induced Fluorescence (PLIF) measurements. The purpose is to get a time evolution and a space
distribution of OH radicals in the post-discharge in air-propane mixtures.
Keywords: nanosecond discharge, air-propane mixtures, ignition, PLIF, OH radical
Introduction
The study of nanosecond scale corona
discharges under high pressure lies within the
scope of the use of non-thermal plasmas for car
engine ignition [1]. In this field, plasma
assisted ignition techniques are closely studied
and developed. In this context, the purpose of
our work is a better understanding of the
physical mechanisms implied in the ignition of
lean mixtures of air and hydrocarbons at high
pressure using nanosecond range discharges.
Such kind of discharges could improve the
energy release in the mixtures, promoting the
creation of radicals and excited species instead
of direct heat, and the ignition efficiency [2-34].The nanosecond scale discharge is generated
in a point-to plane configuration, under very
high, fast and short voltage pulses. In these
conditions and under atmospheric pressure, the
discharge exhibits, in pure dry air, a large and
nearly homogenous pattern corresponding to
the development of a multi-electron avalanche
through a direct field ionization mechanism
[5]. Nevertheless this diffuse pattern
disappears as soon as propane is added and the
discharge becomes filamentary [6]. The
filaments
are
thinner
with
higher
concentrations of propane, and the whole
discharge energy increases and gets saturated.
The study shows that, thanks to the properties
of the nanosecond pulse, the discharge is able
to ignite mixtures not only at the pin electrode
but also all along one plasma filament. The
self-sustained flame which is induced is then
different from the one created by conventional
spark plugs. In such classical systems, ignition
is induced in a very restricted area and flame is
spherical. This study shows that nanosecond
discharges are able to create cylinder-like
flames. In order to investigate the specificity of
our discharge for ignition, time distribution of
OH radicals in the post-discharge in airpropane mixtures at atmospheric pressure is
investigated by Planar LIF technic.
Experimental setup
The nanosecond scale discharge is generated
in a 16 mm gap, between a point of 100 µm
radius and a grounded plane. A positive high
voltage pulse is applied to the point by a homemade generator working on propagation of
signals on 50 lines. This power supply gives
a square pulse of several tens of nanoseconds
with a rise time of about 2-3 ns. The length T
of the pulse can be modified from 10 to 40 ns,
by changing the length of the first line of the
power supply. Its amplitude U can be set
between 40 and 60 kV. The system is placed
into an atmospheric pressure cell, in which
synthetic dry air can be mixed with propane at
different concentrations up to a few percents.
The voltage is monitored with a home made
coaxial high bandwidth probe and the current
is recorded through a low inductance resistive
shunt of 0.2 Ω connected between the plane
and the ground [5].
Line 1
Line 2
Triggering
System
Marx
Generator
Spark gap
Capacitive
voltage
probe
ICCD
camera
Resistive
current
probe
Fig. 1 Experimental arrangement
The energy needed by the discharge to develop
is derived from the voltage and the current
records. All these signals are recorded on a
500MHZ bandwidth digital oscilloscope with 4
Gs s-1 sampling frequency per channel.
A device similar to the voltage probe is placed
to generate a triggering signal for the optical
system to be synchronized and used for
discharge imaging. Time resolved imaging is
done with a 12 bits CCD camera coupled to a
pulsed Fragment intensifier and fitted out with
a 75 mm F/1.9 lens. The lens aperture has been
fully reduced to get the best depth of field. The
shortest intensifier gate is 5 ns but the
reproducibility of the discharge can be used to
resolve much more precisely the dynamics of
the discharge, setting the beginning of the gate
and increasing its length by 1 ns at each
shooting. A multi-exposure mode has been
used for flame propagation imaging, displaying
on a single picture several time resolved
periods of a single event. An example of time
sequence is shown on figure 2. In this mode, R
is the delay time between the discharge and the
first exposure, W is the exposure time and TB
is the burst period between each exposure.
system under study. In this experiment, we will
focus on OH radical which is a very reactive
specie generated in pulsed discharge and
combustion in air-propane mixtures. The PLIF
system consists of a Quantel Dye Laser (TDL
50) pumped by a Nd:YAG laser (Quantel YG
580, 10 Hz, pulse of 13 ns). The excitation of
Q1 (1) OH A-X (1,0) transition at 281 ns is
obtained by doubling the fundamental
frequency of the Dye (Rhodamine 590) with a
KDP crystal mounted on an auto-tracking
system in order to achieve a good stability of
the laser beam. The available energy of the
laser pulse is 12 mJ [7,8]. The PLIF diagnostic
requires the excitation of the discharge zone
with a thin sheet of laser light. The generation
of the sheet from the laser beam is done using
a proper set of cylindrical and spherical lenses.
The fluorescence induced by the laser
excitation for the (0,0) and (1,1) band from 308
to 315 nm is focused onto an ICCD camera
(Princeton Instruments 576 G/1). The camera
is equipped with an UV- Sodern 100mm F/2.8
lens and an UG11 filter. It has a 464 x 384
with a resolution of 29 pixels/mm .The images
captured by the ICCD are transmitted to a
computer for processing. The triggering system
of the nanosecond pulsed discharge, the laser
and the ICCD camera are synchronized by a
Master Pulse (Stanford DG 645). A basic
arrangement of PLIF is illustrated below in Fig
3. [9,10]
Triggering
system
Master Pulse
UV spherical and
cylindrical lenses
Laser sheet
Laser
ICCD
UV lens
Filter
Fluorescence
Computer
Fig. 2 Time sequence of image acquisition in a
multi exposure mode
The Planar Laser Induced Fluorescence (PLIF)
is a derivative of the Laser-induced
fluorescence technique which is an established,
selective and sensitive approach for identifying
species concentration without perturbing the
Fig. 3 PLIF basic arrangement
Results and discussion
The study shows the filamentation effect of
the propane on the discharge. Indeed, from the
time when the propane is added, the diffuse
regime of the discharge changes into the
filamentary one. Figure 4 illustrates this effect
for three different propane concentrations at
atmospheric pressure. The filaments seem to
get thinner as the concentration is higher.
Plane
4%
: 5 mm
0%
14 %
For combustion purposes, the single
nanosecond pulse discharge can ignite a
mixture all along a plasma channel of more
than one centimetre. It gives a cylinder-shaped
flame kernel for an electrical energy release of
70 mJ which is comparable to the energy
release in conventional spark plug systems.
Ignition of propane-air mixtures at atmospheric
pressure is possible for an equivalence ratio
limit of 0.65 with several tens of mJ. In figure
6, we can see an example of flame propagation
in a stoechiometric air-propane mixture at
atmospheric pressure.
Pin
Maximum extention of the
diffuse part (mm)
The diffuse pattern of the nanosecond pulsed
discharge in pure air has been described in a
previous paper [5]. It has been explained by a
diffuse background of seed electrons in the
whole gap and the overlapping of several
avalanches developing during the rise time.
However, the more propane is added, the
smaller is the part of the discharge which
remains diffuse. Figure 5 presents the
maximum extension of the diffuse zone of the
discharge for different propane concentrations.
A linear fitting can be made giving the
extension of the diffuse part of the discharge
for every concentration between 0 and 10 %.
With 4% of propane, the avalanche has only
developed 4mm away from the point compared
to 8 mm in pure air. This constriction promotes
the transition into the filamentary regime.
However, since PLIF measurements are done
between 0.5 and 1.5 % of propane, we can
consider that OH distribution is derived in the
diffuse regime of the discharge.
10
8
6
4
2
0
0%
2%
4%
6%
8%
10%
12%
Propane concentration (%)
Fig. 5 The maximum extension of the diffuse part
of the discharge
: 15 mm
Fig. 4 Filamentation of a diffuse discharge in a
propane-air mixture (U = 45kV, T = 13ns, pictures
integrated over 20 ns)
Fig. 6 Ignitions of a stoechiometric air-propane
mixture (U= 45 kV, T=13ns, 4 % of propane)
In order to investigate the specificity of our
discharge for atmospheric ignition, we present
time and space resolved Planar Induced
Fluorescence measurements. Hydroxyl (OH)
radical is excited by the laser sheet generated
across the nanosecond pulse discharge in airpropane mixtures. The fluorescence images
acquired are processed and displayed by
WinWiew/32 software. The systems are
adjusted so that data have a small offset. This
offset assures that low signals will not be
missed and it can be subtracted after the signal
is acquired to prevent it from having any
influence on the data.
The processed images are obtained by
subtracting the background and dividing the
raw image by the flatfield. An example of
processed image is given on figure 7. For each
image, a sum of all the pixels is applied inside
a defined area (white square in figure 7) in
order to get an average value of the OH
density.
issues, the discharge allows ignition of lean
mixtures and induces a cylinder-like flame
propagation instead of a classical spherical
one. For very small amounts of propane, PLIF
measurements allows to follow the time
evolution of OH radical in the post discharge,
showing propane effect on the delay of
appearance of OH maximum density.
Pin
: 3 mm
References
Plane
Fig. 7 An example of OH fluorescence in the
discharge gap (6.5µs from the discharge, 0.5% of
propane, U=45 kV, T=13ns).
Colors are assigned according to intensity of
fluorescence, low intensity being black and high
being red.
This average value can derived for different
times from the discharge and different
concentrations of propane. Results are shown
on figure 8 where data are normalized to the
maximum value.
Normalized PLIF Intensity of OH
0,5%
1%
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0,9
0,8
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0,7
0,6
0,5
0
2
4
6
8
10
12
14
16
18
20
22
Time ( µs )
Fig. 8 Time evolution of OH radical in the postdischarge with three propane concentrations
The PLIF signal increases up to a maximum in
2-3 µs and slowly decreases reaching a
constant value around 20 µs. The more
propane is added, the earlier OH radical
maximum concentration appears in the postdischarge
Conclusions
The experimental study of a nanosecond range
corona discharge in air-propane mixtures
shows interesting preliminary results. The
addition of propane at atmospheric pressure
prevents the discharge from remaining
completely diffuse. Concerning combustion
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