22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma-assisted combustion: fundamental mechanisms and applications
C.O. Laux1,2, D.A. Xu1,2, D.L. Rusterholtz1,2, G. Pilla1,2, D. Lacoste, M. Simeni Simeni1,2, S. Lovascio1,2, G.D. Stancu1,2
and J. Hayashi3
1
CentraleSupélec, Grande Voie des Vignes, FR-92290 Chatenay-Malabry, France
Laboratoire EM2C, CNRS UPR288, Grande Voie des Vignes, FR-92290 Chatenay-Malabry, France
3
Combustion Engineering Laboratory, Mechanical Engineering Dept., Osaka University, Yamadaoka2-1, Suita,
JP-565-0871 Osaka, Japan
2
Abstract: Studies of nanosecond repetitively pulsed discharges used for plasma-assisted
combustion studies have been conducted. Their thermal, chemical, and hydrodynamic
effects have been evidenced. Applications to methane, propane and kerosene-air flames, at
pressures up to10 bar, have shown that these discharges can efficiently enhance lean flame
ignition and flame stabilization. Their effect on pollutant emissions (NO, CO) is examined.
Keywords: nanosecond repetitively pulsed discharges, flame stabilization, ignition, lean
flames, diluted flames, kinetics, ultrafast heating, pollutant emission, lean extinction limit
1. Introduction
Currently, over 85% of the primary energy conversion
processes are based on the combustion of fossil
hydrocarbons, and this fraction is expected to remain
stable in the foreseeable future.
However, the
environmental impact of fossil fuels has led the
automotive, aeronautical, and power generation industry
to seek new solutions in order to improve combustion
efficiency, reduce pollutant emissions, stabilize lean
flames, ignite lean or diluted mixtures, control
combustion instabilities, and develop novel propulsion
systems. Plasma-assisted combustion is seen as one of
the most promising method to develop new combustion
strategies, and as a growing field for novel applications of
high pressure plasma discharges. Beneficial effects have
been demonstrated in laboratory scale burners under the
action of corona discharges [1, 2], microwave discharges
[3], gliding arc discharges [4], nanosecond repetitively
pulsed discharges [5-7], pulsed nanosecond dielectric
barrier discharges [8-10], or volume nanosecond
discharges [11-12]. Moreover, significant progress has
been made toward understanding the fundamental thermochemical effects of plasma discharges in combustion, and
toward demonstrations in laboratory-scale devices.
Plasma discharges produce thermal effects, with various
degrees of gas heating that have been quantified
depending on the thermal or non-thermal nature of the
discharge, and chemical effects, with the production of
active species such as radicals, ions, or excited atomic and
molecular species. The combined effects of heating and
radical production enhance chain branching and
propagation and may also mitigate chain termination
reactions [13, 14]. Plasma discharges also act on the flow
field via momentum transfer (ionic wind) or via heatinginduced hydrodynamic effects such as shock waves
[10, 15].
Among the various types of discharges,
Nanosecond Repetitively Pulsed (NRP) discharges have
been particularly studied because they present the
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advantages of high power efficiency in the production of
active species, and their high frequency operation
(100-100 kHz) induces a synergy between the effects of
individual pulses. They are particularly well suited for
stabilization applications, but can also be used for ignition
applications where they allow the use of sub-breakdown
fields, hence easier operation. We first discuss the
chemical, thermal and hydrodynamic effects of NRP
discharges in air and then use these results to analyse
illustrative experiments of flame ignition and stabilization
by NRP discharges.
2. Thermal, chemical, and hydrodynamic effects
The nature of the active species created varies with the
applied reduced electric field E/N. At low E/N (10 to
80 Td), discharges excite the rotational, vibrational and
low-energy electronic states of neutral particles, resulting
in the production of species such as ozone or metastable
oxygen (a1Δ g ) [16]. At higher E/N (100-300 Td),
discharges excite electronic levels of nitrogen which
subsequently dissociate oxygen or fuel molecules through
fast dissociative quenching processes. Atomic oxygen
then rapidly produces combustion-enhancing species such
as OH, H 2 and CO, but also nitric oxides. At even higher
fields (300-1000 Td), ion recombination produces even
higher gas heating and radical production [12, 13].
We have conducted a detailed study of the effects of
NRP discharges for a reference case corresponding to
typical conditions of flame ignition and stabilization
applications. In this reference case, a 10 ns, 5.5 kV,
10 kHz NRP spark discharge is applied between two pin
electrodes separated by a distance of 4 mm in an
atmospheric pressure air flow (v = 2.6 m/s) preheated to
1000 K by means of a resistive heater. Advanced optical
diagnostics including absolute Two-photon Absorption
Laser Induced Fluorescence (TALIF), Cavity Ring-down
Spectroscopy (CRDS), and absolute Optical Emission
Spectroscopy (OES) have been used to quantify the
1
Absolute densities [cm-3]
Temperature [K]
30
20
10
0
Voltage (V)
Current [A]
40
6
5
4
3
2
1
0
2500
V
300
250
200
150
100
50
0
Iconduction
E/N [Td]
production of chemical species, particularly atomic
oxygen, as well as the temperature increase resulting from
the application of the NRP discharge [17-19].
This study produced one of the most important insights
into the effects of NRP discharges. The main results are
summarized on Fig. 1. The NRP spark produces a peak
conduction current of about 35 A after 10 ns. After 7 ns,
the voltage across the discharge gap decreases sharply
because the plasma becomes highly conductive. At the
peak of conduction current, the reduced electric field is
about 200 Td, which corresponds to conditions where the
discharge produces significant excitation of the electronic
states of N 2 [20]. The gas temperature, measured from
the rotational distribution of the first and second positive
systems of N 2 increases rapidly during the pulse, from
about 1500 K before the pulse to about 2500 K after
20 ns. As will be seen below, this increase can be
attributed to an ultrafast heating mechanism. The initial
value of 1500 K, higher than the incoming air flow
temperature of 1000 K, is the result of residual heating by
previous pulses. Because the gas heats faster than the
hydrodynamic expansion time scales, the gas pressure
increases proportionally to the gas temperature, i.e., up to
about 1.7 atm.
N2 + e → N2* + e
(1)
N 2 * + e → O + O + heat
(2)
where N 2 * stands for various excited electronic states of
N 2 (A3Σ u +, B3Π g , B’3Σ u -, W3Δ u , C3Π u , a’1Σ u -).
Comparisons with simulations performed by Popov
[22] using the two-step mechanism are shown in Fig. 2.
Their good agreement with the measurements confirms
the two-step mechanism. The main contributor to
ultrafast heating and oxygen production under the
conditions of the reference case are the B3Π g state of N 2 ,
as well as its neighbouring B’3Σ u - and W3Δ u states, which
together are responsible for more than 80% of the oxygen
and heat produced by the discharge.
Temperature
from N2(C-B)
from N2(B-A)
2000
1500
1.2x1018
1.0x1018
8.0x1017
6.0x1017
4.0x1017
2.0x1017
0
1017
1016
1015
1014
1013
1012
-10
O (3P) density
Fig. 2. Comparison of experimental data (squares)
[18-20] and simulations (lines) (Popov [22]) of the
temporal evolution of the temperature and atomic oxygen
density in the reference case (air at 1000 K and 1 atm).
N2(B)
N2(A)
N2(C)
0
10
20
30
Time (ns)
40
50
Fig. 1.
Synchronized measurements of conduction
current, voltage, temperature, O (3P), N 2 (A), N 2 (B),
N 2 (C) densities in air (1 atm, 1000 K) excited by NRP
discharge applied at 10 kHz with 0.67 ± 0.02 mJ/pulse.
The temporal evolution of the absolute density of
ground state atomic oxygen (O3P) was measured by
TALIF [17-19]. In less than 20 ns, the O density
increases to about 1018 cm-3, which corresponds to about
50% dissociation of molecular oxygen. This surprisingly
high dissociation fraction is again the effect of the
ultrafast mechanism. The O density before the pulse,
2
2x1017 cm-3, corresponds to the residual density produced
by the previous pulses.
The temporal evolution of the absolute densities of the
N 2 B3Π g and C3Π u electronic states show a rapid increase
during the pulse with a peak coinciding with the peak of
conduction current, then a fast decay due to dissociative
quenching of O 2 . The change in decay slope after about
20 ns is attributed to the decreasing O 2 density [19]. It is
important to note that the decay phases of N 2 B3Π g and
C3Π u coincide with the ultrafast rise of the atomic oxygen
density and of the gas temperature. This key result
experimentally validates the ultrafast mechanism of
heating and oxygen production as corresponding to the
two-step mechanism initially proposed by Popov [21]:
A consequence of ultrafast heating is the formation of a
heated gas column and shock waves. This was confirmed
in Ref. [15] from time-resolved Schlieren images (see
Fig. 3), and numerically reproduced in 2D simulations
[23].
Fig. 3. Schlieren images of NRP spark discharge in room
air, gap distance = 2 mm, PRF = 1 kHz, flow velocity =
1.6 m/s. The discharge pulse begins at t =0 ns [15].
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22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
The mechanism of nitric oxide production by NRP
discharges was recently investigated in Refs. [24, 25].
Using measurements and simulations of O, N and NO
densities in the afterglow of a nanosecond discharge,
these authors attributed the formation of NO to quenching
reactions of N 2 * by O:
N 2 * + O → NO + N
(3)
Kinetic simulations in remarkable agreement with the
afterglow measurements were obtained by assuming a
near kinetic rate for involving the N 2 A3Σ u +, B3Π g , B’3Σ u , W3Δ u , C3Π u , E3Σ g +, a’1Σ u -, a1Π g , w1Δ u , and a”1Σ g +.
3. Application to ignition and flame stabilization
3.1. Ignition of lean/diluted flames
Two important issues for the ignition of lean or diluted
flames are the reduction of the ignition delay time and the
enhancement of the speed of flame propagation. Many
non-equilibrium discharges can readily reduce the ignition
delay [1, 2, 6], but flame speed enhancement is more
difficult. This is because the plasma kernel can efficiently
enhance reaction kinetics in the interelectrode gap, but
has little effect in the combustion chamber volume where
the flame propagates. An enhancement of the flame
speed by about 10-20% was nevertheless demonstrated by
Xu [26] by applying NRP discharges in a lean propane-air
mixture at 10 bars, as shown in Fig. 4.
Conventional
10 ms
3.2. Stabilization of lean premixed propane-air flames
Several laboratory demonstrations have been made to
stabilize lean premixed flames using NRP discharges. In
a first series of experiments, a lean, premixed, turbulent,
atmospheric pressure propane-air flame of power 11 kW
was stabilized by a discharge generated with a solid state
pulser (FID Tech) producing pulses of amplitude up to
10 kV, 10 ns duration, and pulse repetition frequency
(PRF) up to 30 kHz. The discharge was created between
an aluminium bluff body and a refractory steel electrode
located in the recirculation zone 5 mm downstream of the
bluff body, as shown in Fig. 5. In this experiment,
stabilization was obtained with about 2.5 mJ/pulse,
corresponding to an average power of 75 W at 30 kHz.
This power is less than 1% of the power released by the
stabilized flame. At these conditions, the discharge
produces a so-called NRP spark [15], with a uniform
electric field between the electrodes. In this experiment,
the plasma discharge extended the domain of flame
stability to fuel equivalence ratios about 20% lower than
without plasma. The stabilization was attributed to the
action of the oxygen radicals created by the discharge.
These radicals quickly oxidize the fuel and produce longlived OH radicals, which are then advected in the
recirculating flow toward the shear layer at the edge of the
bluff body, where they promote combustion. A thermal
effect is also likely, although the relative importance of
chemical and thermal effects has yet to be determined.
15 ms
5 ms
32 mm
NRP 5 ms
10 ms
15 ms
Fig. 5. Bluff-body nozzle for the premixed propane- air
flame experiments at P = 1 atm.
Fig. 4. Ignition of a lean (ϕ = 0.7) propane-air mixture at
10 bar by conventional (1 pulse, 3.5 ms, 57 mJ) and NRP
(82 pulses, 10 ns, 30 kHz, 2.7 ms, 55 mJ) discharges [26].
The enhancement effect was attributed to a wrinkling of
the flame front as a result of the hydrodynamic effects
induced by the multiple discharges. Nevertheless, this
effect may be limited in real internal combustion engines
where fuel/air injection creates a strongly turbulent
aerodynamic flow field that may reduce the flame
wrinkling effect. Other possible solutions are currently
being explored include the volumetric excitation of the
combustible gas using diffuse microwave discharges [27].
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Stabilization of a higher power, swirled stabilized flame
was also investigated in a 50 kW two-stage swirled
injector [30]. The NRP discharge was generated at the
outlet of the fuel rich (Φ p = 2.4) primary stage mixing
zone with pulses up to 30 kV in amplitude, 10 ns in
duration, and 30 kHz PRF. Additional air was injected
through the secondary stage at a flow rate four times
higher than that of the primary stage. The global
equivalence ratio in the combustion chamber was thus
Φ g = 0.47. Then, for a constant airflow rate, the global
equivalence ratio was decreased until flame extinction,
with and without discharge. The lean extinction limit
without discharge was reached for Φ g = 0.41. With
plasma, it was lowered down to Φ g = 0.11 (with
3
Φ p = 0:57). Thus the lean extinction was reduced by a
factor of about 4, with a discharge power of only 350 W,
i.e., 0.7% of the flame power P = 53 kW.
Beneficial effects were also obtained at higher power,
higher pressure and with kerosene fuel. For example, a
study was conducted with a 200 kW turbulent
aerodynamic injector working with kerosene/air at 3 bar
[31]. In this case, the lean extinction limit was decreased
from Φ = 0.44 to Φ = 0.21 using a similar NRP discharge,
again with only about 1% of the flame power.
4. Pollutant emissions
The emission of pollutants is an important issue in
plasma-assisted combustion.
In premixed laminar
methane-air flames at 1 atm, Bak et al. [28] showed that
complete combustion could be achieved with NRP
discharges at equivalence ratios above the normal (i.e.,
without plasma assistance) lean flammability limit
Φ = 0.53. At lower equivalence ratios down to Φ = 0.42,
the plasma-assisted combustion was not complete but the
discharge were nevertheless able to hold the flame.
Measurements of CO and NO were also made [31] with
mid-infrared laser absorption spectroscopy at the exit of
the burner depicted in Fig. 5. For lean methane-air
flames, CO emissions remained very low, both with and
without plasma. On the other hand, NO emissions did not
decrease with the equivalence ratio, as would be expected
in lean flames. Rather, the measured NO concentration
remained at the level of a stoichiometric flame. This
effect suggests that NO is produced by a non-thermal
mechanism, likely via the quenching reaction R3
mentioned at the end of Section 2. Future work will
examine how the kinetic pathways can be modified to
reduce NOx emissions, by varying the energy per pulse,
the reduced electric field, or the PRF.
[4]
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[6]
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[9]
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[17]
5. Conclusions
All laboratory demonstrations conducted to date have
shown that NRP discharges can significantly reduce the
ignition and extinction limits of lean flames with high
power efficiency. Thermal, chemical, and hydrodynamic
effects have been clearly identified, and their relative
importance varies depending on the flame configuration.
Plasma assisted combustion thus appears as a highly
promising path for new engine development.
[18]
6. Acknowledgments
This work has been supported by the PLASMAFLAME
and FAMAC ANR programs.
[22]
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