Plasma-Assisted Ignition and Combustion
349
combination of short high-voltage pulse and constant bias allow to provide selective and
extremely nonequilibrium excitation of the gas. Critical high-voltage pulse duration
depends on the gas parameters (density, composition) but for practically important range of
parameters is restricted to few nanoseconds.
Thus the possibility of selective excitation of the gas by electric discharge critically depends
on the possibility of ultra-short high-voltage pulses generation. Figure 19 demonstrate
recent progress of solid-state generators based on “turn-on” FID and “turn-off” DRD
switches according to FID GmbH data [Efanov et al, 2011]. In modern pulsers the pulse rise
time goes down to 80 ps, voltage rise rate reaches 1 MV/ns, maximal voltage 2-10 MV, and
maximal current up to 100 kA. Wide range of possibilities proposed by current progress in
solid-state electronics will lead to the increase of our abilities of nonequilibrium plasma
generation with predicted properties.
Fig. 19. Progress in ultra-short high-voltage pulse generators. A) typical nanosecond pulse
shapes [Roupassov et al, 2008]; b) generators frequency-voltage map [Efanov, 2011].
2.2 Non-equilibrium plasma recombination and energy relaxation
For efficient production of large amount of active particles in the gas discharge it is
necessary both efficient generation in the gas discharge plasma and slow recombination in
collisions with major mixture components.
2.2.1 Rotational relaxation
Due to fast rotational-translational (RT) relaxation, rotational degrees of freedom of the
molecules are quenched rapidly. This process requires few collisions only. For example, for
rotational relaxation in air O
2
(rot) + M → O
2

molecules can be accumulated
in the discharge with intermediate E/n values.
VT relaxation leads to slow thermalization of vibrational energy of the molecules. This
process becomes faster if the mixtures contain hydrocarbons. For example, VT relaxation of
molecular oxygen on methane in stoichiometric methane-air mixture at T = 1000 K and
pressure 1 atm has a characteristic time t ~ 1.3 s. Fast relaxation does not allow to maintain
a significant deviation of vibrational temperature from translational on the long time scale.
From the other hand, VT relaxation of oxygen in H
2
-air mixture lasts ten times longer and
reaches t ~ 15 s for T = 1000 K and P = 1 atm (29% H
2
in the mixture). VT-relaxation of
hydrogen in the same mixture takes approximately 380 s. Thus, vibrational excitation of
hydrogen molecules can be very far from equilibrium during the ignition delay time and
can effect significantly the radical’s production.
Under uncompleted vibrational relaxation conditions chemical reactions between vibrationally
excited molecules play an important role. There are several theoretical models for rate
coefficients of reactions between excited reagents. Almost all these models were developed as
an engineering substitution of time-consuming ab initio calculations [Kovach et al, 2010;
Adamovich et al, 1996; Macheret et al, 1994; Park, 1988]. A model of vibrational energy usage
was developed in [Losev et al, 1996]. The model assumes the decrease of the reaction threshold
by E
vib
. The efficiency of vibrational excitation  can be estimated using activation energy
and thermal effect of the reaction. A model proposed by Macheret [Macheret et al, 1994] allows
to estimate the rate constant of simple exchange endothermic reaction. The model requires the
fraction of energy release in the reverse reaction directed to vibrational excitation and it is
applicable only to a certain type of reactions [Kovach et al, 2010].
It should be noted that almost all analytical models available estimate reaction rate constants

H
2
(v=1) + O(
3
P)  H + OH(w=0) (≤4.7∙10

cm
3
/s)
Experimental measurements show that the averaged factor of vibration energy usage in this
reaction is = 0.31 [Rusanov&Fridman, 1985]. Figure 20,a shows results of calculation of the

Plasma-Assisted Ignition and Combustion
351
reaction rate constant at translation temperature T
tr
= 300 K for various vibration
temperatures with the Boltzmann distribution of molecules over vibrational levels. The
dependence calculated using model [Starikovskii, 2003] is in good agreement with
calculation by -model with experimentally found  = 0.31 at overheating degree T
vib
/T
tr
<
5 (Fig. 20,a). Model [Starikovskii, 2003] predicts the ratio k(v=1)/k(v=0) = 2795, which is in
perfect agreement with experiments [Light& Matsumoto, 1978] (2600). Ratio of channels to
OH(w=1) and OH(w=0) at T = 300 K estimated in [Starikovskii, 2003] is equal to
k(w=1)/k(w=0) = 7.9, which also is in good agreement with experiments [Light&
Matsumoto, 1978] (>2).
The analysis of the reaction rate constant dependence on the vibrational excitation degree for

v=1
= 1.810
-12
cm
-3
s
-1

(k
v=1
/k
v=0
= 620).
Thus, vibrational excitation of reagents can significantly accelerate chemical reactions. The
influence of vibrational excitation is limited by VT-relaxation of the molecules. This process
becomes extremely fast in the presence of hydrocarbons. In mixtures with hydrogen the
efficiency of vibrational excitation increases because of relatively slow vibrational relaxation
of H
2
. Analysis of [Zatsepin et al, 2001] shows the oxidation rate increase in H
2
-air mixture
at T = 300K in 3-5 times.
Fig. 20. Dependence of the rate constant of reaction on non-equilibrium excitation degree
T
vib/
T

K(T , T )/k(T )
vib tr tr
T/T
vi
b
tr
H(v)+OH=HO+H
22

Aeronautics and Astronautics
352
Preliminary excitation of molecular vibrations of H
2
was shown to lead to a noticeable
decrease in the induction zone length and the distance at which the detonation wave was
formed. It was demonstrated that the reason for these effects was an intensification of chain
reactions in the H
2
–O
2
(air) mixture owing to the presence of vibrationally excited hydrogen
molecules in the flow [Bezgin et al, 2006].
2.2.3 Electronic levels excitation and relaxation
At E/n ~ 100-500 Td the main channel of gas excitation is population of electronic degrees of
freedom by electron impact and by energy exchange between vibrationally-excited states.
An important exception from this rule is singlet state of molecular oxygen О
2
(a). This state
has a low excitation threshold and the maximum efficiency of its population corresponds to
E/n ~ 3-10 Td.

2
(b
1

g
+
) [1.64 eV] + H
2
 OH + OH
c. N
2
+e  N
2
(A
3

u
+
) [6.2 eV] + O
2
 N
2
O + O(
3
P)
2. Excitation of the molecule to repulsive or pre-dissociative term leads to molecule
dissociation and formation of two radicals:
a. O
2
+ e  O


g
+
)[11.8 eV]  H(
1
S) + H(
1
S) + e
3. Excitation of the molecule and dissociative quenching of excited state by another
molecule:
a. N
2
+e  N
2
(C
3

u
)[11.02 eV] + O
2
 N
2
+ O(
3
P) + O(
1
D)
b. N
2
+e  N

1
S)
4. Excitation of the molecular electronic state with radiative depopulation, high-energy
photon flux generation and dissociation (ionization) of gas molecules by this radiation:
a. N
2
+e  N
2
(B
1

u
) [12.5 eV]  N
2
+ h  O
2
+ h  O
2
+
+ e
b. N
2
+e  N
2
(B
1

u
) [12.5 eV]  N
2

2
mixtures, in [Zatsepin et al, 2001] for H
2
-O
2
-N
2
mixtures and in [Anikin et al, 2006]
for C
x
H
y
-O
2
mixtures. It should be noted however, that channel branching, rate coefficients
and even products of such reactions are not very well known. The first group of processes
was investigated much better, than second and third. Simultaneous presence in the plasma
of all sorts of excited particles and radicals makes detailed kinetic analysis an extremely
challenging and resource-consuming task. As an example we just mention that mixture
composition variation, very popular approach in combustion chemistry, will not work in
plasma chemistry because simultaneously with afterglow kinetics variation we will change
electron energy distribution function in the discharge phase and kinetics of gas excitation.
Mechanism (I) requires very low electric field to increase the efficiency of the excitation
process because of low energy threshold for oxygen singlet states population. On the
contrary, mechanisms (II)-(IV) require high E/n value and high electron energy for upper
electronic states excitation.
2.3 Low-energy electronic states excitation
Singled oxygen molecules as a tool for ignition and combustion control were proposed by
group of Starik [Smirnov et al, 2008]. The effect of the excitation of oxygen molecules to the
O

significantly (by a factor of 2.5) the velocity of flame propagation for the fuel lean hydrogen–
oxygen mixture. For stoichiometric and fuel rich mixtures the increase in flame velocity due
to an abundance of singlet oxygen molecules in the mixture was found to be significantly
smaller (about a factor of 1.1). Later the same team proposed to use a laser radiation at λ =
762.346 nm for O
2
molecules excitation to the b
1

g
+
electronic state. Experimental observation
of the shortening of the induction zone length in a premixed mode of combustion in a
subsonic H
2
–O
2
low pressure flow due to the presence of oxygen molecules excited to the
singlet a
1

g
electronic state was reported in [Smirnov et al, 2008]. The low pressure electric
glow discharge was used to produce singlet oxygen molecules. The analysis showed that
~1% of O
2
(a
1

g

discharges for plasma assisted ignition and flame stabilization. The idea was to maintain an
extremely high electrical field for a short period of time. This approach allows to generate
highly-excited nonequilibrium plasma with the energy distribution shifted to the electronic
excitation and dissociation. Short pulse duration restricts the plasma conductivity increase
and keeps the energy density in the gas on the relatively low level (equivalent gas heating is
in the range of 10-100 K). Paper [Starikovskiy et al, 2011] summarizes the requirements to
the pulse discharges to maintain the high efficiency of excitation:
1. High-voltage pulse amplitude is limited to set the value of the reduced electric field E/n
> 200-300 Td in the discharge gap which provides optimal conditions for dissociation of
molecular oxygen by electron impact and quenching of nitrogen excited states (in air
and lean fuel-air mixtures).
2. High-voltage rise dU/dt > 3001000 kV/(nsatm) to obtain the field intensity sufficient
for homogeneous ionization wave formation. This condition allows to achieve the
homogeneous gas excitation in the gap and simplifies the analysis of the kinetic data. It
shold be mentioned, however, that for practical applications inhomogeneous excitation
may have specific advantages in some cases (for example, reduction of energy
consumption).
This type of the discharge was used in [Zatsepin et al, 2001] to investigate low-temperature
kinetics in plasma of pulsed nanosecond discharge. Oxidation of molecular hydrogen in
stoichiometric hydrogen-air mixture in the Fast Ionization Wave (FIW) was studied at total
pressures p = 1-8 Torr, and the detailed kinetics of the process has been numerically
investigated. The excitation of the gas in FIW and dynamics of molecular hydrogen
concentration were monitored with the use of measurements of absolute H
2
radiation intensity
(transition a
3

g
+

, H
2
O and OH-radical. The most important
processes in each time interval in plasma afterglow and radicals recombination were
identified. Because the overall picture observed in [Zatsepin et al, 2001] is very typical for
plasma assisted ignition by pulsed discharges, we will analyze it in more details.

Plasma-Assisted Ignition and Combustion
355
2.5 Kinetics of plasma assisted combustion below self-ignition threshold
The mixture compression in the engine before the ignition leads to temperature increase. For
example, in IC engines initial temperature is close to 600 K, in GTEs – 600-700 K, in
SCRAMjets 650-800 K. In these cases the initial temperature of the mixture is below or close
to self-ignition threshold. That is why this range of parameters attracts in attention of
researchers. From the other hand, this temperature interval is poor investigated from the
point of view of chemical kinetic mechanisms. The problem is the lack of data for low-
temperature mechanisms validation. As an example, methane combustion GRIMech-3.0
model was validated in the range 1250 – 2500 K. C1-C4 Konnov’s mechanism was validated
down to ~910 K, hydrogen Popov’s mechanism [Popov, 2008] – to 880 K. Direct
extrapolation of these models down to room temperature conditions or even to intermediate
temperature range below self-ignition threshold, of course, is very questionable (see, for
example, analysis in [Uddi et al, 2011]). Thus the task of kinetics investigations in low
temperature region becomes extremely difficult and complex. We have to take into account
kinetics in gas discharge and plasma afterglow and almost unknown mechanisms of
chemical chains initiation under low temperature conditions.
Another problem of investigations of kinetics in plasma is gas discharge inhomogeneity.
Under low pressure conditions homogeneous gas ionization and excitation can be achieved
even with rather slow voltage increase across the discharge gap. Pressure increase requires a
sharp decrease of the voltage rise time (relations 1)-2) above suggest to keep the voltage rise
rate on the level of ~ 1 MV/ns/atm for room temperature air to achieve homogeneous

(B-A) transitions were carried out in nanosecond
pulsed discharges in air and pure nitrogen. 0-D kinetic simulations coupled with energy
equation are conducted to predict quenching rate coefficients of quenching of N
2
* by N
2
and
dissociative quenching of N
2
* by O
2
by matching the simulated emission curves to the

Aeronautics and Astronautics
356
corresponding measurements. The dissociative quenching was found to be responsible for
82 % of O production whereas the electron-impact dissociation was ~5%.
Papers [Pai et al, 2009, Stancu et al, 2010] reports the results of investigations of nanosecond
repetitively pulsed discharge in atmospheric pressure discharge in air or nitrogen preheated
at 1000 K. The ground state of atomic oxygen was measured by two-photon absorption laser
induced fluorescence, the density of N
2
(A) was measured by cavity ring down spectroscopy
and the densities of N
2
(B) and N
2
(C) were measured by optical emission spectroscopy.
Measurements of O, N
2

auto-ignition and plasma assisted ignition is a high concentration of radicals from the very
beginning of the process and potential influence of non-equilibrium mechanisms with
participation of vibrationally- and electronically- excited particles and ions.
The challenge of high-temperature experiments is the controllable heating of the mixture in
combination with the homogeneous non-equilibrium excitation by gas discharge. The
problem was solved in [Kof&Starikovskiy, 1996-1; 1996-2] where the combined excitation of
the combustible mixture by shock wave and fast ionization wave was proposed.
Experimental installation was based on the shock tube coupled with the discharge section.
Discharge was generated by Marks-type high-voltage pulse generator. The generator
consisted of 10 steps and operated at U = 80-250 kV. Ferrite line with non-linear response

Plasma-Assisted Ignition and Combustion
357
and an impedance of 40 Ohm allowed to decrease the pulse leading front down to 500 ps.
The voltage increase rate on a high voltage electrode was up to 500 kV/ns and allowed a
fast ionization wave formation in the discharge section.
Ignition delay time was analyzed for oxygen-hydrogen mixtures and numerical analysis of
chemical kinetics was performed for simultaneous mixture excitation by shock wave and
high voltage ionization wave. Ionization wave influence (U ~ 250 kV, t
pulse
~ 40 ns) on the
ignition delay time of the mixture H
2
:O
2
:N
2
= 5:19:76 at p = 1 atm was investigated. High
efficiency of the fast ionization wave for spatially-uniform excitation of the chemically-
reacting systems has been found [Kof&Starikovskiy, 1996-1; 1996-2]. The experimental

C
2
H
6
- to C
5
H
12
-containing stoichiometric mixtures with oxygen with 90% Ar dilution as a
function of the gas temperature for autoignition and plasma-assisted ignition. The effect of
gas discharge leads to a drastic decrease in the ignition delay and to ignition of the mixtures
at noticeably lower temperatures and gas number densities.
Good agreement between the measured and calculated ignition delay time after the
discharge in most cases studied shows that the developed kinetic model adequately
describes PAI under the conditions considered. Simulation of discharge processes was also
validated by comparison between calculated and measured temporal evolution in the
discharge current and in the specific energy deposited in the discharge phase.
It should be mentioned that kinetic model of active particles formation in the discharge used
in [Kosarev et al, 2009; Aleksandrov et al, 2009] is significantly simplified and rate
coefficients of number of processes are not very well known. It is related to ions composition
and in part to composition of hydrocarbon radicals. Fortunately under conditions of typical
lean mixtures combustion the atomic oxygen always plays a major role. Atomic hydrogen
and hydrocarbon radicals are less important but processes of their formation are also well
investigated and could be modeled with rather high accuracy. Uncertainty in the radical’s

Aeronautics and Astronautics
358
relative composition is not of critical importance under such conditions because the ignition
delay time and rate of chemical energy release at high temperatures does not significantly
depend on the radical’s nature [Aleksandrov et al, 2009]. Thus even the plasma-chemical

H
12
:O
2
:Ar mixture [Kosarev et al, 2008].
3.1 Vacuum Ultraviolet Emission of the discharge
It is well known that both equilibrium and non-equilibrium plasma are strong sources of
vacuum ultraviolet radiation (VUV). Absorption of VUV radiation by oxygen leads to
molecular oxygen dissociation with quantum efficiency close to one. Thus ultraviolet
sources potentially can generate high concentration of active radicals in the gas.
In [Berezhetskaya et al, 2005] the possibility to utilize the discharge VUV self-emission for
ignition stimulation has been considered. The pulsed microwave radiation was generated by

Plasma-Assisted Ignition and Combustion
359
a MI-389 magnetron. The radiation parameters are the following: peak power P
i
≤ 400 kW,
pulse duration 
f
≤ 50 ms, wavelength 
f
~ 2.5 cm, repetition frequency f ~ 10 Hz. It was
shown that, both in hydrogen-oxygen and in methane—oxygen media, the non-self-
sustained discharge initiates the primary combustion wave with relatively low temperature
and low glow intensity. In contradiction to the literature data, the experiments did not show
a significant difference between the propagation velocities of combustion waves in
hydrogen- and methane-containing media, and considerable (above 1000 K) temperature
jumps were observed behind the front of the primary wave. After the primary wave started
from the initiator passed some distance, a bright burst occurred rapidly and almost

recombination mechanism). Fast termalization of ionization energy leads to effective gas
heating in microsecond time scale [Aleksandrov et al, 2009]. Combination of these two
factors – very high energy price of ionization and very high rate of recombination – makes
the ionization ineffective from the point of view of plasma assisted combustion. Authors of
[Anikin et al, 2001; Starikovskiy, 2003] showed that the efficiency of radicals production in
air-fuel mixtures has a maximum at E/n ~200-400 Td. Further increase of electrical field
value leads to shift of discharge energy distribution to gas ionization and increases the price
of radical’s production.
Of course detailed analysis of the efficiency of gas ionization on the ignition process should
take into account the gas composition, temperature, pressure and plasma density. For
example for high concentration of electrons the main process of plasma recombination is
electron-ion dissociative recombination [Aleksandrov et al, 2009]:

Aeronautics and Astronautics
360
N
2
+
+ e → N(
4
S) + N(
4
S,
2
D)
O
2
+
+ e → O(
3

+ N
2

O
2
+
+ O
2
+ M → O
4
+
+ M
Recombination of cluster ions O
4
+
and N
4
+
is order of magnitude faster process than
recombination of molecular ions and at low temperatures (~300 K) and electron
concentration n
e
~ 10
12
-10
13
cm
-3
is the main process:
N

2
prevails and suppresses the atomic oxygen formation in plasma
afterglow, increasing simultaneously the energy release to translational degrees of freedom.
At low plasma density (n
e
~10
10
cm
-3
) and high oxygen concentration the main channel of
electron losses is an attachment [Aleksandrov et al, 2009]:
O
2
+ e + M → O
2
-
+ M
Ion-ion recombination becomes the most important process:
O
2
-
+ O
2
+
+ M → O + O + O
2
+ M
O
2
-

2
-
+ M
H
2
+ O
2
-
→ OH
-
+ OH

Plasma-Assisted Ignition and Combustion
361
OH + H
2
→ H
2
O + HOH
-
+ H → H
2
O + e
-

H + O
2

3–5
+
, C
3
H
3,5–7
+
and C
4
H
9
+
are observed, with the total cross section reaching a
maximum of ~1.2×10
−15
cm
2
at ∼ 80 eV. It was clearly demonstrated that each ion produces
additional radical in the process of charge transfer reactions. The most important process is
H
-
transfer. This process leads to effective generation of additional radicals and unsaturated
hydrocarbons.
Thus gas ionization plays two ways during plasma decay. Recombination can lead to
formation of molecules and significant heat release to translational degrees of freedom.
Competing mechanism is recombination with radicals (atoms) formation or excited particles
formation. This mechanism produces less heat but more active radicals in the discharge
afterglow. Overall, ionization produces more thermal heat and less radicals than excitation
of electronic degrees of freedom of nitrogen and direct dissociation by electron impact
[Aleksandrov et al, 2010] and not very effective from the point of view of active radicals

combustion, it is necessary to understand the mechanisms of plasma assisted ignition and
combustion and to simulate numerically discharge and combustion processes under various
conditions.
The analysis of discharge processes shows that the discharge energy can be deposited into
desired internal degrees of freedom of molecules when varying the reduced electric field,
E/n, at which the discharge is maintained. The amount of the deposited energy is
controlled by other discharge and gas parameters including electric pulse duration,
discharge current, gas number density, gas temperature, etc. As a rule, the dominant
mechanism of the effect of nonequilibrium plasma on ignition and combustion is associated
with the generation of active particles in the discharge plasma. Numerical simulation of
discharge processes is based on the solution of the Boltzmann equation for electrons and of
the balance equations for active particles. Here, input data are electron-molecule cross
sections and rate constants for reactions with excited and charged particles. These data are
available for simple molecules such as N
2
, O
2
, H
2
, and, to a smaller extent, for simple
hydrocarbons. However, little is known about cross sections and rates for complex
hydrocarbon molecules. The lack of this information does not seem critical when
considering lean and stechiometric mixtures; but this problem is serious for the simulation
of ignition of rich mixtures.
For plasma assisted ignition and combustion in air-containing mixtures, the most promising
active species are O atoms and, to a smaller extent, some other neutral atoms and radicals.
These active particles are efficiently produced in high-voltage nanosecond pulse discharges
due to electron impact dissociation of molecules and due to electron impact excitation of N
2



Plasma-Assisted Ignition and Combustion
363
5. Acknowledgements
The work was partially supported by Russian Foundation for Basic Research under the
project “Nonequilibrium plasma thermalization”, AFOSR under the project “Fundamental
Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted
Combustion”, and DOE Combustion Energy Frontiers Research Center.
6. References
Adamovich I.V., Macheret S.O., Rich J.W., Treanor C.E., Fridman A.A. In: Molecular Physics
and Hypersonic Flows. Ed by M. Capitelli. Kluwer Acad. Publ. 1996. P.85.
Aleksandrov N.L., Kindusheva S.V., Nudnova M.M. and Starikovskiy A.Yu. Mechanism of
ultra-fast heating in a nonequilibrium weakly-ionized air discharge plasma in high
electric fields. J. Phys. D: Appl. Phys. 43 (2010) 255201 (19pp)
Aleksandrov N.L., Kindysheva S.V., Kosarev I.N., Starikovskaia S.M., Starikovskii A.Yu.
Mechanism of ignition by non-equilibrium plasma. Proceedings of the Combustion
Institute 32 (2009) 205–212.
Aleksandrov N.L., Kindysheva S.V., Kukaev E.N., Starikovskaya S.M., Starikovskii A.Yu.
Simulation of the Ignition of a Methane−Air Mixture by a High_Voltage
Nanosecond Discharge. Plasma Physics Reports, 2009, Vol. 35, No. 10, pp. 867–882.
Andreadis D. Scramjet engines enabling the seamless integration of air & space operations.
Pratt & Whitney Space Propulsion, Hypersonics, West Palm Beach, FL, 2005,

Anikin N B, Starikovskaia S M and Starikovski A Yu. Oxidation of saturated hydrocarbons
under the effect of nanosecond pulsed space discharge. J. Phys. D: Appl. Phys. 39
(2006) 3244–3252
Anikin N.B., Pancheshnyi S.V., Starikovskaia S.M., Starikovskii A.Yu. Air Plasma
Production by High-Voltage Nanosecond Gas Discharge. 4 Weakly Ionized Gases
Workshop. Anaheim. CA, 2001. AIAA 2001-3088.
Babich L.P. Analysis of a new electron runaway mechanism and record-high runaway

and CH
4
Containing Mixtures. Combustion and Flame, 2003. V 133, pp. 133-
146.
Capitelli M. Molecular Physics and Hypersonic Flows. Kluwer Acad. Publ. 1996. P.85.
Do H., Im S., Cappelli M.A., Mungal M.G. Plasma assisted flame ignition of supersonic
flows over a flat wall. Combustion and Flame 157 (2010) 2298–2305.
Dutton J. J. Phys. Chem. Ref. Data 4 577 (1975)
Efanov V.M. High-Voltage and High-PRF FID Pulse Generators. FID GmbH. 2011.
Esakov I.I., Grachev L.P., Khodataev K.V., Vinogradov V.A., Van Wie D.M. Efficiency of
propane-air mixture combustion assisted by deeply undercritical MW discharge in
cold high-speed airflow, AIAA-2006-1212, 44th AIAA Aerospace Sciences Meeting
and Exhibit, Reno, Nevada, USA, 9-12 January 2006.
Fridman A.A. Plasma Chemistry. Cambridge University Press. 2008.
Fry R.S. A Century of Ramjet Propulsion Technology Evolution. Journal of propulsion and
power. Vol. 20, No. 1, January–February 2004.
Grisch F., Grandin G A., Messina D., Attal-Trétout B., Laser-based measurements of gas-
phase chemistry in non-equilibrium pulsed nanosecond discharges Comptes
Rendus Mecanique 337 (2009) 504–516
Hayashi M., in: Pitchford L.C., McCoy B.V., Chutjian A., Trajmar S. (Eds.), Swarm Studies
and Inelastic Electron–Molecule Collisions, Springer-Verlag, New York, 1987, pp.
167–187.
Huxley L.G.H. and Crompton R.W. The Diffusion and Drift of Electrons in Gases (New
York: Wiley-Interscience) 1974.
Ionin A.A., Kochetov I.V., Napartovich A.P., Yuryshev N.N., J. Phys. D Appl. Phys. 40 (2007)
R25–R61.
Jiao C Q, DeJoseph Jr C A and Garscadden A. Electron impact ionization and ion reactions
in n-butane. J. Phys. D: Appl. Phys. 40 (2007) 409–414 doi:10.1088/0022-
3727/40/2/018
Kof L.M., Starikovskii A.Yu. Gas-phase ignition initiation by high-voltage ionization wave.

Kosarev I.N., Aleksandrov N.L., Kindysheva S.V., Starikovskaia S.M., Starikovskii A.Yu.
Kinetics of ignition of saturated hydrocarbons by nonequilibrium plasma: CH
4
-
containing mixtures, Combustion and Flame, 154, 569-586 (2008).
Kossyi I.A., Kostinsky A.Yu., Matveyev A.A. and Silakov V.P. Kinetic scheme of the non-
equilibrium discharge in nitrogen-oxygen mixtures. PSST. 1 207 1992
Kovach E.A., Losev S.A., Sergievskaia A.L., Khrapak N.A. Models for Physical-Chemical
Processes. 3. Thermally-equilibrium and Nonequilibrium Chemical Reactions.
Physical-Chemical Kinetics in Gasdynamics. MSU. 2010.
Leonov S.B., Firsov A. A., Yarantsev D. A., Bolshov M. A., Kuritsyn Yu. A., Liger V. V.,
Mironenko V. R. Dynamics of H
2
O Temperature and Concentration in Zone of
Plasma-Assisted High-Speed Combustion. 49th AIAA Aerospace Sciences Meeting
including the New Horizons Forum and Aerospace Exposition 4 - 7 January 2011,
Orlando, Florida AIAA 2011-972.
Light G.C., Matsumoto J.H. // Chem.Phys.Letters. 1978. V.58. N.4. P 578-581.
Losev S.A., Sergievskaia A.L., Rysanov V.D., Fridman A.A., Macheret S.O. Reports of
Russian Academy of Science. 1996. V.346. N 2. P.192.
Macheret S.O., Fridman A.A., Adamovich I.V., Rich J.W., Treanor C.E.// AIAA-Paper 1994-
1984. 1994.
Michael J.B., Dogariu A., Shneider M.N., and Miles R.B., ”Subcritical microwave coupling to
femtosecond and picosecond laser ionization for localized, multipoint ignition of
methane/air mixtures” Journal of Applied Physics. Volume: 108. Issue: 9. Article
Number: 093308. NOV 1 2010.
Nikipelov A.A., Rakitin A.E., Popov I.B., Correale G., Starikovskii A.Yu. Plasmatrons
Powered By Pulsed High–Voltage Nanosecond Discharge For Ultra–Lean Flames
Stabilization. 49th AIAA Aerospace Sciences Meeting including the New Horizons
Forum and Aerospace Exposition 4 - 7 January 2011, Orlando, Florida. AIAA 2011-

Initiation by High-Voltage Nanosecond Discharges, Combustion and Flame, 155,
343-355 (2008).
Roupassov D., Nudnova M., Nikipelov A., Starikovskii A. Sliding DBD for Airflow Control:
Structure and Dynamics. 46th AIAA Aerospace Sciences Meeting and Exhibit 7 - 10
Jan 2008 Grand Sierra Resort Hotel Reno, Nevada. Paper AIAA-2008-1367.
Rusanov V.D., Fridman A.A. Physics of Chemically-Active Plasma. Nayka, Moscow. 1985.
Sagulenko P.N., Khorunzhenko V.I., Kosarev I.N. Ignition by nanosecond surface discharge.
8th Int. Workshop on Magneto-Plasma Aerodynamics, March 31-April 2, 2009,
Moscow, Russia
Serbin S., Mostipanenko A., Matveev I., Tropina A. Improvement of the Gas Turbine Plasma
Assisted Combustor Characteristics. 49th AIAA Aerospace Sciences Meeting
including the New Horizons Forum and Aerospace Exposition 4 - 7 January 2011,
Orlando, Florida AIAA 2011-61.
Shibkov V M, Aleksandrov A F, Chernikov V A, Ershov A P, Konstantinovskij R S and
Zlobin V V 2006 Combined MW-DC discharge in a high speed propane–butane–air
stream 44th AIAA Aerospace Sciences Meeting and Exhibit (Reno, Nevada, USA, 2006)
AIAA-2006-1216
Singleton D., Pendleton S.J. and Gundersen M.A. The role of non-thermal transient plasma
for enhanced flame ignition in C
2
H
4
–air. J. Phys. D: Appl. Phys. 44 (2011) 022001
Smirnov V.V., Stelmakh O.M., Fabelinsky V.I., Kozlov D.N., Starik A.M. and Titova N.S. On
the influence of electronically excited oxygen molecules on combustion of
hydrogen–oxygen mixture. J. Phys. D: Appl. Phys. 41 (2008) 192001 (6pp)

Plasma-Assisted Ignition and Combustion
367
Stancu G.D., Kaddouri F., Lacoste D.A., and Laux C.O. Atmospheric pressure plasma

H
8
/air Plasma assisted
combustion kinetics in a nanosecond discharge. 49th AIAA Aerospace Sciences
Meeting including the New Horizons Forum and Aerospace Exposition 4 - 7
January 2011, Orlando, Florida AIAA 2011-970
Wu L., Fridman A.A. and Starikovskiy A.Yu. Kinetics of Plasma Assisted Combustion At
Low Reduced Electric Fields. 48th AIAA Aerospace Sciences Meeting including the
New Horizons Forum and Aerospace Exposition 4 - 7 January 2010, Orlando,
Florida AIAA 2010-1593
Wu L., Lane J., Cernansky N., Miller D., Fridman A. and Starikovskiy A. Time resolved PLIF
and CRD diagnostics of OH radicals in the afterglow of plasma discharge in
hydrocarbon mixtures. 7
th
US National Combustion Meeting. March 20-23, 2011.
Wu L., Lane J., Cernansky N.P., Miller D.L., Fridman A.A. and Starikovskiy A.Yu. Plasma
Assisted Ignition Below Self-Ignition Threshold In Methane, Ethane, Propane And
Butane-Air Mixtures. Proceedings of the Combustion Institute 33 (2010)
Zatsepin D.V., Starikovskaia S.M., Starikovskii A.Yu. Hydrogen oxidation in a
stoichiometric hydrogen-air mixtures in the fast ionization wave. Combust. Theory
Modeling, 2001. V.5 pp.97-129.

Aeronautics and Astronautics
368
Zatsepin D.V., Starikovskaia S.M., Starikovskii A.Yu. Non-Thermal Decomposition of N
2
O
in Pulsed High Current Discharge. Plasma Physics Reports, 2003. V 29, N 6, pp.
517-527.
13

(Janson, 1994; DeGroot & Oleson, 1996; Mueller, 1997; Bruno, 2001). One of these is the heat
loss through combustor walls due to the much increased surface/volume ratio reducing the
actual energy available for the cycle chosen: this explains the sometimes startlingly low
temperatures observed experimentally (Minotti et al., 2009; Bruno, 2001; Cozzi, 2007; Cozzi &
Caratti, 2007; Bruno et al., 2003; Cozzi et al., 2007). Even when equivalence ratios (Φ) are close
to one, these call for kinetics capable of realistically predicting ignition delays times and
combustion efficiency at a reasonable computational cost.
The requirement to predict with sufficient accuracy combustion performance and heat load
to the chamber walls has lead, in the last decade, the numerical modelling to rapidly become
an essential part of combustion research and development programs, and there has been an
accelerating evolution from the use of single-step empirical kinetics, to the use of lumped
semiglobal (multistep) models (Wesbrook & Dryer, 1981; Bowman, 1986), and finally to the
inclusion of full detailed chemical kinetic mechanisms to better simulate chemistry
interactions. In addition, detailed mechanisms have been developed and validated for the

Aeronautics and Astronautics
370
simplest fuel molecules (Westbrook and Dryer, 1981) and are not available for most
practical fuels. Finally there are many occasions where the great amount of chemical
information produced by a detailed reaction mechanism is not necessary and a simple
mechanism will suffice together with the fact that 3D combustors cannot easily include
detailed kinetic mechanisms because the computational costs of such a treatment would be
much too great.
Several works concerning hydrocarbon kinetics are present in literature (Paczko et al., 1988;
Westbrook Dryer, 1981; Kee et al., 1985; Heffington, 1997; Hautman, 1981; Trevino & Mendez,
1992; Dagaut, 1991), and the work of Gardiner (1999) is important to understand the
hydrocarbon oxidation chemistry, in particular for what concerns differences between methane
and other hydrocarbons. The state of the art for methane reactions is by the Gas Research
Institute, periodically releasing new updated versions of its detailed methane-air reaction
mechanism (GRI-Mech, or ).

i
X
i
Y
Y
X



; this measure tells how
sensitive the output is to a perturbation of the input. If a measure independent from the
units used for Y and X
i
is needed,
i
r
i
X
i
X
Y
S
X
Y







Any simplified reaction mechanism must be capable of reproducing experimental flame
properties over the range of operating conditions under consideration. Hence, in both the
paths the operating conditions definition plays a fundamental rule; they must be previously
decided because the chemistry model, as every model, has a narrow range of validity and
fits real data in a narrow range. It is not uncommon that models which fit data just in some
points are adopted, by means of extrapolation laws, to figure out chemistry behaviours in
ranges wider than their original validity without highlighting the errors percentage
differences in these new ranges. Unfortunately this operation leads to big mistakes which
are often neglected.
Experience shows, and this will be clear in the following sections, that most or almost all
reduced mechanisms are tuned to predict data at high temperatures (where it is easy to
obtain accurate data) but often at low temperatures, and low pressure, (i.e. 1000K-2000K and
for pressures in the range between 1atm and 5atm, typical of non-adiabatic combustion)
they are not accurate or do not predict ignition at all.
In general, for a semiglobal mechanism, the simplest overall reaction representing the
oxidation of a conventional hydrocarbon fuel is:




12 2 2 2 3 2 1 2
3.76 3.76Fuel n O N n CO n H O n N



where n
i
are determined by the choice of fuel.
This global reaction is often a convenient way of approximating the effects of the many
elementary reactions which actually occur but it overestimates the final temperature and

rates involved during the reaction and this is obtained tuning the A, E
a
, n, a and b variables.
3. Reaction mechanisms
The reaction mechanisms presented here are:
1.
Westbrook and Dryer: 4 species and 1 reaction (Westbrook & Dryer, 1981);
2.
Westbrook and Dryer: 5 species and 2 reactions (Westbrook & Dryer, 1981);
3.
Minotti: 6 species and 2 reactions (Minotti et al., 2009);
4.
Kee: 17 species and 58 reactions (Kee et al., 1985);

Aeronautics and Astronautics
372
5. GRI-Mech 12: 32 species and 177 reactions (Gri-Mech 1.2, 1994; Heffington et al., 1997);
These mechanisms have been compared to the predictions given by the detailed GRI-Mech
3.0 (53 species and 325 reactions (GRI-Mech 3.0, 1999; Dagaut et al., 1991)), assumed as the
“reference model” , for a wide range of equivalence ratio (0.3
Φ1.9), and at three different
pressures (P=1, 3 and 5 atm).
In the following sections the ignition delay comparison and the final temperature
comparison are respectively reported.
4. Comparisons – ignition delay times
The ignition delay time is the elapsed time to obtain a temperature increase, from the
injection temperature, of 400K.
The ignition delay time has been compared among the five mechanisms, listed above,
adopting reactants in the temperature range 1000K - 2000K and at pressure 1, 3 and 5atm.
The equivalence ratio (Φ) range tested was from Φ=0.3 to Φ=1.9 (ΔΦ=0.2), plus Φ=1.

1900 0.000062 0.0000622 0.0000623 0.0000627 0.0000638
2000 0.0000374 0.0000376 0.0000384 0.0000385 0.0000392
Table 1. b Ignition Delay, s, P=1atm
O
2
/CH
4
Kinetic Mechanisms for Aerospace Applications at
Low Pressure and Temperature, Validity Ranges and Comparison
373
From Tables 1a-1b, Tables 12a-12b and Tables 23a-23b it is possible to define a reactants
temperature range where reactions might be completed, that is a range in which the
Damkoehler number (residence time/chemical time) is less than 1. For example these tables
indicate that ignition delay times vary between 8.43
10
-6
s and 1.54s. Figure 1 reports the
ignition delay (t
id
), at Φ = 1, as function of reactants temperature and for the different
reaction mechanisms. Fig. 1. Φ=1.0: t
id
comparison
Tables 2 to 11 show the percent differences between the t
id
predicted by GRI-Mech 3.0 and
the reduced mechanisms tested (that is, GRI-Mech3.0 - Reduced Mechanism)/GRI-Mech3.0)

2000 -34.53% -308.63% -1000.72%
Table 2. Φ=0.3: t
id
% differences between reduced and reference mechanisms