Bootstrap Effect in Plasma-Assisted Ignition of Atmospheric
Pressure Methane-Air and Methane-Oxygen Mixtures
at Temperatures Below Auto-Ignition Limit
Mikhail Pekker, Danil Dobrynin, Alex Fridman
A. J. Drexel Plasma Institute, Drexel University, Philadelphia USA
Abstract: In carbon-hydrogen mixtures, the plasma discharge leads to the heating
of the fuel and production of radicals. These two processes stimulate the process
of combustion. In the case of gas temperatures below the auto ignition limit, the
radicals disappear usually in 2-10 microseconds after the ending of discharge.
Therefore, the main role of plasma discharge is traditionally considered to be
related to only heating of the gas mixture. In this paper we investigate the role of
radicals in the shortening of auto ignition delay time (bootstrap effect) for
methane-air and methane-oxygen mixtures at the pressure of one atmosphere. We
show that the radicals created in the plasma discharge in theses mixtures play the
main role in bootstrap effect. In particular we show that annihilation of OH
radicals in these mixtures is accompanied by chain-reactions that lead to
accumulation of CH2O. Our calculations show that taking into account radicals
increases the bootstrap coefficient up 2-3.
Keywords: plasma assisted combustion, bootstrap effect, plasma modeling
Recently, it was shown that non-thermal plasma
may stimulate ignition of H2, CH4 and C2H4 in
temperature region below their auto-ignition
limits, where production of radicals is
suppressed by recombination [1-5], and ignition
is considered impossible in the framework of
conventional combustion kinetics [6].
The role of radicals is traditionally considered to
be related to initiation of chemical chain
reactions, which lead to gradual increase of gas
temperature, and eventually to ignition. The
effect of plasma accelerated fuel ignition is
defined as “bootstrap effect”, and the
corresponding bootstrap coefficient is expressed
as:
K bstr = (Tbstr − T0 ) (Theat − T0 )
(1)
where T0 – the temperature of mixture before
the plasma discharge, Theat – gas temperature
after the discharge, which is increased only due
to plasma heating (without accounting
contribution of radicals), Tbstr – the actual
temperature of mixture at which ignition time
delay is equal to the ignition time delay without
plasma. Kbstr>1 is due to (a) heating due to the
annihilation radicals, and (b) changing the
composition of mixture also due annihilation of
has
three
radicals.
Therefore
Teffective
components:
Teffective = T0 + ∆T1 + ∆T2 + ∆T3 (2)
Here ∆T1 = T heat corresponds to the direct
heating due the plasma discharge, ∆T 2 –
thermal heating due to the annihilation radicals
appeared just after discharge, and ∆T 3
corresponds to the “heating” due to the change
of composition after annihilation the initial
radicals. It is obvious that if time delay after
plasma discharge is reduced to thermal heat,
bootstrap coefficient has to be close to 1.
In [7] it was demonstrated that just a small
change in the composition of fuel mixture can
significantly shorten the delay time of ignition.
Our numerical modeling show that effect of
composition change of mixture plays the main
role in bootstrap effect for methane-air mixture
at atmospheric pressure (Table1 and Fig.1), and
both mechanisms play approximately the same
role for methane-oxygen mixtures. In this work
it is shown that annihilation of OH radicals
accompanied by the chain reaction (3) leading
to accumulation CH2O.
OH + CH ⇒ CH + H O
4
3
2

CH 3 + O 2 ⇒ CH 2O + OH

1
CH 3 + O 2 ( ∆ )⇒ CH 2O + OH
(3)
Fig.2 Dependence of the time delay and production CH2O
due on the initial condition (Table1.).
Table 1. Delay time due to the amount of radicals and
singlet oxygen. (T0=1000K, P=1 atm., [CH4] = 0.103,
[O2]=0.2066, [N2]=0.783).
Initial Initial
Initial
1
Initial [OH], [O2( D)], [CH2O], Delay time,
T0(K)
mole
mole
mole
ms
fraction fraction fraction
1000
1000
0
0
0.0001 0.001
0
178
0
75
Initial Initial
Initial
Initial
[OH], [O2(1D)], [CH2O], Delay time,
T0 + ∆T1
mole
mole
mole
ms
(K)
fraction fraction fraction
Fig.1 Dependence the gas temperature, amount of singlet
oxygen and formaldehyde on time. (Run corresponds to
the same initial conditions as in Table 1, line 2).
Appearance of formaldehyde CH20 is due to the chain
reactions (3).
1008
0
0
0
159
1008
0
0
0.001
84
Our calculation shows that:
1. The influence of plasma discharge on fueloxidizer mixtures at temperatures bellow auto
ignition limit is limited 1-100 µs.
2. Change of mixture composition gives the
main contribution into Bootstrap Effect
(shortening of delay time due to radicals).
3. Exited molecules (for example O2(1∆)) may
increase bootstrap coefficient few times.
4. The chain (3) plays the main role in plasmaassisted ignition of atmospheric pressure
methane-air and methane-oxygen mixtures at
temperatures below auto-ignition limit. It can be
the main reason of shortening of the time delay
in experiments [2-5].
Bootstrap coefficient for methane-oxygen
mixture ([CH4] = 0.0944, [O2] = 0.8905, [OH] =
0.0001, O2(1D) = 0.015) at atmospheric pressure
and temperature interval of 625-1000K equals
2.65-3.14 (Fig. 4).
Fig.4 Effect of additional O2(1D) on effective temperature
(see equation (2)) and bootstrap coefficient (see equation
(1)) in methane-oxygen mixture.
References
Fig.3 Effect of additional O2( ∆) on effective temperature
(see equation (2)) and bootstrap coefficient (see equation
(1)) in methane-air mixture.
1
All numerical simulation were executed on the
base of Konnov’s mechanism [8] in which
reaction CH 3 with singlet oxygen O2(1D) was
added.
Bootstrap coefficient for methane-air mixture
([CH4] = 0.0103, [O2] = 0.2066, [N2] = 0.7830,
[OH] = 0.0001, [O2(1D)] = 0.001) at
atmospheric pressure and temperature interval
of 800-1000K equals 1.52-1.42 (Fig.3).
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