Physicochemical Characteristics of Methane-Air Dielectric Barrier
Discharge at High Pressure
Hidemasa Takana1, Yasunori Tanaka2 and Hideya Nishiyama1
1
2
Institute of Fluid Science, Tohoku University, JAPAN
Graduate School of Engineering, Kanazawa University, JAPAN
Abstract: Computational simulation of methane-air dielectric barrier discharge at
high pressure is conducted for plasma assisted combustion. The effect of ambient
pressure on streamer dynamics is successfully simulated with considering photo
ionization. Furthermore, the chemical kinetics of methane-air mixture is clarified
in detail.
Keywords: Dielectric barrier discharge, streamer, plasma assisted combustion
1. Introduction
The plasma assisted combustion has been especially
paid attention in aerospace and automobile
engineering. Most of the experimental studies and
analyses are typically carried out at pressures close
to 1 atm. For the practical application of the plasma
assisted combustion, it is essential to understand the
radical generating process with streamer propagation
and also to clarify the chemical kinetics under high
pressure, especially for the application to internal
engines.
In this study, a computational simulation of
methane-air dielectric barrier discharge is conducted
to clarify the effect of ambient pressure on streamer
dynamics and chemical kinetics in methane-air DBD.
2. Governing Equations
Following continuity equations for electrons and
ions with drift-diffusion approximation are solved
coupled with Poisson’s equation:
⋅
(1)
,
sgn
⋅
,
,
(2)
(3)
where
,
,
,
,
,
, are the number
density, flux, source term, charge, mobility and
diffusion coefficient for species k, respectively. E,
and
are electric field, permittivity and electric
potential, respectively. Charged particle fluxes are
given by the Scharfetter-Gummel formulation. The
electron transport properties and rate coefficients are
given as a function of reduced electric field and they
are obtained by solving Boltzmann’s equation for
the electron energy distribution [1]. The transport
properties of ions are taken from the reference [2].
The rate of photoionization in a gas volume is
included in the source term in equation (1). In this
study, two-exponential Helmholtz model was
employed for photoionization of oxygen [3]. The
photoionization processes in O2 are caused by the
radiation in the wave range 98 – 102.5 nm. The
radiation in this range is produced by the radiative
transitions
from
three
singlets
of
N2
Π ,
Σ and
Σ ) to the ground state (X Σ
[4].
The self-biased potential on the dielectric
surface is calculated by Gauss’s law, assuming a
constant electric filed in the dielectric. The
secondary electron emission probability by ion
impact on a surface is given as 0.01.
The composition ratio of methane-air mixture is
N : O : CH
15: 4: 1. The chemical kinetics of
methane-air mixture studied in this work includes
processes with participation of species of N 2 ,
Σ ,N
Π ,N
Π ,N
Σ , N2
N
Π , Σ and Σ ) , N , O 2 , O
Δ ,
Σ , O, O , O
Σ , C Δ , and Σ , O 3 ,
O
CH4 CH3, CH2, CH , CH , CH , H , H, H O, OH,
HO2, CH2O, CH3O and electron. The kinetic model
cathode
anode
-3
log10 ne (cm )
15
4.5
10
3.6
2.7
5
1.8
0.9
0
0
Figure 1. Computational domain and initial condition.
Figure 1 shows the computational domain and
initial electron distribution. Only the grounded
3)
electrode is covered with quartz glass ( ⁄
with the thickness of 0.8 mm. The flat powered
electrode (cathode) and grounded electrode (anode)
are separated by 2.05 mm. The voltage on the
powered electrode has a stepwise change to -8.0 kV
for 1 atm and -18.0 kV for 10 atm.
For the initiation of electron avalanche, local
seed charges consist of electron and N were
givenby a Gaussian distribution only in the vicinity
of powdered electrode. The peek number density of
initial seed charges are 1.0 10 1/cm at the
powered electrode on the symmetric axis.
0.05
z (cm)
0.1
(a)
cathode
anode
1000
5.0 ns
10
5.5
6.0
400
5
0
0
600
E/N (Td)
800
15 7.0 6.5
log10 ne (cm-3)
used in this study is based on that developed in the
model described in [6] except for the reactions
associated with argon. Moreover, the O atom decay
processes through recombination with oxygen
M→O
resulting in ozone formation, O O
M and also through recombination with ozone,
O O →O
O are incorporated. It was shown
from the sensitivity analysis that the increased
atomic oxygen loss rate in methane-air occurs
primarily due to reactions of H atoms and CH3
M → HO
M ,
radicals, in particular H O
O HO → OH O , OH CH → CH
H O,
and O CH → H CH O[7]. These reactions are
also taken into account. In addition to that, the
following reactions of OH production and decay are
also considered; OH HO → O
H O , HO
CH → OH CH O , CH O O → HO
CH O ,
and H O → O OH.
0 ns
200
0.05
z (cm)
0.1
0
(b)
Figure 2. Time evolution of electron number density
distributions along z axis at (a) t = 0 – 4.5 ns and (b) t = 5.0 –
7.0 ns.
3. Results and discussion
Figure 2 shows the time evolution of electron
number density distributions along z axis at 1 atm
and 300 K. Electrons drift toward the anode from
cathode with increasing density due to electron
avalanche as shown in Fig. 2 (a). After electron
reaches the barrier surface, a streamer is initiated
near the anode. The streamer propagates toward the
cathode with photoionization ahead of the streamer.
The reduced electric field is quite high around the
streamer head, while the reduced electric field
remains low in the quasi neutral plasma channel
behind the streamer head.
Figure 3 shows the positive streamer
propagation at 1 atm and 300 K with stepwise
applied voltage of -8.0 kV. The propagation of
Figure 3. Positive streamer propagation at 1 atm and 300 K with applied voltage of -8.0 kV. (a) Electron number density, (b) positive space
charge and (c) electric field.
Figure 4. Time evolution of produced chemical species distributions with streamer propagation. (a) Oxygen radical, (b) CH3 radical and
(c) ozone.
streamer accelerates toward the cathode. High
concentration of positive charge is seen around the
streamer head due to the difference in the mobility
and drift direction between positive ions and
electron. The electric field becomes high at the
streamer head because of high concentration of
positive charge.
Figure 4 shows the distribution of oxygen radical,
CH3 radical and ozone during streamer propagation.
These chemical species plays an important role in
plasma assisted combustion. Oxygen radical and
CH3 radical are rapidly produced at the streamer
head mainly due to electron impact dissociation.
Then, they gradually increase behind the streamer
head with quenching of electronically excited N2 by
O2 and CH4. On the other hand, ozone is produced
only behind the streamer head by the three body
reaction; O O
M→O
M.
10-5
Mole fraction
10-6
r = 0 cm,
z = 0.63 cm (center)
10-7 1 atm 3
10-8
10-9
O
O3
CH3
ne
10-10
10-11
10-12
0
Figure 5. Difference in streamer shaper. (a) 1 atm with -8.0 kV
and (b) 3 atm with -18.0 kV.
Figure 5 shows comparison of streamer shapes
at 1 atm and 3 atm. Compared with the case at 1 atm,
streamer shape is rather confined at 3 atm. This
could be due to the larger space charge at the radial
boundaries to create a radially directed ambipolar
electric field[7] and also due to the smaller diffusion
coefficient at higher pressure.
Figure 6 shows time evolution of produced
chemical species at z = 0.63 cm on symmetric axis at
1 and 3 atm at 300 K. Rapid increase around t = 5 –
6 ns is due to the arrival of streamer head. Electron
increases just after the arrival of streamer followed
by oxygen and CH3 radicals at both pressure. Ozone
is generated with delay through recombination of
oxygen radical with oxygen molecule as mentioned
previously. Although electron is constant behind the
streamer head, other chemical species gradually
increase. Because the electric field around the
streamer head is higher at 3 atm, mole fractions of
electron and produced chemical species are higher at
3 atm than 1 atm in spite of lower initial reduced
electric field.
4. Conclusions
Computational simulation of methane-air dielectric
barrier discharge at high pressure was successfully
conducted for plasma assisted combustion. Obtained
results can be summarized as follow.
(1) O and CH3 radicals are rapidly produced at
arrival of streamer head mainly due to electron
impact dissociation.
2
4
t (ns)
6
Figure 6. Time evolution of produced chemical species at z =
0.63 cm on symmetric axis at 1 and 3 atm.
(2) O and CH3 gradually increase behind the
streamer head with quenching of electronically
excited N2 by O2 and CH4, respectively. On the
other hand, ozone is produced behind the
streamer head with delay.
(3) Streamer shape is rather confined at higher
pressure due to the larger space charge at the
radial boundaries and also due to lower diffusion
coefficient.
Acknowledgements
Part of the work was carried out under the
Collaborative Research Project of the Institute of
Fluid Science, Tohoku University.
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