st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Numerical Study on Nano-second Pulse Dielectric Barrier Discharge for
Plasma Assisted Combustion
H. Takana1 and H. Nishiyama1
Institute of Fluid Science, Tohoku University, Sendai, Japan
1
Abstract: Detailed computational simulations of a high energy loading nano-second DC
pulse DBD streamer in a lean methane-air mixture were conducted for plasma assisted combustion in an internal engine. The propagation process of DBD streamer with radical production has been clarified at 10 atm and 600 K. Radicals are produced mostly at the streamer
head with high electric field. In a nano-second DC pulse DBD, the accumulated charged species on the dielectric surface emits into the discharge space without changing the polarity due
to the inverse potential gradient resulting from the rapid fall of applied voltage. Energy is
loaded to the discharge in two stages, which corresponds to the plasma channel formation and
electron emission into the discharge space from dielectric surface.
Keywords: Nano-second pulse DBD, Plasma assisted combustion, Numerical simulation
1. Introduction
The application of non-equilibrium plasma to the plasma assisted combustion of gas mixture has been expected
to bring breakthrough in the field of aerospace and automobile engineering [1][2]. Non-equilibrium plasma, typically generated by dielectric barrier discharge (DBD),
pulsed corona, microwave discharge and volume nanosecond discharge, generates radicals and excited molecules at low input energy through direct electron impact
reactions instead of direct heating. Even relatively small
concentrations of radicals and reactive species, such as
10-5 to 10-3, can sufficiently initiate the chain reactions of
fuel oxidation at low temperature, which leads to the
well-controlled uniform combustion of a lean mixture [3].
There are a lot of numerical and experimental researches of non-equilibrium plasma combustion of
fuel-air mixture; however, most of the reported researches
are conducted at moderate pressures or atmospheric pressure. For the practical application of the plasma enhanced
chemical reactions in an internal engine, it is essential to
understand and characterize the reactive species production at high pressure and high temperature conditions.
In this study, detailed computational simulations of a
high-energy loading nano-second DC pulse DBD streamer in a lean methane-air mixture were conducted at 10 atm
and 600 K for plasma assisted combustion in an internal
engine. The radical production and heating characteristics
associated with streamer propagation during one pulse
have been clarified in detail.
2. Governing equations
Following continuity equations for electrons and ions
with drift-diffusion approximation are solved coupled
with Poisson's equation:
Fig. 1 Computational domain and initial electron
distribution.
(1)
(2)
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. They are obtained by solving
Boltzmann's equation for the electron energy distribution.
In order to clarify the rapid gas heating by nano-second
voltage pulse, the following energy conservation equations are considered for internal energy and average
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
applied voltage -23.0 kV 400
discharge voltage r = 9.5
discharge current (alumina)
0
0
-200
-20
Vapp , Vd (kV)
20
200
I (mA)
Vapp , Vd (kV)
20
40
3 [10-7]
applied voltage -23.0 kV
discharge voltage r = 9.5 2
surface charge (alumina)
1
0
0
-1
-20
-2
-400
-40
0
-40
0
10 20 30 40 50 60
t (ns)
Fig. 2 Wave forms of applied, discharge voltages and
discharge current for r = 9.5 at 10 atm and 600 K.
ave (C/cm2)
40
-3
10 20 30 40 50 60
t (ns)
Fig. 3 Temporal variation of averaged surface charge on
dielectric.
Fig. 4 Time evolution of electric potential distribution in nano second pulse DBD.
vibrational energy of nitrogen molecule [4].
(3)
(4)
The rate of photoionization in a gas volume is included
in the source term in equation (1). In this study,
two-exponential Helmholtz model [5] was employed for
photoionization of oxygen. The photoionization processes
in O2 are caused by the radiation in the wave range 98 102.5 nm.
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 metal surface is given as
0.001.
The composition volume ratio of methane-air mixture is
N2:O2:CH4 = 15:4:1. The kinetic model 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 associated with ozone formation and oxidation are incorporated.
Figure 1 shows the computational domain and initial
electron distribution [7]. Only the grounded electrode is
covered with alumina (r = 9.5) with the thickness of 0.8
mm. The flat powered electrode (cathode) and grounded
electrode (anode) are separated by 2.05 mm. The
nano-second pulse voltage is applied on the powered
electrode at t = 0 s. The wave form of applied voltage is
fitted by Gaussian profile. The slew rate of applied voltage is ~1 kV/ns. For the initiation of electron avalanche,
local seed charges consist of electron and N2+ were given
by a Gaussian distribution as shown in Fig.1. The peak
number density of initial seed charges are 1.0 x 10 8 1/cm3
along the symmetric axis.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig. 5 Number density distributions of electron, oxygen
atom, CH3 and ozone at (a) t = 20.7 ns and (b) 22.7 ns.
6
0.03
4
0.02
2
0
0
0.01
20
40
t (ns)
60
Discharge energy (mJ)
Discharge power (kW)
0.04
power
energy
0
80
Fig. 6 Temporal variations of discharge power and energy.
3. Results and discussion
Figures 2 shows the wave forms of applied and discharge voltages, and discharge current at 10 atm and 600
K, respectively. The discharge voltage corresponds to the
electrical potential difference between the grounded electrode (z = 0 mm) and dielectric surface in a discharge
space (z = 1.25 mm). The peak voltage of -23 kV is
applied in 22 ns and the voltage decreases to 0 kV by
taking another 22 ns. The discharge voltage sharply drops
at t = 24.2 ns and falls to 0 kV. The discharge voltage becomes positive and decreases to 0 kV. 460 mA of
pulse-like negative discharge current flows at 20.7 ns and
then positive current flows.
Figure 3 and 4 show the temporal variation of surface
Fig. 7 distributions of gas temperature and N2 vibrational
temperature at (a) t = 20.7 ns and (b) 22.7 ns.
charge on dielectric and time evolution of electric potential distribution in a nano-second pulse DBD during one
pulse, respectively. During the streamer propagation to
the grounded electrode, electrons drift toward dielectric
and accumulate on the dielectric surface. The electric potential on the dielectric surface decreases due to the accumulation of electron on dielectric as found in Fig. 3 and
4. At t = 24.7 ns, the potential gradient in the discharge
space becomes almost zero as shown in Fig.2. As found
from the potential distribution at 32.2 ns, inverse potential
gradient appears in the discharge space because of the
rapid decrease of applied voltage. The accumulated electrons on the dielectric surface starts to emit into the discharge space by this inverse gradient of electric potential
as shown in Fig. 3. Therefore, in nano-second pulse discharge, accumulated charges are released to the discharge
space from the dielectric surface without changing the
polarity of the applied voltage.
Figure 5 (a) and (b) show number density distributions
of electron, oxygen atom, CH3 and ozone at t = 20.7 ns
and at 22.7 ns, respectively. Oxygen radical and CH3 radical are produced through electron impact reaction at the
steamer head with high electric field. Electron number
density decreases after forming plasma channel in the
discharge space, on the other hand, the number density of
oxygen and CH3 radicals remain the same. Ozone gradually increases after streamer head reaching the ground
electrode. Higher number density of radicals is formed in
the middle of discharge space and near the bare electrode.
Figure 6 shows time variation of discharge power and
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
energy per pulse. The energy is loaded to the streamer
discharge in two stages. Firstly, the energy is loaded to the
streamer by the formation of plasma channel and then
energy loading ceases due to the self-shielding effect. The
energy is loaded to the discharge again after 25.4 ns
through the current carried by the release of accumulated
electrons from dielectric surface.
Figure 7 shows the distributions of gas temperature and
N2 vibrational temperature at (a) t = 20.7 ns and (b) 22.7
ns. According to the energy input shown in Fig.6, rapid
gas heating occurs in two stages. Gas is not uniformly
heated and temperatures are higher around the constriction of streamer channel (z = 0.4 mm) and bare electrode (z = 0 mm) because of the concentration of electric
current. The gas temperature and N2 vibration temperature
are locally heated by 200 K and 2500 K in 60 ns by a
nano-second pulse DBD, respectively.
4. Conclusions
Detailed computational simulations of a high-energy
loading nano-second DC pulse DBD streamer in a lean
methane-air mixture are conducted at 10 atm and 600 K
for plasma assisted combustion in an internal engine. Obtained results can be summarized as follow.
(1) In a nano-second DC pulse DBD, the accumulated
charged species on the dielectric surface emits into
the discharge space due to the inverse potential gradient resulting from the rapid fall of applied voltage.
(2) Energy is loaded to the discharge in two stages,
which corresponds to the plasma channel formation
and electron emission into the discharge space from
the dielectric surface.
References
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Phys. D, 42 (2009)
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