parametric study of plasma-assisted ignition in combustible ......moscow institute of physics and...
TRANSCRIPT
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Moscow Institute of Physics and Technology
Parametric Study
of Plasma-Assisted Ignition
in Combustible Mixtures
N.L. Aleksandrov1, S.V. Kindysheva1,
E. A. Plastinin1, A.Yu. Starikovskiy2
1Moscow Institute of Physics and Technology
2Princeton University PRINCETON
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Outline of talk
- Practically important gas parameters
- Purpose of this work
- How to estimate the effect of PAI
- Method of estimation
- Calculated results and some
experiments
- Conclusions
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Gas parameters of ignition/combustion in engines
SCRAMjet: ~ 0.3-1 atm, ~ 700K, φ ~ 0.8-0.9, H2
IC engine: ~10-12atm, ~ 800 K, φ ~ 0.7, gas
GTE: 20-30atm, ~ 800 K, φ ~ 0.5-0.7, aviation kerosene
Power GTE: 20-30atm, ~ 1000-1100 K, φ ~ 0.3-0.5, methane
HCCI engine: ~ 30-40atm, ~ 1000-1200 K, φ ~ 0.3-0.5, gas
High-pressure NG engine: ~80-100atm, ~ 800 K, φ ~ 0.3-0.5, methane
High-altitude re-light: ~0.2-0.3atm, ~ 250-270К
Practically important ranges of gas parameters:
0.2 – 100 atm 250 – 1500 K φ = 0.3 - 1
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P – T diagram for experimental studies of PAI/PAC after nanosecond discharges
Topical review by Starikovskaia, J. Phys. D (2014)
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Purpose of this work
To numerically estimate the effect of
nonequilibrium discharge plasma on
hydrocarbon ignition in a wide range of gas
parameters
Disadvantages of this approach
- there are no kinetic schemes that could
describe plasma and ignition processes for all
parameters under study
- methods (discharges) of nonequilibrium
energy input are not considered
- ...
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How to estimate the effect of PAI
To compare ignition/combustion characteristics with plasma and
without it under the same conditions (shock-tube observations
(Kosarev et al. 2009))
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The drawbacks are (i) that the specific deposited energy changes
with gas temperature, (ii) that additional energy is deposited in PAI,
as opposed to autoignition, and (ii) that it is impossible to estimate
the effect below autoignition threshold.
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Ratio between autoignition and PAI delay times
Measured data are reduced to
a specific deposited energy
of 0.1 eV/particle
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Kosarev et al.
(Comb. Flame 2015)
tauto/tPAI is higher
- for lower T
- for CH4
(and lower for C2H2)
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Method to estimate the effect of PAI
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Discharge processes: balance equations → [e], …, [O], …, [Ar*], … Boltzmann equation for electrons → kj(E/N(t)) equation for gas temperature → T(t) Ignition processes: balance equations → [O], [OH], … equation for gas temperature → T(t)
Calculation of ignition delay time and deposited
energy during thermal and nonequilibrium
excitations (with fast gas heating)
Numerical zero-dimensional simulation of active
species production in discharge and ignition
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Kinetic schemes for discharge processes and
ignition
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Reactions in discharge and its afterglow
e + O2 → 2e + O2+ … e + O2 → e + O + O …
e + C2H2 → e + C2H + H … e + O2
+ → O + O …
Kinetic schemes for ignition
CH4 (RAMEC); C2H6 – C5H12 (Westbrook)
Good agreement between calculations and shock-
tube observations was obtained for ignition of
these hydrocarbons (Kosarev et al. (2008, 2009))
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Calculated ignition delay time versus energy input
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Bozhenkov et al. (2003)
1. No excitation
2. Thermal excitation
3. Nonequilibrium
excitation at E/N =
300 Td
H2:air φ = 1
1 atm 1000K
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Ignition delay time versus specific deposited energy during thermal
and nonequilibrium excitation
1000 K 1 atm E/N = 300 Td
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10-4
10-3
10-2
10-1
100
101
102
103
104
105
Nonequilibrium excitation
Thermal
excitation
No excitation
Ign
itio
n d
ela
y t
ime
, s
Specific deposited energy, eV/molecule
10-4
10-3
10-2
10-1
100
101
102
103
104
105
Nonequilibrium excitation
Thermal
excitation
No excitation
Ign
ioti
on
de
lay
tim
e, s
Specific deposited energy, eV/molecule
C2H6:air CH4:air
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Ratio between specific deposited energies required to ignite fuel with
equilibrium and nonequilibrium
excitation
Ratio between equilibrium deposited energy and
nonequilibrium one at which the ignition delay
time is 100 μs (E/N = 300 Td)
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600 800 1000 1200 1400 16001
2
3
4
0.1
10
1
100 atm
CH4:air, = 1
wth/w
ne
q
Temperature, K600 700 800 900 1000 1100 12001
2
C5H12:air, 1 atm
C2H6:air, 1 atm
C2H6:air, 10 atm
= 1
wth
/wneq
Temperature, K
The radical-chain mechanism becomes more important with
increasing gas temperature!
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The effect of gas pressure
0.03 eV/molecule E/N = 300 Td
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10-1
100
101
102
1
2
3
4
5
1400
1000 K
C2H
6:air, = 1
t th/t
ne
q
Pressure, atm
tauto
/tPAI
, 1000K
tauto
/tPAI
, 1400K
Pressure dependence of reaction rates can change with pressure.
The more complicated is hydrocarbon molecule, the lower is the
critical pressure; for instance, C2H6 (+ M) ↔ C2H5 + H (+ M) .
10-1
100
101
102
2
3
4
5
6
7
8
9
10
11
1400
1000 K
CH4:air, = 1
t th/t
ne
q
Pressure, atm
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The effect of high-energy O atoms
CH4:air φ = 1 1 atm 0.03 eV/particle
E/N = 300 Td, all O atoms have high
reactivity (energy barrier is removed)
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Chemical reactivity of active species
(O, H, …) is higher for excited and
high-energy particles (Starikovskiy
2014)
e + O2 → e + O(3P) + O(3P, 1D) + ΔE
N2(A,B,C,..) + O2 → N2 + O + O + ΔE
e + CH4 → e + CH3 + H + ΔE
N2(A,B,C,..) + CH4 → CH3 + H + ΔE
… 400 600 800 1000 1200 1400 1600
0.8
1.0
1.2
1.4
1.6
1.8
2.0
t(3P
) / t(
3P
,1D
,1S
,H)
T, K
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The effect of vibrationally excited N2 molecules
CH4:air φ = 1 1 atm 0.03 eV/particle
E/N = 300 Td Tv(N2) = 3000 K
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400 600 800 1000 1200 1400 1600
102
103
104
105
106
107
108
109
PAI with N2(v)
PAI
Tim
e, s
Temperature, K
Starikovskiy (2012)
HO2 + N2(v) → HO2(ν2+ν3) + N2(v-1) HO2(ν2+ν3) → H + O2
The effect of N2(v) is profound at low T. At high T, HO2 radicals
are reactive and their removal can eliminate important reactive channels.
(HO2 + N2 → H + O2 + N2 for T = 3000 K)
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The effect of equivalence ratio in fuel:air mixtures
e + O2 → e + 2O is more efficient than e + CH4 → e + CH3 + H
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1000 1100 1200 1300 1400 1500 1600
4
6
8
10
12
14
0.4
0.7
= 1
t th/t
ne
q
Temperature, K1000 1100 1200 1300 1400 1500 1600
102
103
104
105
Noneq. exc.
Therm. exc. = 1
= 1
= 0.7
= 0.7
= 0.4
= 0.4
t,
s
Temperature, K
CH4:air φ = 1 1 atm 0.03 eV/particle E/N = 300 Td
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The effect of equivalence ratio in hydrocarbon(C2):O2:Ar mixtures
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0,5 0,6 0,7 0,8 0,9 1,0
101
102
103
0,5 0,6 0,7 0,8 0,9 1,0
101
102
103
PAI
Auto
C2H
6
=0.5
Auto
PAI
=1.0
Auto
PAI
Ind
uc
tio
n d
ela
y,
s
1000/T5, [1/K]
PAI
Auto
C2H
4
=0.5
Auto
PAI
=1.0
Auto
PAI
Ind
uc
tio
n d
ela
y,
s
1000/T5, [1/K]
C2H6 (φ = 1) Kosarev et al. (2008)
New shock-tube data for C2H6 (φ = 0.5)
and C2H4 (φ = 0.5 and 1)
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The effect of specific deposited energy
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1000 1100 1200 1300 1400 1500 16000
2
4
6
8
10
12
14
0.01
0.03
t th/tneq
Temperature, K
0.1 eV/particle
1000 1100 1200 1300 1400 1500 1600
102
103
104
105
0.1
0.01 eV/particle
t,
s
Temperature, K
0.03
Closed symbols: thermal excitation
Open symbols: nonequilibrium excitation
CH4:air φ = 1 1 atm E/N = 300 Td
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The effect of reduced electric field
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1000 1100 1200 1300 1400 1500 1600
4
6
t th/tneq
Temperature, K
E/N = 200 Td
300
500
CH4:air φ = 1 1 atm 0.03 eV/particle
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Conclusions
The efficiency of nonequilibrium plasma ignition after a pulsed discharge was estimated with respect to thermal ignition at the same deposited energy.
The effect of nonequilibrium plasma on ignition in hydrocarbon:air mixtures is more profound
- for gas temperatures around self-ignition threshold
- for hydrocarbons with lower rates of thermal production of radicals (lower reactivity)
- for higher specific deposited energy (in the range 0.01-0.1 eV/molecule)
- for lean mixtures
- for not-too-high E/N at which energy is deposited in discharge (in the range 200 – 500 Td)
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Conclusions
The effect of nonequilibrium plasma on ignition becomes more profound with decreasing pressure in CH4-containing mixtures and with increasing pressure in C2H6-containing mixtures.
Additional vibrational excitation of N2 can lead to an orders of magnitude decrease in the induction delay time at temperatures below self-ignition threshold
Formation of high-energy and excited O and H atoms in discharge plasma can affect ignition only below self-ignition threshold; this effect can lead to a decrease in induction time by ~50%.
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Acknowledgments
This work was partially supported
- by the Russian Ministry of Education and Science under
the program “5Top100”,
- by the Russian Foundation of Basic Research under the
projects No. 14-03-31449 and No. 14-08-00400 and
- by the AFOSR MURI program “Fundamental Mechanisms,
Predictive Modeling, and Novel Aerospace Applications of
Plasma-Assisted Combustion”.
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