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.

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)

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

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))

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

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

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!

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

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

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)

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

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)

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

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

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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