plasma assisted combustion...afosr muri november 9 –10, 2011 fundamental mechanisms, predictive...

Plasma Assisted Combustion:Discharge Energy Balance, Bootstrap Effect in Plasma-Chemical Kinetics

1. Nano-Second Pulse Plasma Ignition Below Self-Ignition Threshold• LIF, PLIF, CRD Experiments

2. Energy Efficiency of Plasma Assisted Combustion• Kinetic Modeling of Discharge Energy Balance • Combustion Assisted Plasma • Modeling of Bootstrap Effect in PAC

AFOSR MURI ● November 9-10, 2011Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion

Modified Team:

AlexanderFridman

MikhailPekker

LiangWu

DanilDobrynin

AndreyStarikovskiy

NickCernansky

DavidMiller

Energy Efficiency of Plasma Assisted Combustion (Scramjet)Kinetic Modeling of Active Species Energy Cost in Different Discharges

AFOSR MURI ● November 9 – 10, 2011Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion

7.4 ns

Streamer Discharge (300 Td)

Dielectric Barrier Discharge

PAC Energy Efficiency, Modeling of Transitional DischargesCombustion Assisted Plasma, 1D Methane-Air Modeling

o If W ≈ 10 eV, G~10 (transitional discharges, microwave/gliding arcs, non-premixed: collective effects)

• 0.01 eV/mol (≈ 30 K)• specific enthalpy H ≈ 0.03

MJ/kg • total flow enthalpy 1.5 – 2.5

MJ/kg at M = 5 – 7• power 10 MW/m2

o If W ≈ 0.3 – 1 eV, G~100-300 (transitional discharges, partially premixed, combustion assisted plasma; Gmax, exp ~ 3000 for PO)

• 0.0003 - 0.001 eV/mol (1-3 K)• specific enthalpy H ≈ 1 – 10

kJ/kg • total flow enthalpy 1.5 – 2.5

MJ/kg at M = 5 – 7• power 0.3 – 1 MW/m2


Gliding Arc

GAT

MW

Not only radicals/excited species/ions but easy-to-ignite intermediates (CH2O, CH3OH, etc.) play role in PAI

Nano-Second Pulse Plasma Ignition Below Self-Ignition Threshold

Laser Induced Fluorescence

Methane 300 K

OH Planar LIF

Cavity Ring-Down Spectroscopy

• Three different diagnostic methods

• Homogeneous [OH] distribution along

the discharge channel

• Long life time of OH at low

temperature below ignition threshold

Methane 500 K @2 μs delay


Time resolved PLIF and CRDS diagnostics of OH radicals in the afterglow of plasma discharge in hydrocarbon mixtures

•Methane / Air mixture•Equivalence ratio: Lean (φ=0.1)•Temperature regimes:

Room temperature, 300 KElevated temperature, 500 K

•Premixed and preheated flowFlow rate ~20 cm/s

Liang Wu, J. Lane, N. Cernansky, D. Miller, A. Fridman, A. Starikovskiy, 7th US National Combustion Meeting, Atlanta, March 20-23, 2011

PLIF diagnostics has shown quite homogeneous

distribution of OH formation along the discharge channel

CRDS has provided the absolute [OH] which can be further

used for kinetic model development

Liang Wu, J. Lane, N. Cernansky, D. Miller, A. Fridman, A. Starikovskiy, 7th US National Combustion Meeting, Atlanta, March 20-23, 2011

Time resolved PLIF and CRDS diagnostics of OH radicals in the afterglow of plasma discharge in hydrocarbon mixtures

What causes the oxidation chemistry in afterglow?

T = 300 K 1D CH4-Air Mixture Kinetic Modeling (Konnov & GRI Mechs) with Analysis of Contribution of:1. Plasma generated radicals (O, OH ~10-4 – 10-3)

• Disappear within ~1-10 ms

2. Positive and negative ion catalysis (lifetime ~10 ms, low contribution, Langevin model)• HO2 + H2(M+) → OH + H2O + M+

• O2- + H2 → OH- +OH

• OH- + H(HO2) → H2O (+O2) + e• e + O2(+M) → O2

-

3. Plasma generated NO (~10-3 – 10-2, low contribution)• NO + HO2 → NO2 + OH• H + NO2 → NO + OH

4. Plasma generated excited species (unknown kinetics, alpha model)• HO2(group) + (N2

*) → OH + O + N2

• CH3O2 + (N2*) → CH2O + OH + N2

5. “Bootstrap” effect• Indirect effect of plasma (change of

macroparameters: T, stable components)• Plasma chemistry/reforming add’l heating• Plasma chemistry/reforming change of mixture

T = 500 K

Plasma assisted combustion mechanisms

Plasma catalysis

Chain reactions sustained by ions and excited species

Bootstrap effect

• Indirect effect of plasma (change of macroparameters:

temperature and stable components) to shorten ignition delay

• Plasma chemistry/reforming additional heating due

to chain oxidation by primary plasma species

• Plasma chemistry/reforming change of mixture due

to chain oxidation by primary plasma species (formation of

small amount of easy-to-ignite stable species)

Munchausen effect

Thermal effectNon-Thermal Radical effect

Radical chains terminate below autoignition limit

Bootstrap Effect in Plasma-Assisted Ignition (Atmospheric Pressure Methane-Air Mixture at Temperatures Below Auto-Ignition Limit)

Heating due to radicals: 12K

Kinetic Modeling (Konnov’s Mechanism); T0 = 960 K, direct discharge heating: 40 K, 10-3 [O]

Disappearance of radicals

Accumulation of easy-to-ignite stable species

Ignition delay

Bootstrap effect: plasma induced change of primary ignition macroparameters (T, Comp.)

OH OCHOCH

OH CHCHOH

22 3

2 34

Bootstrap coefficient:

Effective increase of temperature

exceeds direct plasma heating

heat

compRheat

heat

0eff

compRheat0eff

ΔT

ΔTΔTΔT

ΔT

TTK

ΔTΔTΔTTT

For the example above: K=3

DTheat – Direct heat 40KDTR – Heat due to radicals 12KDTcomp – Effective heating due to

change of composition (CH2O) 68K

Bootstrap Effect in Methane-Air Mixture:change of composition (plasma reforming, CH2O) contributes more than radical heating

Direct heating:[CH4]=0.0944, [O2]= 0.189 [N2]=0.7130 [O]=0.001T0=960K, P0=1atm. G-factor for O = 6.0===========================DTheat =40K

[O]=0.001Tin=DTheat+T0=40+960=1000[K]

Temperaturebefore discharge

Direct Heating

Bootstrap effect

Heatingdue to

Radicals

Effective Heatingdue

to change of composition

960K DTheat=40 K

DTR=12 K DTcomp=68K

3ΔT

ΔTΔTΔTK

heat

compRheat

Bootstrap Effect in Methane-Air Mixture:effect of initial temperature, 5·10-4 [O]

Bootstrap Effect in Methane-Oxygen Mixture

Direct heating (estimation):[CH4]= 0.0944, [O2] = 0.9051 [O] = 0.0005T0=979K, P0=1atm.G-factor for O = 8.0

DTheat=15.6K [O]=0.0005T0=DTheat+Tin=15.6+984.6=1000[K]


Direct Heating

Bootstrap effect

Heatingdue to

Radicals

Effective Heating due to change of composition

984.6K DTheat=15.6 K

DTR=21 K DTcomp=83.3 K

D

OH OCHOCH

OH OCHOCH

OH CHCHOH

2

1

2 3

22 3

2 34

[O2(1D)]=0.002

OH OCHOCH

OH CHCHOH

22 3

2 34

Bootstrap Effect in Ethylene-Air Mixturequazi-stable intermediates CH2O, HO2

22

342

HOCOOHCO

OHCOOHCO

HCOCHOHC

3242

223

CHOCHOHHC

OHOCHOCH

Production of radicals and HO2

Chain reactions

0

0.0002

0.0004

0.0006

0.0008

0.001

0 0.00001 0.00002 0.00003 0.00004 0.00005

Time(s)

O

OH

HO2

CO

CH3

CH2O

850

852

854

856

858

860

862

864

0 0.00001 0.00002 0.00003 0.00004 0.00005

T[K

]

Time(s)

Direct heating (estimation):[C2H4]= 0.066, [O2] = 0.198 [N2]=0.735[O] = 0.001 P0=70tor.

DTheat=42.4 K [O]=0.001T0=DTheat+Tin=42.4+807.6=850 [K]

Bootstrap Effect in Ethylene-Air Mixture

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7

1000

1200

1400

1600

1800

2000

2200

2400

2600

Time(s)

T

[K]

T=850 O=0.0001

T=850

T=865

T=865 CH2+Radicals

T=865 H2O

K=3.2Teffect=943K


Direct Heating

Bootstrap effect

Heatingdue to

Radicals

Effective Heating due to change of composition

807.6 K DTheat=42. 4K

DTR=15. 4K DTcomp=77.6K

T[K] [O] [CH3]+[OH] [CH2O] [HO2] Delay time (s)

850 0 0 0 0 6.75

850 0.001 0 0 0 0.675

865 0 0 0 0 4.61

865 0 0.0003+0.000016 0.000537 0 1.052

865 0 0 0 0.000232 0.756

Direct heating (estimation):[C2H4]= 0.066, [O2] = 0.198 [N2]=0.735[O] = 0.001 P0=70tor.

DTheat=42.4 K [O]=0.001T0=DTheat+Tin=42.4+807.6=850 [K]

quazi-stable intermediates CH2O, HO2

Plasma assisted combustion mechanisms

Plasma catalysis

Chain reactions sustained by ions and excited species

Bootstrap effect

• Indirect effect of plasma (change of macroparameters:

temperature and stable components) to shorten ignition delay

• Plasma chemistry/reforming additional heating due

to chain oxidation by primary plasma species

• Plasma chemistry/reforming change of mixture due

to chain oxidation by primary plasma species (formation of

small amount of easy-to-ignite stable species)

Munchausen effect

Thermal effectNon-Thermal Radical effect

Radical chains terminate below autoignition limit

Plasma Fuel Reforming vs Plasma Assisted Ignition

CH4

[L/min]

Air

[L/min]

[O/C] W

[watt]

From Discharge

[eV/molec]

From Reaction

[eV/molec]

Total

[eV/molec]

Add Air

[L/min]

Total

[eV/Molec]

5.39 20.09 1.55 480 0.26 0.074 0.34 (DT≈876K) 31.42 0.15 (DT≈390K)

Ignition temperature

about 700K

H2 CO CO2 N2 CH4 C2H4 C2H2 O2

12.15 5.97 0.50 60.74 12.043 0.21 0.79 9.59

Reforming Mixture

CH4 + Air Reforming Mixture +Air

T=1025K(delay ⋍ 0.1s) T= 850 K(delay 0.1s)


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plasma assisted combustion...afosr muri november 9 –10, 2011 fundamental mechanisms, predictive...

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