Fundamental Mechanisms, Predictive Modeling,

and Novel Aerospace Applications of Plasma

Assisted Combustion

Yiguang Ju

AFOSR MURI Review Meeting

Ohio State University

Nov 9-10, 2011

Princeton Team members:

Wenting Sun, Joe Lefkowitz, Mruthunjaya Uddi, Sang Hee Won

Collaborators

AFRL: Campbell Carter, Timothy Ombrello

International: Fei Qi, Huijun Guo (USTC) 1

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Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18

Motivation

Hypersonic propulsion system

2

Hypersonic propulsion system

X-51A

Ignition time (~10ms)

Flow residence time (~1ms)Da= >>1

Challenges:• Ignition time, Ignition energy

• Flame stabilization

• Combustion completion

F135 engine: (F35, 2011)

Mach 6-8

Ignition instability

Plasma assisted combustion

Plasma

Ions/electrons

Excited species

Kinetic enhancement

Fuel fragmentsTemperature

increase

Transport enhancementThermal enhancement

RadicalsH2, CO

CH4

Understanding: Good poor

O, NO

O2(a∆g)

marginal

3

Change of ignition and extinction diagram: the S-curve transition

Residence time

Te

mp

era

ture

Scramjet

Plasma generated species:

O, H, O2(a∆g) …

4

Research goals

Understand the fundamental enhancement

mechanism of plasma-flame chemistry

Develop new experimental tools to validate

plasma flame kinetic mechanism

Develop numerical methods to achieve efficient

modeling of detailed plasma flame chemistry

5

Outline

1. Background

2. Experimental investigations

• Effects of plasma assisted fuel oxidation on

flame extinction

• Effects of in situ plasma discharge on

ignition enhancement

• Molecular beam mass spectrometry study of

low temperature chemistry

3. Conclusion and future work

6

Background and previous study: flame extinction

Air Air

1

2

5

15

16

3

14

N2

H2 &

N2

7

10

11 12

13

8 6

9

Fuel Fuel 4

N2 N2

1. Silicon Controlled Rectifier, 2. Silicon carbide heater, 3. R-typethermocouple, 4. Fuel injection spacer 5. MGA plasma power supply, 5.MGA device, 6. MGA power supply, 7. Cathode, 8. Anode, 9. Magnets,10. Gliding arc initiation wire, 11. MGA, 12. Insulator, 13. Nozzle withN2 co-flow, 14. K-type thermocouple & FT-IR probe, 15. Diffusionflame, 16. Water-cooled nozzle with N2 co-flow.

0

50

100

150

200

250

300

350

19 20 21 22 23 24 25 26

Str

ain

Rate

, 1

/s

Percent Methane Diluted in Nitrogen

Bundy et al.

Puri & Seshadri

No Plasma

33 Watts

44 Watts

60 Watts

78 Watts

0.00E+00

6.00E+15

1.20E+16

1.80E+16

-0.4 -0.2 0 0.2 0.4

Distance Between Nozzles, cm

Nu

mb

er

De

nis

ty o

f O

H0 Watts, a=83.3 1/s

48 Watts, a=183 1/s

78 Watts, a=127.7 1/s

Computation

Only thermal effect!Ombrello, et al, AIAA J, 2006

7

Previous work - Ignition study

NOx

catalytic effect

1. non in situ discharge

2. Short life times of radicals and excites species

CH4/air counterflow diffusion flame

2323 NOOCHNOOCH

NOOCHNOCH 323

H2/air counterflow diffusion flame

22 NOOHNOHO

NOOHNOH 2

Ombrello, et al, IEEE Plasma Sci, 20088

Previous researches – O3

u

bliftedL SS

0.2

0.3

0.4

0.5

0 0.005 0.01 0.015

Mixture fraction gradient dY F /dR

Sli

fted

[m

/s]

0

2

4

6

8

10

12

14

En

ha

nce

men

t [%

]

0 ppm O3 592 ppm O3

1110 ppm O3 1299 ppm O3

1299 ppm O3

1110 ppm O3

592 ppm O3

(~ 1/axial distance)

Flame speed extraction

Ombrello, et al, CNF, 20109

Previous researches – O2(a1∆g)

[O2(a1Δg)], ppm ΔHL, mm

3137 4.76

4470 6.82

4627 6.83

5098 7.31

0

1000

2000

3000

4000

5000

6000

4 5 6 7 8

Change of Flame Liftoff Height, ΔHL [cm]

Co

nce

ntr

ati

on

[p

pm

]

SDO (w/ NO)

SDO (w/o NO)

O3 (w/o NO)

Energy Coupling Into Flow

≈ 1 eV to produce O2(a1Δg)

≈ 5000 ppm O2(a1Δg) 2-3 % Lifted Flame Speed Enhancement

Nozzle Tip

O2 (a1Δg) + H = OH+O fast

O2 + H = OH +O slow

Ombrello, et al, CNF, 201010

Previous researches – Atomic oxygen effect

O quenched even at 60 Torr:

How to utilize radicals efficiently?

discharge

O2/Ar

CH4/Ar

0 5 10 15 20 25 30 35 40 45

0

2

4

6

8

10

12

14

O co

ncen

trat

ion

(10

15 cm

-3)

Pulse repetition frequency (KHz)

Crossover T: 900 K

Sun, et al, PCI, 2010

5000 ppm

1200 ppm

11

Research focus in the second year

Thrust 1. Kinetic effects of non-equilibrium plasma-assisted fuel

oxidation on diffusion flame extinction limits

Thrust 2. Direct ignition and the S-curve transition by in situ nano-

second pulsed discharge

Thrust 3. Plasma flame chemistry study in a flow reactor with

Molecular Beam sampling Mass Spectrum (MBMS)

Thrust 4. Development of a plasma assisted jet stirred reactor with

molecular beam sampling and a high pressure ignition

chamber

12

Thrust 1. Kinetic effects of non-equilibrium plasma-assisted fuel

oxidation on diffusion flame extinction limits

13

Experimental setup

14

FWHM= 12 ns

f = 0~50 kHz

20 & 28 mm ID

15 mm × 22 mm

10 mm

E/N~10-15 Vcm2

10 mm away from exit

Power~1.3 mJ/Pulse

FTIR/GC sampling

(heated)

-40 -20 0 20 40 60 80 100-3000

-1500

0

1500

3000

4500

6000

Vo

lta

ge (

V)

Time (ns)

O2/Ar/He/CH4

The thermocouple was coated with MgO and covered with grounded Nickel-Chrome sheath to remove EMI

OO

ffO

OU

U

L

Ua

1

2

P= 60 Torr

Laser diagnostics schematic

225nm mirrors

Filters

840nm

Collection lens

UV focusing lens

Photodiode

1064nm

225nm mirrors

225.7nm

Nd:Yag SHGTunable

Dye

Laser

BBO

DoublingBBO

Mixing

UV

Separator

Pulser

Boxcar

SRS272

PMT

Flow

direction

15

Numerical model

16

Kinetic model: OSU air

plasma model [1,2] with USC

mech II in addition of

Ar/He/CH4 related reactions.

Physical model: quasi-one

–dimensional flow equation +

steady two-term expansion

Boltzmann equation [1] Species concentrations

from simulation

ReactionsRate Const

(cm3s-1)

Ar(+) + CH4Ar +CH3 (+) + H 6.5×10-10

Ar(+) + CH4Ar +CH2 (+) + H2 1.4×10-10

Ar* + CH4Ar +CH3 + H 5.8×10-10

Ar* + CH4Ar +CH2 +H2 5.8×10-10

He(+) +O2 O(+) + O + He 0.6×10-11T0.5

Ar* + O2Ar+2O 2×10-10

He(+) +O2(a) O(+) + O + He 0.6×10-11T0.5

He+2O He* + O2 1×10-33

He* + CH4 CH + H2 + H+ He 5.6×10-13

Reference:

[1]. A. Bao, Ph.D thesis (2008) OSU [2]. M. Uddi et al, PCI 32(2009) 929 [3]. I.N. Kosarov et al, C&F 156(2009) 221 [4]. A. Hicks et al, JPD, 38(2005) 3812 [5]. D. S.

Stafford et al, JAP, 96(2004) 2451 [6]. M. Tsuji et al, JCP, 94(1991) 277 [7]. A.M. Starik et al, C&F, 157(2010) 313 [8]. I.N. Kosarev et al, C&F 154(2008) 569

Ar/He/O2/CH4

(0.32/0.4/0.26/0.02)

Counterflow nozzle exit

22 mm 10 mmelectrode

discharge

Reactions [1-8]Rate Const

(cm3s-1)

e+ O2 e+2O f(E/N)

e+ O2 e + O + O(D) f(E/N)

e + CH4 CH3 + H + e f(E/N)

e + ArAr* + e f(E/N)

e + ArAr(+) + 2e f(E/N)

e + He He* +e f(E/N)

e + He He(+) + 2e f(E/N)

Ar* + CH4Ar +CH2 +2H 3.3×10-10

Ar* + CH4Ar +CH +H2 + H 5.8×10-10

Exp

erim

enta

l observ

atio

ns o

f dis

cha

rges

f=30 k

Hz

O2 (0

.26

)/Ar(0

.32

)/He

(0.4

)/CH

4 (0.0

2)

Stro

ng

est e

mis

sio

n: A

r*, O*

Em

issio

ns: H

e*, O

H*, H

CO

*, an

d C

H*1

7~ ~ < ~ -~ = ~ .... =--= 9 '-"

w 0 0

~ 0 0

U"l

0

Intensity (a.u) ~ N w ~ U"l 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

L- CH (43 1.4 nm) and 0 (431.2 nm)

0 I ?" He (501.6 nm) and 0 (502 nm) 0

0\ 0 0

';==-----He (587.5 nm)

~---.;.....--~ He (667.8 nm)

0\ -.....) 0 0 0 0 0 0 0 0

~ Ar (696.5 nm) 0 Ar (707 nm) and He (706.5 nm)

QO 0 0 0 0

QO 0 0

Ar (727 nm) and He (728.1 om)

§i~~i~~~~~~~~A~r~(7~3~8~.4~n~n~l):a:o:d::O (738.7 om) Ar (750 nm) Ar (772.4 nm) Ar (763.5 nm)

.r (7~5 ~ (777.4 nm) Ar (80r.~ nm) and 0 (801.7 nm)

:.\r (811 .4 nm) Ar (826 nm)

L 0 (844.7 om) 0 (853.4 nm)

Discharge repetition effect on species concentrations

4000 3500 3000 2500 2000 1500 10000.00

0.25

0.50

0.00

0.25

0.50

0.00

0.25

0.50

Wavenumber (cm-1

)

no plasma

2% CH4

Ab

sorb

an

ce f=4 kHz

f=10 kHz

4000 3500 3000 2500 2000 1500 1000

Wavenumber (cm

-1)

f=20 kHz

f=30 kHz

f=40 kHz

FTIR spectrum with different pulse frequency

H2O

CH4

CH4

H2O

CO2

COCH2O

18

Discharge repetition effect on species concentrations

0 5 10 15 20 25 30 35 40 450

4000

8000

12000

16000

20000

Concentration

Oxidization rate

Pulse repetition frequency (kHz)

CH

4 c

on

cen

tra

tio

n (

pp

m)

0

20

40

60

80

100

CH

4 ox

idiz

atio

n r

atio

(%)

0 5 10 15 20 25 30 35 40 45

0

4000

8000

12000

16000

20000

24000

28000

Pulse repetition frequency (kHz)

Co

ncen

tra

tio

n (

pp

m)

CO (exp)

CO2 (exp)

CH2O (exp)

H2O (exp)

H2 (exp)

CO (sim)

CO2 (sim)

H2O (sim)

H2 (sim)

CH2O (sim)

Carbon deficiency: 5%

Relative uncertainties:

<1% for CH4, CO, CO2

5% for H2O and H2

The uncertainty of CH2O measurement is 80 ppm

Under prediction: CO2

Over prediction: CO, H2, H2O

423 K

613 K

743 K

843 K

933 K

19

Reaction path analysis-CH4&H2

CH4

CH3

CH2OCH3O

HCO

CO

CO2

OH, O,

H, e,

Ar(+),

Ar*

98.5%

66%

0.2%

1.4%CH2 CH

1.3%

5.3%

27.3%

1.3%

23.4%

68.6%

5.3%

100%

100%

100%

100%

H2

H2O

98.8%

1.2%

21.6%

OH + H

H + M

Ar* + CH4

M=CH4, CH2O, HO2,

HCO, CH2, CH3

77.4%

e, Ar*

f = 40 kHz

P = 60 Torr

T = 300 to 933 K

20

Reaction path analysis-H&O

H CH4 +Ar*/e/Ar(+) 11.7%

CH3 + O

51%

OH + H2

11.5%

CH2* + H2

10.5%

CO + OH

4.1%

HCO + H2O/O5.6%

O + H2

3.2%

O

O2 + e33.5%

H + O2

33.6%

He(+) + O2

Ar* + O2

20.4%

O2(a1Δg)/O2(b

1Σ) + H 9.7%

2.8%

OH H O2

0.6%54.3% 47.6%

+ CH4, HO2,

CH2O, H2,

HCO

+ CH3, H2,

HCO, CH2,

the reaction rate at 300 K for O(1D) + H2 = H + OH (4.4×1010 /cm3s)is much larger than O + H2 = H + OH (2.6×103 /cm3s).

f = 40 kHz

P = 60 Torr

T = 300 to 933 K

Mechanism was not validated below 700 KLarge uncertainty at low temperature

21

Extinction limit measurement & calculation

0.34 0.35 0.36 0.37 0.38 0.39 0.400

100

200

300

400

500

600

700

simulation (To= 336 K)

simulation (To= 423 K)

simulation (To= 613 K)

Fuel mole fraction

Ex

tin

cti

on

str

ain

ra

te (

1/s

)

no plasma

with plasma (f=4 kHz)

with plasma (f=10 kHz)

0.0 0.2 0.4 0.6 0.8 1.0

150

200

250

300

350

400

450

500

550

40% CH4 was reformed

to CO and H2

E

xti

ncti

on

str

ain

ra

te (

1/s

)Oxidization or reforming ratio

Reference (2% CH4)

Fuel oxidization

Fuel reforming

40% CH4 was oxidized

to CO2 and H

2O

Simulations were performed with experimentally measured boundary conditions.

OH, H concentrations were estimated from simulation by matching O concentrations.

Case 1: fuel was oxidized to CO2 & H2OCase 2: fuel was reformed to CO & H2

Fuel reforming enhancement: fast H2

chemistryFuel oxidization enhancement: extracting chemical enthalpy rapidly

Faster fuel oxidization, larger extinction extension

22

Extinction limit measurement & calculation

10 15 20 25 30 35 40300

400

500

600

700

800

900

1000 Experiments

Simulation

Xf=0.2

Pulse repetition frequency (f)

Ex

tin

cti

on

str

ain

ra

te (

1/s

)

Simulations were performed with experimentally measured boundary conditions.

OH, H concentrations were estimated from simulation by matching O concentrations.

CH4 oxidization ratio (or f) increased, extinction limits increased significantly

The dominant enhancement mechanism is plasma introduced rapid fuel oxidization.

Deviation is due to additional reaction paths, but not significant (10%).

5.3% enhancement from H2

23

Thrust 2. Direct ignition and the S-curve transition by in situ nano-

second pulsed discharge

24

Experimental setup

25

25.4 mm

P = 72 Torrf = 24 kHz

Power ~ 17 W

Laser beam

ICCD images

26

OH* emission ~310 nm30 ms gate

Single shot Single shot

(a) ICCD image, He/O2 (0.6:0.4) and He/CH4 (0.75:0.25), 50 ns gate

(b) ICCD image, He/O2 (0.6:0.4) and He/CH4 (0.86:0.14), 50 ns gate

(c) direct photo of (a), 50 ms exposure time

(d) direct photo of (b), 50 ms exposure time

P = 72 Torr, f = 24 kHz, a = 175 1/s

Classical S-curve

27

Relationship between OH* emission intensity, local maximum temperature and fuel mole fraction, To=650 K, Tf=600 K He/O2 = 0.66:0.34 , P = 72 Torr, f = 24 kHz, a = 400 1/s

hysteresis between ignition and extinction: S curve

Rayleigh Scattering[1,2]

method for T measurement at 532 nm from Nd:YAG laser

[1] R.B. Miles, W.R. Lempert, J.N. Forkey, Meas. Sci. Technol. 2001 [2]J.A. Sutton, J.F. Driscoll, Exp Fluids 2006

S curve transition

28

Relationship between OH* emission intensity, local maximum temperature and fuel mole fraction, P = 72 Torr, f = 24 kHz, a = 400 1/s

He/O2 = 0.45:0.55 He/O2 = 0.38:0.62

ignition and extinction points were pushed to lower fuel concentrations

monotonic ignition and extinction curve (monotonicS curve)

Can the hysteresis be removed ?

Numerical modeling

29

e + O2 reactions Rate (cm3s-1)

e + O2 2O + e f(E/N)

e + O2 O + O(D) + e f(E/N)

e + O2 O2(+) + 2e f(E/N)

e + O2 O2(a) + e f(E/N)

He related reactions Rate (cm3s-1)

He + e He* +e f(E/N)

He + e He(+) + 2e f(E/N)

He* + O2 O2(+) + He + e 1.5×10-11T0.5

He(+) +O2 O(+) + O + He 0.6×10-11T0.5

He* + CH4 CH + H2 + H+ He 5.6×10-13

e + CH4 reactions Rate (cm3s-1)

e + CH4 CH3 + H + e f(E/N)

e + CH4 CH2 + H2 + e f(E/N)

e + CH4 CH4(+) + 2e f(E/N)

Recombination reactions Rate (cm3s-1)

e + O2(+) 2O 5.6×10-6T-0.5

He(+) + e + M He + M 1.4×10-8

e + O2 + M O2(-) + M 4.2×10-27T-1

e + CH4(+) CH3 + H 1.0×10-8

OPPDIF + electron impactKinetic mechanism: USC mech II + OSU air plasma model[1]

Rate constants: Boltzmann equation solver[1, 2]

[1]. A. Bao, Ph.D thesis (2008) OSU [2]. M. Uddi et al, PCI 32(2009) 929

E: electric field, N: particle density

Simulation results

30

XO2 = 0.34, XCH4 = 0.16, P = 72 Torr, f = 24 kHz, a = 400 1/s

no flame, but reaction zone was built up by radicals generated from plasma

e + CH4CH3 + H + e

fuel oxidizer

e + O2O+O(D) + eIn situ discharge, increased T, increased E/N, increased rate const

Path flux analysis

31

O

O2 + e47.7%

He(+) + O2

23.3%O2(a1Δg)/O2(b

1Σ) + He

21.7%

OH H O2

0.2%43.7% 56.1%

O2(+) + e 5.4%

1.9%

other paths

O

OH H27.5%

59.4%

O e

53.6% 37.9%

CH4

CH3

OH: 39.9%

e: 25.2%

O: 12.7%

H: 9.6%

(a)

CH4(+)11.5%

100%

CH3O6.3%

23.1%

CH2O

45.9%

HCO

CO

CO2

C2H6

24.5%

CH2*

14.8%

CH3OH

1.1%7.3%

17.9%

11.5%

82.1%

65.4%

(b)

O

O2 + e47.7%

He(+) + O2

23.3%O2(a1Δg)/O2(b

1Σ) + He

21.7%

OH H O2

0.2%43.7% 56.1%

O2(+) + e 5.4%

1.9%

other paths

Change of branching ratio

Change of the branching ratio at the reaction zone!

S curve transition

Increased productivities of radicals

76% of O production by e and ions from plasma

Radical generation initiated the reaction zone and controlled the transition!!

ReactionsNormalized

branching ratio

H + O2 = O + OH 1

e + O2 = O + O(D) + e 0.48

e + O2 = O + O(+) + e 0.42

e + CH4 = CH3 + H + e 0.22

He(+) + O2 = O + O(+) + He 0.52

e + O2 = 2O + e 0.06

H + O2 + M = HO2 + M 0.2

1.7

32

Thrust 3. Plasma flame chemistry study in a flow reactor with

Molecular Beam sampling Mass Spectrum (MBMS)

33

Characteristic of low T chemistry

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

600

900

1200

1500

1800

2100

2400

1050 K

Te

mp

era

ture

(K

)

Time (sec)

650 K

j=1, n-heptane/air, 1 atm

ignition

Ignition delay,

H2O2 was stableH2O2 2OH

H2O2: low T chemistry indicator

Difficulties:

Absorption - overlap with H2O

GC – decomposition/low reactivity

Transition from low T

to high T ignitionLow T ignition

34

Molecular Beam Mass Spectrum

pump

Sampling system Time of fly

HeHeHeHe D

D

S

S

1111

S : signal intensity

D : mass discrimination factor

: cross sections

: mole fractions35

Schematic of experiments with MBMS

Molecular beam

Rea

cto

rex

itR

eact

ion

pro

du

cts

0.1-5 atm

Quartznozzle

Skimmer

1st Turbopump

2nd Turbopump

Chargedion separation

Mass analyzer

10-4

Torr10-6

Torr

Molecular beam

Rea

cto

rex

itR

eact

ion

pro

du

cts

0.1-5 atm

Quartznozzle

Skimmer

1st Turbopump

2nd Turbopump

Chargedion separation

Mass analyzer

10-4

Torr10-6

Torr

Laser beam

MBMS analysisFuel

Preheated air

High pressure, high temperature chamber

DBD discharge

MixingJet stirred reactor

Laser beam

MBMS analysisFuel

Preheated air

High pressure, high temperature chamber

DBD discharge

Mixing

Laser beam

MBMS analysisFuel

Preheated air

High pressure, high temperature chamber

DBD discharge

MixingJet stirred reactor

ovenJacket heater

MBMS

14 inch2 inch 36

480 520 560 600 640 680 720 760

0

500

1000

1500

2000

2500

3000

H2O

2 C

on

cen

tra

tio

n (

pp

m)

Temperature (K)

experiments

simulation

DME: 1%

O2: 5%

He: 94%

residence time: 0.2 S

Flow tube experiments

H2O2 measurement

DME: rich low temperature chemistry

Pressure: 1 atm

DME model: Zhao et al., Int. J. Chem. Kinet., (40) 200837

Flow tube experiments

DME: rich low temperature chemistry

Pressure: 1 atm

25 30 35 40 45 50 55 60 65

0

500

1000

1500

2000

2500

3000

3500

6000

6500

7000

7500

8000R

ela

tive

Sig

na

l

m/z

x2

0

60 (CH3OCHO)

46(DME)

45(CH3OCH

2)

(CO2)44

34(H2O

2,34

O2)

32(O2)

30(HCHO)

38

1. Plasma can significantly accelerate the fuel oxidization at low temperature to extend

the extinction limit dramatically.

2. Major kinetic pathways in plasma assisted combustion were identified .

3. A new counterflow burner with in situ discharge was developed. This burner

provides a new platform to study kinetic effect of plasma assisted combustion.

4. The In situ discharge can maximize E/N at high T flame region, therefore, maximize

the electron energy and effect on reaction zone, and enhance ignition and extinction.

5. The In situ discharge can dramatically enhance the ignition and modify the classical

S-curve to be a monotonic curve.

6. MBMS was developed and H2O2 was successfully measured directly for the first time

in reacting system, enabling diagnostics of intermediate species in plasma assisted

combustion at low T.

Conclusions

39

Future work

MBMS part:

1. Develop a JSR to study the low temperature and high pressure chemistry

2. Integrate JSR with plasma discharge to investigate plasma chemistry

3. Develop advanced light source to ionize the molecular beam

Plasma part:

1. OH PLIF for counter flow diffusion flame with in situ discharge and compare

with simulations

2. Low temperature plasma assisted combustion for large alkanes

3. Flow reactor experiments on liquid fuel with QCL diagnostics on H2O, H2O2

and HO2

4. Develop validated plasma flame models

40

Thanks the support from AFOSR!

Questions?

41

Flow tube experiments

0 2 4 6 8 10 12 14

510

540

570

600

630

660

690

720

750

780

T/K

Distance/inch

upstream downstream

ovenJacket heater

MBMS

14 inch2 inch

42

H2O2 calibration

Dissociation:

do it quickly

changing H2O2 concentrations

monitor O2 peak

Syringe pump Vaporizer

H2O2 solution

Dilution gas

MBMS

450 K

43

0 2 4 6 8 10 12 14 16400

600

800

1000

1200

1400

1600

1800

2000

Tem

pera

ture (

K)

Position (mm)

w. discharge

w.o discharge

w. discharge

w.o discharge

O2 = 53.5%, CH4 = 20%, a = 400 1/s

44