Plasma Assisted Combustion: Flame Regimes and Kinetic Studies

Yiguang Ju, Joseph Lefkowitz, Tomoya Wada, and Sanghee Won

Department of Mechanical and Aerospace Engineering, Princeton University Princeton, NJ 08544, USA

AFOSR MURI Program Review 2015.01.05

MURI Facility Summary and collaborative team structure

3000K

1000K

300K

0.01atm 1atm 100atm

Flame chemistry (Ju, Sutton)

Flow Reactors ( Yetter,

Adamovich)

Shock Tube (Starikovskiy)

RCM (Starikovskiy)

MW+laser (Miles)

JSR/Flow reactor Species and kinetics

(Ju)

1. Plasma activated low temperature combustion & cool flames (liquid fuels: dimethyl ether, n-heptane)

Today’s Presentation (2014)

3. In-situ and time accurate multispecies diagnostics in a plasma flow reactor (kinetics)

4. Development of low temperature and high pressure plasma combustion mechanism (HP-MECH/plasma) (collaboration)

2. Plasma assisted mild combustion (flame regimes)

τ1 τ2

Hot ignition Low temperature ignition

0.0 0.1 0.2300

600

900

1200

1500Te

mpe

ratu

re (K

)

Time (sec)

R+O2=RO2

HCO+O2=CO+HO2

2HO2=H2O2+O2

H2O2=2OH

H+O2=O+OH O+H2=H+OH

RO2→QOOH →R’+OH O2QOOH →R’’+2OH

Thermal effect dominant Kinetic effect

1. Plasma Activated Low Temperature Combustion and cool flames for liquid hydrocarbon fuels

>1100 K High temperature ( better understood)

800-1100 K Intermediate

500-800 K Low

Plasma has more kinetic enhancement effect in lower temperature combustion However, poorly studied and understood…

Two-stage ignition charateristics

Large molecules Fuel fragments Small molecules

CH2O+X=HCO+XH

1.1 Plasma activated low temperature combustion of liquid fuels: flame regime changes

5

Residence time

Tem

pera

ture

LTC

Ignition

Extinction

Flame

LTC

t2<< t1 So it occurs in ms or even without an extinction limit!

t2 t1

Plasma assisted low temperature

0.00 0.02 0.04 0.06 0.08 0.10 0.12

1x105

2x105

3x105

4x105

5x105

6x105

Extinction

increase decrease

CH2O

PLI

F (a

.u.)

Fuel mole fraction

Hot Ignition

P = 72 Torr, a= 250 1/s, f = 24 kHz

XO2=40%, varying Xf

LTC

HTC

0.00 0.02 0.04 0.06 0.08 0.10 0.12

1x105

2x105

3x105

4x105

5x105

6x105 increase decrease

CH2O

PLI

F (a

.u.)

Fuel mole fraction

LTCHTC

P = 72 Torr, a= 250 1/s, f = 34 kHz,

XO2=60%, varying Xf

DME

Sun et al. 2014, Combustion & Flame.

1400K

• Fixed O2 molar fraction (XO2 = 0.3) and stretch rate (a = 150 s-1)

0.3 0.25 0.2 0.175 0.15 0.125 0.1 Plasma: on

Plasma: off

0.0125 0.025 0.0375 0.05 0.0625 0.075 0.1

Flame Extinction Fuel molar fraction, XF Low

Fuel molar fraction, XF High Ignition

1.1 Plasma activated low temperature combustion: n-heptane

OH-PLIF measurement with varied XF (n-heptane)

• Hysteresis (S-Curve, thin and thick reaction zones) • Flame: Combustion chemistry dominated regime at high

temperature and, • Ignition: Plasma chemistry dominated regime at low temperature

7

Flame

Ignition

Species measurements in plasma assisted low temperature combustion

• Probe O.D.: 363 µm • Adjust position (Vert. & horiz.) • Negligible influence on the flame

Tomoya Wada 8

25.4 mm

2/3/2015

N-heptane

near extinction

(XF = 0.1, XO = 0.3, and a = 150 s-1)

near ignition

Species distribution near ignition and extinction

High temperature chemistry Low temperature chemistry

Providing validation targets

1.2 Experimental study of plasma assisted diffusional cool flames

• A heated counterflow burner integrated with vaporization system1

• n-heptane/nitrogen vs. oxygen/ozone

• Ozone generator (micro-DBD) produces 2- 5 % of ozone in oxygen stream, depending on oxygen flow rate

• Speciation profiles by using a micro-probe sampling with a micro-GC.2

10

Heated N2 @ 550 K

N2 @ 300 K

Stagnation plane

O2 + O3 @ 300 K

Fuel/N2 @ 550 K

Pressure chamber

Micro-GC

Positioning stage

Ozone generator O2 @ 300 K

1) S. H. Won, et al., Combust. Flame 157 (2010) 2) J. K. Lefkowitz, S. H. Won, et al., Proc. Combust. Inst. 34 (2013)

(b) Normal diffusion flame

(a) Cool diffusion flame

Stability diagram of diffusional cool flames

• Lower Xf, higher a; no flame initiated. • Higher Xf, lower a; normal diffusion flames • Intermediate Xf and lower a; cool diffusion flames

• Unstable regime extended • As increasing both a and Xf • Continuous ignition and

extinction of cool flames

11

40

60

80

100

0.02 0.06 0.1 0.14 0.18

Stra

in ra

te a

[s-1

]Fuel mole fraction Xf

no flame

hot diffusion flame

4 % O3

Cool flames extends the auto-ignition limit!

Sensitivity Analysis near Extinction

Reactions • Importance of low temperature

chemistries • RH + OH (~ 15% heat

production) • R + O2 reactions (~40%) • QOOH reactions • HO2 reactions

Transport • Very sensitive to ozone diffusion

• O3 + N2 → O2 + O + N2 for initiation of radical pool.

• Thus, fuel diffusion is important as well.

• Strong sensitivity to CH2O • Indicator of low temperature

reactivity1

12 -0.2 -0.1 0 0.1 0.2 0.3 0.4

nc7h16+oh=c7h15-2+h2o

o3+n2=>o2+o+n2

c7h15o2-3=c7h14ooh3-5

c7h15o2-2=c7h14ooh2-4

ho2+oh=h2o+o2

c2h5+ho2=c2h5o+oh

c7h15o2-1=c7h14ooh1-3

ch2o+oh=hco+h2o

c7h14ooh1-3o2=nc7ket13+oh

c7h15o2-4=c7h14ooh4-2

c7h15o2-2=c7h14ooh2-3

pc4h9o2=c4h8ooh1-3

Logarithmic senstivity efficient

Xf = 0.05, XO3 = 0.03Tf = 550 K, To = 300 K

-0.1 0 0.1 0.2 0.3 0.4

o3

nc7h16

o2

n2

ch2o

h2o

c7h14o2-4

ch3cho

Logarithmic senstivity efficient

Xf = 0.05, XO3 = 0.03Tf = 550 K, To = 300 K

1) S. H. Won et al, Combust. Flame 161 (2014) 475-483

Speciation Profiles and validation of kinetics • Reasonable prediction of acetaldehyde and CH2O • Significant over-estimation of C2H4 and CH4 formation

• Factor of 10.

13

0

2000

4000

6000

8000

6 10 14 18

Spec

ies

mol

e fr

actio

n [p

pm]

Distance from fuel side nozzle [mm]

acetaldehyde, expacetaldehyde, modelch2o, expch2o, model

0

200

400

600

800

1000

6 10 14 18

Spec

ies

mol

e fr

actio

n [p

pm]

Distance from fuel side nozzle [mm]

c2h4, exp.

c2h4/10, model

ch4, exp

ch4/10, model

(a)

(b)

R +

O2

RO2

QOOH

Olefin +

HO2Propagation

Olefin +

Carbonyl

Olefin +

HO2QO

+ OH

O2QOOH

Ketohydroperoxide + OH

CH2O +

R +

CO +

OH

Branching

+ O2

- O2

Propagation

+ HO2

1.3 Plasma assisted premixed cool flames

14

• Lean Flammability Limit: Normal flame vs. cool flame

Flam

e sp

eed

Equivalence ratio Φ0 Φ’0 Φ’’0

? cool flame

15

1.3a. Numerical results of Freely propagating 1D planar cool flames

• Geometry 1D freely propagating flames • Mixture and Kinetic model Fuel: Dimethyl ether Oxidizer= (1-x)O2 + xO3, x=0 - 0.1, p=1 atm Ozone chemistry & Dimethyl ether model Ombrello, et al., Combustion and Flame, Vol. 157, 2010 Zhao et al., Int. J. Chem. Kinet., 40 (2008) Liu et al., Combustion and Flame, 160 (2013) • Numerical method Modified Chemkin with arc-continuation method Radiation (Optically thin model for CO2, H2O, CO, CH4) Ju et al. JFM, 1997

SL

16

• Lean Flammability Limit Extension by formation of cool flames

– Lean limit of ϕ = 0.078 w & WO 5% ozone addition – Ozone promote cool flames – Three flame regimes – Cool flames significantly extends the lean burn limit of normal flames – Cool flames can have a high flame speed between (~15 cm/s)

transition

– Temperature of N2= 600K – Temperature of DME/O3/O2=300 K – Strain rate=80 s-1 – Ozone concentration: 3%

17

Experimental observation of premixed cool flames

Heated N2 @ 600 K

N2 @ 300 K

Stagnation plane

DME+ O2 + O3 @ 300 K

N2 @ 600 K

Pressure chamber

Micro-GC

Positioning stage

Ozone generator O2 @ 300 K

Premixed Cool Flame stability/regime diagram

– Three flame regimes found: • Unburned mixture past lean

limit • Stable cool flames • Transition regime to hot

flame

– Lean limit slightly increases with strain

– Width of stable cool flame region doubles from 75 s-1 to 85 s-1

18

m

Conventional Mild combustion

High temperature combustion

Conventional combustion

Unstable combustion region

21% 10% 3% Oxygen mole fraction in diluted air

Dilu

ted

air

Tem

pera

ture

Tig

Tpig

New plasma assisted mild combustion (PAMiC)

∆Tf

∆Tig

2. Plasma assisted mild combustion

Can plasma extend the boundary of mild combustion to lower temperature?

Mild combustion: co-axial burner

1

2

3

4

4

1. Electrodes 2. Insulation 3. Preheat burner 4. Oxidizer flowing sec.

Center burner (Fuel/N2) Plasma reactor (lean mix.)

Preheated Oxidizer

Plasma

Center burner Plasma reactor

Electrode

6/27/2014 Tomoya Wada (Princeton University) 20

MILD combustion w/ and w/o plasma

• Condition • Preheat gas temp.: 1050 K • Preheat gas O2: 12% • Center burner vel.: 20 m/s • Center burner CH4/N2: 10% • Plasma reactor vel.: 5 m/s

• Plasma reactor • CH4/air ratio: 0% and 3%

Shorter and wider reaction zone

6/27/2014 Tomoya Wada (Princeton University) 21

3. In Situ time accurate Mid-IR LAS Diagnostics in plasma/flow reactors (CH4/O2)

Fig. 1 CH2O time history measurements and modeling of a 300 pulse burst at 30 kHz in a stoichiometric CH4/O2/He with 75% dilution.

0

50

100

150

200

250

0 5 10 15 20

Mol

e Fr

acti

on (p

pm)

Time (ms)

CH2O Experiment

CH2O Model

CH2O

Ge Etalon

Reactor

CollimatingLenses

Mirror

Flip Mirror

Quartz Wall

MacorWall

Vacuum Chamber

Species Wavelength (nm)

Wavenumber (cm-1)

Line strength @ 300 K (cm/molecule)

CH4/Temp 7442.91 1343.56 1.898x10-22

7442.52 1343.63 1.78x10-22

CH2O 5791.09 1726.79 6.47x10-20

CH4, H2O, C2H2…

CH2O,…

0

25

50

75

100

125

150

175

200

100 1000 10000 100000

Mol

e Fr

actio

n (p

pm)

x 10

00

Frequency (Hz)

O2 Experiment O2 ModelCH4 Experiment CH4 ModelH2O Experiment H2O Model

0

2000

4000

6000

8000

10000

12000

14000

100 1000 10000 100000M

ole

Frac

tion

(ppm

)Frequency (Hz)

CO ExperimentCO ModelCO2 ExperimentCO2 ModelH2 ExperimentH2 Model

0

200

400

600

800

1000

100 1000 10000 100000

Mol

e Fr

actio

n (p

pm)

Frequency (Hz)

CH2O ExperimentCH2O ModelCH3OH ExperimentCH3OH ModelC2H2 ExperimentC2H2 ModelC2H4 ExperimentC2H4 ModelC2H6 ExperimentC2H6 Model

Continuous Plasma – CH4/O2/He

23

• Stoichiometric, 75% helium dilution, 30 kHz pulse rep. freq.

• Fuel consumption and major species agree well with model

• Disagreement with minor species Intermediate species

Figure 6: Path flux analysis of fuel consumption integrated over a single pulse period during continuous discharge at 30 kHz repetition frequency and steady state temperature conditions. Bold species represent those which are measured in Figure 5, red arrows refer to reactions from the combustion model, and blue arrows are from the plasma model.

CH4

CH3 + OH

CH3 + H2O + OH 15%

CH3+ + H

+ e- 13%

CH2OH + H CH3 + H CH4+

+ O

2 + M

94%

CH3O2 + M

+ O 6% CH2O + H

CH3O + O2/OH CH3OH + O2

+ e-

100

%

CH2 + H/H2

+ O

2 100

%

CH4 + O2+

+ O

2 100

%

CO + OH + H

CO2 + H + H

CH2O + O CO2 + H2

+ O

2 100

%

CH2O + HO2

Large uncertainty in low temperature oxidation pathways

0

100

200

300

400

500

1

10

100

1000

10000

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

Tem

pera

ture

(K)

Mol

e Fr

actio

n (p

pm)

Time from last pulse (ms)

C2H2, Exp. C2H2, HP C2H2, USCCH4, Exp. CH4,HP CH4, USCH2O, Exp. H2O, HP H2O, USCT, Exp. T, HP T, USC

In Situ Mid-IR Diagnostics and kinetic study in plasma/flow reactors (c2h4/o2)

Fig. 2 Comparison of measured and predicted species (H2O, CH4, C2H2 formation in C2H4 oxidation: HP-Mech vs. USC Mech

In-situ Steady state species measurements

C2H4

C2H5

+ H +M 31%

+ Ar* 5%

C2H2

+ Ar(+) 13% C2H3+

+ e- 30% CH2CH2OH

+ OH 15%

CH3+ HCO

+ O 13%

H + CH2CHO

+ O 11%

+ e- 65%

C2H

CO + CH2O + OH

+ O2 46%

+ H 21%

CH20 + HCO

+ O 21%

CH3O2

+ O2 + M 85%

CH3O

+ X 95%

CH2O

+ X 96%

C2H5O2

+ O2 + M 97%

C2H5O2H

+ HO2 98%

HCO + CO

+ O2 100%

O2C2H4OH

+ O2 100%

2 CH2O + OH

100%

M = Third body collider X = Radical

Blue = Plasma Red = High temperature, Green= Low temperature

Ethylene Oxidation Pathways (C2H4/O2/Ar)

LTC HTC

PAC activates C2H4 low temperature chemistry Large uncertainty in low temperature oxidation pathways

CH2O

Key reaction pathways in combustion kinetics at high pressure and low temperature: HO2/RO2

Bretfield et al., JPC letters, 2013.

blue arrow: Below 700K; yellow arrow: 700-1050 K; red: above 1050K

•Strong spectra overlap between HO2, H2O2, RO2 in UV and with H2O in mid-IR •Unstable •OH detection is limited by linebroading.

Paramagnetic (radical) species

Absorption

Dispersion

ν

ν

HO2 energy levelsZeeman splitting

New diagnostics: HO2/OH using mid-IR Faraday Rotational Spectroscopy

Laser Lock-In Amplifier

+Bfield

( )0( ) sin 2RMS RMSV GPν θ= Θ

Bremfield et al., 2013, JPC letters, 2013; Kurimoto et al. 2014

Experimental results: HO2/OH measurements

Implication: RO2→QOOH→O2QOOH uncertainty HCO+O2=HO2+CO reaction uncertainty and HCO formation pathway?

Signal

DME flow reactor model validation

Sensitivity OH HO2

• Base mechanism: high pressure combustion mechanism: HP-Mech H2/O2 sub-mechanism: Burke et al. 2012 (PU and ANL) CO/CH2O/CH3OH sub-mechanism: Labbe et al. 2014 (ANL and PU in CEFRC) • O3 sub-mechanism: (PU, Ombrello et al. 2010) O3 decomposition updated (J. Michael, 2013) • O(1D) reaction pathways O(1D) + Fuels/N2/O2/CO/CO2/H2O/CH2O updated • O2(singlet) reaction pathways O2(singlet) + Fuels/H/OH/CH3/H2/CH4 updated • NOx reaction pathways

Mueller et al., Intl. J. Chem. Kin. (1999), Vol. 31, pp. 705-724 Allen et al., Combust. Flame (1997), Vol. 109, pp. 449-470 Dean and Bozelli (2000, Gardiner ed.)

Klippenstein, Stephen J.; Harding, Lawrence B.; Glarborg, Peter; Miller, James (2011)

4. High Pressure Mechanism for Plasma Assisted Combustion (HP-Mech/plasma) H2/H2O2/O3/CO/CH2O/CH3OH/CH4

0.0E+00

2.0E-03

4.0E-03

6.0E-03

8.0E-03

1.0E-02

1.2E-02

0 500 1000 1500

CH3O

H M

ole

Frac

tion

Time [μs]

1266 K and 2.5 atm

1368 K and 2.4 atm

1458 K and 2.3 atm

1610 K and 2.2 atm

Tests of NOx chemistry in various fuel oxidation systems

H2 system

0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2%

0 1 2

H2 M

ole

Frac

tion

Time [s]

1.0 atm 3.0 atm

0.E+00

2.E-05

4.E-05

6.E-05

8.E-05

1.E-04

0 2

NO

Mol

e Fr

actio

n

Time [s] Tini = 807 K 1% H2, 2% O2, 108 ppm NO, balance N2 Experimental measurements at various points in flow reactor (Mueller et al., 1999)

CO system

0.0% 0.1% 0.2% 0.3% 0.4% 0.5% 0.6%

0 0.5 1 1.5

CO

Mol

e Fr

actio

n

Time [s]

3.0 atm 6.5

0.E+00 2.E-05 4.E-05 6.E-05 8.E-05 1.E-04

0 0.5 1 1.5 NO

/ N

O2 M

ole

Frac

tion

Time [s]

N

NO

Tini = 952 K 0.5% CO, 0.75% O2, 0.5% H2O, 108 ppm NO, balance N2 Experimental measurements at various points in flow reactor (Mueller et al., 1999)

•Mueller et al., Int. J. Chem. Kin. 31 (1999), pp. 705-724

Collaborating with Richard Yetter, 2014

Plasma Modeling Tool Development

… E/N

Time 0

32

ZDPlasKin CHEMKIN II - SENKIN

1𝛾𝛾 − 1 𝑘𝑘𝐵𝐵

𝑑𝑑(𝑁𝑁𝑇𝑇𝑔𝑔𝑔𝑔𝑔𝑔 )𝑑𝑑𝑑𝑑 = 𝑃𝑃𝑒𝑒𝑒𝑒𝑑𝑑 − 𝑃𝑃𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 − 𝑃𝑃𝑒𝑒ℎ𝑒𝑒𝑒𝑒

𝜌𝜌𝑑𝑑𝑌𝑌𝑘𝑘𝑑𝑑𝑑𝑑 = 𝜔𝜔𝑘𝑘𝑊𝑊𝑘𝑘

0%

2%

4%

6%

8%

10%

12%

14%

0.6 1.1 1.6Incr

ease

of f

lam

e sp

eed,

%

Equivalence ratio

O3-2330ppm-exptsO3-2330ppm-exptsO3-2330ppm-HPMechO3-3730ppm-HPmechKonnov-Simulation-3730ppmKonnov-Simulation-2330ppm

HP-Mech/plasma validation: Ozone effect on flame speeds

Conclusions

1. This MURI program is a very exciting exploration of knowledge frontier.

2. Plasma activated Self-Sustaining diffusion and premixed Cool Flames & mild combustion were established for the first time. Creating exciting opportunities in engine and fuel applications.

3. Plasma has a strong kinetic effect in low temperature combustion. A direct ignition transition to flame without extinction limit was observed.

4. New diagnostic method (e.g. FRS) for in-situ and time accurate measurements

of intermediate species and HO2 radicals was developed. Plasma active low temperature chemistry via CH2O and RO2 is an important fuel oxidation pathway at low temperature.

5. Plasma combustion chemistry remains a big challenge, especially at low

temperature. The existing plasma kinetic mechanism is not able to predict appropriately the plasma activated low temperature kinetics.

Publications and Awards:

1. Distinguished Paper Award of the 35th International Symposium on Combustion: “Self-Sustaining n-Heptane Cool Diffusion Flames Activated by Ozone”

2. Plenary Lecturer, The 8th International Conference on Reactive Plasmas, Fukuoka, Japan, 2014.

Awards:

Journal Publications 1. Ju, Y. and Sun, W., (2015), Plasma Assisted Combustion: Dynamics and Chemistry, Progress of Energy

Science and Combustion, 2015. 2. Ju, Y. and Sun, W., (2015), Plasma Assisted Combustion: Challenges and Opportunities, Combust.

Flame, 2015. Invited opinion paper. 3. Peng Guo; Timothy Ombrello, Sang Hee Won, Christopher A Stevens, John L Hoke, Frederick Schauer,

Yiguang Ju, Schlieren Imaging and Pulsed Detonation Engine Testing of Ignition by a Nanosecond Repetitively Pulsed Discharge, submitted to Combust. Flame, 2015.

4. Lefkowitz, J.K., Uddi, M., Windom, B., Lou, G.F., Ju, Y. (2015), In situ species diagnostics and kinetic study of plasma activated ethylene pyrolysis and oxidation in a low temperature flow reactor, Proceedings of Combustion Institute, 35, 2015.

5. Won, S.H., Jiang, B., Diévart, P., Sohn, C.H., Ju, Y., (2015), Self-Sustaining n-Heptane Cool Diffusion Flames Activated by Ozone, Proceedings of Combustion Institute, 35, 2015

6. Brumfield, B., Sun, W., Wang, Y., Ju, Y., and Wysocki, G. (2014), Dual Modulation Faraday Rotation Spectroscopy of HO2 in a Flow Reactor, Optics Letters, Vol. 39, Issue 7, pp. 1783-1786 (2014).

5. Future research

• Low temperature Fuel oxidation kinetics involving O(1D), HO2, O3, O2(1Δ) in photolysis and flow reactor (0.1-2 atm)

• High pressure plasma assisted cool flames (1-10 atm)

• Plasma combustion kinetic mechanism development • Time accurate species and plasma property measurements

3. Plasma assisted low temperature combustion Methane vs. Dimethyl ether (DME)

38

25.4 mm

P = 72 Torr f = 24 kHz

Power ~ 17 W (repetitive pulses)

Laser beam OH, CH2O PLIF

E = 7500 V/cm, E/N ~ 900 Td Peak Voltage = 7.8 KV

1. Plasma assisted Cool Flames and Mild Combustion:

(a) Hot diffusion flame

(b) Cool diffusion flame

Fig. 1 Plasma assisted normal and cool diffusion flames

N-heptane Normal diffusion flame Tf~1900 K

Cool diffusion flame Tf~650 K

Fig.3 Plasma assisted mild combustion (methane diluted by N2)

Direct chemi-luminescence image of cool premixed flame by ICCD camera for DME/O2/O3 mixture (φ = 0.104)

Heated N2

DME/O2/O3

Fig.2 Plasma assisted cool premixed flame (DME)

1. Plasma activated Cool Flames: n-heptane-air

(a) Hot diffusion flame

(b) Cool diffusion flame

Fig. 1 Hot and cool n-heptane diffusion flames at the same condition

Tf~1900 K

Tf~650 K

400

800

1200

1600

2000

2400

0.1 1 10 100 1000 10000

Max

imum

tem

pera

ture

Tm

ax[K

]

Strain rate a [s-1]

nC7H16/N2 vs O2 or O2/O3in counterflow burner

Xf = 0.05,Tf = 550 K, and To = 300 K

Extinction limit ofconventional hot diffusion flame

(HFE)

Extinction limit ofcool diffusion flame

(CFE)

without O3

with O3

HF branch

CF branchHTI

LTI

40

60

80

100

0.02 0.06 0.1 0.14 0.18

Stra

in ra

te a

[s-1

]

Fuel mole fraction Xf

no flame

hot diffusion flame

Fig. 2 Ozone (red line) extends the burning liit of cool flames

Fig. 3 Diagram of hot flame (pink), stable cool flame (blue), and unstable cool flame (white)

Plasma makes cool flame to be observed at 1 atm at 10 ms timescale.

2. Plasma assisted flameless (MILD) combustion

1

2

3

4

4

1. Electrodes 2. Insulation 3. Preheat burner 4. Oxidizer flowing sec.

• Tested conditions – Preheat: 1050 K (including 12% O2) – Center burner CH4/N2 and vel.: 10-70% and 5-40 m/s – Flame structure change with CH4% in plasma reactor

0%

w/o

Pla

sma

w/ P

lasm

a

3%

0% 3% 70%

Flameless combustion Regular combustion

70%

0

100

200

300

400

500

1

10

100

1000

10000

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

Tem

pera

ture

(K)

Mol

e Fr

actio

n (p

pm)

Time from last pulse (ms)

C2H2, Exp. C2H2, HP C2H2, USCCH4, Exp. CH4,HP CH4, USCH2O, Exp. H2O, HP H2O, USCT, Exp. T, HP T, USC

Detector

QCL Laser

Diluents

Diluents Oxidizer

Fuel

Vacuum Pump

Electrode Heated Vacuum Chamber Nanosecond -

Pulsed Power Supply

Pulsed Signal Generator

Digital Delay Generator

Function Generator

Oscilloscope Ge Etalon

Detector

Observation Window

Beam Splitter

Collimating Lenses

3. In Situ Mid-IR Diagnostics and kinetic study in plasma/flow reactors

Fig. 1 Experimental setup of plasma reactor and IR-Herriot cell

Fig. 2 Comparison of measured and predicted species (H2O, CH4, C2H2 formation in C2H4 oxidation: HP-Mech vs. USC Mech

Fig. 3 OH and HO2 diagnostics in DME flow reactor by using Faraday rotational spectroscopy. Predicted and measured signals.

In situ diagnostics of H2O, CH4, C2H2, OH, and HO2 measurements were conducted by using mid-IR absorption and FRS.

4. Development of high pressure mechanism (HP-Mech) for plasma assisted combustion

0%

2%

4%

6%

8%

10%

12%

14%

0.6 0.8 1 1.2 1.4 1.6

Incr

ease

of f

lam

e sp

eed,

%

Equivalence ratio

O3-2330ppm-expts O3-2330ppm-expts O3-2330ppm-HPMech O3-3730ppm-HPmech Konov-Simulation-3730ppm Konov-Simulation-2330ppm

Fig.1 Comparison of predicted flame speed increase (percentage) by O3 addition in methane/air flame (HP-Mech vs. Konov)