Laminar Flames and the Role of Chemistry and Transport

Yiguang Ju and Sang Hee Won Princeton University, USA

Zheng Chen

Peking University, China

1st Flame Chemistry Workshop

Advanced Engines require fuel flexibility and work near kinetic limit

HCCI

Plasma assisted combustion

Synfuels in gas turbine engine

•Low temperature,

•High pressure,

•Near ign./ext . limit,

•Multiple fuels,

Kinetic limiting

“Validated“mechanisms?

•Hundreds of fuels •Different structures elements H, C, O, N, S…) •Thousands species •Extreme conditions

•Homogeneous ignition/reactor

•Inhomogeneous flames

Two validation targets

Reactive flow

dy

duLaser (u’, v’),PIV

PLIF

Turbulent fuel stream

Flame

Turbulent flow reactor

OH PLIF

Propagating edge flame in a mixing layer

Premixed flame front

Flame regimes in combustion How does chemistry affect flames?

in non-uniform flow field with complex transport and chemistry coupling

•Thin flame •Thickening flame •Local extinction •Re-ignition

Flames “Flame” is a ignition/reaction front supported by thermal and species transport

Fuel

•Radicals

•Heat release

Oxidizer Fuel

fragments

Diffusion Fuel/Oxygen Radicals/Fragments, (e.g. H and C2H4)

Heat release rate OH + CO =CO2 +H HCO+OH= CO2+H2O

Fuel decomposition • H abstraction by H • Radical termination

Branching/Termination reactions

Non-uniform species/temperature distribution

Then, what is the role of transport on kinetics? • Is flame chemistry different from that of

homogeneous ignition?

• How does transport and flame chemistry govern flame extinction?

• How does transport and flame chemistry affect unsteady flame initiation and propagation?

• How does low temperature chemistry change flame regimes?

1. Why is flame chemistry different from ignition?

Flames: Different fuels have different extinction limits

0

100

200

300

400

500

0 0.05 0.1 0.15 0.2

Ex

tin

cti

on

str

ain

ra

te a

E[1

/s]

Fuel mole fraction Xf

n-decane

n-nonane

n-heptane

JETA POSF 4658

Princeton Surrogate

iso-octane

nPB

toluene

124TMB

135TMB

n-alkanes

aromatics

Tf = 500 K and To = 300 K

How to decouple chemistry from transport and fuel

heating value?

50

150

250

350

450

0.02 0.04 0.06 0.08 0.1 0.12 0.14

Ex

tin

cti

on

str

ain

ra

te a

E[1

/s]

Fuel mole fraction, Xf

Tf = 500 K and To = 300 K

nC7

nC9nC10nC16

increaing carbon #

Ignition vs. flames: n-alkanes and esters

Westbrook, 2010

Won et al., 2010

50

150

250

350

450

0.05 0.09 0.13 0.17 0.21 0.25 0.29

Exti

nc

tio

n s

tra

in r

ate

aE

[1/s

]

Fuel mole fraction, Xf

??

Tf = 500 K, Tox = 298 K

Methyl Formate

Methyl Ethanoate

Methyl Propanoate

Methyl Butanoate

Methyl Pentanoate

Methyl Hexanoate

Methyl Octanoate

Methyl Decanoate

?

Kinetic coupling between alkanes and aromatics Blending toluene into n-decane: Extinction Limits

50

100

150

200

250

0.02 0.06 0.1 0.14

Ex

tin

cti

on

str

ain

ra

te a

E[1

/s]

Fuel mole fraction Xf

n-decane (present)

n-decane (Seshadri, 2007)

toluene (present)

toluene (Seshadri, 2007)

calculation (present)

80% n-decane + 20% toluene

60% n-decane + 40% toluene

40% n-decane +60% toluene

LIF tested condition

Won, Sun, Dooley, Dryer, and Ju, CF 2010

N-decane

Toluene

0

300

600

900

1200

1500

1800

-2.00E-04

-1.50E-04

-1.00E-04

-5.00E-05

0.00E+00

5.00E-05

1.00E-04

1.50E-04

0.85 0.9 0.95 1 1.05 1.1

Te

mp

era

ture

[K]

Re

ac

tio

n r

ate

[m

ole

/cm

3s

]

Axial coordinate [cm]

C10H22=C2H5+C8H17-1C10H22=nC3H7+C7H15-1C10H22=CH3+C9H19-1C6H5CH3+H<=>C6H5CH2+H2C6H5CH3+OH<=>C6H5CH2+H2OC6H5CH3+H<=>A1+CH3

60 % n-decane + 40 % toluene near extinction, Xf = 0.1, a = 176 1/s

C6H5CH2+ H = C6H5CH3

overall decomposition of n-decane

overall decomposition of toluene

Kinetic coupling between n-decane and toluene in diffusion flames

0.1 mm

Won, Sun, Dooley, Dryer, and Ju, CF 2010

H Diffusion loss

High pressure hydrogen kinetics: ignition and flames

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 5 10 15 20 25 30

Pressure (atm)

Ma

ss

bu

rnin

g r

ate

(g

/cm

^2

s)

Present experiments

Li et al. (2007)

Davis et al. (2005)

Sun et al. (2007)

Konnov (2008)

O'Connaire et al. (2004)

Saxena & Williams (2006)

H2/O2/Ar, φ=2.5

Tf ~1600K

Uncertainty of HO2 related kinetics at high pressure,

•Ignition governed more by chain initiation and branching rates •Flames governed more by branching rate and heat release rate •Different radical pool concentration (H, OH, …)

Bulke, Marcos, Dryer, Ju, CF, 2010

0.5 0.6 0.7 0.8 0.9 110

-3

10-2

10-1

100

101

102

1000/T (K-1

)

[O2] ig

(m

ol lite

r-1

s)

Skinner and Ringrose (1965) - 5 atm

Schott and Kinsey (1958) - 1 atm

Petersen et al. (1995) - 33 atm

Petersen et al. (1995) - 64 atm

Petersen et al. (1995) - 57 & 87 atm

Present model

Li et al. (2004)

Burke, Chaos, Ju, Dryer, Klippenstein, IJCK 2011

H2/O2/Ar mixtures

% consumed by radical reaction

component OH HO2 H O

n-decane 86% 6.0% 3.6% 2.0%

iso-octane 82% 6.5% 5.8% 2.3%

Toluene 88% 2.8% 4.5%

Fuel consumption by radicals

OH is r the most significant radical in fuel consumption

1. Won et al. CNF 159 (2012) 2. Dooley et al. CNF 159 (2012) 1371-1384.

Difference of kinetics in homogeneous reactor and flames

component % by uni-molecular

decomposition

% consumed by radical reaction

H OH CH3 O

n-decane [1] 19.3% 67.2% 6.1% 5.8% 0.1%

Methylbutanoate [2] 6.6% 70.8% 10.3% 8.1% 3.7%

Fuel consumption by Radicals

Homogeneous reactor (800 K)

Diffusion flames (~1600K)

2. How does transport and flame chemistry govern diffusion flame extinction?

0

0

0~ dxdx

dYDQ i

Fi N2

O2H2

H2OH

OHO

COCO2CH3CH4

C2H4C2H6C2H2

CH2COpC3H4aC3H4C4H81

A1C6H5OH

C3H8C3H6

C10H22C5H10-1

C6H5CH3

-0.1 0.0 0.1 0.2

Diffusion sensitivity

0.9N2+0.09n-decane+0.01toluene

Fuel

•Heat release

OxidizerFuel

fragments

Fuel decomposition• H abstraction by H• Radical termination

1. Why is flame chemistry different from ignition?

Flames: Different fuels have different burning limits

0

100

200

300

400

500

0 0.05 0.1 0.15 0.2

Ex

tin

cti

on

str

ain

ra

te a

E[1

/s]

Fuel mole fraction Xf

n-decane

n-nonane

n-heptane

JETA POSF 4658

Princeton Surrogate

iso-octane

nPB

toluene

124TMB

135TMB

n-alkanes

aromatics

Tf = 500 K and To = 300 K

How to decouple chemistry from transport and fuel

heating value?

i

fp

FF

F

e RTTC

QY

MMa *

)(/

1 ,

A generic correlation for extinction limit: Transport weighted Enthalpy & radical index

Theoretical analysis of Extinction Damkohler number

Transport Heat release/heat loss Fuel chemistry Radical production rate

32

, 3

2

,

1 2 1( , , ) ( , ) exp

fO aF F F O F F

E F f a f

TY TLe P Le Le L Le

Da e Y T T T T

Extinction Strain Rate

Won et al. CNF 159 (2012)

Transport weighted Enthalpy *Radical index

A General Correlation of Hydrocarbon Fuel Extinction vs. Transport Weighted Enthalpy (TWE) and Radical Index

15

R² = 0.97

0

100

200

300

400

500

0.5 1 1.5 2

Ex

tin

cti

on

str

ain

ra

te a

E[1

/s]

Ri[Fuel]Hc(MWfuel/MWnitrogen)-1/2 [cal/cm3]

n-decane

n-nonane

n-heptane

iso-octane

n-propyl benzene

toluene

1,2,4-trimethly benzene

1,3,5-trimethly benzene

Tf = 500 K and To = 300 K

Enabling rapid fuel screening!

50

150

250

350

450

0.02 0.06 0.1 0.14 0.18

Ex

tin

cti

on

str

ain

ra

te a

E[1

/s]

Fuel mole fraction Xf

n-decaneiso-octanetoluene1st generation surrogate for Jet-A POSF 4658Prediction from correlationPrediction from correlation

Tf = 500 K and To = 300 K

TWE and radical index for predicting extinction limits and synfuel fuel ranking and screening

Fuel Radical Index

n-dodecane 1.0

iso-octane 0.7

toluene 0.56

n-propyl benzene 0.67

1,2,4-trimethylbenzene 0.44

1,3,5-trimethylbenzene 0.36

JetA POSF 4658 0.79

S8 POSF 4734 0.86

JP8 POSF 6169 0.80

HRJ Camelina POSF 7720 0.82 50

100

150

200

250

300

350

400

450

0.5 1 1.5 2 2.5

Ex

tin

cti

on

str

ain

ra

te [

s-1

]

Transport-weighted enthalpy [cal/cm3]

[fuel]Hc(MWf/MWn)-0.5

JP8 POSF 6169

SHELL SPK POSF 5729

HRJ Camelina POSF 7720

HRJ Tallow POSF 6308

SASOL IPK POSF 7629

n-alkane

iso-octane

Extinction of diffusion flame in counterflow configuration

Tf = 500 K and Tair = 300 K @ 1 atm

Synfuels

Single fuel Real fuel

Representing radical pool High temperature reactivity

Radical Index

Ignition Delay vs. Radical Index (real fuel)

17

100

1000

10000

0.8 1 1.2 1.4 1.6

Ign

itio

n d

ela

y t

ime

[u

s]

1000/T [1/K]

S8 surrogate

2nd Gen surrogate

Ignition delay of stoichiometric fuel/air mixture at 20 atm

2nd Gen POSF 4658 surrogate : Ri = 0.80

(nC12, iC8, nPB, 13TMB)S8 POSF 4734 surrogate : Ri = 0.86

(nC12, iC8)

50

100

150

200

250

300

350

400

450

0.5 1 1.5 2 2.5

Ex

tin

cti

on

str

ain

ra

te [

s-1

]

Transport-weighted enthalpy [cal/cm3]

[fuel]Hc(MWf/MWn)-0.5

S8 POSF 4734 surrogate

2nd Gen POSF 4658 surrogate

Extinction of diffusion flame in counterflow configuration

Tf = 500 K and Tair = 300 K @ 1 atm

Measurements from RPI (Oehlschlaeger), Dooley et al., CNF 159 (2012), Dooley et al., CNF (2012) in press

Consistent in high temperature reactivity

Dooley et al., CNF 159 (2012)

50

150

250

350

450

0.05 0.09 0.13 0.17 0.21 0.25 0.29

Exti

nc

tio

n s

tra

in r

ate

aE

[1/s

]

Fuel mole fraction, Xf

Tf = 500 K, Tox = 298 K

Methyl Formate

Methyl Ethanoate

Methyl Propanoate

Methyl Butanoate

Methyl Pentanoate

Methyl Hexanoate

Methyl Octanoate

Methyl Decanoate

Scaling high temperature reactivity of methyl esters: Using TWE and Radical Index

0

100

200

300

400

500

0.5 1 1.5 2

Exti

nc

tio

n s

tra

in r

ate

aE

[1/s

]

Transport-Weighted Enthalpy [cal/cm3]

Different heating values Transport properties

Diévart et al, 2012 to presented on Monday at 34th Symposium

??

CH3OH+

CO

CH2O+

HCO

CH3O+

CO

CH3

+CO2

H + CO HO2 + CO

35% 18%

42%

+R/-RH

+R/-RH

62%38%

81%

9%

88% 12%

+M +O2

Impact of alkyl chain length on methyl ester reactivity Methyl Formate, R0C

Higher reactivity

CH2OCH3OCH2CO CH3CO

CH3 + CO

+ +

-H

+R/-RH +R/-RH

47% 47%

5%

95%

HCCO

CO + CO

+H35%

56%

+OH+O

Methyl Acetate, R1C

Lower reactivity

Diévart et al, 2012 to presented on Monday at 34th Symposium

H abstraction reactions, CH3OCO and CH3OC(O)CH2 decomposition reaction rates: large discrepancies (Xueliang, 2012)

3. How does transport and flame chemistry affect flame initiation and propagation?

Puzzle of high altitude relight: an unresolved ignition problem or a flame problem?

Altitude [1/p]

Flight speed

Flow speed

Is the flame speed really a problem for relight?

Flame speed ~ pn/2-1

Engine instability Flow

speed

Q ?

• What governs the ignition & Eig?

• What are the chemistry and

transport effects?

•Eig,min: Defined by stable “flame ball” size?

Zeldovich et al. (1985), Champion et al. (1986)

LeTTCRE adpZig ~)(3

4 3

Larger fuel molecules larger Eig

•Eig,min: Defined by flame thickness, δ (make a guess)?

B. Lewis and Von Elbe (1961), Ronney, 2004, Glassman (2008)

11)(

3

42/33

3

LeSTTCE

u

adpig

Larger fuel molecules smaller Eig volume heat capacity

Ignition spark to a flame

δ

fuelJet

oxygenLe

ydiffusivit Mass

ydiffusivit Thermal

Theory: Critical Ignition Size vs. Flame Speed

Q ?

Assumptions and simplification: • 1D quasi-steady state, Constant properties • One-step chemistry • Center energy deposition

f

f

R

RR

R

R

fT

TZ

de

eReR

de

eRT

)1(

1

2exp

Le

1Q

ULe2

ULe2U2

U2

U2

Flame radius, R

Fla

me

pro

pa

ga

tin

gsp

ee

d,U

10-1

100

101

102

0.0

0.4

0.8

1.2

1.6

Le=0.5

0.8

1.01.2

2.0

OO O O O

adiabatic(h=0.0)

O

1.4U=0:

Flame

ball Extinction

limit

Increase of fuel molecule size

Small fuel

Large fuel

The critical ignition size and energy is governed by two different length scales: •Flame ball size (small Le) •Extinction diameter (large Le)

Chen & Ju, Comb. Theo. Modeling, 2007

24

Ignition by heat and radical deposition (qt=0.05)

LeF = 2.2

R

U

10-1

100

101

102

10-3

10-2

10-1

100 Le

Z= 1.0

LeF

= 1.0

qt

= 0.0

12

3

4

5

1: qc

= 0.0

2: qc

= 0.4

3: qc

= 0.8

4: qc

= 1.0

5: qc

= 1.2

RU

0 0.05 0.1 0.15 0.20

0.05

0.1

4

5

3

Radical Only

R

U

10-1

100

101

102

10-3

10-2

10-1

100

LeF

= 2.2

LeZ

= 1.0

qt

= 0.05

1: qc

= 0.0

1

(b)

R

U

10-1

100

101

102

10-3

10-2

10-1

100

LeF

= 2.2

LeZ

= 1.0

qt

= 0.05

1: qc

= 0.0

2: qc

= 0.5

2

21

(b)

R

U

10-1

100

101

102

10-3

10-2

10-1

100

LeF

= 2.2

LeZ

= 1.0

qt

= 0.05

1: qc

= 0.0

2: qc

= 0.5

3: qc

= 0.675

2

2

3

3

3

1

(b)

R

U

10-1

100

101

102

10-3

10-2

10-1

100

LeF

= 2.2

LeZ

= 1.0

qt

= 0.05

1: qc

= 0.0

2: qc

= 0.5

3: qc

= 0.675

4: qc

= 0.7

2

2

3

4

3

3

44

1

(b)

1st

flame

bifurcation

R

U

10-1

100

101

102

10-3

10-2

10-1

100

LeF

= 2.2

LeZ

= 1.0

qt

= 0.05

1: qc

= 0.0

2: qc

= 0.5

3: qc

= 0.675

4: qc

= 0.7

5: qc

= 0.73

2

2

3

4

3

3

44

5

5

1

(b)

2nd

flame

bifurcation

R

U

10-1

100

101

102

10-3

10-2

10-1

100

LeF

= 2.2

LeZ

= 1.0

qt

= 0.05

1: qc

= 0.0

2: qc

= 0.5

3: qc

= 0.675

4: qc

= 0.7

5: qc

= 0.73

6: qc

= 1.0

2

2

3

4

3

3

6

44

5

5

6

1

(b)

Chen et al. 2011

Cube of critical flame radius, RC

3

Min

imu

nig

nitio

np

ow

er,

Qm

in

0 500 1000 1500 2000 25000

0.5

1

1.5

2

Z = 10

Cube of critical flame radius, RC

3

Min

imu

nig

nitio

np

ow

er,

Qm

in

0 500 1000 1500 2000 25000

0.5

1

1.5

2

Z = 13

2.0

1.4

1.6

1.7

1.8 = Le

1.9

1.51.4

2.5

2.0

1.9

1.8

1.7

1.6

1.5

Le = 2.1

2.3

2.4

2.2

Ignition energy: impacts of flame chemistry and transport

Chen, Burke, Ju, Proc. Comb. Inst. Vol.33, 2010

Activation energy

0

0.5

1

1.5

2

2.5

3

0.6 0.7 0.8 0.9 1

Cri

tic

al r

ad

ius

[c

m]

Equivalence ratio

JP8 POSF 6169

SHELL SPK POSF 5729

@ 1 atm

Unburned Temperature = 450 KFuel/Air (21% O2) mixture

Fuel Mean molecular

weight

Radical Index

JP8 POSF 6169 153.9 0.80

SHELL SPK POSF 5729

136.7 0.85

Won, Santer, Dryer, Ju, 2012

Unsteady flame initiation Three different flame regimes (n-heptane/air)

– Regime I

• Spark assisted ignition kernel

– Regime II

• Weak flame regime from sparked driven ignition

kernel to normal flame

– Regime III

• Self-sustained propagating normal flame

Regime II

Regime I Regime III

Ignition

Ignition Linear

extrapolation Regime II

Rapid rise 2 ms 5.7 ms

Critical radius

0 100 200 300 400 500 60080

100

120

140

160

180

200

220

Fla

me

sp

ee

d,

Sb [

cm

/s]

Stretch rate K [sec-1]

0.0 0.5 1.0 1.5 2.0 2.580

100

120

140

160

180

200

220

(b)

Flame radius, Rf [cm]

(a)

Kim et al, 2012, to be presented on Wednesday at 34th Symposium

150 200 250 300 350

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Fla

me

th

ick

ne

ss

[m

m]

Stretch rate, K [sec-1]

150 200 250 300 350

0.000

0.001

0.002

0.003

0.004

0.005

0.006

H

C2H4

Ma

x s

pe

cie

s m

ole

fra

cti

on

Stretch rate, K [sec-1]

OH

Rapid change of flame structure in flame initiation process

Kim et al, 2012, to be presented on Wednesday at 34th Symposium

ignition

Weak flame

Normal flame ignition

Completely different flame structures!

(lean n-heptane/air)

Will a model for flame speed predict the unsteady flame initiation? An example: Lean n-heptane flame initiation

0 200 400 600 800 1000 12000.5

0.6

0.7

0.8

0.9

1.0

RHOT

3.5mm

RHOT

3.0mm

RHOT

2.5mm

No

rma

lize

d f

lam

e s

pe

ed

, Sb /

Sb

0

Stretch rate, K [sec-1]

Experiment

Chaos et al. model (2007)

Wang et al. model (2010)

Varying the Hot spot size

Accurate transient

flame speed

prediction

Turbulent modeling

ϕ=0.9,

Critical flame initiation radius (Rc) >10 mm

Rc~ 12 mm

4. How does low temperature flame chemistry affect flame initiation and propagation, and stabilization?

•Low temperature chemistry (multi-stage ignition) •Plasma assisted low temperature ignition. extinction

0.000 0.005 0.010 0.0150.0

0.2

0.4

0.6

0.8

1.0

Hot ignition

LTI

at wall

Low temperature flame dominated

double flame (decoupled)

Single high temperature

flame front

Lo

ca

tio

n o

f m

axim

um

he

at

rele

ase

(cm

)

Time (s)

High temperature flame

dominated double flame (coupled)

Low temperature ignition

TransitionSf=15.3 cm/s

Sf=27.5 cm/s

Sf=25.6 m/s

Movie

Multi flame regimes in HCCI ignition n-heptane:40 atm, T=700 K

Ju et al. 33rd symposium 2011

0.02 0.04 0.06 0.08 0.10 0.120

5000

10000

15000

20000

25000

30000

35000

O2

=55%

Fuel mole fraction

OH

PL

IF s

ign

al

(a.u

.)

different

emissions

Same OH

LIF signal

New low temperature flame regime in

Plasma assisted combustion (Counterflow DME/O2/He ignition)

CH3O + OH CH2O* + H2O

RCH2O + OH CH2O* + ROH

R: organic radicals 31

Nano-sec discharge

Sun et al., Friday 34th symposium on combustion, 2012

S-curve

He/O2 = 0.66:0.34 and 0.38:0.62 , P = 72 Torr, f = 24 kHz, a = 400 1/s

Kinetic enhancement of plasma assisted ignition: Change of S-curve

CH4

0.05 0.10 0.15 0.20 0.25 0.30 0.35

1x1015

2x1015

3x1015

4x1015

5x1015

6x1015

7x1015

Smooth

Transition

Extinction

Ignition

O2

=34%

O2

=62%

Fuel mole fraction

OH

nu

mb

er d

en

sity

(cm

-3)

32

Residence time Te

mp

era

ture

Plasma assisted LTC

Plasma generated

species:

O, H, O2(a∆g) …

Today’s combustors Classical S-curve

Role of kinetics on PAC at low temperature? Sun et al., 34th symposium on combustion, 2012

Conclusion

•Flames chemistry differs from homogeneous ignition in diffusion, fuel decomposition, radical pool production/consumption.

•Low temperature and unsteady combustion processes lead to new different flame regimes and structures.

•Flame initiation and extinction are strongly affected by both transport and chemical kinetics.

•Transport weight enthalpy and radical index are developed for predicting extinction limit and ranking fuel reactivity

•Large uncertainties in elementary reaction rates of kinetic mechanisms for simple fuels exist in extreme conditions.

•A validated mechanism using flame speeds fails to predict unsteady flame transition and the critical flame radius.

Acknowledgement of financial support

Prof. Chung K. Law (CEFRC-Princeton)

Dr. Atreya, Arvind (NSF)

Dr. Abate, Gregg (EOARD, AFOSR)

Dr. Li, Chiping (AFOSR)

Dr. Wells, Joe (ONRG)

Welcome to The 1st International Flame Chemistry Workshop

•Invited lecture (11) speakers and Session (4+1+2) chairs

•Committee and advisory board members

•All participants, especially poster (10) contributors

•Prof. TamásTurányi & Incoming Inc. for local organization

Sincere Thanks To: