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Laminar Flames and the Role of Chemistry and Transport Yiguang Ju and Sang Hee Won Princeton University, USA Zheng Chen Peking University, China 1 st Flame Chemistry Workshop Advanced Engines require fuel flexibility and work near kinetic limit

slide-1
SLIDE 1

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

slide-2
SLIDE 2

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

slide-3
SLIDE 3

Reactive flow

dy du

Laser

(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
slide-4
SLIDE 4

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

slide-5
SLIDE 5
  • 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?
slide-6
SLIDE 6

Flames: Different fuels have different extinction limits

100 200 300 400 500 0.05 0.1 0.15 0.2

Extinction strain rate aE [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?

slide-7
SLIDE 7

50 150 250 350 450 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Extinction strain rate aE [1/s] Fuel mole fraction, Xf

Tf = 500 K and To = 300 K

nC7 nC9 nC10 nC16

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

Extinction strain rate 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

?

slide-8
SLIDE 8

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

Extinction strain rate aE [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

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SLIDE 9

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

Temperature [K] Reaction rate [mole/cm3s] Axial coordinate [cm]

C10H22=C2H5+C8H17-1 C10H22=nC3H7+C7H15-1 C10H22=CH3+C9H19-1 C6H5CH3+H<=>C6H5CH2+H2 C6H5CH3+OH<=>C6H5CH2+H2O C6H5CH3+H<=>A1+CH3 60 % n-decane + 40 % toluene near extinction, Xf = 0.1, a = 176 1/s C6H5CH2+ H = C6H5CH3

  • verall decomposition
  • f n-decane
  • verall decomposition
  • f 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

slide-10
SLIDE 10

High pressure hydrogen kinetics: ignition and flames

0.00 0.20 0.40 0.60 0.80 1.00 1.20 5 10 15 20 25 30 Pressure (atm) Mass burning rate (g/cm^2s) 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 1 10

  • 3

10

  • 2

10

  • 1

10 10

1

10

2

1000/T (K-1) [O2] ig (mol liter-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

slide-11
SLIDE 11

% 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)

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SLIDE 12
  • 2. How does transport and flame chemistry govern diffusion flame

extinction?

  

  ~ dx dx dY D Q

i F i

N2 O2 H2 H2O H OH O CO CO2 CH3 CH4 C2H4 C2H6 C2H2 CH2CO pC3H4 aC3H4 C4H81 A1 C6H5OH C3H8 C3H6 C10H22 C5H10-1 C6H5CH3

  • 0.1

0.0 0.1 0.2 Diffusion sensitivity 0.9N2+0.09n-decane+0.01toluene

Fuel

  • Heat release

Oxidizer Fuel fragments Fuel decomposition

  • H abstraction by H
  • Radical termination
slide-13
SLIDE 13

Flames: Different fuels have different burning limits

100 200 300 400 500 0.05 0.1 0.15 0.2

Extinction strain rate aE [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?

slide-14
SLIDE 14

i f p F F F e

R T T C Q Y M M a * ) ( / 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

3 2 , 3 2 ,

1 2 1 ( , , ) ( , ) exp

f O a F F F O F F E F f a f

T Y T Le 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

slide-15
SLIDE 15

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

15

R² = 0.97 100 200 300 400 500 0.5 1 1.5 2

Extinction strain rate aE [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!

slide-16
SLIDE 16

50 150 250 350 450 0.02 0.06 0.1 0.14 0.18

Extinction strain rate aE [1/s] Fuel mole fraction Xf

n-decane iso-octane toluene 1st generation surrogate for Jet-A POSF 4658 Prediction from correlation Prediction 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

Extinction strain rate [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

slide-17
SLIDE 17

Ignition Delay vs. Radical Index (real fuel)

17

100 1000 10000 0.8 1 1.2 1.4 1.6

Ignition delay time [us] 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

Extinction strain rate [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)

slide-18
SLIDE 18

50 150 250 350 450 0.05 0.09 0.13 0.17 0.21 0.25 0.29

Extinction strain rate 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

100 200 300 400 500 0.5 1 1.5 2

Extinction strain rate aE [1/s] Transport-Weighted Enthalpy [cal/cm3]

Different heating values Transport properties Diévart et al, 2012 to presented on Monday at 34th Symposium

??

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SLIDE 19

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

CH2O CH3O CH2CO CH3CO CH3 + CO

+ +

  • H

+R/-RH +R/-RH 47% 47% 5% 95%

HCCO CO + CO

+H 35% 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)

slide-20
SLIDE 20
  • 3. How does transport and flame chemistry

affect flame initiation and propagation?

slide-21
SLIDE 21

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

Altitude [1/p] Flight speed

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SLIDE 22

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)

Le T T C R E

ad p Z ig

~ ) ( 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)

1 1 ) ( 3 4

2 / 3 3 3

Le S T T C E

u ad p ig

   

 

Larger fuel molecules  smaller Eig volume heat capacity

Ignition spark to a flame

δ

fuel Jet

  • xygen

Le y diffusivit Mass y diffusivit Thermal  

slide-23
SLIDE 23

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 R R R R f

T T Z d e e R e R d e e R T ) 1 ( 1 2 exp Le 1 Q

ULe 2 ULe 2 U 2 U 2 U 2

     

 

Flame radius, R Flame propagating speed, U 10-1 100 101 102 0.0 0.4 0.8 1.2 1.6

Le=0.5 0.8 1.0 1.2 2.0

O O O O O

adiabatic (h=0.0)

O

1.4

U=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

slide-24
SLIDE 24

24

Ignition by heat and radical deposition (qt=0.05)

LeF = 2.2

R U

10

  • 1

10 10

1

10

2

10

  • 3

10

  • 2

10

  • 1

10

LeZ = 1.0 LeF = 1.0 q t = 0.0

1 2 3 4 5

1: q c = 0.0 2: q c = 0.4 3: q c = 0.8 4: q c = 1.0 5: q c = 1.2

R U 0.05 0.1 0.15 0.2 0.05 0.1

4 5 3

Radical Only

R U

10

  • 1

10 10

1

10

2

10

  • 3

10

  • 2

10

  • 1

10

LeF = 2.2 LeZ = 1.0 q t = 0.05 1: q c = 0.0 1 (b)

R U

10

  • 1

10 10

1

10

2

10

  • 3

10

  • 2

10

  • 1

10

LeF = 2.2 LeZ = 1.0 q t = 0.05 1: q c = 0.0 2: q c = 0.5

2

2 1 (b)

R U

10

  • 1

10 10

1

10

2

10

  • 3

10

  • 2

10

  • 1

10

LeF = 2.2 LeZ = 1.0 q t = 0.05 1: q c = 0.0 2: q c = 0.5 3: q c = 0.675

2

2 3 3 3 1 (b)

R U

10

  • 1

10 10

1

10

2

10

  • 3

10

  • 2

10

  • 1

10

LeF = 2.2 LeZ = 1.0 q t = 0.05 1: q c = 0.0 2: q c = 0.5 3: q c = 0.675 4: q c = 0.7

2

2 3 4 3 3 4 4 1 (b)

1

st flame

bifurcation R U

10

  • 1

10 10

1

10

2

10

  • 3

10

  • 2

10

  • 1

10

LeF = 2.2 LeZ = 1.0 q t = 0.05 1: q c = 0.0 2: q c = 0.5 3: q c = 0.675 4: q c = 0.7 5: q c = 0.73

2

2 3 4 3 3 4 4

5

5 1 (b)

2

nd flame

bifurcation R U

10

  • 1

10 10

1

10

2

10

  • 3

10

  • 2

10

  • 1

10

LeF = 2.2 LeZ = 1.0 q t = 0.05 1: q c = 0.0 2: q c = 0.5 3: q c = 0.675 4: q c = 0.7 5: q c = 0.73 6: q c = 1.0

2

2 3 4 3 3 6 4 4

5

5 6 1 (b)

Chen et al. 2011

slide-25
SLIDE 25

Cube of critical flame radius, RC

3

Minimun ignition power, Qmin 500 1000 1500 2000 2500 0.5 1 1.5 2

Z = 10

Cube of critical flame radius, RC

3

Minimun ignition power, Qmin 500 1000 1500 2000 2500 0.5 1 1.5 2

Z = 13

2.0 1.4 1.6 1.7 1.8 = Le 1.9 1.5 1.4 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

0.5 1 1.5 2 2.5 3 0.6 0.7 0.8 0.9 1

Critical radius [cm] Equivalence ratio 

JP8 POSF 6169 SHELL SPK POSF 5729 @ 1 atm Unburned Temperature = 450 K Fuel/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

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SLIDE 26

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

100 200 300 400 500 600 80 100 120 140 160 180 200 220

Flame speed, Sb [cm/s]

Stretch rate K [sec

  • 1]

0.0 0.5 1.0 1.5 2.0 2.5 80 100 120 140 160 180 200 220

(b) Flame radius, Rf [cm] (a)

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

slide-27
SLIDE 27

150 200 250 300 350 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Flame thickness [mm] 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 Max species mole fraction 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)

slide-28
SLIDE 28

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

200 400 600 800 1000 1200 0.5 0.6 0.7 0.8 0.9 1.0 RHOT 3.5mm RHOT 3.0mm RHOT 2.5mm

Normalized flame speed, Sb / Sb 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

slide-29
SLIDE 29
  • 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
slide-30
SLIDE 30

0.000 0.005 0.010 0.015 0.0 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

Location of maximum heat release (cm) Time (s)

High temperature flame dominated double flame (coupled) Low temperature ignition Transition

Sf=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

slide-31
SLIDE 31

0.02 0.04 0.06 0.08 0.10 0.12 5000 10000 15000 20000 25000 30000 35000

O2=55%

Fuel mole fraction OH PLIF signal (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

slide-32
SLIDE 32

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 1x10

15

2x10

15

3x10

15

4x10

15

5x10

15

6x10

15

7x10

15

Smooth Transition Extinction Ignition

O2=34% O2=62%

Fuel mole fraction OH number density (cm

  • 3)

32

Residence time Temperature 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

slide-33
SLIDE 33

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.

slide-34
SLIDE 34

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: