Yiguang Ju

Princeton University

Dynamics of Near-Limit Cool Flames

The 1st International Workshop on Near Limit Flames

June 21 – 23, 2017Boston University

1

Sang Hee Won

Associate professor,

Univ. South Carolina

Wenting Sun

Assistant professor,

Georgia Tech

Prof. Chae Hoon SohnSejong University

Chris Reuter

Graduate student

Acknowledgement

Omar Yehia

Graduate student

Weiqi Sun

Graduate student

Rui Zhang

Visiting Scholar

500-950 K

1300-2200 K

0.0 0.1 0.2300

600

900

1200

1500

Te

mp

era

ture

(K

)

Time (sec)

cool flame ignition

Hot flame ignition

N-heptane

Ignition Won & Ju et al., Proc. Combust. Inst. 35, 2015Reuter, Won, & Ju, Proc. Combust. Inst. 36, 2017Nayagam, D.L. Dietrich, P.V. Ferkul, M.C. Hicks, F.A. Williams,

Combustion and Flame 159 (2012) 3583-3588.

(b) Hot diffusion flame

(a) Cool diffusion flame

Diffusion Flames

Tmax~700 K

Tmax~2000 K

Premixed Flames

(a) Cool flame

(b) Hot flame

Flames

Cool flames vs. Hot flames

CH2O/CO/C2H4

3• Cool flame is cool (600-900 K)• No NOx and soot emissions

1. Dynamics and Regimes of Premixed Cool Flames

2. Dynamics and Regimes of Diffusion Cool Flames

3. Cool Flames as a platform for mechanism validation

4. Conclusion

Content

4

1970

Detailed kinetic mechanism

2000

2010

1950

1D Kinetic-transport-radiation model

Thermo-kinetic models

R+ O2 = RO2 mechanism

Unified chain-reaction theory

1990

Pre-1930 Observation of premixed cool flames in

(flow tube, hot surface…) 1817 H. Davy

P-T ignition diagram (unstirred closed reactor)

Oscillatory cool flames

Cool flame chemistry

CH2O formation

0D kinetic modeling

2D turbulent modeling

1D knocking model

1D kinetic-Transport model

Extinction limits

Flammability limit diagram

Cool flames with high pressure fuel injection

Micro-gravity premixed cool flames (Pearlman et al.)

Micro-gravity droplet diffusion cool flames (Williams et al.)

Plasma/O3 assisted stable diffusion cool flames

Diffusion, premixed, partially premixed cool flames

Diffusion cool flames for fuel reactivity studies

Micro-tube premixed cool flames (Maruta et al.)

Oscillatory cool flames in CSTR

Rapid compression machine

Years Experiments/Observation Kinetics-theory-models

Cool flame ignition in RCM, shock tube…

Pre

ssu

re

Temperature

E

Cool flame(LTI)

High temperature ignitionA

B

C

D

F G

History of cool flame studies

(Ju, Law, and co-workers, 2010-2017)

Premixed double flame

6

A puzzle on cool flame propagation at elevated temperature

Y. Ju et al., Proc. of Combust. Inst., 33, 2011

0.1 1

1000

1500

2000

2500

Te

mp

era

ture

dis

trib

utio

n (

K)

Flame radius, r (cm)

0.1ms

3.0ms

9.3ms

n-heptane

p=20atm, T=700K

Cool flame

Spark ignition

Spark assisted HCCI flame propagation:(High pressure, elevated temperature)

hot flame

Q

• Why is there a double flame structure with coupled hot/cool flames?

• How fast and how can a cool flame can propagate?

• What is the effect of pressure on cool flame burning limit?

Hot flame by a spark1 atm, 300 K

1. Dynamics and Regimes of Premixed Cool Flames

How fast a cool flame propagate?How lean/rich can it burn?

High temperature flames (1940s-201x)

w. cool flame

Flam

e t

em

pe

ratu

re, K

Equivalence ratio

Φ0,C=0.02 Φ0=0.077

Cool flameCool flame

1200

500

5 100

Ju et al., Combustion and Flame, 2015.

Lean limit Rich limit

7

DME/O2 1atm530 K

Cool flame Lean limit Cool flame rich limit

Cool flame can burn much leaner and richer at hot flames at elevated temperature

How does pressure affect flame regimes and structure?

8

0 2 4 6 8 10

500

1000

1500

2000

2500

E

Cool flame

Double flame

Cool flame

Hot flame

F

D

C

B

A

1.5atm 1atm

4atm

2atm

P=20atm

DME/air

Fla

me

tem

pera

ture

(K

)

Equivalence ratio

• Pressure dramatically changes flammable region of lean cool flames, but not the rich cool flame

• The cool flame to hot flame transition limit is sensitive to pressure.

Y. Ju, Combustion and Flame, 2017

1. Why do we have three different flame regimes on lean side?

2. What is the cool flame structure on the rich side?

3. Do we still have a cool flame at high pressure?

3 Questions?

Different flame structures (2 atm)

-0.25 0.00 0.25500

1000

1500

2000

2500

3000

X (cm)

Tem

pe

ratu

re (

K)

1E-3

0.01

0.1Normal

flame

T

H2OCO2DME

CO

T CH

2O

Mole

fractio

n

DME/O2

=0.1

-0.5 0.0 0.5 1.0 1.5

400

600

800

1000

1200

1400

1600

X (cm)

Tem

pe

ratu

re (

K)

1E-3

0.01

0.1Double

flame

T

H2O

CO2

DME

CO

T

CH2O

Mole

fractio

n

DME/O2

=0.1

-0.5 0.0 0.5 1.0 1.5

400

600

800

1000

1200

1400

1600

X (cm)

Tem

pe

ratu

re (

K)

1E-5

1E-4

1E-3

0.01

0.1Cool flame

H2O2

HCOOH

HO2

CH4T

T

Mole

fractio

n

DME/O2

=0.1

0.00 0.05 0.10 0.15 0.20 0.25 0.30

1000

1500

2000

dc

b

a

300 K

T0=530 K

DME/O2

Fla

me

tem

pera

ture

, K

Equivalence ratio

Normal

flame

Cool flame

CH2O

Flame size=1.5cm

Hot flame

Cool flameDouble

flame

0.0 0.5 1.0400

600

800

1000

1200DME/air

P=20atm

CODME

T

X (cm)

Te

mp

era

ture

(K

)

H2O2

0.000

0.005

0.010

0.015

Mo

le fra

ctio

n

Double flame structure of high pressure lean/rich cool flames

-0.1 0.0 0.1 0.2 0.3

-500

0

500

1000

QCond

QReac

P=20 atm

Lean cool flame

Ch

em

ica

l a

nd

co

nd

uctive

he

at

flu

x (

J/c

m3)

X(cm)

0.04 0.06 0.08 0.10 0.12

-10000

0

10000

20000

30000

QCond

QReac

P=20 atm

Rich cool flame

Ch

em

ica

l a

nd

co

nd

uctive

he

at

flu

x (

J/c

m3)

X(cm)

δc,h

0 2 4 6 8 10

500

1000

1500

2000

2500

E

Cool flame

Double flame

Cool flame

Hot flame

F

D

C

B

A

1.5atm 1atm

4atm

2atm

P=20atm

DME/air

Fla

me

tem

per

atu

re (

K)

Equivalence ratio

Fuel lean

Fuel Rich

Effects of pressure and mixture dilution on cool flame speeds and burning limits

5 10 15 200.0

0.5

1.0

Cool flame,

Hot flame, SL

Normalized pressure, P/1atm

No

rmalz

ied fla

me tem

pe

ratu

re, T

f

Tf

Cool flame,

0.5

1.0

1.5

No

rma

lize

d fla

me s

pe

ed

, SL

• Lean and rich cool flame speed and temperature have distinctive pressure dependence due to the NTC effects.

0.1 1 10

10

100

DME/(0.1O2+0.9N2)

DME/air

F

A EDC

B Cool flame

Hot fla

me

Cool flame

P=2atm

(b)

Fla

me

sp

ee

d (

cm

/s)

Equivalence ratio

• Dilution and EGR enhance cool flames and suppress hot flames

Y. Ju, Combustion and Flame, 2017

12

0.0 0.5 1.0 1.50

5

10D

C

E B

A

Hot flame

extinction limit

Cool flame to hot flame

transition limit

Cool flame

extinction limit

DME/air

Pre

ssu

re (

atm

)

Lean burn limits (equivalence ratio)

A generic flammability limit diagram: the K-Curve

(premixed flames)

for both cool and hot flames at elevated pressure

Y. Ju, Combustion and Flame, 2017

13

Recent experimental studies of cool flames(w plasma & ozone sensitization)

Heated N2 @ 600 K

N2 @ 300 K

Stagnation

plane

DME+ O2 + O3 @ 300 K

N2 @ 600 K

Pressure

chamberMicro-GC

Positioning stage

Ozone

generatorO2 @ 300 K

Sun, W., Won, S.H. and Ju, Y.,

2014. Combustion and Flame, 161.Won, S.H., et al., 2015, Proceedings of the Combustion Institute, 35.

Nano-sec plasma assisted cool flames Ozone assisted cool flames

Hot Flame to Cool Flame Transition: 3 flame regimes

14

Premixed Flame Extinction transition

from hot flame to cool flame

• Constant strain rates

• Reducing fuel concentration

Hot flame

Hot flameCool flame

Coolflame

0.1 1

1000

1500

2000

2500

Tem

pera

ture

dis

trib

ution

(K

)

Flame radius, r (cm)

0.1ms

3.0ms

9.3ms

n-heptane

p=20atm, T=700K

Cool flame formation

Spark ignition

Spark assisted HCCI combustion

0

2

4

6

8

10

12

14

16

10 15 20 25 30

Ma

xim

um

Fla

me

In

ten

sit

y (

a. u

.)

Time (s)

Hot

Flame

Cool

Flame

ϕ = 0.102 ϕ ~ 0.090

Reuter, C.B., Won, S.H. and Ju, Y., 2016. Combustion and Flame, 166, pp.125-132.

400

800

1200

1600

2000

2400

0.1 1 10 100 1000 10000

Maxim

um

tem

pera

ture

Tm

ax [

K]

Strain rate a [1/s]

nC7H16/N2 vs O2 or O2/O3

in counterflow burner

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

Extinction limit of

conventional hot diffusion flame

(HFE)

Extinction limit of

cool diffusion flame

(CFE)

without O3

with O3

HF branch

CF branchHTI

LTI

2. Dynamics of Diffusional Cool Flames

• Existence of cool diffusion flames in counterflowconfiguration with n-heptane– cool flame regime exists regardless of addition of ozone– Addition of ozone extends cool flame regime.

16

(b) Hot diffusion flame

(a) Cool diffusion flame

Won, S.H., et al., 2015, Proceedings of the Combustion Institute, 35.

Self-Sustaining partially premixed Cool

Flames

• Cool partially premixed flames: DME/O2/O3

17Reuter et al., Proc. Combust. Inst. 2017

(a)

Hot partially premixed flame

Transition from cool flames to hot flames• Start w/ partially premixed cool flame at a = 73 s-1,

ϕ = 0.11, XF = 0.44

• Gradually add fuel to diffusion side up to XF ~ 0.48

• At a higher fuel concentration cool flame transfer to hot flames

18

500

1000

1500

2000

0.1 1 10 100

Fla

me t

em

pera

ture

, K

Strain Rate, 1/s

DME-N2/O2-N28 atmXfuel= 0.06Tfuel = 550 KToxi = 300 K

No radiation XO2 = 0.30

Radi. XO2 = 0.30

Radi. XO2 = 0.26

Radi. XO2 = 0.24

A

F

B

C

E

D

G

H

I

Cool flame

Z

(a)

0.1

1

10

100

1000

0.2 0.25 0.3 0.35 0.4 0.45 0.5E

xti

ncti

on

Str

ain

Rate

, 1/s

Oxygen mole fraction in oxidizer stream

DME-N2/O2-N28 atmXfuel = 0.06Tfuel = 550 KToxi = 300 K

B

H

ZD

F

D’

F’

(b)

Diffusion flame flammability limit diagram: € - Curve:

for cool, mild, and hot flames at elevated pressure

A “mild flame” regime may exist at very low stretch rate?

Eric Lin and Y. Ju et al., 2017

2. Dynamics of Regimes of Diffusion Cool Flames

n-C7H16-30-70% O2-He, 1atm

Droplet ISS

a. Droplet extinction: diffusion vs. radiation(Flex experiment)

Relationship

Small D0

Large D0

Model cross-validation

Counterflow Cool flame

Fuel

O2/He

Stretch rate, s-1

Flam

e t

em

per

atu

reCool flame

Hot flame

Cool flame

Stretch Extinction (SE)

Radiation Extinction (RE)

aH,SE

aC,SE

aC,RE

aH,RE

Lower YO2

0

b. Counterflow l flame extinction: stretch vs. radiationTop: Experiment cool flame, Bottom: Schematic

Cool flame

Hot flame

Competition between radiation/diffusion heat loss with chemical heat release

20

Effect of air temperature on diffusion cool flame burning limits

400

900

1400

1900

2400

0.1 1 10 100 1000 10000

Fla

me T

em

pera

ture

(K

)

Strain Rate (1/s)

DME/N2 vs Airp = 8 atmXF = 0.10TFuel = 550 K

TO = 900 K

500 K300 K

300 K

The increase of air temperature dramatically affects the cool flame regimes and burning limits:

• Cool flame shifts to higher stretch rate• mild flame branch disappear on high stretch side.• Radiation extinction limit disappear on low stretch side.

3. The role of cool flames in rating low temperature

fuel reactivity and model validation

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

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

Diffusion Flame Extinction Limit: n-Alkanes, iso-Alkanes, Aromatics

Co-relation with radical index and enthalpy weighted transport

Won, Sang Hee, et al. "A radical index for the determination of the chemical kinetic

contribution to diffusion flame extinction of large hydrocarbon fuels." Combustion and

Flame 159.2 (2012): 541-551.

30

50

70

90

110

130

150

170

0.01 0.10 1.00

Ex

tin

cti

on

Str

ain

Ra

te (

s-1

)

Fuel Mass Fraction, YF

nC7

Flames0% O3

Exp HFE Exp CFENUI-S131 HFE NUI-S131 CFENUI-S238 HFE NUI-S238 CFENUI-S526 HFE NUI-S526 CFENUI-Full CFE

-5.0E-5

0.0E+0

5.0E-5

1.0E-4

1.5E-4

2.0E-4

2.5E-4

3.0E-4

3.5E-4

C0 C1 C2 C3 C4 C5 C6 C7 C8Heat

rele

ase

ra

te (

kca

l/c

m2-s

)

Reaction Size

nC7 Cool FlameYF = 0.284a = 38 s-1

O3 = 0%

S131-CF

S238-CF

S526-CF

Full-CF

Will a model, which captures low temperature ignition well, be able to predict extinction limits of cool flames?

• The answer is no! we need to capture the low temperature heat release rate correctly.

• Cool flames can play an important role in model validation for heat release rate.

Reuter, C.B., Lee, M., Won, S.H. and Ju, Y., 2017. Combustion and Flame, 179, pp.23-32.

30

50

70

90

110

130

150

170

0.01 0.10 1.00

Ex

tin

cti

on

Str

ain

Ra

te (

s-1

)

Fuel Mass Fraction, YF

Fuel/N2 vs O2

Cool Flames0% O3

nC12 (Exp) nC10 (Exp) nC8 (Exp) nC7 (Exp)

nC12-S145 nC10-S149 nC8-S133 nC7-S141

PRINCETONMechanical and Aerospace Engineering

• Laminar Flame Speed

• Flame Extinction

Cool Flames as Mechanism

Validation Experiments

23

Hig

h T

em

p

Full-Strength

(Heat release rates)

Low

Tem

p

Laminar

Flame

Speed

?

S L

φ

a Ext

XF

Flame

Extinction

Diluted

(Kinetics)

Validation in systems with chemistry-transport-heat release coupling:

No validation of heat release rate at low temperatures!

“Experimental blind spot” – can be addressed in cool flame measurements

• Shock tube• RCM• JSRT• µ tube

Cool flames

24

0.050 0.075 0.100 0.12550

75

100

125

150

Cool flame

Exti

ncti

on

str

ain

rate

(s

-1)

100% DME

50% DME + 50% CH4

Hot flame

Fuel Mass fraction, YFuel

CH4/DME Cool flames and mechanism validation

Cool flames

500 600 700 800 900 1000 11000.000

0.005

0.010

0.015

0.020

0.025 JSR Exp. (Rodriguez et al.)

HP-Mech/Liu et al./Zhao et al.

OCH2OCHO=HOCHO+HCO (Wang/Dames et al.)

QOOH/O2QOOH ratio (Kurimoto/Burke et al.)

CH2OCH2O2H reactions (Burke et al.)

DM

E m

ole

fra

cti

on

Temperature (K)

1. Zhao, Zhenwei, Marcos Chaos, Andrei Kazakov, and Frederick L. Dryer, IJCK, 40, no. 1 (2008): 1-18.

2. Kurimoto, N., Brumfield, B., Yang, X., Wada, T., Diévart, P.,

Wysocki, G. and Ju, Y., 2015. Proc. Combust. Inst., 35, p.457.3. Wang, Z., Zhang, X., Xing, L., Zhang, L., Herrmann, F., Moshammer, K.,

Qi, F. and Kohse-Höinghaus, K., 2015. Combust. Flame, 162(4), p.1113.4. Burke, U., Somers, K.P., O’Toole, P., Zinner, C.M., Marquet, N., Bourque,

G., Petersen, E.L., Metcalfe, W.K., Serinyel, Z. and Curran, H.J., 2015. Combustion and Flame, 162(2), pp.315-330.

5. Dames, E.E., Rosen, A.S., Weber, B.W., Gao, C.W., Sung, C.J. and Green, W.H., 2016. Combustion and Flame, 168, pp.310-330.

6. Liu, D., Santner, J., Togbé, C., Felsmann, D., Koppmann, J., Lackner, A., Yang, X., Shen, X., Ju, Y. and Kohse-Höinghaus, K., 2013. sures. Combustion and Flame, 160(12), pp.2654-2668.

Conclusion

•Cool flames lead to many new flame regimes, dynamics, and insights in combustion research and engine development.

•Cool flames extend the burning limits and add new physics and time/length scales in combustion dynamics.

•Cool Flames provide a new platform in validation of low temperature chemistry.

•Experiments of high pressure/turbulent cool flames, and ignition assisted cool flames are interesting and challenging.

25

26