fuel effect on soot formation in diffusion flames · • temperature c 2 h 4 > c 2 h 4 +c 3 h 8...

Clean Combustion Research Center

Fuel Effect on Soot Formation in

Diffusion Flames

Suk Ho Chung

Named Professor in Mechanical Engineering


King Abdullah University of Science and Technology

Saudi Arabia

KAUST Future Fuels Workshop, March 7-9, 2016


Contents

• PM issues in IC engines

• Soot zone structure in diffusion flames

• Chemical Cross-linking Effect on Soot Formation

– Ethylene/Propane mixture

– Gasoline surrogate fuels Toluene/n-heptane/iso-octane

– PAH kinetic modeling

• Soot growth rate

• Soot modeling

• Soot oxidation modeling


o Visible Emission: Public awareness

o Health : Carcinogenic and Mutagenic

o Black Carbon : Global Warming

o Strictly Regulated Emission

o Radiation Heat Transfer

o Incomplete Combustion : Efficiency

o Deposit : Burner Lifetime / Performance

Importance of Soot

Fuel Oil Burner (VKW) Gulf War (1991) Oil Well Fire, NG Wiki

Lee, Argonne NL


PM Issues in IC Engines

• DISI Engines – Spray-guided / Multiple injection

– Stoich. homogeneous/Lean stratified

– PM number emission (PN)

• Diesel Engines – Multiple injection

– Partially premixed flames

– Autoignition

– DPF: PM regeneration

• Low-temperature combustion (LTC) – High level of dilution

– Autoignition

– Partially-premixed compression ignition

• Common aspects on combustion

– Autoignition

– Soot

– Partially premixed flames

– Best fuel for LTC engines?


Soot Formation Pathway

Odd carbon atom pathways

of CH3, C3H3, C5H5, C7H7 for

PAH formation and growth

Fuel

C4Hx + C2H2

C3H3 + C3H3

Ax + C2H2

Ax + C3H3

Surface HACA

+ C2H2

Hydrogen-abstraction-C2H2-

addition (HACA)

Ring

formation PAH growth Inception Growth Oxidation

Role of CH3, C2H, C3H3 on soot

growth (HACA)

Soot + PAH Condensation

Coagulation of Soot+Soot

+ PAH

+ Soot

Issues

Incipient ring formation: C2+C4 vs C3+C3

PAH growth: HACA vs aromatic addition

Inception: A4 + A4 vs extension

Soot growth: Radical site

PAH condensation to soot

Oxidation of soot: O2 & OH


KAUST Soot Lab

LE/LS, LII, PAH LIF, PIV, (VUV)


Soot Zone Structure

Motivation

Models developed based on PF & DF are not cross-applicable

Soot structure

KT Kang, JY Hwang, SH Chung, W Lee, "Soot zone structure and sooting limit in diffusion flames: Comparison of counterflow and coflow flames," Combust. Flame 109 (1997) 266-281.

Fuel density effect

Yuan Xiong, Min Suk Cha, Suk Ho Chung, “Fuel density effect on near nozzle flow field in small laminar coflow diffusion flames,” Proc. Combust. Inst. 35 (2015) 873–880.


Configurations for Soot Studies

NDF

Coflow DF Premixed Flames

Fuel Oxidizer

Soot models depend on flow configurations they are based on

Hai Wang. PCI 2011

Hadef et al. I J Thermal Sci 2010

Counter flow DF

O x i d i z e r

F u e l

F l a m e


Experiment

• Counterflow flames

Mirror

Mirror

Argon-ion laser

Chopper

Half-wave

plate

Pinhole

Pinhole

Convex lens

Convex

lens

Counter-flow

burner

Iris

Polarization

filter Pinhole

Narrow-band

pass filter

Photo-

multiplier

2-dimensional

positioner PC

Lock-in amplifier

Neutral-density

filter

Photodiode

SF

O F

lam

e

SF

Fla

me

• Position relative to stagnation

• Laser-based measurement

– LE/LS

– LII

– PAH fluorescence

– LDV/PIV

– CARS

XC2H4 = 0.106 XO2 = 0.833

XC2H4 = 0.745 XO2 = 0.257


Sooting Zone Structure

Soot Formation Flame (SF) Soot Formation/Oxidation Flame (SFO)

Inverse Diffusion Flame (IDF) Normal Diffusion Flame (NDF)

Rich Premixed F

Hadef et al. I J Thermal Sci 2010

Hai Wang. PCI 2011

Kang et al. CNF 1997

Counterflow


Counterflow Flame Structure Soot Formation Flame

Thermophoretic velocity 34(1 /8)tp

TVTa

Soot Formation/Oxidation Flame

Thermophoretic Effect : Particle motion in temperature gradient by

molecular collision


Soot Zone Structures

0

0.1

0.2

0.3

0

0.05

0.10

0.15

Maximum

temperature

Particle

stagnation

Qf

Conv.

XF,o

= 0.25

XO,o

= 0.9

So

ot

vo

lum

e f

ractio

n

x 1

06 PA

H flu

ore

sce

nce

Qf [a

.u.]

0

1.0

2.0

3.0

0

0.5

1.0

1.5

So

ot

vo

lum

e f

ractio

n

x 1

06

Maximum

temperature

PA

H flu

ore

sce

nce

Qf [a

.u.]

Particle

stagnation

Qf

Conv.

XF,o

= 1.0

XO,o

= 0.24

0

0.02

0.04

0.06

0.08

0.10

109

1010

1011

1012

1013

0 2 4 6 8 10 12 14

Maximum

temperature

Particle

stagnationN

Conv.

XF,o

= 1.0

XO,o

= 0.24

So

ot

pa

rtic

le s

ize

D

63

[m

]

So

ot n

um

be

r de

nsity

N [c

m-3]

Distance from fuel nozzle Z [mm]

D63

0

0.02

0.04

0.06

0.08

0.10

109

1010

1011

1012

1013

0 2 4 6 8 10 12 14

Maximum

temperature

Particle

stagnation

Conv.

XF,o

= 0.25

XO,o

= 0.9

N

So

ot

pa

rtic

le s

ize

D

63

[m

]

So

ot n

um

be

r de

nsity

N [c

m-3]


D63

• Soot formation flame • Soot formation/oxidation flame

SootZone

Flame

Stagnation

Oxidizer

Fuel

Stagnation

Oxidizer

Fuel

FlameSootZone


Chemical Cross-linking Effect on

Soot Formation

Synergistic effect of gas mixture fuel (Hwang et al., CNF 114, 1998)

Effect of O2 addition to fuel (Hwang et al., PCI 27, 1998)

Synergistic effect with benzene addition (Lee et al., CNF 136, 2004)

Synergistic effect for various gaseous fuels (Yoon et al., PCI 30, 2005)

DME mixing (Yoon et al., CNF 154, 2008)

Synergistic effect on soot size and number (Choi et al., IJAT 12, 2011)

Gasoline surrogate fuel (Choi & Chung, PCI 33, 2010)


• Incipient ring formation mechanism: Issue in late 1990’s – C2 path (C2 + C4 C6)

– C3 path (C3 + C3 C6)

• Importance of C3 path draws attention

– Propagyl radical C3H3

• To identify relative importance of C2 & C3 paths:

Fuel mixture of C2H4 + C3H8 has been tested.

Role of C3 Species

Fuel

C4Hx + C2H2

C3H3 + C3H3

Ax + C2H2

Ax + C3H3

+ C2H2

Ring

formation PAH growth Inception Growth

Coagulation

(Oxidation)

+ C2H2

+ PAH

+ Soot

+ PAH


Synergistic Effect in C2H4/C3H8 Mixtures

• Temperature

C2H4 > C2H4+C3H8 > C3H8

• Soot

C2H4+C3H8 > C2H4 > C3H8

• PAH

C2H4+C3H8 > C3H8 > C2H4

0

1.0

2.0

3.0

4.0

2000

2200

2400

2600

2800

3000

0 0.2 0.4 0.6 0.8 1

Ad

iab

atic

flam

e te

mp

era

ture

Ta

d [K]

Tad

Propane ratio

Qf,max

Ma

x.

PA

H f

luo

resce

nce

Q

f,m

ax

[a.u

.]

(Ethylene) (Propane)

max

Ma

x.

so

ot

vo

lum

e f

ractio

n

x 1

06

• C2 : Growth of PAH and Soot

• C3 : Ring Formation

• C2H4 : High C2 conc. + Low C3 conc.

– Relatively low conc. of rings

• C3H8 : Low C2 conc. + High C3 conc.

– Relatively slow growth rate of PAH and soot

• C2H4 + C3H8 : Synergistic

– Increase in PAH and soot formation

• Role of propargyl (conjectured through dehydrogenation)


PAH and Soot in Various Mixture Flames

C2H4

ethylene

0

1

2

3

4

0 0.2 0.4 0.6 0.8 1

45

0

Mixture ratio

Norm

aliz

ed m

axim

um

PA

H L

IF [

a. u

.]

C3H6

C3H8

CH4

C2H6

PAH LIF

C2H4

ethylene

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 0.2 0.4 0.6 0.8 1

me

tha

ne

Mixture ratio

Norm

aliz

ed m

axim

um

LII

[a.

u.]

C3H6

C3H8

CH4

C2H6

0.8

1.0

1.2

1.4

0 0.02 0.04 0.06 0.08

meth

ane

LII

• Role of methyl & methylene radicals

CH3 CH2

C2H2 + CH2 C3H3 + H

Yoon et al., PCI 30, 2005


Mixture Flames of DME

Mixture ratio

Max

. PA

H L

IF s

ign

als

[a. u

.]

Max

. LII

sig

nal

s [a

. u.]

0

0.5

1.0

1.5

2.0

2.5

0 0.2 0.4 0.6 0.8 1

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

C2H4+DME

C3H8+DME

C3H8+DME

C2H4+DME

C2H6+DME

CH4+DME

• Mixture flames of C2H4 + DME

– Increases PAH and soot formation at 0 < β

< 0.1, and decreases at 0.1 < β < 1

– Max. LII and PAH LIF signals at β = 0.1

are 1.3 and 2 times larger than those of

pure C2H4

• Mixture flames of CH4, C2H6, C3H8 +

DME

– Decrease PAH and soot formation

monotonically

Yoon et al., CNF 154, 2008


Gasoline Surrogate Fuels

BC Choi, SK Choi, SH Chung, “Soot formation characteristics of gasoline surrogate fuels in counterflow diffusion flames,” Proc. Combust. Inst. 33 (2011) 609-616.


Direct, LII, and LIF Images

• iso-Octane/Toluene Mixtures

– Toluene ratio RT: Liquid volume ratio of toluene in the binary mixture

– As the RT increases

> Negligible LII & LIF signal (RT = 0)

> LII & LIF intensity become stronger

– SFO flame: LIF images exhibiting double-layer

• Other Mixtures

– n-heptane/toluene mixture: similar tendency

– iso-octane, n-heptane, and their mixtures: non-sooting

• Flames of iso-octane/Toluene and n-Heptane/Toluene Mixtures


Soot formation (SF) flame

• iso-Octane/Toluene Mixtures

– Data are normalized with the max. value at RT = 1.0

– Blue flame position: near 4.15 mm (insensitive to RT)

• Max. LII Signal

– Increases rapidly for RT > 0.4

• Max. LIF Signal

– RT = 0.6 & 0.8 > RT = 1.0

– Synergistic behavior

• n-Heptane/Toluene Mixtures

– Similar trends


Normalized Max. LII & LIF Signals

• Normalized Max. LII Signals

– Monotonic with RT

– However, minimal up to RT = 0.4 for iso-octane(n-heptane)/toluene

– Increases reasonably linearly

– Tolerance in terms of toluene mixing for soot formation

• Normalized Max. LIF Signals

– Non-monotonic behavior

– Max. value at certain

range of RT > RT = 0.0 & 1.0

(synergistic effect)

– iso-octane/toluene mixtures:

> Higher tendency in

producing soot and PAHs


[1]. G. Blanquart, P. Pepiot-Desjardins, and H. Pitsch, Chemical mechanism for high temperature

combustion of engine relevant fuels with emphasis on soot precursors, Combust. Flame, vol. 156, pp. 588–

607, 2009.

[2]. C. Marchal, J. Delfau, C. Vovelle, G. Moreac, C. Mounaim-Rousselle, and F. Mauss, Modelling of

aromatics and soot formation from large fuel molecules, Proc. Combust. Inst., vol. 32, pp. 753–759, 2009.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.5

1.0

1.5

2.0

2.5

3.0

A2

A1

A1

A3

A3

A4

A4 x 0.3

MEC1 MEC2

A1

A2

A3

A4

SF flame

iso-octane / toluene

No

rma

lize

d m

ax

. m

ole

fra

cti

on

RTRT (Toluene fraction in fuel mixture)

Simulation of SF flame of iso-octane – toluene mixture [1]

A4 (Pyrene)

A1 (Benzene)

A2 (Naphthalene)

A3 (Phenanthrene)

MEC 1[2] MEC2[3]

Flame simulation


PAH Kinetics Modeling

Raj et al, KAUST PAH Mech 1 (CNF 2012)

Wang et al, KAUST PAH Mech 2 (CNF 2013)

Raj et al, Free-edge oxidation (CNF 2012)

Raj et al, Soot oxidation (CNF 2013)

Raj et al, PAH growth by propargyl (J Phys Chem A, 2014)

Fuel

C4Hx + C2H2

C3H3 + C3H3

Ax + C2H2

Ax + C3H3

+ C2H2

Ring


Coagulation

(Oxidation)

+ C2H2

+ PAH

+ Soot

+ PAH


Literature search

Quantum calculations

Density functional theory using Gaussian 09 on PAHs Transition state theory with appropriate corrections

Validation

Comparison of species profiles, ignition delay times, and laminar flame speeds

Mechanism Development

• ABF Mech: based on ethylene flame data, HACA

– J. Appel, H. Bockhorn, M. Frenklach (2000)


KAUST PAH mechanism 1 (for gasoline surrogate fuels)

PAH Mechanisms


PAH Mechanism Development

Benzene Naphthalene Phenanthrene Pyrene

(A1) (A2) (A3) (A4)

Benzyl[e]pyrene Benzyl[ghi]perylene Coronene

(A5) (A6) (A7)

Mechanism of Marshal et al. (2009)

154 species and 1404 reactions

Up to pyrene

New mechanism up to coronene

231 species and 2126

PAHs


n-heptane premixed laminar flames at 1 atm : Flame A

Experiment: F. Inal, S.M. Senkan, Combust. Flame 132 (2002)


n-heptane premixed laminar flames at 1 atm : Flame A

Concentrations of PAHs larger than pyrene (A4)


Soot formation (SF) flames


Soot Growth Rate

Hwang & Chung, CNF 125, 2001

Fuel

C4Hx + C2H2

C3H3 + C3H3

Ax + C2H2

Ax + C3H3

+ C2H2

Ring


Coagulation

(Oxidation)

+ C2H2

+ PAH

+ Soot

+ PAH


Surface HACA & Empirical Model

C2H2 C 4 2 22 [C H ]m k

• HACA: Frenklach (1990)

– Cs-H + H = Cs*+ H2 (H abstraction to form radical site)

– Cs*+ H Cs-H

– Cs*+ C2H2 Cs-H + H (surface growth controlling)

– Cs*+ O2 products (Oxidation)

– Cs-H + OH products (Oxidation)

– Cs-H : arm-chair site on soot particle surface

– Cs*: radical site

*s

HACA C C2H2 2 2 C2 [C H ] /

Am k N

Avogadro No

Surface density • Empirical: Linstedt (1994)


Soot Mass Growth Rates

-1.0

-0.5

0.0

0.5

1.0

2 3 4 5 6 7


Particle

stagnation

Max

temp.

C2H2

HACA

OH

+O2

G

XF,o

= 0.25

XO,o

= 0.9

Conv.

So

ot

ma

ss g

row

th r

ate

G

[103

g/c

m2/s

]

Figure 8

-1.0

-0.5

0.0

0.5

1.0

4 5 6 7 8 9


Max.

temp.

Particle

stagnation

C2H2

HACA

OH

+ O2

G

XF,o

= 1.0

XO,o

= 0.24 Conv.

So

ot

ma

ss g

row

th r

ate

G

[103

g/c

m2/s

]

Figure 9

• Growth Models : C2H2 addition to soot

– HACA : under-predicted for soot formation flame

– Empirical : over-predicted for soot formation/oxidation flame

• Soot formation flame • Soot formation/oxidation flame


soot + M* soot* + M

soot* + C2H2 soot + H

M* : H, CH3, C2H, C3H3,

0

0.2

0.4

0.6

0.8

4 5 6 7 8 9


Max.

temp.

Particle

stagnation

XF,o

= 1.0

XO,o

= 0.24

Mo

le f

ractio

n

[%]

H

C3H

3 (x10)

CH3 (x10)

Soot Zone

W A k msHACA s* C2H2 C 2 2C H 2 [ ]

s* H CH3 3 C2H 2

H

H

H CH C H

H

H

k k k

k

k

[ ] [ ] [ ]

~ [ ]

[ ]

a f(Premixed flames)

(Diffusion flames)

HACA Reactions : PF Based

H-Abstraction suggested

Pitsch (1996)

Cs-H + C2H Cs*+ C2H2

Cs-H + CH3 Cs*+ CH4

Cs-H + C3H3 Cs*+ C3H4


-0.2

0.0

0.2

0.4

0.6

4 5 6 7 8 9


Max.

temp.

Particle

stagnation

WHACA

WG

XF,o

= 1.0

XO,o

= 0.24 Conv.

So

ot

ma

ss g

row

th r

ate

W

G

[103

g/c

m3/s

]

WHACA

mod

W k

W k k k k

HACA s* H

HACA

mod

s* H CH3 3 C2H 2 C3H3 3 3

H

H CH C H C H

: [ ]

: [ ] [ ] [ ] [ ]

a f

Role of HC Radicals in C2H2 Addition

Soot formation flame


• High temperature (> 1000 K)

– Modifies HACA for soot growth

• Low temperature :

– High PAH concentration

– PAH-soot coagulation

-0.2

0.0

0.2

0.4

0.6

4 5 6 7 8 9


Max.

temp.

Particle

stagnation

WPAH

WG

Conv.

So

ot

ma

ss g

row

th r

ate

W

G

[103

g/c

m3/s

]

WHACA

mod

XC2H4,o

= 1.0

XO2,o

= 0.24

Soot Mass Growth Mechanism


Soot Modeling Predictive tool development

Wang Y, Raj A, Chung SH, “Soot modeling of counterflow diffusion flames of ethylene-based binary mixture fuels” CNF 162 (2015) 586-596

Fuel

C4Hx + C2H2

C3H3 + C3H3

Ax + C2H2

Ax + C3H3

+ C2H2

Ring


Condensation

(Oxidation)

+ C2H2

+ PAH

+ Soot

+ PAH


Soot Models

• Previous models

– Chemistry : up to A4 (Pyrene)

– Inception : Collision efficiency = 1

– Soot growth : Surface HACA

• Present model

– Chemistry : up to A9 (Coronene)

– Inception : 8 PAH species

– Collision efficiency

– Soot growth : Modified surface HACA

+

A. Raj, M. Sander, V. Janardhanan, M. Kraft, Combust. Flame 157 (2010) 523–534.

+


Target flames

• Counterflow diffusion flame of

binary mixtures

• Tested for fuel mixing effect in

counterflow diffusion flames

25% O2 + 75% N2

100% C2H4

95% C2H4 + 5% C3H8

95% C2H4 + 5% C2H6

95% C2H4 + 5% CH4

V0 = 20cm/s, Burner separation L = 8 mm


Soot Modeling

• Ethylene & binary mixtures with methane, ethane, propane


Reduced Mechanism

• KAUST-Aramco PAH Mech 1.0

– AramcoMech 1.3 C0-C2 + KAUST PAH Mech I

• Reduced from 397 to 99 species

– DRG-X + Sensitivity analysis

– Method of moments with interpolative closure (MOMIC)

Prabhu Selvaraj, Paul G. Arias, Hong G. Im, Yu Wang, Yang Gao, Sungwoo Park, S. Mani Sarathy,

Tianfeng Lu, Suk Ho Chung, “A computational study of ethylene-air sooting flames: Effects of lar

ge polycyclic aromatic hydrocarbons,” Combust. Flame 163 (2016) 427-436.

fuel effect on soot formation in diffusion flames · • temperature c 2 h 4 > c 2 h 4 +c 3 h 8...

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