Post on 24-Aug-2020
Clean Combustion Research Center
Fuel Effect on Soot Formation in
Diffusion Flames
Suk Ho Chung
Named Professor in Mechanical Engineering
Clean Combustion Research Center
King Abdullah University of Science and Technology
Saudi Arabia
KAUST Future Fuels Workshop, March 7-9, 2016
Clean Combustion Research Center
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
Clean Combustion Research Center
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
Clean Combustion Research Center
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?
Clean Combustion Research Center
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
Clean Combustion Research Center
KAUST Soot Lab
LE/LS, LII, PAH LIF, PIV, (VUV)
Clean Combustion Research Center
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.
Clean Combustion Research Center
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
Clean Combustion Research Center
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
Clean Combustion Research Center
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
Clean Combustion Research Center
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
Clean Combustion Research Center
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]
Distance from fuel nozzle Z [mm]
D63
• Soot formation flame • Soot formation/oxidation flame
SootZone
Flame
Stagnation
Oxidizer
Fuel
Stagnation
Oxidizer
Fuel
FlameSootZone
Clean Combustion Research Center
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)
Clean Combustion Research Center
• 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
Clean Combustion Research Center
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)
Clean Combustion Research Center
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
Clean Combustion Research Center
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
Clean Combustion Research Center
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.
Clean Combustion Research Center
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
Clean Combustion Research Center
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
Clean Combustion Research Center
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
Clean Combustion Research Center
[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
Clean Combustion Research Center
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
formation PAH growth Inception Growth
Coagulation
(Oxidation)
+ C2H2
+ PAH
+ Soot
+ PAH
Clean Combustion Research Center
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)
Clean Combustion Research Center
KAUST PAH mechanism 1 (for gasoline surrogate fuels)
PAH Mechanisms
Clean Combustion Research Center
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
Clean Combustion Research Center
n-heptane premixed laminar flames at 1 atm : Flame A
Experiment: F. Inal, S.M. Senkan, Combust. Flame 132 (2002)
Clean Combustion Research Center
n-heptane premixed laminar flames at 1 atm : Flame A
Concentrations of PAHs larger than pyrene (A4)
Clean Combustion Research Center
Soot formation (SF) flames
Clean Combustion Research Center
Soot Growth Rate
Hwang & Chung, CNF 125, 2001
Fuel
C4Hx + C2H2
C3H3 + C3H3
Ax + C2H2
Ax + C3H3
+ C2H2
Ring
formation PAH growth Inception Growth
Coagulation
(Oxidation)
+ C2H2
+ PAH
+ Soot
+ PAH
Clean Combustion Research Center
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)
Clean Combustion Research Center
Soot Mass Growth Rates
-1.0
-0.5
0.0
0.5
1.0
2 3 4 5 6 7
Distance from fuel nozzle Z [mm]
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
Distance from fuel nozzle Z [mm]
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
Clean Combustion Research Center
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
Distance from fuel nozzle Z [mm]
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
Clean Combustion Research Center
-0.2
0.0
0.2
0.4
0.6
4 5 6 7 8 9
Distance from fuel nozzle Z [mm]
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
Clean Combustion Research Center
• 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
Distance from fuel nozzle Z [mm]
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
Clean Combustion Research Center
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
formation PAH growth Inception Growth
Condensation
(Oxidation)
+ C2H2
+ PAH
+ Soot
+ PAH
Clean Combustion Research Center
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.
+
Clean Combustion Research Center
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
Clean Combustion Research Center
Soot Modeling
• Ethylene & binary mixtures with methane, ethane, propane
Clean Combustion Research Center
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.
Clean Combustion Research Center