Laminar Flames and the Role of Chemistry and Transportyju/1st-workshop summary and... ·...
Transcript of Laminar Flames and the Role of Chemistry and Transportyju/1st-workshop summary and... ·...
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)
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: