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Med Colket and Steve Zeppieri
4th Annual Fuel Research Meeting
Multi-Agency Coordinating Council for Combustion Research
Argonne National Laboratories, Chicago, Ill
September 20-22, 2011
Use of Kinetic Models to Predict Fuel
Impact on Engine Performance
2
Background
Secure supply
Military needs guaranteed fuel supply in time of war
Global Warming:
Combustion of fossil-derived fuels
Combustion emissions (soots/particulates)
Physics-based combustor models
Existing models barely sufficient for design predictions
Physical vs. chemical effects
Physical effects presumed to dominate
Complexity of fuels
Large number of species
Unknown/uncertain reaction rate laws
DoD needs
AF – 50% utilization of alternative fuels by 2016 (in US)
Navy – Sailing the „Great Green Fleet‟ by 2016
SERDP – Understand impact on emissions
3
Objectives and Outline
Objectives:
Apply methods/tools that can incorporate both physical and chemical effects to predicting impact on engine performance
Outline:
Recommendations from 2010!!!!
Review general modeling approach
Applications to
Lean blow-out (review)
Altitude Relight
Emissions Unburned hydrocarbons
Soot
Summary
Recommendations
Challenge Problems
4
Recommendations/Needs – from 2010!!!!!
Validated mechanisms that predict ignition and extinction
particularly aromatics and cycloalkanes
Broader/validated tool for selecting surrogates that match properties of
petroleum fuels and synthetic fuels
Soot growth models incorporating effect of PAH surface addition
Models that describe HC emissions fingerprint and changes with fuels
Inclusion of ignition criteria for certification of alternative fuels
What should be the standard?
Incorporate physical effects?
New ignition data at low pressure needed
Fuel components
Jet Fuel
Alternative jet fuels
5
Tools for Assessing Fuel Impact on Performance
Alternate Fuel:
Phys & Chem Prop
Surrogate for
alternate fuel
Detailed Kinetics
for Surrogate
Selection of physics-
based tools sets
Simulations Probable impact
on combustor
Interpretations and
Engine Impact
Testing
Combustor Simulation
H. Pitsch
Emissions
Quenching Due to Fresh Reactant
Mixing with Reacting Mixture
0
500
1000
1500
2000
2500
3000
3500
-1.0E-06 9.0E-06 1.9E-05 2.9E-05 3.9E-05
Time (sec)
Te
mp
era
ture
(K
)
Ymix = 0
Ymix = 2,000
Ymix = 6,000
Ymix = 20,000
Ymix = 30,000
Ymix = 35000
Ymix = 40,000
Ymix = 60,000
Ymix = 100,000
Ymix = 200,000
Ignition (Ymix=0)
Ignition (Ymix=2,000)
Ignition (Ymix=6,000)
Ignition (Ymix=20,000)
Ignition (Ymix=30,000)
Ignition (Ymix=35,000)
tignition =
4.92 microsec
phi=1
all vapor
Ignition Modeling
6
Modeling Methods – Physical Models
Lean Blow Off – Extinction of a „Perfectly Stirred Reactor‟
Altitude Relight – Revised SENKIN with added mixing and vaporization
Initial energy provided by prescribed spark
Soot – (PSR) Network Reactor Code with coupled soot equations
Simplified, kinetics-based model of physical problem (CHEMKIN based)
1600
2100
2600
3100
3600
1.00E-06 1.00E-05 1.00E-04 1.00E-03
Tau (sec)
Tem
pera
ture
(F
)
n-Heptane/Air
Ti = 1857 F
P = 6 atm.
Phi = 2.0
PSR
Extinction Time
PSR Ignition Time
Extinction
Time
Ignition
Time
Te
mp
era
ture
Tau (sec)
Volume, V
Residence time, Temperature, T
Enthalpy, hk
Mass Fraction, Yk
inlet outletm
Y*k
h*k
m
Yk
hk
Heat loss
Q
. .Volume, V
Residence time, Temperature, T
Enthalpy, hk
Mass Fraction, Yk
inlet outletm
Y*k
h*k
m
Yk
hk
Heat loss
Q
Volume, V
Residence time, Temperature, T
Enthalpy, hk
Mass Fraction, Yk
inlet outletm
Y*k
h*k
m
Yk
hk
Heat loss
Q
. .
Y*S YSSoot Sections, YS
Network Reactor Simulation
Fuel-spray shear layer
Recirculation zones
Quench
zones
Burn-out
zones
Volume, V
Residence time,
Temperature, T
Enthalpy, hk
Mass Fraction, Yk
inlet outletm
Y*k
h*k
m
Yk
hk
Heat loss
Q
. .Volume, V
Residence time,
Temperature, T
Enthalpy, hk
Mass Fraction, Yk
inlet outletm
Y*k
h*k
m
Yk
hk
Heat loss
Q
Volume, V
Residence time,
Temperature, T
Enthalpy, hk
Mass Fraction, Yk
inlet outletm
Y*k
h*k
m
Yk
hk
Heat loss
Q
. .
7
Surrogate Fuel Identification - Demonstration
1) Kinetics model (ignition surrogate)
selected from Westbrook, et al
2) Compared to surrogates and kinetics
proposed by others
3) Benchmarked model against Vasu et al
(C&F) expts
4) Kinetics models/surrogates predict
ignition times a factor of two too long
Liq vol
fraction
Mass
fraction
Molar
fraction
n-heptane 25 0.227 0.226
iso-octane (2,2,4-trimethylpentane) 20 0.184 0.161
methylcyclohexane 35 0.358 0.364toluene 20 0.231 0.249 Min Max Min Mean Max
grams/cc 0.752 0.775 0.84 0.796 0.817 0.837
MW 99.712
H/C 1.899 1.85 1.85 1.9 2.1
SP(mm) 21.14 19.00 19 21 26
HtComb (MJ/kg) 43.31 42.60 42.90 43.11 43.40
ONR JP-5 Spec PQIS DatabaseMil Spec PQIS Database
Surrogate – blend of reference
Fuels obtained by matching H/C,
Smoke point, Ht. of combustion
1
10
100
1000
10000
0.5 0.7 0.9 1.1 1.3 1.5
Ign
itio
n T
ime
(µs)
1000/T (1/K)
Models Correctly Predict Trends
(but a factor of two higher than Vasu et al, C&F 2008)
Vasu et al [1]
Vasu et al [1] current work
Dean et al [2]
Dean et al [2] current work
Ignition Surrogate
Dooley, et al (2010)
CSE - Jet-A
Model: P = 20 atm; φ = 1.0
Shock tube data at φ = 1.0 and scaled to 20 atm(time ~ P-1)
Based on HC classes
8
Observations from LBO Simulations
Small differences due to
physical effects JP-8 to JP-5
One fuel component had
longer PSR extinction times
– and controlled surrogate
extinction time. (Perhaps
due to uncoupled reaction
mechanism)
Most mechanisms are not (or
poorly) validated against
flame extinction
Better reaction models for
cycloalkanes are needed
Comparison of Extinction Behavior for Various
Fuels & Representative Fuel Blend
(10 atm, 750K Inlet Conditions)
0.000
0.001
0.002
0.003
1400 1800 2200 2600 3000
Adiabatic Flame Temperature (K)
Ex
tin
cti
on
Tim
e (
s)
HeptaneIso-octaneMCHToluene4 Component Fuel Blend
Mole Fraction
n-Heptane 22.63%
iso-Octane 16.06%
Toluene 24.95%
MCH 36.36%
Based on PSR Extinction Times
9
AutoIgnition: Strong pressure dependence
NTC shifts to higher temperatures with increased P
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
0.6 0.8 1 1.2 1.4
Ign
itio
n D
ela
y [
sec.]
1/Temperature (x1000) [K-1]
Stoichiometric Heptane-Air MixturesLLNL n-Heptane MechanismConstant Volume Calculations
0.5 atm
2.5 atm
10 atm25 atm
50 atm
(tau ~ P-1)
in „hot‟ ignition regime
NTC
Review from
last year
10
AutoIgnition: Strong pressure dependence
NTC relatively region unimportant for present day engines
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
0.6 0.8 1 1.2 1.4
Ign
itio
n D
ela
y [
sec.]
1/Temperature (x1000) [K-1]
Stoichiometric Heptane-Air MixturesLLNL n-Heptane MechanismConstant Volume Calculations
0.5 atm
2.5 atm
10 atm25 atm
50 atmNTC
Altitude Relight and
Augmentor Ignition
11
AutoIgnition: Strong pressure dependence
NTC of increased interest with increased P
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
0.6 0.8 1 1.2 1.4
Ign
itio
n D
ela
y [
sec.]
1/Temperature (x1000) [K-1]
Stoichiometric Heptane-Air MixturesLLNL n-Heptane MechanismConstant Volume Calculations
0.5 atm
2.5 atm
10 atm25 atm
50 atm
Regime of interest in advanced,
high efficiency (next gen) engines
– and reciprocating engines!
12
Challenge Problem: Altitude Relight
Spark survival of >10 msec followed by ignition is inconsistent with
mixing/cooling rates
Spark energy
should
dissipate in 1-3
milliseconds !
Explanation may
be fuel-type
dependent
Spark Trajectories from Reid, Rogerson and Hochgreb, AIAA 2008-957
13
Igniter
Spark kernel
Flame kernel Flame propagation/
stabilization (not
modeled)
Fuel Droplets
Size Distribution,
fuel/air ratio
Air
Temperature,
Pressure
Droplet Evaporation
At Tkernal, and
Detailed kinetics
Spark
Energy added
Ignition? Ymix
„fresh‟
reactant
Reacting, vaporizing
mixture
t = 0
Coupled Kinetics/Mixing/Vaporization Model:
Flame Kernel Model
Full detailed kinetics (SERDP surrogate)
Specify SMD (drop size distribution)
Specify mixing rate Ymix (1/sec)
Added energy – after early, rapid relaxation
14
Altitude Relight/Ignition Modeling – Mixing Effect
Turbulent Mixing can quench chemical reactions and inhibit ignition
Quenching Due to Fresh Reactant
Mixing with Reacting Mixture
0
500
1000
1500
2000
2500
3000
3500
-1.0E-06 9.0E-06 1.9E-05 2.9E-05 3.9E-05
Time (sec)
Te
mp
era
ture
(K
)
Ymix = 0
Ymix = 2,000
Ymix = 6,000
Ymix = 20,000
Ymix = 30,000
Ymix = 35000
Ymix = 40,000
Ymix = 60,000
Ymix = 100,000
Ymix = 200,000
Ignition (Ymix=0)
Ignition (Ymix=2,000)
Ignition (Ymix=6,000)
Ignition (Ymix=20,000)
Ignition (Ymix=30,000)
Ignition (Ymix=35,000)
tignition =
4.92 microsec
phi=1
all vapor
Example conditions:
Phi = 1
FracL=0 (fuel vaporized)
P=1.422 atm
T=440.9 K
E/M=943 J/gm
Gaseous
fuel: no
vaporization
15
Altitude Relight/Ignition Modeling – Mixing/Vaporization
Coupled Mixing with Vaporization complicates inhibition (thermal quench)
250
750
1250
1750
2250
2750
0 2 4 6 8 10
Tem
pe
ratu
re [K
]
Time [msec.]
SMD-20
SMD-40
SMD-60
SMD-80
SMD-100
SMD-120
SMD-140
SMD-160
SMD-180
SMD-200
SMD-220
All fuel initially liquid (variable Sauter Mean Diameter); Total phi = 3.5
Tinit = 265K; P = 0.36 atm; Ymix = 625 sec-1
20
220
16
Altitude Relight/Ignition Modeling – Mixing/Vaporization
Coupled Mixing with Vaporization complicates inhibition (vapor quench)
All fuel initially liquid (variable Sauter Mean Diameter); Total phi = 3.5
Tinit = 265K; P = 0.36 atm; Ymix = 625 sec-1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 2 4 6 8 10
Vap
or
Ph
i
Time [msec.]
SMD-20SMD-40SMD-60SMD-80SMD-100SMD-120SMD-140SMD-160SMD-180SMD-200SMD-220
20
220
17
Prediction of Fuel Effects of Altitude Relight
SERDP Mechanism*
Three fuels
77% Dodecane/23% m-Xylene
Dodecane
m-Xylene
* UIC + USC + UTRC
SMD fracL phi E/M
10 0.9 0.5 1300
30 0.93 0.75 1400
50 0.96 1 1400
70 0.99 1.25 1750
90 1.5
110 1.75
130 2
150 2.25
170 2.5
190 2.75
3
3.25
3.5
3.75
4
4.25
4.5
4.75
5
5.25
5.5
Use fuel with range in ignition
characteristics – accentuate
effects
Identify conditions suitable for
sustained kernel
18
Final Kernel Temperatures for 1500J/gm
n-dodecane
Fuel: nC12
Boiling Point: 533 K
Spark Energy/Mass: 1500 J/gm
Fuel: 64.38% nC12 / 35.62 m-Xylene
Boiling Point: 533 K
Spark Energy/Mass: 1500 J/gm
Fuel: m-Xylene
Boiling Point: 533 K
Spark Energy/Mass: 1500 J/gm
n-dodecane/
m-xylene m-xylene
Tboiling = 463K
(lower limit for
Jet-A)
Fuel: nC12
Boiling Point: 463 K
Spark Energy/Mass: 1500 J/gm
Fuel: 64.38% nC12 / 35.62 m-Xylene
Boiling Point: 463 K
Spark Energy/Mass: 1500 J/gm
Fuel: m-Xylene
Boiling Point: 463 K
Spark Energy/Mass: 1500 J/gm
Tboiling = 533K
(upper limit for
Jet-A, e.g., JP-5)
19
Sustained Kernels at 1400J/gm
Blend, 1400 J/gm, 463 K boilnC12, 1400 J/gm, 463 K boil
Blend, 1400 J/gm, 533 K boil
nc12, 1400 J/gm, 533 K boil
n-Dodecane n-Dodecane/
m-xylene
463K
533K
20
„Statistical‟ Analysis (arbitrary and unvalidated)
Statistics suggest dodecane will light more easily than the JP-8
surrogate (blend) and that lower boiling point fuels will light
slightly easier
Fuel
E/M (J/gm) (463 K) (533 K) (463 K) (533 K) (463 K) (533 K)
1300 0.0% - 0.0% - - -
1400 55.4% 54.1% 39.6% 38.6% - -
1500 62.9% 61.3% 61.2% 59.6% 0.0% 0.0%
1750 63.8% 62.3% 63.1% 61.5% 59.9% 58.0%
(trial size: 840 cases)
n-Dodecane Blend Fuel m-Xylene
21
Challenge Problem: HC Emissions from Gas turbine Engines
Empirical evidence that emissions scale linearly – independent of engine or power
• What physical/chemical processes control this scaling?
• Can impact of alternative fuels on atmospheric pollution be anticipated?
Results from
APEX 2 and 3
22
Approach
Identify chemistry and physics controlling scalability of HC emissions
Methodology:
• Utilize reactor-network formulation to simulate flowfield of burner
• Fuel used: 77% dodecane, 23% m-xylene (vol %) – SERDP mechanism
• Initially assumed “bulk flow” conditions (e.g., total flow rates, total burner
volume, etc.)
• Modified to focus on conditions with measurable emission levels and
those exhibiting scaling
• Approximate “Idle”/low-power conditions:
P3 = 4.08 atm,
T3 = 478 K
overall ~ 0.25
23
Network Construction for Refocused Analysis
Based on fluid streaks
Front-End Residence time: .2-1.4 msec
Phi: 1.6-2.2
Quench Region Residence time: ~0.05-0.005 msec
Exit Phi: 0.1-0.35
Burn-out Residence time: na
Exit Phi: 0.1-0.35
Fuel
Primary Air
Secondary/Dilution Air (evenly added over 20 reactors)
Relaxed constraints relative to burner dimensions
(Emission levels expected to be high)
24
Results of Streak Analysis
0
20
40
60
80
100
120
140
160
180
200
0 20 40 60 80
HC
Exi
t EI
[gm
/kg/
fue
l]
CH2O Exit EI [gm/kg fuel]
CH4 EI
C2H4 EI
CH3CHO EI
C3H6 EI
C4H6 EI
C6H6 EI
m = 3.31
m = 0.58
m = 0.36
m = 0.12 m = 0.03
‘Correlations’ enable estimating HC sensitivity to CH2O EI values
25
Results of Streak Analysis (for two surrogates)
Reasonable agreement for methane, ethene, propene.
Butadiene, benzene, acetaldehyde not as well predicted
Linearity not always perfect
Uncertainties in scaling factors needs determination
• particularly large for propene
n-Dodecane/m-Xylene n-Dodecane/MCH Spicer (JP-5/CFM-56/idle) Aerodyne (AAFEX/JP-8)
CH4 0.36 0.29 0.43 -
C2H4 3.31 4.68 2.69 1.33
C3H6 0.58 0.55 0.79 0.51
C6H6 0.12 0.00 0.32 0.16
C4H6 0.03 0.10 0.30 -
CH3CHO 0.07 0.09 0.47 0.36
26
Summary of Simulation Conditions that Yielded Trend Data
One scenario: rapid quenching of rich pockets to very lean conditions
Critical conditions:
• Fuel rich phi: 1.6 to 2.2
• Quench residence time – 2.2 to 12 microseconds!
• Final phi - 0.1 to 0.2
Typical quench times:
In traditional RQL burners > 0.1 milliseconds!
Possible mechanisms discussed at ESS/CI – October (Storrs, CT)
27
Simulation of PM emissions from RQL combustor
Full gas-phase and soot kinetics with aerosol
dynamics model coupled into perfectly stirred
reactor (PSR) network model
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0.38 0.4 0.42 0.44 0.46 0.48 0.5
Soo
t m
ass
((m
g/m
^3
)
Equivalence ratio
FT (100% n-dodecane)
JP8 (77%n-dodecane/23%m-xylene)
1SNETPSR model
Fuel-rich
front endQuench
ZoneLean, Burn-
Out Zone
Reduced-order model developed and
installed into LES codes for predicting soot
at combustor exit plane
Mass fraction
Size (cm)
Number density
(cc-1)
Reduced PM emissions with FT fuels Predictions of mass, size, and number
28
Summary
Coupling of chemistry amongst fuel molecules may be
needed for simulation of extinction/LBO
Physical effects are likely as/more important as
chemical effects
Kinetic models need to be accurate for phi variations
Fuel rich chemistry (especially for aromatics) may
need additional studies
Preliminary coupled ignition model coupling
kinetics/mixing/vaporization explains qualitative
observations
HC emissions fingerprint likely due to rapid quenching
of reacting gas element
28
29
Recommendations/Needs – 2011
Validated mechanisms that predict ignition and extinction
particularly aromatics and cycloalkanes
Inclusion of ignition criteria for certification of alternative fuels
What should be the standard?
Incorporate physical effects?
Soot models that predict fuel effects – (isoalkanes)
Statistical methods for simulation of relative (physical/chemical) effects
may be needed
Review rich side chemistry, especially for aromatics
30
Thank You!!!!!
Acknowledgements:
Office of Naval Research – Dr. Sharon Beermann-Curtin
Strategic Environmental Research and Development Program
Air Force Research Laboratories – Dr. Mel Roquemore and Dr.
Tim Edwards
United Technologies Research Center – Heidi Hollick
Pratt and Whitney – Dr. Jeff Lovett