LES Modeling for IC Engines - Department of Energy · LES Modeling for IC Engines Chris Rutland...
Transcript of LES Modeling for IC Engines - Department of Energy · LES Modeling for IC Engines Chris Rutland...
LES Modeling for IC Engines
Chris Rutland Engine Research Center
University of Wisconsin - Madison
LES Introduction • Large Eddy Simulation
– Next generation of turbulence modeling
• Concept:
– RANS uses ensemble averaging – LES uses local spatial averaging
(filtering) – Achieved by different sub-models
• Intent
– Retain more flow structure on grid – Better predictive capability – Capture more details by simulating
more flow structures
LES RANS
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Better Predictive Capability
• Ignition chemiluminescence compared with temperature predictions
RANS LES Experiment
CA =
-12
o a
TDC
Temp (K)
(Experimental images courtesy Singh et al., 2006) 3
LES Advantages Increased design sensitivity
Capture more flow instabilities Better response to geometric details Address wider range and more subtle design conditions
Study new phenomenon Cycle-to-cycle variability
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Practicalities of LES in Engines • LES is:
– NOT: Just an LES turbulence model – NOT: Turn-key for IC engine applications
• Need reasonable computational times
– Limits grid density – Requires accurate sub-models
• Complex physics
– Sub-grid models for
Turbulence Combustion Scalars Sprays Walls, injectors, etc.
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LES versions required
LES Sub-Models for Engines • Tier 1 models: Available for use
– Turbulence * – Combustion – Scalars – Sprays: far field effects
• Tier 2 models: Need work
– Emissions – Wall effects and wall heat transfer – Sprays, sprays, sprays
• Nozzle effects, breakup, atomization
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• 0 – equation • 1 – equation
• Viscosity • Non-viscosity
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LES Modeling Technologies • Scale similarity
– Use big ‘whorls’ to help model small ‘whorls’
• Dynamic procedure – Obtain sub-model coefficients from solution
• Approximate de-convolution
– Reverse filter: estimate small scales
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Examples
• Combustion modeling
• Spray modeling
• Wall heat transfer modeling
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Mixing-Controlled Detailed Chemistry (MCDC) Model
• Modifies chemistry solution with mixing time scale • Based on ‘Kong-Reitz’ model formulation [Kong et al, 2002]:
• Damkohler number:
• Mixing time scale:
DCS = Direct Chemistry Solver (Chemkin or equivalent) DCS reaction rate for species i:
Dynamic scale similarity model: χsgs
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1mixing sgsτ χ≈
Subgrid χ Model Validation • Engine comparisons
– PLIF experiments of Peterson and Ghandhi
125-cycle experimental 10-cycle LES simulations
Fuel – N 2 N 2 Fuel – N 2 N 2
Ensemble PDF of resolved and subgrid scalar dissipation
DEER 2012: Rutland-LES 9 Tin = 323 K, 240 bTDC. Sample domain: 19.5 x 15.2 mm.
MCDC Combustion Model Validation
-30 -20 -10 0 10 20 300
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4
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10 Experiment DCS Model
Pres
sure
[MPa
]
Crank [°aTDC]
0
100
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600
700 Experiment DCS Model
HRR
[J/D
eg](a)
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Sandia Cummins N-14 optical engine
Engine speed [RPM] 1200 IMEP [bar] 4.4 Injection pressure [bar] 1200
Intake temperature [oC] 111
SOI [o ATDC] -7
MCDC Combustion Model Validation
-40 -30 -20 -10 0 10 20 30 40
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9 Experiment DCS Model
Pres
sure
[MPa
]
Crank [°aTDC]
0
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700
HRR
[J/D
eg]
Experiment DCS Model
(b)
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Engine speed [RPM] 1200 IMEP [bar] 4.6 Injection pressure [bar] 1600
Intake temperature [oC] 90
SOI [o ATDC] -22,+15
Sandia Cummins N-14 optical engine
Spray Modeling
Symbols: Experiments Solid: LES Dashed: RANS
Vapor penetration
Liquid penetration
Exp
LES
RANS
LES
RANS • Injection pressure: 135 MPa • Nozzle diameter: 246 μ • Ambient density : 30.2 kg/m3 • Ambient temperature: 1000 K
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• Key component for LES spray modeling: spray source term in sub-grid kinetic energy equation: used approximate de-convolution
• Liquid and vapor penetration: LES and RANS show similar global results
High Flame Lift-off Case • Coherent structures: yellow • Cool flame: blue (CH20) • High temperature reactions: red (OH)
Cool flame (CH2O) evolves with the CS Long cool flame period
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Fuel Injection 1163 bar
Fuel, temperature C14H30 , 408 K
Ambient pressure 22.7 bar, 1100 K
Ambient density [kg/m3] 7.17
Low Flame Lift-off Case • Coherent structures: yellow • Cool flame: blue (CH20) • High temperature reactions: red (OH)
Cool flame is inside high temperature
reaction zone Cool flame has a steady penetration
(~1 ms ASOI) High temperature reaction zone evolves
with the CS
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Fuel Injection 1163 bar
Fuel, temperature C14H30 , 408 K
Ambient pressure 64 bar, 1200 K
Ambient density [kg/m3] 18.58
Engine Case: LTC Early Injection
Blue: Cool flame (CH20) Red: High temperature reaction zones (OH) Grey: Droplets Black: Velocity vectors on CS
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Engine Case: Short Ignition Delay
Blue: Cool flame (CH20) Red: High temperature reaction zones (OH) Grey: Droplets Black: Velocity vectors on CS
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RANS-LW
0 5 10 15 20 25 30 35 40 45 500
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LES-WW
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0 5 10 15 20 25 30 35 40 45 500
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Friction Factor
Nusselt Number
Wall Modeling: Channel Flow
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Wall Heat Flux
LES-LW LES-WW
EXP RANS-LW
Engine Wall Heat Transfer
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TC: 6
TC: 8
Summary • LES can increase accuracy and sensitivity • LES for engine modeling is here now
– Requires accurate sub-models – Requires very knowledgeable users
• Current efforts on improvements – Sprays: nozzle interior, near nozzle – Appropriate use: multiple cycles, new analysis
techniques, validation • Additional information:
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“Large Eddy Simulations for Internal Combustion Engines – A Review,” Rutland, International Journal of Engine Research, 2011, Vol. 12 (5), pp 421-451 DOI: 10.1177/1468087411407248.
Acknowledgements
Chalearmpol Plengsaard Noah Van Dam
Yuxin Zhang Siddhartha Banerjee Nidheesh Bharadwaj
Bing Hu Rahul Jhavar
Sergei Chumakov Eric Pomraning
Department of Energy (DE-EE0000202)
General Motors R&D, CRL Caterpillar
NSF, AFOSR, ONR
http://www.erc.wisc.edu/
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Thank You
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