LES Combustion Modeling for Diesel Engine Simulations
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Transcript of LES Combustion Modeling for Diesel Engine Simulations
LES Combustion Modeling for Diesel Engine Simulations
Bing Hu Professor Christopher J. Rutland
Sponsors: DOE, Caterpillar
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Background Motivation
Better predictive power: LES is potentially capable of capturing highly transient effects and more flow structures
New analysis capability: LES is more sensitive to initial and boundary conditions than RANS such that it is better suitable for studying cyclic variations and sensitivity to design parameters.
Primary components Turbulence model: a one-equation non-viscosity model called
dynamic structure model for subgrid scale stresses Scalar mixing models: a dynamic structure model for subgrid
scale scalar flux and a zero-equation model for scalar dissipation
Combustion model: a flamelet time scale model
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Large Eddy Simulations
u
x
Actual u
LES averaged u
RANS averaged u
Spatial filtering
Filtering of non-linear terms in Navier-Stokes equations results in subgrid scale terms needed to be modeling
i i iu u u
ij i j i ju u u u
Smagorinsky model use eddy viscosity
Dynamical structure modelone equation modelk: sub-grid turbulent kinetic energyCij :dynamically determined tensor coefficient
ij t ijS ij ijc k
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Flamelet Time Scale Combustion Model
Overview Flamelet mixture fraction approach: each species is a function of
mixture fraction and stretch rate , this functional dependence is solved using a 1-D flamelet code prior to the CFD computation
Use probability density function (PDF) to obtain mean values
Modification for slow chemistry using a time scale
Additional features PDF of mixture fraction is constructed from its first and second
moment which are solved from LES transport equations LES sub-grid model for scalar dissipation helps to construct PDF of
stretch rate
*i i iY YY
t
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*
0 0
( , ) ( , )q
i iY Y P d d
( , )i iY Y
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Jet Flame Tests (Sandia Jet Flames)• Sandia piloted flames are simulated to validate models• A coarse grid is used: 15cm x 15cm x 60cm, about 230,000 cells• Instantaneous temperature fields are presented below• Black curves represent stoichiometric mixture fraction• Reynolds number at fuel jet for flame D = 22,400• Reynolds number at fuel jet for flame E = 33,600
flame D A Relatively stable flame
flame E Significant local extinctions result in lower temperature
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Engine Test Case (Caterpillar Diesel Engine)
Cylinder bore X stroke (mm) 137.6 X 165.1Displacement volume (L) 2.44Compression ratio 15.1Engine speed (rpm) 1600% Load 75START OF INJECTION -9 ATDCDuration of injection (degree) 21
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-10 -5 0 5 10 15 20 25 30 35 40
CA [oATDC]
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Experiment
Simulation
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-10 -5 0 5 10 15 20 25 30 35 40CA [
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Experiment
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Mixture fraction Mixture fraction variance
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Summary and Future Work A flamelet time scale combustion model was
integrated with LES dynamical structure turbulence and scalar mixing models
Model results agreed well with experiments of jet flames and a diesel engine
More accurate spray models are to be integrated with LES turbulence and scalar mixing models
More precise initial and inflow conditions are to be generated for LES simulations