LES Combustion Modeling for Diesel Engine Simulations

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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

• Spatial filtering
• Filtering of non-linear terms in Navier-Stokes
  equations results in subgrid scale terms needed
  to be modeling

• Dynamical structure model
• one equation model
• k sub-grid turbulent kinetic energy
• Cij dynamically determined tensor coefficient

• Smagorinsky model
• use eddy viscosity

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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

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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 E Significant local extinctions result in
lower temperature
flame D A Relatively stable flame
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Engine Test Case (Caterpillar Diesel Engine)

• Cylinder bore X stroke (mm) 137.6 X 165.1
• Displacement volume (L) 2.44
• Compression ratio 15.1
• Engine speed (rpm) 1600
• Load
  75
• START OF INJECTION -9 ATDC
• Duration of injection (degree) 21

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