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LES Combustion Modeling for Diesel Engine Simulations

LES Combustion Modeling for Diesel Engine Simulations. Bing Hu Professor Christopher J. Rutland Sponsors: DOE, Caterpillar. Background. Motivation Better predictive power: LES is potentially capable of capturing highly transient effects and more flow structures

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LES Combustion Modeling for Diesel Engine Simulations

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  1. LES Combustion Modeling for Diesel Engine Simulations Bing Hu Professor Christopher J. Rutland Sponsors: DOE, Caterpillar

  2. 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

  3. Actual u u LES averaged u RANS averaged u x 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

  4. 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

  5. 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

  6. 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

  7. 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

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