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

2. 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. 3 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 Smagorinsky model  use eddy viscosity Dynamical structure model  one equation model  k: sub-grid turbulent kinetic energy  Cij :dynamically determined tensor coefficient

4. 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. 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 D A Relatively stable flame flame E Significant local extinctions result in lower temperature

6. 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. 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