A mixing controlled direct chemistry (MCDC) model for diesel engine combustion modelling using large eddy simulation

Abstract

A mixing controlled direct chemistry (MCDC) combustion model with sub-grid scale (SGS) mixing effects and chemical kinetics has been evaluated for Large Eddy Simulation (LES) of diesel engine combustion. The mixing effect is modelled by a mixing timescale based on mixture fraction variance and sub-grid scalar dissipation rate. The SGS scalar dissipation rate is modelled using a similarity term and a scaling factor from the analysis of Direct Numerical Simulation (DNS) data. The chemical reaction progress is estimated from a kinetic timescale based on local internal energy change rate and equilibrium state internal energy. An optical research engine operating at conventional operating conditions and Low Temperature Combustion (LTC) conditions was used for evaluation of the combustion model. From the simulation results, the effect of SGS scalar mixing is evaluated at different stages of combustion. In the context of LES, the new approach provides improved engine modelling results compared to the Direct Chemistry Solver (DCS) combustion model.

Keywords:

• scalar dissipation rate
• detailed chemistry
• large eddy simulation
• diesel engine combustion
• kinetic timescale

Acknowledgements

This study is supported by the US Department of Energy under contract DEFC26–06NT42628. The authors also heartily thank Joshua Leach and Dr. Youngchul Ra for computer support and inspiring discussions respectively.