Diesel combustion modelling using LES turbulence model with detailed chemistry

Abstract

Diesel spray combustion and emissions are modelled in this study using large eddy simulation (LES) turbulence models coupled with spray breakup and detailed chemistry models. The objective of this study is to develop numerical models that can be used to predict the diesel spray combustion process. The LES filtering procedure yields unknown subgrid scale terms that need to be modelled. A one-equation, non-viscosity dynamic structure model is used to model the subgrid stress tensor and the gradient method is used to model the subgrid scalar flux. The model uses a tensor coefficient determined by a dynamic procedure and the sub-grid kinetic energy that is to enforce a budget on the energy flow between the resolved and unresolved scales. A skeletal n-heptane reaction mechanism is used to simulate the diesel fuel chemistry. A reduced NO _x mechanism is incorporated into the n-heptane mechanism to simulate NO _x formation, and the soot emission process is simulated by a phenomenological model that uses a competing formation and oxidation rate formulation. The present models were validated by comparing predicted results against experimental data of a diesel engine. The predicted and measured cylinder pressure history and heat release rate data are in good agreement. Trends of NO _x and soot emissions are also captured with respect to different injection timings and exhaust gas recirculation (EGR) levels. Results indicated that the present models can capture the overall combustion process and can be further developed into a useful tool for engine combustion simulations.

Keywords:

• diesel spray combustion
• LES
• detailed chemistry

Acknowledgements

The authors acknowledge the financial support of Ames Laboratory, U.S. Department of Energy. The authors also thank Dr Christopher J. Rutland of University of Wisconsin–Madison for his contribution and help in the present LES models.