Study of low temperature combustion using large eddy simulations

Banerjee, S. Study of Low Temperature Combustion Using Large Eddy Simulations. University of Wisconsin-Madison, 2011.

Low Temperature Combustion (LTC) diesel was investigated using non-viscosity dynamic structure Large Eddy Simulations (LES) turbulence model. In modeling engine flows, LES model is better at providing temporal and spatial details in the mixing field than Reynolds-averaged Navier-Stokes (RANS). The objective of this study is to validate LES sub-models and to investigate an advanced diesel combustion strategy using LES sub-models.

The LES model exhibits sensitivity towards varying Computational Fluid Dynamics (CFD) mesh sizes. To validate LES models, spray induced turbulence is studied. Coherent structures (CS) of this turbulent flow were visualized using eigenvalues of a symmetric tensor derived from deformation strain rate. The temporal evolution of the CS helps to explain the mechanism of fuel-air mixing and importance differences between the LES and the RANS sub-models. It is found that the fuel-air mixing of a fine mesh LES is governed by larger CS breaking down into number of smaller structures over time, while the RANS model fails to predict such mechanism altogether. Additionally the RANS approach fails to provide the intermittencies in the CS that is otherwise obtained from the fine mesh LES results. Based on the CS obtained from the spray induced turbulence study, a set of criteria on CFD grid is proposed for the LES models.

Using LES sub-models, an advanced diesel combustion using mixed mode operation was examined for single cycle diesel mode transition at a moderate load condition. Mode switches were performed from a traditional mixing controlled to an early injection pre-mixed combustion mode and vice versa. Various aspects of combustion during the single cycle mode switch were investigated. It was found that in-cylinder mixture charge preparation and gas exchange processes play a vital role in the combustion control during the mode transition. By moderating the injection schedule and employing internal EGR using VVT, a safe mode transition may be achieved. Additionally by applying intake port water injection to offset intake oxygen, an intermediate mode between traditional diesel and LTC may be obtained that can then be used for an improved mode transition for mixed mode engine operations.