Large Eddy Simulation of Turbulent Flow and Combustion in HCCI Engines

Sammanfattning: This thesis deals with numerical simulations of the turbulent combustion process in Homogeneous Charge Compression Ignition (HCCI) engines. An accurate and computationally efficient Large Eddy Simulation (LES) model was developed and used throughout this thesis to investigate the development of in-cylinder turbulence, temperature stratification, onset of auto-ignition, and development of the reaction fronts. Compared with the conventional Reynolds averaged Navier-Stokes (RANS) approaches, LES has the potential of capturing the fine spatial and temporal structures in engine combustion chambers. Yet, there are several difficulties when applying LES to engine flows. It is often not possible to have the fine resolution needed for some of the flow scales in the cylinder, for example in the near-wall regions. In addition to the difficulty of resolving the flow scales there is a lack of a numerically accurate and affordable method for coupling the detailed auto-ignition chemical kinetic to the flow field simulations. An efficient auto-ignition model is developed based on parameterization of auto-ignition history obtained from detailed chemical kinetics calculations. The model is implemented to the LES solver and used to improve the understanding of the fundamental physical and chemical process in HCCI engines. First, an experimental engine with a rectangular shaped combustion chamber operating at low speed was chosen as a test case for validation of the LES model. Fairly good agreement between the LES results and the PIV (particle image velocimetry) experiments are found with respect to the cycle averaged mean flow field and turbulence fluctuations. Several spatial and temporal average methodologies were examined based on one single cycle LES data to characterize the mean flow and turbulence. The LES model is then used to study several HCCI experimental engines that have realistic cylinder geometry and engine speed. With very high spatial and temporal resolution the LES model successfully simulated the development of flow structures during the different strokes of the engine cycle. The effect of intake gas, residual gas, wall temperature, and piston geometry on the turbulent flow and temperature stratification was quantified. The LES revealed fundamental aspects of HCCI combustion, and its dependence on engine cylinder geometry, cooling, and operational conditions such as intake gas preheating. The LES results indicated that the effect of the geometry is not to alter the production of in-cylinder turbulence, but instead, to affect the heat transfer between the in-cylinder gas and the bowl wall. Compared with flat piston engines, the bowl-in-piston engine generates a high level of temperature stratification in the cylinder which leads to an earlier auto-ignition. By controlling the intake gas temperature to obtain the same auto-ignition timing, it was found that the combustion duration was increased with the bowl-in-piston as compared to a flat piston. This phenomenon was first observed in HCCI engine experiments. With the present LES study a clearer understanding of the flow-heat transfer-reaction coupling was obtained. Systematic LES studies carried out in this thesis showed that the effect of turbulence can be important for the formation and destruction of the temperature stratification in the engine cylinders. Turbulence can decrease the temperature inhomogeneity in the bulk flow far away from the walls. Therefore residual gas/intake gas induced temperature stratification is suppressed by strong turbulence eddy motion. On the other hand, turbulence eddy motion in the wall boundary layers is responsible for the generation of temperature stratification in the cylinder. Turbulence was found to be able to affect the reaction front propagation directly in HCCI engines under conditions that the large eddies and the hot reaction zones are comparable in size.