Large Eddy Simulation Of Turbulence Characteristic Through Intake Process Of Internal Combustion Engines

The transient three-dimensional turbulent in cylinder flow of internal combustion engine has been widely studied since it affects directly and strongly the charging efficiency, heat and mass transfer, fuel atomization, mixture formation and combustion as well as emission characteristics of the engine, which in turn determine the dynamic performance and fuel economy.The purpose of this thesis is to characterize the fluid dynamic behavior of the intake process of IC engines by the large eddy simulation (LES) approach. Firstly, the flow through a valve into a steady combustion chamber was simulated by using three different sub-grid scale (SGS) models, namely, the Smagorinsky-model, Dynamic Smagorinsky-model and Kinetic-Energy Transport model. LES results exhibited a finer flow structure and significantly better agreement with velocity measurement than the RANS solutions. It is also shown that the Kinetic-Energy Transport sub-grid scale model is better than the other two SGS models in the simulation and the Samagorinsky model is the worst and could scarcely predict the microstructure of the flow field.Subsequently, three-dimensional simulation of the transient inlet flows in the inlet-valve-cylinder assemblies of two model IC engines, namely an axis-symmetrical one-valve cylinder model and a two valve test engine, were conducted using the LES method. An unstructured dynamic mesh approach and the dynamic Smagorinsky SGS model were adopted. The in-cylinder flow structure and turbulence viscosity distribution as well as their temporal evolutions were obtained through numerical computations.Comparisons between experimental data and predicted results of the one-valve LES show a satisfactory agreement. Compared with RNAS model, LES is superior in predicting more accurately the transient structures and random characteristics of complex in-cylinder turbulent flows. The prediction of the two-valve model also presents good performance of the LES, however, compared with the experimental data, the computed mean velocity shows some delay in the phase of the peak value., The reason for this disagreement should be attributed to the differences between the simulated and measured engine configurations.The predicted results of this thesis demonstrate the superiority of the LES, which should be a powerful tool for designing diesel intake system, organizing and optimizing fuel injection and combustion of internal combustion engines.