Numerical Simulation Of In-Cylinder Turbulent Flows Of Internal Combustion Engines

The in-cylinder flow field and the turbulent characteristics are the base for the research of working processes of internal combustion engines. Gas flows in the cylinder of internal combustion engines are invariably complicated three-dimensional and turbulent. Numerical simulation and accurate prediction of turbulent flows are a problem that has not still been solved satisfactorily so far. Through comparing the modified eddy-viscosity models (EVM) with algebraic stress models (ASM), investigations have indicated that the modified eddy-viscosity models (EVM) are simpler in formulation, and need lower cost for computation than the algebraic stress models (ASM). Furthermore, they can provide reasonable predictions, and hence are suitable to engineering applications. This thesis performs a comparative study on three versions of eddy-viscosity models (EVM), i.e. conventional k-εmodel, RNG k-εmodel and realizable k-εmodel. 2-D and 3-D numerical computations are carried out for two different configurations of combustion chamber with the three k-εmodels. Emphasis is put on the realizable k-εmodel. The main works completed in the thesis is as follows:A concise overview of the turbulence models applicable to internal combustion engines is given. The theoretical basis of the three versions of eddy-viscosity models (EVM) is presented. Based on the engine CFD code KIVA-3V, a subroutine for implement of computation for the realizable k-εmodel and corresponding modifications are introduced into the original computer program.Computational results and comparisons among the three models show that the RNG k-εmodel is improved from the conventional k-εmodel, while realizable k-εmodel produced the best results among the three models. This should be attributed to fact that the realizable k-εmodel contains a non-constant expression for turbulent viscosity, in which to the effects of swirling and curvature are taken into account.It is observed that the geometry of the combustion chamber has a negligible effect on the flow characteristics in the initial phase of the compression stroke, but exerts a significant influence as TDC is approached. Different swirl rates have some effects on turbulent flows. There is some influence in the operating mode of internal combustion engines clue to different rpm.