Semester

Spring

Date of Graduation

2000

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Ismail Celik.

Abstract

Turbulence and turbulent mixing are two of the most important factors that influence the efficiency and emissions level in internal combustion (IC) engines, particularly for diesel engines. This study has been performed with the premise to accurately predict in-cylinder turbulence by employing the large eddy simulation (LES) technique. In order to assess the turbulence scales involved correctly, a review of measured and computed scales relevant to IC engines is conducted. An assessment of these is made in comparison with the self-imposed scales of the engine itself. This assessment focuses on the influence of combustion, compression ratio, initial conditions and numerical mesh on predicted turbulence scales. It was found that the turbulence scales predicted by employing the commonly used k-epsilon turbulence model were in good qualitative agreement with experimental observations and could be used as a guide to determine the degree of resolution needed in LES.;To establish a base to improve existing Reynolds averaged Navier-Stokes (RANS) models, a comparative study of the commonly used RANS models applied to IC engines was conducted, using an experimental benchmark case, which is an isothermal, incompressible flow within a piston-cylinder arrangement motored without combustion. This study has lead to a new hybrid turbulence model, namely, the Smagorinsky based eddy viscosity (SEV) model, which is self-adjusting between an eddy viscosity model and subgrid-scale model, depending on the grid size, continuously from RANS to LES. It was tested against the above-mentioned experimental benchmark. The predicted velocity profiles and streamlines are in good agreement with experiments. The new model is a viable alternative to the k-epsilon model in predicting the mean flowfield in IC engines.;Furthermore, the relative importance of the turbulence generation mechanisms in IC engines has been studied using LES. First, the compression and expansion strokes of a piston-bowl configuration are thoroughly investigated using a fine 2-D axisymmetric grid and a 3-D grid. Next, the flow dynamics during the intake stroke has been examined using a full 3-D configuration of an engine with valves and a typical bowl. The results show that a significant portion of the inertial sub-range in the energy spectra can be resolved using a moderately fine grid with about 300,000 vertices. The calculated spectra display about the same energy content up to f ≈ 104 Hz, which is on the same order of maximum frequency resolved in typical experiments. It is shown via the LES technique, that significant turbulence is likely to be generated by a carefully designed bowl; and that the more energetic intake turbulence generated during the intake stroke decays rapidly during the compression stroke.

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