k- ε- τ 亂流모델을 利用한 엔진 실린더내 流動에 관한 數値적 硏究

Abstract

A Numerical Study on In-Cylinder Flow Fields of an Engine

Using Turbulence Model

This thesis describes and discusses the applicability of the turbulence model for calculation of the in-cylinder flow fields of an engine. The thesis also discusses the effects of swirl and valve seat angle to the characteristics of in-cylinder flow fields.

The equations are solved by finite difference method on a computational mesh which is made to always lie between the cylinder head and the moving piston head by defining a coordinate transformation which allows the axial grid line position to be expressed in terms of a time independent coordinate. The transformed conservation equations are integrated over the finite difference cells to provide algebraic equations. A hybrid differencing scheme is employed for numerical stability and PISO algorithm is used for the velocity-pressure coupling. turbulence model which considers the compressibility effect due to the compression and expansion of the piston was used.

The predicted results using turbulence model of the turbulent flow fields in a model engine are compared to those from the modified turbulence model and the experimental data. The results obtained with the turbulence model are in much better agreement with the experimental data than the modified turbulence model, as far as the mean velocity and the turbulence intensity are concerned.

Finally the effects of swirl on the in-cylinder flow structure are examined through the parametric study of swirl numbers 0.0, 0.6, 1.2 and 2.4, then the effects of valve seat angle are examined. As the swirl number increases the center of the main vortex moves to the cylinder wall and the counterclockwise vortex increases near the intake valve. The turbulence intensity increases with swirl number during intake stroke, but it has a maximum value at swirl number 1.2 during compression stroke.

For the valve seat angle of 45° or more the flow pattern remains same but for the valve seat angle of 30° an alternative structure appears, in which the outer edge of the jet does not separate from the cylinder head wall, thus diminishing the corner vortex.