Numerical Analysis of Developing Turbulent Flow in a Square-Sectioned Bend. Prediction of Turbulent Flow with the Secondary Flow of the First and the Second Kind.

Abstract

A numerical analysis has been performed on developing turbulent flow in a square-sectioned 180 deg bend. The ratio of bend mean radius of curvature to hydraulic diameter is 3.35 and straight ducts of 31 hydraulic diameter long are attached to the inlet and outlet planes of the bend. In calculation, an algebraic stress model was adopted in order to predict Reynolds stresses precisely, and a boundary fitted coordinate system was introduced to briefly set boundary conditions. Calculated results were compared with the experimental data available. Moreover, the transition from the secondary flow of the second kind to that of the first kind and vice versa are examined. It was found that the present method predicted the mean flow velocity with a minimum at 90 deg of bend without much discrepancy. In a straight duct of a bend inlet, the secondary flow of the second kind transforms into that of the first kind at 1.0∼1.6 hydraulic diameter upstream from the bend inlet. In contrast, in a straight duct of a bend outlet, the secondary flow of the first kind is accelerated downstream from bend outlet by increasing the pressure gradient of the square cross section. After this acceleration, the secondary flow of the first kind decays and gradually transforms into the secondary flow of the second kind.