STEADY NATURAL CONVECTION IN A TILTED LONG CYLINDRICAL ENVELOPE WITH LATERAL ADIABATIC SURFACE, PART 1: THEORETICAL MODELING AND NUMERICAL TREATMENTS

Abstract

The steady natural-convection heat transfer in a tilted cylindrical envelope with constant but different end temperatures (300 and 80 K) is investigated numerically with the lateral surface being adiabatic. The inner diameter of the envelope is 27.8 mm and its length/diameter ratio is 9. This is supposed to be a simplified model for the pulse tube in a pulse-tube cryocooler when the pulse tube is positioned at different orientations. The problem studied is a typical nonlinear one in that the thermophysical properties of the working fluid (helium) vary significant om the hot end to the cold end. Three-dimensional steady-state governing equations are solved with fully variable thermal properties. The high nonlinearity of the problem leads to many special characters of the convergence process, and a very peculiar convergence process is found. Initial-field dependence is also revealed. After quite a few preliminary computations, a series of convergence criteria are proposed. Grid-independence examination is conducted for inclination angle of 110°. It is found that the grid system of 20 ( r ) 2 20 ( } ) 2 80 ( Z ) with grids in the z direction being nonuniformly positioned can obtain a grid-independent solution. Preliminary computations are conducted for the horizontal position with 70°C of end temperature difference. The predicted velocity and temperature distributions are compared with available measured data. Good agreement between the predicted and measured results provides strong support for the physical model and numerical treatments developed in this article.