Numerical and experiment studies on convective phenomena in materials processing systems.

摘要

A fundamental study on convective transportation phenomena in material processing systems is presented. The study is focused on two aspects of the convection, Rayleigh-Benard-Marangoni instability of convective flow in Czochralski crucibles and magnetic damping of natural convection in rectangular boxes.; Changes in melt flow patterns, such as transition from steady state flow to time dependent flow and breaking up from axisymmetric base flow, have a strong effect on the crystal structure. A linear stability analysis of the convective flow in Czochralski crystal growth systems is carried out to investigate the changes in the flow patterns. A numerical model of the melts flow in the Czochralski crucible is developed using high order finite difference method. The stability analysis is based on the solution of linearized governing equations in a cylindrical coordinates system. The perturbation equations are discretized using the high order finite difference scheme and the resulting eigenvalue problem is solved by linear fractional transformation. Both radiation participating and non-participating fluids are considered in the stability analysis to reveal how the internal radiation may affect the stability of the melt flow. The radiative transfer equation is solved using discontinuous finite element method. The discontinuous finite element model is coupled with the high order finite difference model via the heat source term resulting from the internal radiation. The results suggest that the internal radiation changes the temperature field and therefore the convective flow structure significantly.; The damping effect of a magnetic field on the thermally-induced convection in a rectangular box is investigated experimentally using molten gallium as working fluid. The velocity and temperature fields are measured using constant-temperature hot-film anemometry and a thermocouple. The hot-film anemometer is calibrated over a narrow temperature range using a rotating container filled with gallium following the technique outlined by Sajben. The velocity and temperature profiles are measured with and without the magnetic field. Numerical simulations are also performed and found to be in good agreement with the experimental results. The damping effect of the magnetic field occurs in both the temperature and the velocity profiles and increases as the strength of the magnetic field is increased.