Effects of buoyancy-driven convection on melting within spherical containers

A computational study of the effects of buoyancy-driven convection on const rained melting of phase change materials within spherical containers is pre sented. The computations are based on an iterative, finite-volume numerical procedure using primitive-dependent variables, whereby the time-dependent continuity, momentum and energy equations in the spherical coordinate syste m are solved. A single-domain enthalpy formulation is used for simulation o f the phase change phenomenon. The effect of phase change on convection is accounted for using a Darcy's law-type porous media treatment. Early during the melting process, the conduction mode of heat transfer is dominant, giv ing rise to concentric temperature contours. As the buoyancy-driven convect ion is strengthened due to the growth of the melt zone, melting in the top legion of the sphere is much faster than in the bottom region due to the en hancement of the conduction mode of heat transfer. When buoyancy effects ar e very marked, as many as three time-dependent recirculating vortices are o bserved. In comparison to the diffusion-controlled melting, buoyancy-driven convection accelerates the melting process markedly. The Prandtl number pl ays an important role in the melting process. With the Rayleigh number fixe d, changing the Prandtl number from 0.03 to 1.0 and 50 brings about totally different flow and melting patterns. The computational findings are verifi ed through qualitative constrained melting experiments using a high-Prandtl number wax as the phase change material. (C) 2001 Elsevier Science Ltd. Al l rights reserved.