An enthalpy method is employed to solve transport processes associated with the effect of density change on melting of unfixed rectangular phase change material (PCM) under low-gravity environment. This method permits the phase-change problems to be solved within fixed numerical grids, hence eliminating the need for coordinate transformation. The PCM, initially and its melting temperature, is placed inside a rectangular enclosure. The lower surface of the enclosure is then exposed to a uniform temperature higher than the PCM melting temperature. The difference in densities of solid and liquid causes a force imbalance on the solid. In the case where the density of the solid phase exceeds that of the liquid, the solid continually moves downwards as melting progresses and hence generates a flow field within the liquid. The problem is formulated as a one-domain problem and the effect of density change at the solid/liquid interface is treated via source and sink terms at interface and the outflow boundaries. The governing equations are discretized by using a control-volume-based finite difference scheme. The results are presented in form of parametric study of the effects of solid/liquid density ratio, Archimedes number, Stefan number, Prandtl number and the geometric parameters on the melt thickness, the downward solid velocity, the elevation of the top surface and the volume of the solid PCM. In general, the effect of solid/liquid density ratio is small on melting characteristics in low-gravity environment.

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