AbstractSimultaneous melting and solidification of a phase-change material around two horizontal cylinders spaced vertically have been investigated experimentally. Experiments have been performed to examine the effects of the cylinder surface temperature difference and the spacing between the cylinders. The solid/liquid phase-change process has been clarified from observations of timewise variations in the solid/liquid interface, flow visualization, and measurement of temperature distributions in the phase-change material. The results show that the melt shape and the solid/liquid interface motion are very sensitive to the surface temperature of the lower cylinder and the spacing between the cylinders