Experimental investigation on combined close-contact and natural convection melting in horizontal cylindrical and spherical capsules

Melting and solidification of phase change material (PCM) in a capsule is of practical importance in the latent heat thermal energy storage (LHTES) systems which are useful for leveling of electricity demand in the urban areas and also for energy conservation. Two heat transfer modes occur with melting in capsules. One is close-contact melting mode between solid bulk and capsule, and another is natural convection heat transfer in the liquid region. The paper reports the experimental results on combined close-contact and natural convection melting in both horizontal cylindrical and spherical capsules immersed in a high temperature environment. Close-contact melting heat-transfer characteristics including natural convection in the liquid region were studied experimentally. Elucidation of such a heat-transfer mechanism is useful for practical heat storage systems employing the LHTES. The melting shape and the complete melting time under various ambient temperatures were observed. In addition the effect of variation of the inner wall temperature and molten mass fraction on melting characteristics were investigated experimentally. Efforts have been devoted to clarify the mechanism of close-contact heat transfer for a single element with various capsule shapes (e.g. Bareiss and Beer, Moallemi and Viskanta, Saitoh and Kato). However, there is no exact numerical simulation considering both close-contact and natural convection melting processes within the capsule. A comparison is made with some numerical and analytical results