Natural convection onset during melting of phase change materials: Part I – Effects of the geometry, Stefan number, and degree of subcooling

Abstract

Natural convection plays a dominant role in the melting dynamics of phase change materials (PCMs). However, natural convection is not present at the beginning of the melting process of a fully solid PCM. Instead, it starts playing a role after some PCM has melted by conduction. This study was undertaken to explore the conditions for the onset of natural convection in subcooled PCMs in a 2-dimensional experimental system. Melting of n-octadecane and dodecanoic acid in horizontal cylindrical enclosures with center-tube diameters of 18, 27, and 36 mm were studied for PCMs initially subcooled by 2.5, 7.5, 15, and 22.5 °C. At these conditions, the difference between the melting temperature of the PCMs and the center-tube wall temperature, ΔTm, was maintained at 8.4, 16.9, 25.3, 33.8, and 42.2 °C. The melt volume at the onset of natural convection was found to increase with the center-tube diameter, with shorter times and less liquid PCM required for the onset of convection as the ΔTm -based Stefan number increased. Subcooling of the PCMs delayed the occurrence of convection onset, with a geometric, ΔTm -based Stefan number, and PCM dependence observed for the impact of subcooling on melt volume needed for the onset of convection.