A Study of Natural Convection Onset and Melting Process of Phase Change Materials in Horizontal Cylindrical Enclosures

Abstract

Phase change materials (PCMs) are a promising medium to store thermal energy in latent heat energy storage systems. While these materials offer a large storage density, their thermal conductivities are low and offer challenges for practical applications. It has been reported by researchers that natural convection plays a dominant role in the melting of the PCMs, the energy-storing phase. However, natural convection does not exist at the beginning of the heating process but rather comes into existence after some PCM melts by conduction first. This study was undertaken to explore the conditions for the onset of natural convection.

Melting of n-octadecane and dodecanoic acid in horizontal cylindrical enclosures with center-tube diameters of 18, 27, and 36 mm were studied. The PCMs were initially subcooled by 2.5, 7.5, 15, and 22.5 °C. The melting temperature differentials (the difference between the melting temperature of the PCMs and the heating temperature) were 8.44, 16.9, 25.3, 33.8, and 42.2 °C for n-octadecane and 8.44, 25.3, and 42.2 °C for dodecanoic acid.

The results indicate that the melt volume at the onset of natural convection increases with the diameter of the center-tube. Also, a lesser amount of liquid PCM is required for the onset of convection when the melting temperature differential is high. It was found that the convection onset occurred faster at higher Stefan numbers. Subcooling of the PCMs delayed the occurrence of convection onset. No such clear trend was observed for the center-tube diameters.

Correlations to predict the moment of onset of convection and the volume of liquid PCM required for the onset of convection were developed. The correlations developed separately for each PCM comparable. A generic correlation was developed from the combined results of the two PCMs that can be used to predict the convection onset occurrence for different geometric and thermal conditions.