Natural convection onset during melting of phase change materials: Part III – Global correlations for onset conditions

Abstract

The melting of n-octadecane for ΔTm (the difference between the melting temperature of the PCM and the heating temperature) of 8.44, 16.9, 25.3, 33.8, 42.2 °C and dodecanoic acid for ΔTm of 8.44, 25.3, and 42.2 °C in horizontal annular enclosures heated from 18, 27, and 36-mm center-tubes was studied experimentally. The phase change materials (PCMs) were initially subcooled by 2.5, 7.5, 15, and 22.5 °C. The effects of geometry, Stefan number, and subcooling of the PCMs on the onset of natural convection have been reported in Part I of this paper. In Part II, the effects of Fourier, Grashof, and Rayleigh numbers have been presented. In this last part, a thorough dimensional analysis was carried out to identify the dimensionless groups that would affect the onset of natural convection. Global correlations were then developed to directly estimate the amount of liquid PCM required for the onset of convection based on PCM properties, geometry and operational conditions. The other global correlation presented in this paper can be used to determine how fast the onset of convection would occur.