Natural convection onset during melting of phase change materials: Part II-Effects of Fourier, Grashof, and Rayleigh numbers

The effects of geometry, Stefan number based on melting temperature differential, and subcooling of a phase change material (PCM) on the onset of natural convection have been reported in Part I of this paper. In this second part, the effects of Fourier, Grashof, and Rayleigh numbers are reported. The melting of n-octadecane and dodecanoic acid in horizontal cylindrical enclosures heated from the center-tubes with diameters of 18, 27, and 36 mm was studied experimentally. The PCMs were initially subcooled by 2.5, 7.5, 15, and 22.5 degrees 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 degrees C for n-octadecane and 8.44, 25.3, and 42.2 degrees C for dodecanoic acid. The experimental results show that the melt volume at the onset of natural convection decreases nonlinearly as the Grashof number increases. Also, the Fourier number at the onset of convection decreases nonlinearly as the Delta Tm-based Stefan number increases. Correlations were developed for the Fourier and Rayleigh numbers at the onset of natural convection. These correlations would allow an estimation of the elapsed time or melt volume at the onset of natural convection for a given geometry and operating temperature.