MELTING OF PHASE CHANGE MATERIALS INSIDE A VERTICAL CYLINDER, WITH NATURAL CONVECTION IN THE MELT

The melting of a phase change material (PCM) inside a vertical cylinder and its possible application to problems in the design of heat storage units using PCMs was studied. The melting process is almost always accompanied by convective motion in the melt, with convective heat transfer becoming the dominant mode after a very short initial period during which conduction is the dominant mechanism.;A correlation of the form.;(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI).;for the average heat flux for the overall melting process was developed from the experimental and theoretical values of Q(,av).(' ).;A model describing the unsteady state melting heat transfer to a vertical cylinder with an adiabatic top and bottom was developed. The finite difference forms of the vorticity-stream function equations, the energy equation and th equation for the energy balance at the solid-liquid interface in transformed cylindrical co-ordinates, were solved using a successive overrelaxation method. The timewise variation of the average dimensionless heat flux to the cylinder, the isothermals and the interface shapes clearly demonstrate the important role played by natural convection in the phase change process, which was neglected in the models developed earlier. Numerical solutions for Rayleigh numbers between 2 x 10('4) and 80 x 10('4), and aspect ratios 1, 2 and 4 were obtained. Prandtl numbers between 31 and 140 affected the heat flux curves very little. Incorporation of the Stefan number into the dimensionless time reduced the number of parameters in the study by one.;The experimental measurements of the total melting times of.;SUNOCO P-116 was agreed well with the values predicted by the.;model. A qualitative verification of the model results was also done.;by visual studies and the transient responses of the thermocouple probes embedded in the wax and in sodium thiosulfate pentahydrate.;A simplified model for the transient behavior of a heat storage unit (HSU) with a PCM inside vertical cylinders was also developed. The correlation for the overall melting process predicted quite well, the transient response of the HSU. The conditions inside the HSU were more like mixed flow, characterised by low Peclet number. The correlation also was used successfully to predict the total melting times in the HSU.;The primary conclusion reached in this work is that the free convective motion in the liquid is the most important mechanism in the melting of solids, and hence can not be neglected in the design of heat storage systems using PCMs.