A comparative study of the effect of wall heat flux on melting and heat transfer characteristics in phase change material thermal energy stores arranged vertically and horizontally

An experimental investigation was carried out to analyse the heat transfer characteristics during melting of phase change material (PCM) RT44HC in thermal energy stores arranged vertically and horizontally to assess the effect of different values of wall heat flux on the development of melt fraction, solid/liquid interface location and temperature distribution. A test cell consisting of a rectangular cross section enclosure was constructed from polycarbonate sheet, copper plates and mica heaters. Both left and right sides of the store were subject to uniform wall heat fluxes of 675, 960 and 1295 W/m2. Thermocouples were used to measure the temperature at different locations inside the phase change material (PCM) and on the surface of the copper plates. The study included visualization of the melting process and measurement of the temperature distribution at the vertical mid-plane of the store. Images of the melting process were analysed with an image processing technique to determine the melt fraction at selected times. It was observed that as the heat flux input increased the total melting time reduced, the reduction in melt time for the horizontally oriented store was about 12.5-15.0% when compared to the vertically oriented store as a result of the intensification of natural convection flows. Heat conduction was the dominant mode of heat transfer during the early stage of melting, followed by short transition period after which convection dominates during the rest of the melting process. The experimental results provide a set of benchmark data for validation of numerical codes.