Thermal interaction between water and liquid paraffin wax inside an intertwined enclosure: The role of natural convection in design of latent heat storage units

This work numerically investigated free convective heat transfer between water in a tank and melted paraffinwax in an enclosure mounted inside the tank. The mediums were separated by thermally conductive walls. Three aspect ratios (AR = 0.3, 0.5, and 0.7) and three Rayleigh numbers (Ra = 104, 105, and 106) were examined for side- and bottom-heating orientations. The computations were conducted separately for the convection and conduction cases to observe the effects of natural convective heat transfer between the enclosures of the storage tank. The results showed that the heating orientation had a remarkable impact on the heat transfer between two fluids inside the intertwined enclosures. In the side-heating orientation, the variation of the average Nusselt number (Nu) with respect to Ra did not exhibit a consistent trend. It increased by 23.1 % when Ra increased from Ra = 104 to 105, but it decreased by 52.8 % when Ra was further increased from Ra = 105 to 106, at AR = 0.5. The Nu along the bottom wall of the inner enclosure was up to 24-folds of that along its side walls for the sideheating orientation and was 2.45-folds in the bottom-heating orientation. Furthermore, accumulation of hot fluid around the inner cavity walls, which occurred in side-heating cases, reduced convective heat transfer because of the low temperature gradient in these regions. Considering the outcomes of the present work, the bottom-heating orientation with AR = 0.5 can be recommended for future applications involving phase change processes as it ensured an effective thermal interaction between the two fluids.