Thermal analysis in phase change materials (PCMs) embedded with metal foams
The heat transfer enhancement for phase change materials (PCMs) has received increasing attention nowadays, since most of PCMs have low thermal conductivities which prolong the charging and discharging processes. Metal foams, as a sort of novel material with high thermal conductivity, are believed to be a promising solution to enhance the heat transfer performance of PCMs for thermal energy storage systems. The effects of natural convection on heat transfer enhancement for PCMs embedded with metal foams are investigated in this paper. The numerical investigation is based on the two-equation non-equilibrium heat transfer model, where the coupled heat conduction and natural convection in PCMs are considered at phase transition and liquid zones. The numerical results are validated by experimental data. In order to investigate the effect of metal foams on heat transfer, two different cases are compared in this study, which are the Case A (PCMs embedded with metal foams) and the Case B (pure PCMs). At the solid zone, heat conduction plays a dominant part because of natural convection's absence, thus metal foams achieve much higher heat conduction rate than pure PCMs, and this can be attributed to the high thermal conductivity of metal foams skeleton and the heat can be quickly transferred through the foam solid structure to the whole domain of PCMs. At the two-phase zone and liquid zone, natural convection takes place and becomes the dominant heat transfer mode, but metal foam structures suppress the natural convection inside the PCMs owing to big flow resistance in metal foams. In spite of this suppression caused by metal foams, the overall heat transfer performance of Case A is still superior to the counterpart of Case B (pure PCMs), implying the enhancement of heat conduction offsets or exceeds the natural convection loss. The results show that the heat transfer enhancement due to the natural convection in PCMs embedded with metal foams is not as strong as expected, since metal foams have big flow resistance and the natural convection is suppressed. It also shows that better heat transfer performance can be achieved by using the metal foams of smaller porosity and bigger pore density. Last but not least, a series of detailed velocity and temperature profiles are given through numerical solutions, in order to present a vivid evolution of flow field and temperature profiles in the whole melting process.