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Natural convection plays a crucial role in latent heat storage system. Determining how natural convection affects the melting of phase change material (PCM) will provide a better understanding of the melting process and give helpful advice on strengthening techniques. The research content of this paper is a mathematical model is established to explain the melting process of PCM affected by natural convection. The mathematical model of phase change melting process is given by introducing the natural convection and the temperature gradient. A coordination factor (Co) based on the mathematical model, which is an instantaneous quantity and proportional to the heat exchange rate and the intensity of natural convection, is introduced to explain the melting process. A square-shaped and shell-tube (concentric, eccentric) PCM heat storage unit were numerically analyzed in more detail. It is found that the Co factor is closely related to the melting rate. It will not promote the melting rate when Co less than 0 while it will promote the melting rate when Co higher than 0, and the higher the Co value, the faster the melting rate. This model gives a clear and quantitative explanation for the problem of melting interface movement affected by natural convection, as well as that increasing the heating temperature and changing only the shape of the heat storage unit can greatly enhance the heat storage rate in this study. The model also provides theoretical guidance for the study of the phase change heat storage enhancement.

The natural convection of phase change material (PCM) in a two-dimensional square cavity is numerically analyzed. The cavity consists of heat surface with a constant total heated area and adiabatic wall. The Grashof and Prandtl numbers for the PCM (n-eicosane with the melting temperature, Tmelt=36°C) in basic LHS system are 9 × 10^5 and 62.7, respectively, at 350.15K. The mass, momentum and energy balance equations of the system were considered. Three basic heating surface strategies were considered (discrete heat sources): single, side-side and side-bottom heating surface. The results show that the transient Nusselt number, mean kinetic energy at the surface and energy storage rate of the fluid are effectively enhanced by proper arrangement of the discrete heat source location and heating from the bottom half of the left and right sides requires the least time for 300 kJ energy storage. The results indicate that optimally placed discrete heat sources in PCM could be a promising alternative for high-efficient thermal energy storage.