A Numerical Study to Investigate the Heat Transfer and Thermodynamic Performance of a Natural Convection Driven Thermal Energy Storage System

In this work numerical experiment was performed for studying the heat transfer and thermodynamic performance of the melting process in a bottom heated square cavity. The bottom wall is maintained at a temperature higher that the melting temperature of the PCM, while all other walls are perfectly insulated. The transient numerical simulations were performed for melting Gallium, (a low Prandtl number PCM with high latent heat to density ratio) at moderate Rayleigh number (Ra ≊ 10⁵). The transient numerical model consist of solving coupled continuity, momentum and energy equation in the unstructured formulation using the PISO algorithm. In this work, the fixed grid, source-based enthalpy-porosity approach has been adopted. The heat transfer performance of the melting process was analyzed by studying the evolution of global fluid fraction, Nusselt number at the hot wall, volume averaged normalised flow kinetic energy with time. The thermodynamic performance is analyzed by calculating local entropy generation rates considering both irreversibility due to finite temperature gradient and viscous dissipation. The values of second law efficiency clearly shows that the current thermal design of the phase-change heat accumulators are very close to the ideal design.