Direct numerical method for studying heat transfer behaviors in an enclosure with dispersed thermal conductive particles

Numerical simulations in a two-dimensional (2-D) thermal energy storage (TES) unit were carried out to investigate the thermal behaviors of phase change materials (PCMs) - sodium nitrate (NaNO3) inside porous media. The porous media are composed of randomly distributed particles with high thermal conductivity. The effects of heat conduction through particles, natural convection of liquid PCM, and the geometric parameters such as porosity and particle shape were numerically examined. The results show that for the TES dispersed with square particles, the heat transfer coefficient of the system can be increased up to 27-310% for the porosity ranging from 60% to 90%; while for the particles of expanded graphite (EG)-like structure, the heat transfer coefficient of the TES at 60% porosity can be raised as much as 93.3 times. The high weight fraction of particles in the TES could depress the happening of natural convection, especially for EG-like particles. The Rayleigh number considering the permeability of material (RaK) was proposed to evaluate the roles of heat conduction and natural convection. When RaK is lower than 20.6, the heat conduction starts to dominate the heat transfer inside the composite that leads to the total heat performance of the composite with square particles is lower than that of the system with pure liquid PCM.