Effects of gradient porous metal foam on the melting performance and energy storage of composite phase change materials subjected to an internal heater: A numerical study and PIV experimental validation

This work aims to explore the effects of gradient porous metal foam on the 3D melting heat transfer as well as the heat storage performance of a latent heat thermal energy storage (LHTES) unit inside an internal heated cubic cavity. A modified structure of metal foam with gradient porosity of three stratification layers is proposed to optimize the melting characteristic of the composite phase change materials. The enthalpy-porosity method is adopted to model melting process, and the Darcy-Forchheimer law and local thermal non-equilibrium model are assumed for metal foam. The numerical code of proposed model is in good consistency with the experimental results of particle image velocimetry (PIV) visualization. The effects of porosity gradient with equal and non-equal stratification heights, as well as pore per inch (PPI) on the melting performance, heat transfer and heat storage are investigated. The results indicate that the competitive relation between the conduction and convection induced by the gradient porosity influences the heat transfer and energy storage. The energy storage of negative gradient is greater than that of positive gradient, but the average rate of energy storage of the former is lower than that of the latter. In the middle melting stage, the total average Nusselt number increases with larger porosity gradient for positive cases and decreases for negative cases. With the increase of the stratification height of middle layer, the heat transfer efficiency of positive cases weakens, while it is enhanced for negative cases. Moreover, the increase of PPI can reinforce the conduction effect and improve the average rate of energy storage of the LHTES unit for both positive and negative cases. (c) 2021 Elsevier Ltd. All rights reserved.