Melting of phase change materials inside metal foams with uniform/graded porosity: Pore-scale simulation

Abstract

Latent heat thermal storage system has the potential for industrial applications to reduce excessive consumption of fossil fuels. However, the commonly employed phase change materials (PCMs) have the drawback of poor heat transfer capability, causing slow charging/discharging rates for thermal storage systems. To enhance the melting rate of PCM, metal foams (MFs) with good thermal conductivity were exploited, focusing on the contribution of porosity gradients in foam structures to the melting characteristics. Based on tetradecahedron cells, geometric models of MFs with uniform porosity or porosity gradients were created, including negative gradient foam (NGF), homogeneous foam (HF), and positive gradient foam (PGF). Pore-scale numerical simulations (PNS) based on the tetradecahedron cells with porosity gradients were carried out. Results of PNS demonstrated that the complete melting time for the NGF, the HF and the PGF were 92 s, 92 s and 132 s, respectively. However, the NGF had the fastest integrated average temperature response rate (IATRR). Specifically, compared with the HF, the IATRR of NGF increased by 15.4%, while that of the PGF decreased by 36.1%. In addition, natural convection enhanced the heat transfer efficiency between the PCM and the metallic skeletons of the MF. The impact of natural convection on the phase transition process was analyzed from the microscopic pores. Under the influence of natural convection, the top PCM melted faster than the bottom PCM in the z-direction.