Three-dimensional numerical study on the melting of composite PCMs with gradient skeleton

The solid-liquid phase change is a highly promising energy storage technology. However, the low thermal conductivity of phase change materials (PCMs) significantly limits the efficiency of energy storage. The addition of metal skeletons is very important in improving the heat transfer properties of the phase change process. In this study, the complex skeleton models with multi-gradient pore structures are generated using the Quartet Structure Generation Set (QSGS) method. Employing a two-zone model of the mushy zone, the three-dimensional lattice Boltzmann method (LBM) is utilized to investigate the solid-liquid phase change of PCMs containing porous skeletons with different directional gradients. The influences of skeleton gradients, including porosity gradient and pore density, on heat and mass transfer and the migration evolution characteristics of the mushy zone are discussed in detail. Results indicate that the direction of the porosity gradient has a significant impact on the phase change process. The horizontal negative gradient enhances thermal conductivity, resulting in a higher melting rate and a thinner mushy zone, whereas a horizontal positive gradient leads to a thicker mushy zone and a lower melting rate. The complete melting time for the horizontal negative gradient skeleton is reduced by 12.8% compared to the horizontal positive gradient skeleton. Additionally, variations in the vertical porosity gradient also affect the melting rate of PCMs, with a vertical negative gradient reducing the complete melting time by 11.4% compared to a vertical positive gradient. The gradient variations in pore density also influence heat transfer in phase change process, and the negative gradient in the horizontal direction enhances heat transfer more effectively than the positive gradient. The enhancing effect of the gradient pore density is further strengthened with increasing porosity. This study will provide a foundation for further exploration of the impact of QSGS skeletons on phase change process.