Investigation of the effects of the hollow skeleton on the melting peocess in copper foam

Abstract

Compositing the solid-liquid phase change material with the metal foam is an effective way to improve the heat transfer performance of the latent heat thermal energy storage system. In this paper, the three-dimensional numerical structure of the copper foam is reconstructed by using the micro CT, and then the pore-scale numerical simulation of the melting process in a cubic cavity filled with the phase change material composited with the copper foam is performed via the lattice Boltzmann method. The effects of the hollow skeleton on the melting process are discussed in detail under different Rayleigh numbers and ratios of thermal conductivity between the copper foam and the phase change material. The results show that, compared with the solid skeleton copper foam, the hollow skeleton copper foam leads to a lower average Nusselt number along the left wall at the early stage of the melting process, together with a slower melting rate and a higher energy storage efficiency η. Compared with the skeleton region of the copper foam, the heat transfer rate entering the cubic cavity through the hollow region of the skeleton is almost negligible. Because of the competition between heat conduction and natural convection, the heat transfer enhancement efficiency of copper foam ζ first increases, then decreases, and then increases again with the increase of the Fourier number. When the Rayleigh number decreases, the energy storage efficiency η increases, and the natural convection also weakens. Meanwhile, the fluctuation of the heat transfer enhancement efficiency ζ decreases as the Fourier number increases, and the gap of the heat transfer enhancement efficiency ζ between the hollow and solid skeleton copper foams tends to be smaller. When the ratio of the thermal conductivity between the copper foam skeleton and the phase change material kλ increases, the energy storage efficiency η is relatively high at the early stage of the melting process but becomes relatively low when the melting process is completed. With a larger thermal conductivity ratio kλ , the heat transfer rate entering the cubic cavity through the skeleton region of the copper foam becomes dominant, which reduces the effect of the hollow skeleton on the heat transfer, and thus the gap of the heat transfer enhancement efficiency ζ between the hollow and solid skeleton copper foams becomes relatively small.