Periodically unsteady thermal response of metal-foam solid composite

Abstract

With the development of materials and manufacturing techniques, using highly conductive metallic porous materials, such as metal foam, metal fiber felt, lattice-frame material, and so on, to enhance heat transfer becomes possible. As a highly-conductive multi-functional material, metal foam owns many good features including high porosity, light weight, large surface area, high strength, etc. To date, the thermal response characteristics of porous media with transient heat transfer have been investigated to some extent. For metal foam, the local thermal non-equilibrium (LTNE) effect can be found during the unsteady heat conduction process. However, for the unsteady heat conduction of the highly-conductive metal foam saturated with solid with a low thermal conductivity, the LTNE effect exists, but the thermal response characteristics of the metal foam and the infill solid with the periodically changing thermal boundary condition are rarely investigated. In this paper, based on the two equations model, the thermal response of the composite material which is made up of the metal foam and the saturated paraffin wax was numerically investigated with the periodic fluctuating temperature boundary condition, in which the temperature difference between metal foam and the infill paraffin wax was considered. The finite volume method (FVM) was used to discrete the coupled governing equations including the porous solid equation for metal foam and the infill solid equation for paraffin wax. The governing equations were solved by Gauss-Seidel iteration method. The present simulation method was validate by comparing with the analytical results under a constant temperature boundary. Then, the effects of various parameters on the thermal response characteristics were discussed. It was found that the LTNE effect exists in the metal foam and paraffin wax and thus the two equations model must be employed. When the ambient temperature periodically fluctuates with time, the temperature field inside the metal foam also reflects the cyclical fluctuations, and with the increasing of temperature fluctuation cycles, the LTNE effect increases firstly and then decreases. This means that there is a resonant time cycle corresponding to the most significant LTNE effect. When the fluctuant amplitude of ambient temperature is fixed, the temperature amplitudes of the metal foams increase with the increasing of temperature fluctuation cycles. The decay degrees of temperature amplitudes are different at different positions. The farther the position away from the heating surface, the more lagging the temperature amplitudes. The thermal diffusivity of the metal foam has a significant impact on the temperature amplitude and decay degree of temperature, while the porosity, pore density and nano-particles additive have less notable effect on them. For copper foam, pore density has a larger impact on LTNE phenomena, compared with porosity. When porosity and pore density are constant, the thermal diffusivity of metal foam has main impact on LTNE phenomena. Therefore, these factors should be considered synthetically so that the heat transfer rate of the composite is optimal. This paper reveals the temperature differences between porous solid phase and the infill solid phase during the metal foam heat conduction process, which shows direct scientific significance for the LTNE effect of an unsteady heat conduction in porous media.