Thermal performance analysis of phase change materials (PCMs) embedded in gradient porous metal foams

Abstract

In this work, a comprehensive analysis is presented on utilizing non-homogeneous porous metal foams as thermal conductivity enhancers (TCEs) in energy storage systems and heat sinks working with phase change materials (PCMs). The transient phenomena inside the PCM-based systems with TCEs of gradient pore structure is numerically investigated and compared to the conventional uniform porous TCEs. The metallic foam is assumed to be made of copper, and paraffin is employed as the phase change material. Simulations are performed using finite-volume discretization, and Darcy-Brinkman-Forchheimer model is employed to represent the porous medium with variable properties. First, the effect of a positive and negative gradient porous structure in various spatial directions is discussed in detail and thermal response of the energy storage system to different thermal loads is compared to the case with uniform structure. The results show that gradient porous TCEs can be designed to improve the thermal performance of PCMs, and provide more uniform melting profile and heat transfer distribution throughout the system. The improvement has been shown for various heating configurations and at different melting stages. A general melting pattern is found for the left-heated layouts while comparatively, the bottom-heated layouts have more distinct melting profile. Also, the cases with lower porosity at the heat source have relatively higher melting rate at the beginning but a lower one at the final melting stages. Afterwards, variations of the porosity parameters such as average porosity, porosity change rate, and pore density in the gradient porous TCEs is studied for selected designs under different heat load conditions. The results show that when these porous foam features are changed, the thermal response and increase or decrease of the melting rate for the various structures is different. The differences in the melting and natural convection behavior is thoroughly discussed. At the final stage of this work, the thermal performance of the uniform and gradient porous structures is evaluated and compared at different widths and heights of the energy storage system. The results show that depending on the heat source location and the direction of the gradient foam with respect to the gravity, the improvement is more eminent when the size of the enclosure changes at certain directions. When increasing the height of the enclosure, a positive foam gradient in y direction has a better melting for the left-heated layouts while a positive foam gradient in x direction is superior for the bottom-heated layouts.