Gradient design of pore parameters on the melting process in a thermal energy storage unit filled with open-cell metal foam

•Metal foam with gradient in porosity markedly affected the melting process than pore density.•Metal foam with positive gradient in pore parameters showed a better temperature uniformity.•An optimal gradient porosity (0.89–0.95–0.98) could reduce the full melting time by 21.1%.•Positive gradient design exhibited a prospective capacity for enhancing melting heat transfer. In this paper, a novel latent heat thermal energy storage unit filled by metal foams with gradient in pore parameters was proposed to enhance thermal energy storage performance. Two kinds of gradients, namely positive and negative gradients, in porosity and pore density for metal foams were designed. Physical and numerical models were established for this problem to predict the melting front evolution, temperature and velocity field during the melting process. An experimental test rig was built and measurements on temperature variation in paraffin/metal foam composite were conducted to validate the numerical model, achieving good agreement. The temperature variations and evolution of melting front were explored and recorded. The numerical results demonstrated that the positive gradient in porosity can provide the better heat transfer enhancement than the uniform and negative gradient cases. A 17.9% reduction in full melting time can be achieved with the positive gradient design of porosity and simultaneously a better temperature uniformity was also obtained in comparison with the other arrangements. Improving the arrangement of pore density contributed little to the melting process but the temperature uniformity for the case with positive gradient in pores density increased by 9.1% compared to the uniform arrangement. An optimization recommended on a gradient in porosity of 0.89–0.95–0.98 to further reduce the full melting time by 21.1%.