Performance evaluation on the gradient design of pore parameters for metal foam and pin fin-metal foam hybrid structure

•Natural convection contributes little to formation and evolvement of solidification front.•Insertion of pin fins greatly improve the solidification rate and a maximum of 30.5% is found.•Graded porosity other than material promotes solidification for fin-foam hybrid structure.•Gradient in either porosity or material improves solidification heat transfer for metal foam.•Fin-foam hybrid structure with graded porosity increases solidification rate by 34.7%. This paper reported a study on the heat transfer in solidification of phase change materials (PCMs) embedded in metal foam. Heat transfer enhancement techniques are addressed in solidification including the insertion of pin fins and gradient design of pore parameters. Numerical models to describe the transient phase change heat transfer are established through the volume-averaged theory. One-temperature model is accounted for under the core assumption of local thermal equilibrium state. An experimental test rig is designed and established to verify the feasibility of the built numerical models by means of comparing solidification fronts and temperatures at PCM. The effects of gradients in parent materials, porosity and pore density for metal foam upon solidification behavior in metal foam and pin fin-metal foam hybrid structures are quantified. The contribution of local natural convection to the solidification behavior is justified and found this effect can be safely neglected for simulation on solidification in metal foam. Results demonstrate that the insertion of pin fins notably improve the solidification in metal foam regardless of gradient in pore parameters. The gradient in porosity rather than parent materials for the pin fin-metal foam hybrid structures can further improve the solidification rate. As for metal foam, both gradient in parent materials and graded porosity can significantly promote the solidification. The best heat transfer structure is recommend to be a pin fin-metal foam hybrid structure with gradient in metal foam porosity, outperforming other competing heat transfer techniques including pin fins or metal foam.