Experimental and numerical investigations on the effect of porosity and PPI gradients of metal foams on the thermal performance of a composite phase change material heat sink

This paper reports the results of an experimental and numerical study on the thermal performance of a metal foam based composite phase change material (PCM) heat sink cylindrical in shape with porosity and PPI (pores per inch) density gradients. Studies are conducted on different configurations of composite PCM made of an open-cell aluminum metal foams with porosities of 0.9, 0.94 and 0.97 and PPI of 8, 14 and 20 encapsulated with n-eicosane as the PCM. Experiments are carried out on the PCM heat sink heated from the bottom for different configurations of metal foams with a uniform porosity and non-uniform porosity created with layer wise arrangement of metal foams from the bottom to the top. Complementary three-dimensional numerical simulations have been conducted using the enthalpy porosity methodology with a non-thermal equilibrium model to understand the melting and solidification dynamics of PCM while melting (charging) and solidification (discharging) respectively. Heat sink configurations with uniform porosity, and porosity gradient created with bi-layer arrangement of metal foams have been simulated numerically. Further, the numerical simulations have been extended to study heat sink configurations containing metal foams with uniform PPI density and PPI density gradient. From the results, it is seen that the case of non-uniform variation in porosity (decreasing from the bottom to the top) with constant PPI density and the case of non-uniform PPI density (increasing from the bottom to the top) with constant porosity show superior performance up to 28 and 45% over the heat sink configurations with uniform porosity and PPI density respectively in the charging cycle in terms of the time to reach a setpoint temperature. From the numerical simulations, it is seen that the melt fraction of PCM significantly changes the convection velocity cells, which affects the melting dynamics of PCM. Additional numerical simulations have been conducted on PCM heat sink with non-uniform porosity (i.e. decreasing porosity from the bottom to the top) and non-uniform PPI density (i.e increasing PPI density from the bottom to the top) gradients created with three layers (tri-layer) of metal foams. The results show that the PCM heat sink with non-uniform porosity and non-uniform PPI density gradients created with tri-layer metal foams have almost the same performance as a bi-layer metal foam with enhancement ratio up to 4 and 4.4 times respectively over the base line case (i.e. PCM heat sink without metal foams). In the discharging cycle, it is seen that the porosity and PPI gradients do not have any effect on the thermal performance of the heat sink. © 2020