Experimental Investigation of Buoyancy-Induced Convection in High-Porosity Open-Cell Aluminum Metal Foams Under Different Orientations

This investigation deals with buoyancy-induced convection of air in an open-cell aluminum foam under different orientations. Metal foam samples with a porosity of 93% and pore densities of 2, 4, 8, and 16 pores per cm (PPC) were used. The average heat transfer coefficient was determined for several values of the angle of inclination of the base plate, ranging from the vertical to the horizontal, with the foam facing upwards as well as downwards. The heat transfer coefficient was found to depend on the pore density, the thickness of the foam, the orientation of the base plate, and the difference in temperature between the base plate and the ambient. In all cases, the average heat transfer coefficient was found to be higher than that of the base plate without the foam. For a given angle of inclination and foam thickness, the thermal performance of samples with lower pore density was found to be superior. Two empirical correlations for predicting the effective Nusselt number have been proposed, one for the cases where the foam faces upwards and the other for cases where the foam faces downwards, relating the Nusselt number to Rayleigh number, Darcy number, the ratio of the thickness of the foam to the length of the square base plate and the angle of inclination from the vertical in the range of -90 deg (foam facing down) to +90 deg (foam facing up). The correlation predictions were found to match with experimentally determined Nusselt numbers within +/- 5% when the Rayleigh number ranged from 2500 to 6500.