Effect of Metal Foam Thickness and Pore Density on Array Jet Impingement Heat Transfer

Abstract

Abstract High porosity metal foam is a popular option for high performance heat exchangers as it offers significantly larger area per unit volume for heat dissipation as compared to other cooling techniques by convection. Further, metal foams provide highly tortuous flow paths resulting in thermal dispersion assisted by enhanced mixing. This paper reports an experimental study on jet array impingement onto high-porosity (ε∼0.95) thin aluminum foams. Our goal was to study the effect of foam thickness on convective transport and determine the optimum combination of foam thickness and pore density for maximum gain in thermal-hydraulic performance. To this end, three different pore-density foams (5, 10 and 20 pores per inch, ppi) were tested with three different jet array (5 × 5) impingement configurations (x/dj = 2,3 and 5), where “x” is the distance between any two adjacent jets and “dj” is the jet diameter. For the three pore densities selected, six values of foam thickness — 6.35 mm, 12.7 mm and 19.05 mm for the 20 ppi foam, 12.7 mm and 19.05 mm for the 10 ppi foam, and 12.7 mm for the 5 ppi foam — were deployed. The minimum thickness for each of the ppi value was dictated by the vendor’s manufacturing constraint. The thermal performance of these foams was compared against the orthogonal jet impingement onto a smooth heated surface, for which the distance between the jet exit plane and the heated surface was maintained at the foam thickness level. The data indicates that for a given pore density, thin foams have higher heat transfer rates compared to those for thicker foams, especially with jet configurations with larger open area ratios. The gain is due to the increased jet penetration and foam volume usage in thin foams compared to those for thick foams. Of the different pore density and foam thickness combinations, a 12.70 mm/20 ppi combination was found to have the highest thermal hydraulic performance.