Pool Boiling of FC-72 over Metal Foams: Effect of Foam Orientation and Geometry

Abstract

The effects of heat sink orientation and metal foam geometry on the pool boiling characteristics of FC -72 in high porosity aluminum metal foam heat sinks are reported. Experiments are conducted on two different foam samples with almost identical porosities but different numbers of pores per inch (PPI). The height of the metal foam in each case is gradually reduced and for each height the foam is tested with the heater surface oriented horizontally and vertically. The effects of varying foam height and different orientations on the onset of nucleate boiling, heat transfer performance and the critical heat flux are studied. Hysteresis and incipience excursion are almost absent when the foam is oriented vertically. The heat transfer performance of the high PPI sample in nucleate boilin g deteriorates in the vertical orientation while that of the low PPI sample shows a two tailed trend, first decreasing and then increasing with reduction in foam height. The temperature jump, marking transition from nucleate to film boiling, and the critic al heat flux both increase with decreasing height in the low PPI sample for both orientations. This temperature jump is however seen to decrease in the vertical orientation as the PPI is increased. At 40 PPI, no temperature jump is observed and transition is seen to occur very smoothly indicating an optimum pore spacing.