Experimental study of pool boiling on pin-finned and straight-finned surfaces on an inclined plate in FC-72

Pool boiling experiments were conducted on copper surfaces in a saturated dielectric fluid, FC-72, at three orientations at a saturation temperature of 50°C. A plain surface, four types of straight-finned surfaces, and six types of pin-finned surfaces were tested to investigate the effects of fin dimensions. Fins were 0.1-0.4 mm thick and the fin lengths varied from 0.1 to 1.6 mm. For the plain surface, the heat transfer coefficient increased as the inclination angle increased at low heat fluxes for the increasing heat flux tests. No marked effect on CHF and heat transfer coefficient during the decreasing heat flux tests was observed for all surfaces in the present tests. The pin fins made by wire cutting yield higher boiling performance than the wet-etched surfaces because of the advantage of larger surface roughness and sharper fin base corners. The heat transfer coefficient increased with increasing fin height/width ratio. The best surface was the pinfinned surface having a fin width of 0.4 mm, a fin height of 1.6 mm, and a fin pitch of 0.8 mm. It enhanced the boiling heat transfer coefficient about eightfold. For the same fin height and width, the straight fins and the pin fins yield similar performance at q" < CHF (critical heat flux). However, the CHF of the pin fins was greater than that of the straight fins because the opening area on the top of the pin fins was greater than that of the straight fins. For both straight fins and pin fins, the boiling heat transfer efficiency was mainly enhanced by increasing the total surface area at q" < CHF. A CHF correlation of the finned surfaces has been proposed. It predicts the CHF data of FC-72 from four sources with an rms deviation of 14.95%.