Boiling of saturated FC-72 on square pin fin arrays

Abstract

Pool boiling heat transfer from finned copper surfaces immersed in a saturated dielectric fluid (Fluorinert FC-72) has been experimentally studied. Twelve extended surfaces with different geometrical configurations were tested. Each extended surface consisted of an array of pin fins with square cross-section. Fins were 3 or 6 mm long and their width varied from 0.4 to 1.0 mm. Fins were uniformly or non-uniformly spaced on the base surface: starting from the uniform configuration, in which the width and the spacing of fins were equal, non-uniform surfaces were obtained by regularly removing some rows of fins. For each extended surface, boiling curves were obtained at three different saturation pressures: 0.5, 1.0 and 2.0 bar. The effects of fin dimensions, spacing and pressure on heat transfer in saturated pool boiling were examined. In particular, the effect which the non-uniform distribution of fins produces on boiling behaviour was analysed. When fins thin out, the overall heat transfer coefficient based on the total area of the extended surface increases, but the heat transfer rate does not improve, even in the boiling region close to the maximum heat flux. The better wetting of the boiling surface by the liquid refrigerant, as a result of the sparser grouping of fins, is apparently offset by the reduction in the heat transfer area. If pressure is increased, the boiling curves of finned surfaces move towards lower wall superheats, as already observed in previous studies with regard to both plane and finned surfaces.