Enhancement of liquid forced convection heat transfer in microchannels dueto the release of dissolved noncondensables

An experimental investigation addressing the effect of the release of disso lved noncondensables on the heat transfer in a long, heated microchannel su bject to subcooled liquid forced convection was conducted. The convection h eat transfer coefficients near the exit of a copper microchannel with 0.76 mm inner diameter and 16 cm heated length, subject to forced-flow cooling b y subcooled water, were measured. The range of experimental parameters were : wall heat flux = 0.5-2.5 MW/m(2); liquid velocity = 2.07 to 8.53 m/s; cha nnel exit pressure = 5.9 bar. Experiments were performed with degassed wate r and water saturated with air. The convection heat transfer coefficients obtained with degassed water were systematically under predicted by the widely-used Dittus-Boelter correlati on for turbulent pipe flow. The presence of dissolved air in water could in crease the heat transfer coefficients by as much as 17%, despite the fact t hat the maximum increase in the coolant velocity due to the noncondensable gas release was only a few percent. The heat transfer enhancement increased with increasing the heat flux and decreasing the liquid velocity. (C) 1999 Elsevier Science Ltd. All rights reserved.