Area-Averaged Void Fraction Analysis of Flow Boiling in a Microscale Branching Channel Network

Abstract

Area-averaged void fraction images of convective boiling in a branching channel heat sink were acquired with a high speed high resolution camera at a rate of 1,000 frames per second for one second. Data sets are limited by the buffer size of the camera. Test conditions include a flow rate of 30 g/min, 30 W of energy added, and a subcooling of approximately 11°C. Time-varying area-based void fraction data were estimated using an image processing algorithm designed to minimize noise. Conditions for upstream bubble growth are reported as are liquid momentum, evaporation momentum, and surface tension forces for two extreme mass fluxes through the channels. Mass fluxes vary for each branching level as well as with the amount of vapor present in the heat sink. The heat sink is 38.1 mm in diameter with a radial branching flow pattern. The ratio of daughter-to-mother branching lengths is equal to 1.4, which is in contrast to a previous investigation in which the length scale ratio was 0.70.