Effect of gravitational orientation on flow boiling of water in 1054 x 197 micron parallel minichannels

Abstract

Microchannels and minichannels are being considered for high heat flux applications under microgravity environment in space missions. Due to the small hydraulic diameter, the effect of gravitational forces on flow boiling heat transfer is expected to be negligible. In an attempt to validate this assumption, an experimental study is undertaken to determine the effect of gravitational orientation on the flow boiling characteristics of water in a set of six parallel minichannels, each 1054 microns wide by 197 microns deep and 63.5 mm long with a hydraulic diameter of 333 μm. Three orientations are investigated under the identical operating conditions of heat flux and mass flux: horizontal, vertical downflow and vertical upflow. High-speed images are obtained to reveal the detailed two-phase flow structure and liquid-vapor interactions. The experimental data and high speed flow visualization indicate that there are no major differences between the horizontal and the vertical upflow cases. In the vertical downflow case however, the flow reversal phenomenon was found to be more pronounced, with vapor flowing back into the inlet manifold, leading to channel flow maldistribution, heat transfer degradation, and an increase in pressure drop.