EFFECT OF GRAVITATIONAL ORIENTATION ON FLOW BOILING OF WATER IN 1054 × 197 µm PARALLEL MINICHANNELS

Microchannels and minichannels are being considered for high heat flux applications under microgravity environment in space missions. An experimental study is undertaken to determine the effect of gravitational orientation on flow boiling characteristics of water in a set of six parallel minichannels, each 1054 µm wide by 197 µm deep and 63.5 mm long with a hydraulic diameter of 333 µm. Three orientations – horizontal, vertical downflow and vertical upflow- are investigated under identical operating conditions of heat and mass fluxes. 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 compared to the horizontal case, the flow becomes less chaotic for vertical upflow, while the reversed flow becomes more pronounced in vertical downflow case. In the vertical downflow case, the flow reversal phenomenon was found to be more pronounced. The resulting increase in the back-flow is responsible for channel-to-channel flow maldistribution and heat transfer degradation