The effect of heating area orientation on flow boiling performance in microchannels heat sink under subcooled condition

Subcooled flow boiling heat transfer experiments were conducted in this work in order to investigate the effect of the heating area orientation of microchannels heat sink on flow boiling heat transfer and pressure drop characteristics. The flow boiling heat transfer experiments were conducted in a 31 parallel “U” shaped microchannels (width 305 µm and depth 290 µm) heat sink with deionized water as the working fluid. The tests were conducted for different orientations such as, Horizontal upward facing (HU), Horizontal with heating area vertically aligned (HV), Vertical with up flow (VUF), Vertical with downflow (VDF) and Horizontal downward facing (HD) in a forced convection loop with volume flow rate of 50 ml/min, 100 ml/min and 150 ml/min. From the experimental results, it was observed that the performances of the heat sink under all orientation conditions were found to be almost identical except the vertical downflow orientation (VDF). The critical heat flux values are found to be less in the case of vertical downflow orientation. In the case of vertical downflow orientation, the critical heat flux values corresponding to 50, 100 and 150 ml/min flow rate were 44.1 W/cm2, 74 W/cm2 and 99.3 W/cm2 respectively. For VDF orientation the buoyancy force acts on bubbles against the flow direction. Consequently, the bubbles were built up and merge each other due to the difficulty in draining and reversed flow was created with less heat flux input. The total pressure drop observed to be more for vertical downflow orientation compared to other orientations. Significant pressure fluctuations were observed during flow boiling in microchannels with VDF and HD orientations at low flow rates. The percentage reduction in effective heat flux value at the incipience of critical heat flux (CHF) in VDF orientation for flow rate of 50, 100 and 150 ml/min were 13%, 10.30% and 7.40%, and the corresponding percentage reduction in maximum outlet heat transfer coefficient was 30%, 23% and 19% respectively.