Heat transfer, pressure drop, and dissolved gas effect during flow boiling in microchannels

Microchannels are being considered in many advanced heat transfer applications, including automotive, fuel cells, and electronics cooling. However, there are a number of fundamental issues still unresolved with respect to heat transfer and fluid mechanics perspective. An experimental investigation of the heat transfer, pressure drop, and flow patterns during flow boiling in microchannels is performed. Six parallel microchannels with a mean hydraulic diameter of 207 micron are manufactured and tested. Flow patterns have been observed in the channels under diabatic conditions. Observations suggest that the conventional flow patterns also occur in microchannels, however, the Reynolds number range is significantly lower in microchannels than in conventional channels (hydraulic diameter of 3 mm or higher). The effect of dissolved gas in water has also been investigated. A novel method for the removal of dissolved air has been applied and used to achieve several different levels of degassing. It was found that if the water is degassed to oxygen levels of 5.4 ppm, 3.2 ppm, and 1.8 ppm, behaves as predicted by correlations. The water that contained dissolved gas with an oxygen level of 8.0 ppm first exhibits a decrease in heat transfer and then an enhancement. The range of parameters are: mass flux - 160 to 1827 kg/m2s, heat flux ? 5 to 930 kW/m2, inlet temperature ? 22 ?C, and exit quality - subcooled to almost 1. The corresponding single-phase, all-liquid flow Reynolds number range was 116 to 1813. The pressure drop and local heat transfer coefficient have been obtained as a function of heat and mass fluxes and local quality. The highest value of the local heat transfer coefficient achieved was 195 kW/m2K. The single phase pressure drops are in agreement with the conventional theory, and the heat transfer data obtained here represent one of the first local data sets obtained under such low Reynolds number conditions