The effect of the cross-sectional geometry on saturated flow boiling heat transfer in horizontal micro-scale channels

In the present paper, convective boiling heat transfer results of R134a for circular, square and triangular tubes are presented. The evaluated channels present the same external perimeter and equivalent diameters of 1.100, 0.977 and 0.835 mm, respectively. Experiments were performed for mass velocities ranging from 200 to 800 kg/m2 s, heat fluxes from 15 to 85 kW/m2, saturation temperatures of 31 and 41 °C, and vapor qualities from 0.05 to 0.95. In order to perform reasonable comparisons among the test sections, the tests were run under similar mass velocities for the three geometries. The experimental data were carefully analyzed and discussed focusing on the effect of the cross-sectional geometry. It was found that for low heat fluxes, the heat transfer coefficient for the circular channel is higher. While for high heat fluxes, the heat transfer coefficient for the triangular channel is higher than for circular and square channels. Subsequently, the experimental data were compared with predictive methods from literature which are usually developed based only on data for single circular channels. Statistically, Kanizawa et al. [24] and Kim and Mudawar [34] provided reasonable predictions of the overall database. However, none of the methods captured adequately the experimental trends for the triangular channel.