Heat transfer coefficient of boiling flows in minichannels with a semicircular cross-section

The boiling heat transfer characteristics of vertically upward flows in small-diameter semicircular channels heated with hot water were experimentally investigated to characterize the influences of channel cross-section and thermal diffusivity of a heat transfer wall on heat transfer characteristics. The inlet subcooling degree was set to 10 K at a saturated temperature of 30 degrees C with a mass flux of 100-400 kg/(m2 s). Two types of hydraulic channels (stainless steel and copper) with diameters of 1.04 and 0.55 mm were used with refrigerant R-245fa. Moreover, the average heat transfer coefficient was evaluated considering the fin efficiency of the heat transfer wall. As a result, the heat transfer coefficient for high mass flux increased with the outlet quality owing to the nucleate boiling in the channel corners, causing a strong fluctuation of the gas-liquid interface. For the 0.55-mm channel, as the liquid film was thicker in the corners than in other regions due to surface tension, the increase in thermal resistance at the corners promoted nucleate boiling. A new correlation equation was derived to express the heat transfer coefficient for semicircular minichannels considering the fin efficiency; the mean absolute error between the experimental data and calculated values was 11.3 %.