Pressure drop and flow patterns of boiling flows in Mini-Channels with Semi-Circular Cross-Section

Frictional pressure drop and flow patterns of vertically upward boiling flows in semi-circular mini-channels with the hydraulic diameters of 1.04 mm and 0.55 mm were experimentally investigated using R-245fa as the working fluid. Measurements were conducted at a saturation temperature of 30 degrees C with a mass flux of 100 to 400 kg/(m(2) s) at an inlet subcooling degree of 10 K. The working fluid was heated with hot water flowing in the counter-direction. It was found that the gas Weber number at the flow pattern transition to annular flow increased with an increasing mass flux and a decreasing hydraulic diameter. While the film thickness in the corner of the cross-section became thick in the 0.55 mm channel, nucleate boiling leading to strong fluctuation of the gas-liquid interface was observed owing to the higher heat flux. For the 0.55 mm channel, the gas-liquid interface was relatively smooth. Moreover, the increase in the frictional pressure drop was suppressed at a moderate outlet quality with a low mass flux. A new correlation equation based on a separate flow model could estimate the frictional pressure drop with mean absolute errors of 21.4% and 34.0% for the 1.04 mm and 0.55 mm channels, respectively.