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

Abstract

• Boiling flow characteristics in a semi-circular mini-channel are demonstrated. • Suppression of the increase in pressure drop was observed for the 0.55 mm channel. • Bubble nucleation at the corners affects the flow patterns and pressure drop. • An annular flow transition occurs at a high gas Weber number. • A new equation based on a separated flow model is proposed for the pressure drop. 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 °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.