Mechanistic modeling of the liquid film shape and heat transfer coefficient in annular-regime microchannel flow boiling

A methodology is proposed for predictive modeling of the liquid-gas interface shape and saturated flow boiling heat transfer coefficient in two-phase microchannel flows within the annular regime. The mechanistic model accounts for the effects of surface tension and interface curvature, gravity, and shear stress in determining the liquid film shape; one-dimensional conduction is assumed to occur across the variable-thickness film to calculate wall heat transfer coefficients locally along the channel length. Model performance is benchmarked against 251 experimentally measured heat transfer coefficient values taken from the literature for annular-regime flow boiling in microchannels of a rectangular cross-section. These data are successfully predicted with a mean absolute error of 21.7%, and 72.1% of the points lie within an error band of ±30%. The match to data is poorest at lower vapor qualities corresponding to the onset of the annular regime, for which the heat transfer coefficient is underpredicted; an experimental investigation is performed to better understand the disparity under these operating conditions. Liquid film shapes are measured during adiabatic annular flow through microchannels of square cross-section for a range of channel hydraulic diameters (160 µm, 510 µm, 1020 µm) and operating conditions, so as to control the void fraction and Weber number of the flow. Using air and water as the working gas and liquid, respectively, trends in film behavior are identified and compared against model predictions. The experimental findings reveal the non-negligible impact of capillary pumping on the interface morphology at the onset of the annular regime.