Theoretical model for local heat transfer coefficient for annular flow boiling in circular mini/micro-channels

This study examines two-phase heat transfer characteristics associated with annular flow boiling in circular mini/micro-channels with circumferentially uniform heat flux. A theoretical control-volume-based model is developed based on the assumptions of smooth interface between the annular liquid film and vapor core, and uniform film thickness around the channel's circumference. Droplet entrainment and deposition effects are incorporated in the model with the aid of new correlations for initial liquid droplet quality at the onset of annular flow, and deposition mass transfer coefficient, respectively. The model also accounts for interfacial suppression of turbulent eddies due to surface tension with the aid of an eddy diffusivity model specifically tailored to shear-driven turbulent films. The model shows excellent predictive capability against 149 convective boiling dominant data points for saturated flow boiling in circular mini/micro-channels, evidenced by an overall mean absolute error of 16.5%, with 91.3% and 98.0% of the data falling within ±30% and ±50% error bands, respectively.