Predicting liquid film dryout is crucial for the safety design of Boiling Water Reactors (BWRs). While correlations have been developed for droplet entrainment caused by the shear force of gas flow, the phenomenon caused by nucleate boiling, especially under forced convection, is not yet fully understood. This research reports on an experimental investigation into the onset conditions and boiling entrainment rate in a falling liquid film under forced convection. For droplet entrainment caused by nucleate boiling, we have identified three distinct entrainment mechanisms through visualization with a high-speed camera. They are the Jet-type and Filament-type, which are initiated by bubble collapse, and the Wave-type, which arises from disturbances at the gas-liquid interface caused by bubble generation. Based on these observations, we developed new empirical correlations to predict the onset conditions for each mechanism using dimensionless numbers, including the Wenb, H*, Wel and Weg. Furthermore, experiments were conducted to measure the boiling entrainment rate, and it was shown that the rate increases significantly upon the occurrence of the Wave-type mechanism. Using these results, a correlation for the droplet entrainment rate was proposed. This model was shown to successfully correlate the entrainment rate with the excess boiling Weber number (Wenb − Wenb,c), which represents the driving force beyond the critical onset condition.
NADE et al. (Wed,) studied this question.