Experiments on the onset of boiling entrainment from a falling liquid film with gas sheared flow

Abstract

The process of droplet entrainment caused by nucleate boiling within a falling liquid film was visualized using a high-speed camera to investigate the mechanism and the onset conditions of the boiling entrainment. In the experiments, air was flowed along the liquid film to explore the potential effects of vapor core flow on the droplet generation process in diabatic annular two-phase flow. The two types of boiling entrainment mechanisms called the jet-type and the filament-type were observed in the present work. The jet-type boiling entrainment commenced at relatively low wall heat flux and produced small droplets. The filament-type was observed at higher wall heat flux and it produced larger droplets. Hence, the filament-type was supposed to have a greater impact on the liquid film flow rate in diabatic annular two-phase flow. The experimental data accumulated in this work were used to develop correlations for the onset conditions of the jet-type and filament-type boiling entrainments.