Investigation on Influencing Factors of Film Bubbles in Vertical Upward Annular Flow Based on Fluorescence Imaging

Gas liquid annular flow extensively exists in the heat transfer applications, including the condenser, the boiling water reactors, and the refrigeration system. The study of the film bubbles is essential to enhance the thermal transmission between the tube wall and the liquid film. However, film bubble has received the insufficient attention in the field of gas liquid annular flow. In this article, planar laser-induced fluorescence coupled with the local distortion correction is applied to investigate the film bubbles. Based on the fluorescence imaging method, the distribution characteristics of the film bubbles are investigated. In particular, the size and instantaneous axial cumulative distribution of the film bubbles follow the unimodal distribution and the multimodal distribution in the radial direction, respectively. Moreover, the diverse relationships between the film bubbles and the liquid film features including the curvature and thickness of the liquid film are analyzed in the wave region. Furthermore, combined with the radial migration characteristics of the film bubbles, the effects of the distance from the film bubbles to gas-liquid interface on the film bubbles are revealed. Finally, the influencing factors on the maximum size of the film bubbles are quantitatively investigated. These results support the further development of the existing knowledge of film bubbles and provide a key solution to investigate the performance optimization of heat exchangers.