The Behavior Of Gas Bubbles Entrained In The Liquid Film Of Horizontal Annular Flow

Small entrained vapor bubbles can have a significant impact on the evaporation or condensation of a thin liquid film as may be found in the annular flow of refrigerant in a heat exchanger. For example, past work has indicated that unattached small vapor bubbles serve as sites for vaporization within a film that, on average, would be too thin to support nucleate boiling. Vapor entrained in a thicker film during condensation will effectively lower the resistance the film poses to heat transfer. In this work, the behavior of small air bubbles (on the order of 100 microns or less) in annular water films is studied using a variety of visualization techniques. The ultimate purpose is to understand the process of vapor entrainment and transport in the film. The mean size and concentration of bubbles are presented for a variety of stratified and annular flow conditions. In addition, particle tracking velocimetry (PTV) is used to find the velocities of gas bubbles moving within the film. PTV results show very complex behaviors, such as rapid accelerations and decelerations and corkscrew trajectories.