Experimental study of three-dimensional trajectory and dynamic characteristics of rising bubbles

Understanding the bubble motion characteristics in the two-phase flow is crucial for mass, momentum, and heat transfer processes between gas and liquid phases in various engineering systems. Despite decades of studying the phenomenon of bubbles rising in quiescent water, there is no unified conclusion regarding the three-dimensional motion characteristics of bubbles larger than 3 mm, where bubbles exhibit zigzag or spiral trajectories with shape oscillations. Meanwhile, the zigzag and spiral motions are sustained by the induced additional lift caused by the wake vortices acting on the bubbles, however, the evolution of forces acting on the bubbles during the stable rising phase has not been further studied by considering the combined effects of forces. This paper presents experimental research on the three-dimensional motion characteristics of bubbles during the stable rising phase. The results show that the bubbles transition from ellipsoidal shape to irregular shapes as the diameter increases, and the trajectory of the bubbles changes from regular spiral motion to random motion coupled with straight lines, zigzags, and spiral. The motion of bubbles during the stable rising phase is influenced by the combined effects of buoyancy, drag, and induced lift by the wake vortex. Additionally, the aspect ratio and terminal velocity of the bubbles were investigated using empirical relationships and dimensionless numbers, Eotvos, Weber, and Tadaki number. It was found that using the Tadaki number to predict the aspect ratio yielded the most accurate results, with the experimental values well predicted by an empirical correlation.