Quantitative characterization of elongated bubble breakup dynamics in intermittent flow using Stereo-PIV/PLIF

The deformation and detachment of elongated bubbles substantially impact the heat and mass transfer efficiency in intermittent flow. To investigate the criterion of elongated bubble breakup induced by flow conditions, a high-speed optical system integrating monocular stereo particle image velocimetry (Stereo-PIV) and planar laser-induced fluorescence (PLIF) is proposed to monitor synchronously the interfacial structures and three-dimensional flow parameters of intermittent flow. High-speed photography is utilized to judge the presence of small bubbles and determine the critical flow conditions leading to elongated bubble breakup. Meanwhile, the PLIF technique is applied to capture the interfacial structure of elongated bubbles and analyze their temporal variations with flow conditions, while monocular Stereo-PIV is employed to obtain the liquid velocity field. However, the diversity of the full-component velocities with flow conditions makes it difficult to directly reveal the criterion for the transition of elongated bubbles from an intact state to a broken state. To address this limitation, vorticity and vorticity stretching terms are employed to analyze elongated bubble deformation, revealing a strong correlation between the variation of elongated bubble length and vorticity as well as vortex stretching. Furthermore, statistical analyses of turbulent kinetic energy and Weber number are performed, revealing that the critical turbulent kinetic energy for bubble breakup is 0.02 m²/s², and the critical Weber number is 4, consistent with the findings of F. Lehr. Turbulent normal stress, turbulent shear stress, and their combinations are identified as potential destructive forces for elongated bubble breakup, primarily induced by turbulent fluctuations and interfacial instability. These findings provide new insights and data support for theoretical research and engineering application of elongated bubble breakup in horizontal intermittent flow.