Bubble Trapping and Clustering Dynamics in an Upward Liquid-Gas Flow around a Cylinder

Two-phase wakes from bluff bodies in liquid-gas flows can have non-uniform distributions of void fraction due to bubble trapping and clustering. We have conducted an experiment to characterize these bubble dynamics using a water-air mixture in an adiabatic upward flow channel. In this experiment, the air superficial velocity of the two-phase mixture was 0.061 m/s and the liquid Reynolds number (Re), based on a cylinder diameter of 9.5 mm, was 3,000. Additionally, the equivalent bubble diameters ranged from 1 mm to 4 mm. We measured the bubble diameters and calculated bubble trajectories using shadowgraphs of the flow obtained at 2,000 fps and Lagrangian Particle Tracking Velocimetry (PTV). In addition to the bubbles, we injected liquid flow tracers and used Particle Shadow Image Velocimetry (PSIV) to analyze the liquid flow fields. The two-phase tracking results allowed us to evaluate the effect of bubble size on buoyancy, inertial, drag, lift, and pressure gradient forces. The force balance analysis showed inertial, lift, and pressure gradient forces contributed to the formation of the bubble trapping wakes. While the pressure gradient forces were the strongest, the results suggested inertial and lift forces are comparably important.