Bubble coalescence and breakup in turbulent bubbly wake of a ventilated hydrofoil

Auto-venting turbines have been proposed as a promising solution to the problem of low oxygen content in the discharged downstream water of an electric power plant. The current design of these turbines relies primarily on computational simulation. The experimental studies that focus on the physical processes occurring in turbulent bubbly wake are urgently needed to improve the performance of these simulations in predicting the bubble size distribution behind auto-venting turbines. Therefore, in the current study, we conducted detailed experimental investigations into the bubble size distributions in the wake of a ventilated hydrofoil. The mean bubble statistics is measured at different liquid velocities and air entrainment rates, and then the variation in mean bubble statistics is studied at different downstream locations in the wake. The bubble size distributions at different downstream locations have revealed the presence of distinct coalescence-dominant and breakup-dominant regimes. Analytical expressions are derived for the prediction of maximum stable diameter and Sauter mean diameter of bubbles, in the breakup and coalescence regimes, respectively. The observations from high speed imaging provide support for the measurements of bubble statistics, and physical insights into different mechanisms of bubble breakup and coalescence in turbulent wake. It is hoped that these insights will aid in developing generic model of bubble size distribution, and will help researchers improve bubbly flow simulations for auto-venting turbines.