Hydrodynamic modifications in vertical bubble plumes by a grid-screen

Abstract

Air injection through a single circular nozzle has been used for aeration enhancement, oxygen transfer, and mixing in natural lakes and reservoirs. A series of laboratory experiments with different hydrodynamic conditions was carried out to improve mixing capacity and entrainment in vertically discharged bubble plumes. Hydrodynamic modifications were performed by inserting a grid-screen with different openings and distances from the nozzle and hydrodynamic modifications were examined for different air discharges. Bubble plume characteristics such as bubble concentration and velocity were measured along the vertical axis of the plume using an accurate Refractive Bubble Index (RBI) probe. Utilizing bubble concentration and bubble velocity data, the hydrodynamic forces such as buoyancy, drag, added mass, and surface tension were calculated before and after the grid-screen to evaluate modification by a grid-screen. The bubble induced force acting on the grid-screen was then calculated by implementing the momentum transfer at the vicinity of the grid-screen. The effects of grid-screen openings, the location of grid-screen respect to the nozzle, and air discharge on the contributed forces were examined to design the optimum operation condition. The results indicated that the grid-screen size and its distance from the nozzle decreased the buoyancy and drag forces after the grid-screen by 49% and 42%, respectively. Non-linear correlations between the hydrodynamic forces and bubble Reynolds number were proposed indicating that all contributing forces increased with increasing bubble Reynolds number. Empirical models were also developed for prediction of surface tension and bubble induced forces on the grid-screen.