Kinematics of droplets and bubbles flowing in a liquid stream

The effect of the insertion of a bubble swarm in the separation of finely dispersed oil droplets in a liquid flow was investigated experimentally. The generation of gas bubbles and oil droplets in a liquid stream allowed the analysis of the efficiency of offshore separation processes, which has shown to be strongly dependent on hydrodynamic forces and physical interactions of the dispersed phases in flows governed by viscous dissipation as well as hydrodynamic effects when inertia forces dominate. Despite its importance, studies in this field are scarce and a lack of information about the physics of the problem persists. The goal of this study is to ascertain which of these phenomena govern the separation kinematics. To achieve this objective, the high-speed shadowgraph technique allowed the determination of the trajectories of the dispersed phases, equivalent diameters and their rising velocity. The experiments suggested that the presence of the bubble swarm enhances the separation of the oil droplets from the liquid because of the increased gravitational effects associated to the liquid-induced flow, where the droplets may rise along the wake of the bubbles, in addition to the observed momentum exchange that arises from a bouncing effect. Furthermore, the insertion of the bubbles reduced the random circulation of the droplets inside the bubble column. Buoyancy effects were increased, leading to a smaller retention time of the droplets. Thence, the hydrodynamic interactions promoted by the insertion of the bubbles govern the separation of droplets from the liquid stream, leading to an enhanced separation process.