Simulation of Microbubble Dynamics in Turbulent Channel Flows

This work investigates microbubble dynamics in four-way (with coalescence) coupled microbubble-laden turbulent channel flows. Upward and downward flows of water at a shear Reynolds number of Reτ = 150 are predicted using direct numerical simulation (DNS). Microbubbles, assumed to be non-deformable and spherical, are injected into the water flow and tracked using a Lagrangian approach. One-way and two-way coupled predictions were successfully compared against other available DNS-based results and used to demonstrate different trends in bubble preferential motion, with bubbles pushed by the lift force towards the wall in upflow and towards the centre of the channel in downflow. Four-way coupled simulations with bubble coalescence clearly demonstrate that the presence of the bubbles, and collisions between them, have a non-negligible effect on the fluid phase. Analysis of bubble collision behaviour highlights that binary collisions most frequently occur at very small approach angles and with low relative approach velocities. Once a collision is detected, the occurrence of bubble coalescence is evaluated, with special attention given to the performance of different bubble coalescence models. The film drainage model returns a 100% coalescence efficiency, while on the other hand the energy model returns a 0% coalescence efficiency, with this large discrepancy requiring further investigation and model development. The knowledge gained from the present results on the mechanisms that underpin bubble collisions is of value to the further development of more advanced coalescence closure models.