Direct Numerical Simulation of Microbubble Dynamics in Turbulent Channel Flow

This work investigates microbubble dynamics in four-way (with coalescence) coupled microbubble-laden turbulent channel flows. Both upflow and downflow of water at a shear Reynolds number Reτ = 150 are considered, where microbubbles, assumed to be non-deformable and spherical, are injected and tracked using a Lagrangian approach. One-way and two-way coupled predictions, used to successfully validate the model, demonstrate different trends in bubble preferential motion. Four-way coupled simulations with coalescence clearly demonstrate that the presence of the bubbles, and collisions between them, have a non-negligible effect on the fluid phase. Simulation results indicate that binary bubble collisions occur at very small angles between the two velocity vectors, and with low relative approaching velocities generally favoured. Special attention is paid to the performance of different bubble coalescence closures, with the film drainage model returning a 100% coalescence efficiency. In contrast, the energy model returns a 0% coalescence efficiency, and this large discrepancy between available coalescence models requires further investigation. The knowledge gained on the mechanisms that underpin bubble collisions is of value for the further development of coalescence closure models.