Buoyancy-driven exchange flow between pure and particle-laden fluids in a vertical tube

The buoyancy-driven exchange flows of particle-laden fluids are commonly observed in a variety of natural and industrial processes, ranging from persistently active volcanoes to fluid mixing in petroleum refineries. Previous laboratory experiments of exchange flows between pure liquids have comprehensively elucidated the role of the liquid viscosity ratio on the overturn styles in a vertical tube. However, little attention has been paid to the variability in the exchange flow caused by the presence of particles. This study demonstrates that some components of the core–annular flow that occur at low or intermediate liquid viscosity ratios are the consequence of a high solid fraction. The rising speed of the ascending flow first increases and then decreases as the solid fraction increases due to competition between buoyancy and the drag force on the interface between the suspension and the pure liquid. Moreover, larger particles generate a higher rising speed for a given solid fraction. Direct numerical simulations confirm the hypothesis that larger particle clusters form around the interface between the ascending and descending flow in the case of larger particle sizes, which lead to instability of the flow field and cause a higher particle–particle force. It suggests that particle clusters alter the pattern and dynamics of the exchange flow, which implies that the realized flow field can only be predicted by considering particle–particle interaction. The numerical simulation successfully reproduced the flow behaviors observed in the experiments, shedding light on how the liquid–liquid, liquid–particle, and particle–particle interactions affect the stability of the exchange flow.