Preferential concentration and rise velocity reduction of bubbles immersed in a homogeneous and isotropic turbulent flow

Abstract

The behavior of small spherical bubbles immersed in a homogeneous isotropic turbulent carrier flow of a heavier fluid has been experimentally studied. Air bubbles with diameters between 10 and 900 μm were injected in the test section of a horizontal water channel and allowed to interact with the turbulence induced by a grid located at the entrance to the test section. Point measurements of the bubble diameter and convective and rise velocities were taken from light interferometry data, together with flow visualizations that showed the instantaneous concentration field of bubbles in the carrier flow. The effect of the turbulence on the bubbles was found to alter the concentration field of bubbles leading to preferential accumulation at small scales, a phenomenon referred to as clustering, and to a decrease in the rise velocity of bubbles in the flow below the value measured and predicted for bubbles in a stationary fluid. These results are interpreted in terms of the different forces acting on the bubble in an inhomogeneous flow and in particular as the effect of pressure fluctuations that drive the bubbles preferentially to the core of vortices.