Dynamics of bubbles rising in pseudo-2D bubble column: Effect of confinement and inertia

Abstract

2D Rectangular columns, formed within a thin gap between two enclosed parallel plates, are often used in visualisation and CFD studies of multiphase flows. In this study, a numerical analysis is performed on the dynamics of a bubble rising in such setup (D = 5–7 mm, Re ≈ 1000–2000), with the aim of determining the effect of the size of the gap (h) on the velocity field around it. For most cases, the bubble adopts an oblate spheroidal shape, with chaotic rise behaviour and continual fluctuations in both aspect ratio, orientation, and velocity. The bubble aspect ratio generally decreases with increasing level of confinement, because of increased level of resistance provided by the walls. With increasing Reynolds number, however, the bubble (D = 7 mm, h/D = 1) is found to display a more elongated oblate spheroidal shape and a rise behaviour that is largely rectilinear. This change in behaviour has been attributed towards severe disruptions in the development of toroidal vortex formation at the walls, confining the formation of hairpin vortices towards the side interface of the bubble at the centreline of the column. This prevents the development of large-scale fluctuations and confines the path oscillation of the bubble towards high-frequency fluctuations. Further, the formation of a region of recirculating flow is also evident, as a result of rapid detachment of vortices at the bubble equator. Intense fluctuation in velocity in the horizontal direction parallel to the wall is developed, and the dissipation in this fluctuation, as well as in the time-averaged velocity below the bubble, is found to occur more gradually than in the case of columns with larger gaps.