Homogeneous swarm of high-Reynolds-number bubbles rising within a thin gap. Part 1. Bubble dynamics

The spatial distribution, the velocity statistics and the dispersion of the gas phase have been investigated experimentally in a homogeneous swarm of bubbles confined within a thin gap. In the considered flow regime, the bubbles rise on oscillatory paths while keeping a constant shape. They are followed by unstable wakes which are strongly attenuated due to wall friction. According to the direction that is considered, the physical mechanisms are totally different. In the vertical direction, the entrainment by the wakes controls the bubble agitation, causing the velocity variance and the dispersion coefficient to increase almost linearly with the gas volume fraction. In the horizontal direction, path oscillations are the major cause of bubble agitation, leading to a constant velocity variance. The horizontal dispersion, which is lower than that in the vertical direction, is again observed to increase almost linearly with the gas volume fraction. It is however not directly due to regular path oscillations, which are unable to generate a net deviation over a whole period, but results from bubble interactions which cause a loss of the bubble velocity time correlation.