Mixing induced by a bubble swarm rising through incident turbulence

This work describes an experimental investigation on the mixing induced by a swarm of high Reynolds number air bubbles rising through a nearly homogeneous and isotropic turbulent flow. The gas volume fraction a and the velocity fluctuations u(0)' of the carrier flow before bubble injection are varied, respectively, in the ranges 0 <= alpha <= 0.93% and 2.3 cm/s <= u'(0) <= 5.5 cm/s, resulting in a variation of the bubblance parameter b in the range [0, 1.3] (b = v(r)(2)alpha/u'(2)(0) where V-r is the relative rising velocity). Mixing in the horizontal direction can be modelled as a diffusive process, with an effective diffusivity D-xx. Two different diffusion regimes are observed experimentally, depending on the turbulence intensity. At low turbulence levels, D-xx. increases with gas volume fraction alpha, while at high turbulence levels the enhancement in D-xx is negligible. When normalizing by the time scale of successive bubble passage, the effective diffusivity can be modelled as a sole function of the gas volume fraction alpha* = alpha/alpha(c), where alpha(c) is a theoretically estimated critical gas volume fraction. The present explorative study provides insights into modeling the mixing induced by high Reynolds number bubbles in turbulent flows. (C) 2019 Elsevier Ltd. All rights reserved.