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Abstract

his 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.