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 α and the velocity fluctuations u ₀ ^′ of the carrier flow before bubble injection are varied, respectively, in the ranges 0 ≤ α ≤ 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=[Formula presented], 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 α 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 α* ≡ α/α _c , where α _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.