Mixing induced by a bubble swarm rising through incident turbulence

TL;DR

This experimental study investigates how a swarm of high Reynolds number bubbles enhances mixing in turbulent flows, revealing two diffusion regimes and modeling the effective diffusivity based on gas volume fraction and turbulence intensity.

Contribution

It introduces a model for bubble-induced mixing in turbulence, identifying two diffusion regimes and relating effective diffusivity to gas volume fraction and turbulence levels.

Findings

Two diffusion regimes depending on turbulence intensity.

Effective diffusivity increases with gas volume fraction at low turbulence.

Normalized diffusivity as a function of critical gas volume fraction.

Abstract

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 $\alpha$ and the velocity fluctuations $u'_0$ of the carrier flow before bubble injection are varied, respectively, in the ranges $0\leq \alpha \leq 0.93\%$ and 2.3 cm/s $\leq u'_0 \leq 5.5$ cm/s, resulting in a variation of the bubblance parameter $b$ in the range [0, 1.3] ($b = \frac{V_r^2 \alpha}{{u'}_0^2}$, 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 $\alpha^* \equiv \alpha/\alpha_c$, 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.