Mixing by bubble-induced turbulence

This work reports an experimental investigation of the dispersion of a low-diffusive dye within a homogeneous swarm of high-Reynolds-number rising bubbles at gas volume fractions ${\it\alpha}$ ranging from 1 % to 13 %. The capture and transport of dye within bubble wakes is found to be negligible and the mixing turns out to result from the bubble-induced turbulence. It is described well by a regular diffusion process. The diffusion coefficient corresponding to the vertical direction is larger than that corresponding to the horizontal direction, owing to the larger intensity of the liquid fluctuations in the vertical direction. Two regimes of diffusion have been identified. At low gas volume fraction, the diffusion time scale is given by the correlation time of the bubble-induced turbulence and the diffusion coefficients increase roughly as ${\it\alpha}^{0.4}$ . At large gas volume fraction, the diffusion time scale is imposed by the time interval between two bubbles and the diffusion coefficients become almost independent of ${\it\alpha}$ . The transition between the two regimes occurs sooner in the horizontal direction ( $1\,\%\leqslant {\it\alpha}\leqslant 3\,\%$ ) than in the vertical direction ( $3\,\%\leqslant {\it\alpha}\leqslant 6\,\%$ ). Physical models based on the hydrodynamic properties of the bubble swarm are introduced and guidelines for practical applications are suggested.