Mixing by bubble-induced turbulence

Abstract

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 α 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 α 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 α. The transition between the two regimes occurs sooner in the horizontal direction (1 % α 3 %) than in the vertical direction (3 % α 6 %). Physical models based on the hydrodynamic properties of the bubble swarm are introduced and guidelines for practical applications are suggested.

Mixing induced by a bubble swarm rising through incident turbulence

“…This is a diffusive process and can thus be described by an effective diffusivity [18,24,25]. Following Taylor theory, Alméras et al [24] proposed to model the diffusion coefficient, as D ii = u 2 i T , where u 2 i is the variance of the velocity fluctuations of the liquid phase, and T is a characteristic time scale of the turbulent diffusion that requires to be reinterpreted for two-phases flows. Namely, at low gas volume fractions, this characteristic time scale is given by the Eulerian integral length scale λ of the bubble-induced turbulence, T = λ u , while it is the time between two bubbles passages T 2b at larger gas volume fractions [24].…”

Twente mass and heat transfer water tunnel: Temperature controlled turbulent multiphase channel flow with heat and mass transfer

“…The objective of future studies is to obtain the horizontal diffusion coefficient by measuring concentration of the dye by means of quantitative laser induced fluorescence. For further details on this experimental technique we refer to Alméras et al 31 .…”

Influence of Surfactants on the Motion of a Pair of Bubbles in Quiescent Liquids

“…気泡流には様々なスケールがあり、気泡が生成 する乱れは多くの研究者の注目を集めている。そ の乱れは、流れ場と相互作用し、単相流とは異な る複雑な流動構造を示す [1][2][3][4][5][6]。特に気泡クラスタ ーは、乱流の秩序渦構造よりも大きく、大規模な 流動構造そのものを変化させる [7,8]。 そのため気 泡クラスター形成の有無について様々な報告が 行われている [9][10][11]。 気泡のクラスター形成は、まず理論的に予測さ れた。ポテンシャル流れを仮定し、複数の気泡が 上昇する際、気泡間相互作用によって水平面に気 泡 が クラスターを 形成することが報告された [12,13]。一方で、実際の気泡流では、気泡同士の 合体によって気泡径差が生まれ、通常気泡のクラ スター構造は観察されない。さらに前述のポテン シャル流れを用いた理論でも、気泡径差を考慮す るとクラスターは形成されない [14]。しかし、実 験的研究において、界面活性剤や電解質を混入さ せることで気泡クラスターの形成が報告されて いる [15,16]。 界面活性剤や電解質の混入は、気泡表面の境界 条件を大きく変化させる。界面活性剤では気泡の 抗力の増加や合体を防ぎ、電解質ではやクリーン な気泡を保ちながら気泡合体を防ぐ [17][18][19][20] /01 02 0/%/ü ! 1 ö 3 4 5 6 7 8 9 6 % !ú ÷ ù ' ü ü ö !…”