Experimental investigation of the turbulence induced by a bubble swarm rising within incident turbulence

Abstract

This work reports an experimental characterisation of the flow properties in a homogeneous bubble swarm rising at high Reynolds numbers within a homogeneous and isotropic turbulent flow. Both 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% < alpha < 0.93% and 2.3 cm s(-1) < u '(0) < 5.5 cm s(-1). The so-called bubblance parameter (b = V-r(2)alpha/u '(2)(0) 0, where V-r is the bubble relative rise velocity) is used to compare the ratio of the kinetic energy generated by the bubbles to the one produced by the incident turbulence, and is varied from 0 to 1.3. Conditional measurements of the velocity field downstream of the bubbles in the vertical direction allow us to disentangle three regions that have specific statistical properties, namely the primary wake, the secondary wake and the far field. While the fluctuations in the primary wake are similar to that of a single bubble rising in a liquid at rest, the statistics of the velocity fluctuations in the far field follow a Gaussian distribution, similar to that produced by the homogenous and isotropic turbulence at the largest scales. In the secondary wake region, the conditional probability density function of the velocity fluctuations is asymmetric and shows an exponential tail for the positive fluctuations and a Gaussian one for the negative fluctuations. The overall agitation thus results from the combination of these three contributions and depends mainly on the bubblance parameter. For 0 < b < 0 : 7, the overall velocity fluctuations in the vertical direction evolve as b(0.4) and are mostly driven by the far-field agitation, whereas the fluctuations increase as b(1.3) for larger values of the bubblance parameter (b > 0.7), in which the significant contributions come both from the secondary wake and the far field. Thus, the bubblance parameter is a suitable parameter to characterise the evolution of liquid agitation in bubbly turbulent flows

Liquid velocity fluctuations and energy spectra in three-dimensional buoyancy-driven bubbly flows

“…, we find that w is indeed comparable to the viscous dissipation in the fluid phase µ,f (see table 1). (Riboux et al 2010;Almras et al 2017;Prakash et al 2016). Our results are consistent with the recently proposed stochastic model of Risso (2016) and the turbulent agitation because of flow instabilities together lead to the observed velocity distributions.…”

“…Riboux et al (2010) investigated turbulence in the wake of a bubble swarm and found the k −3 scaling for length scales larger than the bubble diameter d (i.e., k < 2π/d), whereas Mercado et al (2010); Prakash et al (2016) observed this scaling for scales smaller than d in a steady state bubble suspension. Experiments on bouyancy driven bubbly flows in presence of grid-turbulence (Lance & Bataille 1991;Prakash et al 2016;Almras et al 2017) observe Kolmogorov scaling for scales larger than the bubble diameter and smaller than the forcing scale and a much steeper k −3 scaling for scales smaller than the bubble diameter and larger than the dissipation scale. Lance & Bataille (1991) argued that, assuming production because of wakes to be local in spectral space, balance of production with viscous dissipation leads to the observed k −3 scaling.…”

Mixing induced by a bubble swarm rising through incident turbulence

“…These have been characterised by looking at the statistical properties of the liquid phase in the absence of bubbles (see Ref. [28] for more details). Table I summarises the flow characterisation of the eight single phase turbulence cases investigated in the present study.…”

“…We expect this to be comparable to the Lagrangian integral time scale, which is relevant for turbulent mixing [32]. A homogeneous bubble swarm is produced using 621 capillary tubes of inner diameter 0.12 mm positioned on 9 islands, which are located below the active-grid [28]. By varying the gas flow rate, the gas volume fraction α can be adjusted from 0 to 0.93%.…”

“…These have addressed the problem from both Eulerian and Lagrangian viewpoints. Alméras et al [28] showed that the liquid agitation in such flows are the outcome of a complex interaction between the bubble swarm and external turbulence at the large scales. Recently, in a Lagrangian investigation of 2 mm bubbles in homogeneous turbulence, Mathai et al [29] reported that bubbles contribute to a fast short term spreading, but a slower large scale spreading.…”