Velocity fluctuations in a homogeneous dilute dispersion of high-Reynolds-number rising bubbles

Abstract

An experimental investigation of a homogeneous swarm of rising bubbles is presented. The experimental arrangement ensures that all the bubbles have the same equivalent radius, a = 1.25 mm. This particular size corresponds to high-Reynolds-number ellipsoidal rising bubbles. The gas volume fractions α is small, ranging from 0.5 to 1.05%. The results are compared with the reference situation of a single rising bubble, which was investigated in a previous work. From the use of conditional statistics, the existence of two regions in which the liquid velocity fluctuations are of a different nature are distinguished. In the vicinity of the bubbles, the liquid fluctuations are the same as those measured close to a single rising bubble. They therefore do not depend on α. Far from the bubble, the liquid fluctuations are controlled by the nonlinear interactions between the wakes of all the bubbles. Their probability density function scales as α0.4, exhibiting a self-similar behaviour. The total fluctuation combines the contributions of these two regions weighted by the fraction of the liquid volume they occupy. The contribution of the bubble vicinity is thus shown to vary linearly with α while the wake contribution does not. Both are non-isotropic since strong upward vertical fluctuations are more probable.

Voronoï analysis of bubbly flows via ultrafast X-ray tomographic imaging

“…At low Morton numbers, there is increased clustering leading to coalescence in bubble swarms and thus creating larger bubbles, while at high Morton numbers coalescence has not been observed and therefore fosters a larger interfacial area, which is more advantageous regarding mass transfer. This is in accordance with the experimental study of Risso and Ellingsen (2002), who examined rising bubbles bearing a high Reynolds number and could not observe any cluster formation. It should be noted that, since the Morton number is inversely proportional to the fourth power of the Reynolds number, there is some tradeoff between a large interfacial area and high Reynolds numbers for individual bubbles.…”

Velocity fluctuations generated by the flow through a random array of spheres: a model of bubble-induced agitation

“…Most of its energy comes from variations between regions that include wakes, where the timeaveraged velocity is low, and regions that surround the wakes, where the time-averaged velocity is close to the bulk velocity. At this stage, it is important to recall that the wake of a sphere surrounded by many spheres is strongly attenuated compared to that of an isolated sphere, as is the case for a bubble in a swarm of bubbles (Risso & Ellingsen 2002;Roig & Larue de Tournemine 2007). Risso et al (2008) have shown that both the wake of a sphere or the wake of a bubble exponentially decay with the distance, and have almost vanished before 3 diameters.…”

“…In this context, the description of the liquid agitation is a key issue. In the most general situation, such as that encountered in an industrial bubble column, the agitation has different sources: (1) the bubble-induced agitation that results from the flow disturbances generated by bubble motions (Lance & Bataille 1991;Cartellier & Rivière 2001;Zenit, Koch & Sangani 2001;Risso & Ellingsen 2002), (2) the buoyancy-driven recirculation cells that develop as soon as the volume fraction is not uniform (Climent & Magnaudet 1999;Mudde 2005), and (3) the turbulence produced by mean shear (Larue de Tournemine & Roig 2010;Prakash et al 2016). The present work focuses on the bubble-induced agitation at large Reynolds number, i.e.…”

Wake attenuation in large Reynolds number dispersed two-phase flows

“…However, the latter contribution is only significant close to the bubble and the wake decay is predicted well by the steady axisymmetric wake around a rectilinear rising bubble, provided the local orientation of the wake is taken parallel to the bubble trajectory (figures 15 and 20 in Ellingsen & Risso 2001). Risso & Ellingsen (2002) investigated a homogeneous swarm of bubbles of the same size rising in water otherwise at rest at dilute gas volume fractions (5!10 K3 %a%10 K2 ). Concerning the bubbles, dual optical probe (DOP) measurements showed that their positions were randomly distributed (no clustering) and that their fluctuating velocity remained controlled by the path oscillations-bubble interactions had thus negligible effects.…”