Interaction and coalescence of large bubbles rising in a thin gap

We present accurate measurements of the relative motion and deformation of two large bubbles released consecutively in a quiescent liquid confined in a thin-gap cell. Although the second injected bubble was smaller, we observed that, in all cases, it accelerated and caught up with the leading bubble. This acceleration is related to the wake of the leading bubble, which also induces significant changes in the width and curvature of the trailing bubble. On the contrary, the velocity of the leading bubble is unaltered during the whole interaction and coalescence process. Shape adaptation of the two bubbles is observed just prior to coalescence. After pinch-off, the liquid film is drained at a constant velocity

Experimental study of liquid spreading in structured packings

Optimization of industrial gas-liquid columns dedicated to CO2 capture requires prediction of liquid distribution within packed beds. In this context, liquid hold-up as well as liquid spreading from a source point have been investigated for Mellapak 250.X structured packing. Local liquid hold-up measurements have been achieved in a 400 mm diameter column by means of gamma-ray tomography with operation in the counter-current mode at different positions downstream the source point injection. Liquid hold-up and retention map measurements have been performed for two fluid systems: Air / Water and Air / MEA 30wt.%. A correlation that relates global liquid hold-up and liquid load taking into account liquid viscosity is proposed. This correlation has been further used to determine spread factors using a simple dispersion model for all investigated operational conditions. Liquid dispersion model is found to well reproduce experimental data in the range of operational conditions that were tested which enables to determine spread factors for various operating conditions. The spread factor is observed not to vary with liquid load, gas capacity factor in the range of 20% to 80% of flooding nor liquid viscosity. This led us to stipulate that liquid dispersion is controlled by packing geometry only. Nevertheless, the effect of surface tension on liquid hold-up and dispersion is discussed since its effect is not fully understood and calls for further experiments if one wants to apply those results for hydrocarbons

Comparison of modern packings : assessing proper choice for post-combustion carbon capture absorption columns

CO2 capture from industrial flue gases is expected by IEA [1] to contribute to up to 19% of carbon mitigation by 2050. Absorption of CO2 into chemical solvents used in post-combustion capture processes is widely recognized as a reference path due to its high selectivity and CO2 recovery rate [2]. However, absorption and solvent regeneration columns required for that purpose are of huge sizes and further induce high investments impacting avoided CO2 cost. This impact is important enough so that it cannot be neglected when compared with operating unit costs [3]. We are presenting in this paper what is required in terms of packing characteristics, that is pressure drop, mass transfer performances and also liquid dispersion properties. This latter property, even if little discussed in the literature, is of great importance, since it will be used for determining the maximum height for packed beds as well as for column redistribution internals design. All these properties are presented for both random and structured packings and a discussion about packing choice is proposed, especially based mass transfer performances and on original dispersion results obtained for Mellapak and IMTP packing

Motion of a single bubble rising in a countercurrent flow in a Hele-Shaw cell

We investigate experimentally the motion of isolated bubbles rising in a vertical Hele-Shaw cell in the presence of a downward flow. The bubbles are strongly flattened in the plane of the cell, their equivalent diameter d being large compared to the gap of the cell e . Furthermore, their dynamics is strongly influenced by the confinement which imposes thin liquid films between the bubble and the walls and strongly attenuates the flow perturbation in the liquid due to wall friction

Mélange d’un constituant peu diffusif dans un nuage homogène de bulles

Les écoulements à bulles sont utilisés dans de nombreuses applications industrielles. Dans les opérations de transformation de la matière en colonne à bulles, pour réaliser des réactions chimiques il est souvent important d’assurer un mélange efficace des constituants dissous en phase liquide. En présence de couplages inverses forts, l'agitation au sein de tels écoulements est complexe à analyser et les mécanismes qui régissent le mélange ont été peu étudiés. Ce séminaire se propose de présenter des résultats récents concernant le problème générique du mélange d’un traceur peu diffusif par l’agitation induite au sein d'un nuage de bulles en ascension dans un liquide au repos à grand nombre de Reynolds particulaire. Nous montrerons que, tout comme l’agitation induite qui résulte d’interactions entre les sillages des bulles, le mélange gagne à être analysé en considérant que le mouvement fluctuant au sein du liquide est une combinaison de deux types d'agitation : (a) une agitation du liquide liée aux perturbations des sillages moyens qui est contrôlée par le positionnement aléatoire des bulles et (b) une agitation turbulente issue de l'instabilité de Navier-Stokes. Les propriétés du mélange ont été étudiées dans notre équipe sur plusieurs dispositifs expérimentaux définis pour contraster les mécanismes mis en jeu et favoriser leur analyse : (i) nuages de bulles réels non confinés, (ii) nuage de bulles en cellule de Hele-Shaw et (iii) écoulement traversant un réseau aléatoire de sphères fixes. Des résultats génériques commencent à émerger de ces études et permettent une meilleure compréhension du mélange engendré

Dynamics of a high-Reynolds-number bubble rising within a thin gap

We report an experimental analysis of path and shape oscillations of an air bubble of diameter d rising in water at high Reynolds number in a vertical Hele-Shaw cell of width h. Liquid velocity perturbations induced by the relative movement have also been investigated to analyze the coupling between the bubble motion and the wake dynamics. The confinement ratio h/d is lower than unity so that the bubble is flattened in between the walls of the cell. As the bubble diameter is increased, the Archimedes and the Bond numbers increase within 10 6 Ar 6 104 and 6 × 10−3 6 Bo 6 140. Mean shapes become more and more elongated. They first evolve from in-plane circles to ellipses, then to complicated shapes without fore-aft symmetry and finally to semi-circular capped bubbles. The scaling law Re = 0.5Ar is however valid for a large range of Ar, indicating that the liquid films between the bubble and the walls do no contribute significantly to the drag force exerted on the bubble. The coupling between wake dynamics, bubble path and shape oscillations evolves and a succession of contrasted regimes of oscillations is observed. The rectilinear bubble motion becomes unstable from a critical value Ar1 through an Hopf bifurcation while the bubble shape is still circular. The amplitude of path oscillations first grows as Ar increases above Ar1 but then surprisingly decreases beyond a second Archimedes number Ar2. This phenomenon, observed for steady ellipsoidal shape with moderate eccentricity, can be explained by the rapid attenuation of bubble wakes caused by the confinement. Shape oscillations around a significantly elongated mean shape starts for Ar > Ar3. The wake structure progressively evolves due to changes in the bubble shape. After the break-up of the fore-aft symmetry, a fourth regime involving complicated shape oscillations is then observed for Ar > Ar4. Vortex shedding disappears and unsteady attached vortices coupled to shape oscillations trigger path oscillations of moderate amplitude. Path and shape oscillations finally decrease when Ar is further increased. For Ar > Ar5, capped bubbles followed by a steady wake rise on a straight path

Dynamique d'un nuage de bulles homogène confiné

De nombreuses applications industrielles mettent en jeu des écoulements à bulles dans des échangeurs de masse et de chaleur ou des réacteurs. Les mouvements des bulles génèrent de l'agitation dans le liquide qui, en retour, influence la distribution spatiale des bulles et leur vitesse. La compréhension générique de ce problème de couplage inverse total est fondamentale mais délicate. Des travaux expérimentaux dédiés dans des configurations d'écoulements bien définies sont donc nécessaires pour atteindre cet objectif. Ce travail explore la dynamique d'un nuage de bulles en ascension à grand nombre de Reynolds dans une cellule de Hele-Shaw ([1]). Cette configuration apporte une contribution à une compréhension générale car elle permet d'étudier l'agitation générée par des bulles à grand nombre de Reynolds possédant des sillages instables tout en empêchant, par les effets de confinement, la production de turbulence. La comparaison avec la dynamique de nuages de bulles non confinés ([2]) est également éclairante. Par ailleurs, la détection des interfaces est considérablement facilitée par le confinement: une description complète et précise de la répartition spatiale et de la dynamique des bulles peut être ici obtenue directement par ombroscopie avec une seule caméra. De même, la mesure par PIV du champ de vitesse du liquide intégré dans l'épaisseur de la cellule permet de caractériser de manière pertinente la dynamique du liquide ([3]) (Fig.1-a). La dynamique des deux phases a ainsi été explorée pour des fractions volumiques de gaz α comprises entre 1% et 14% dans un régime où l'inertie est importante (Re≈500). Les bulles étudiées possèdent un sillage instable avec des lâchers tourbillonnaires réguliers et suivent une trajectoire ascendante oscillante tout en gardant une forme elliptique constante. Le frottement aux parois impose néanmoins une décroissance très forte des sillages ([4]). Les résultats montrent que l'on peut expliquer les statistiques associées au mouvement des bulles dans le nuage à partir de deux mécanismes élémentaires: (i) les oscillations induites par le sillage associées aux lâchers tourbillonnaires et (ii) la forte perturbation de vitesse localisée à l'arrière des bulles. Le mécanisme dominant dans la direction verticale est l'entrainement dans le sillage alors que celui qui régit la dynamique des bulles dans la direction horizontale est associé aux oscillations générées par les sillages dont l'intensité est indépendante de α (Fig.1-b). L'auto-dispersion des bulles a également été étudiée. Elle peut être caractérisée par des coefficients de dispersion qui évoluent linéairement avec α. En ce qui concerne l'agitation dans le liquide, comme en écoulement non confiné, les deux composantes des fluctuations de vitesse évoluent proportionnellement à αn avec ici αn=0.38 et 0.46 dans les directions horizontales et verticales respectivement. Le spectre spatial des fluctuations de vitesse évolue, sur une gamme de nombres d'ondes k bien définie, proportionnellement à k-³. Dans cette configuration où la turbulence ne peut se développer, cette évolution s'explique très clairement par la superposition linéaire de perturbations de vitesses aléatoires ([5]), il s'agit donc d'un effet statistique associé aux passages de perturbations convectées par les bulles

Attenuation of the wake of a sphere in an intense incident turbulence with large length scales

We report an investigation of the wake of a sphere immersed in a uniform turbulent flow for sphere Reynolds numbers ranging from 100 to 1000. An original experimental setup has been designed to generate a uniform flow convecting an isotropic turbulence. At variance with previous works, the integral length scale of the turbulence is of the same order as the sphere diameter and the turbulence intensity is large. In consequence, the most intense turbulent eddies are capable of influencing the flow in the close vicinity of the sphere. Except in the attached region downstream of the sphere where the perturbation of the mean velocity is larger than the standard deviation of the incident turbulence, the flow is controlled by the incident turbulence. The distortion of the turbulence while the flow goes round the sphere leads to an increase in the longitudinal fluctuation and a decrease in the transversal one. The attenuation of the transversal fluctuations is still significant at 30 radii downstream of the sphere whereas the longitudinal fluctuations relax more rapidly toward the incident value. The more striking result however concerns the evolution of the mean velocity defect with the distance x from the sphere. It decays as x−2 and scales with the standard deviation of the incident turbulence instead of scaling with the mean incident velocity

PIV with volume lighting in a narrow cell: An efficient method to measure large velocity fields of rapidly varying flows

In this work we test a methodology for PIV measurements when alargefield of view is required in planar confined geometries. Using a depth of fieldlarger than the channel width, we intend to measure the in-plane variations of the velocity of the fluid averaged through the width of the channel, and we examine in which operating conditions this becomes possible. Measurements of the flow through anarrow channel by PIV are challenging because of the strong velocity gradients that develop between the walls. In particular, all techniques that use small particles as tracers have to deal with the possible migration of the tracers in the direction perpendicular to the walls. Among the complex mechanisms for migration, we focus on the so called Segré–Silberberg effect which can lead to transverse migration of neutrally buoyant tracers of finite size. We report experimental PIV measurements in a Hele-Shaw cell of 1 mm gap, which have been carried out by using neutrally buoyant tracers of size around 10 μm. By considering steady flows, we have observed, in particular flow regimes, the effect of an accumulation of the tracers at a certain distance to the wall due to the so called Segré–Silberberg effect. The particle migration is expected to occur at any Reynolds numbers but the migration velocity depends on the Reynolds number. A significant migration therefore takes place each time the observation duration is large enough compared to the migration time. For a given observation duration, the tracers remain uniformly distributed at low Reynolds numbers whereas they all accumulate at the equilibrium position at large ones. When using volumelighting, the PIV algorithm provides the average velocity of the flow through the gap at low Reynolds number, while it leads to the velocity of the flow at the equilibrium position of the tracers at large Reynolds numbers. By considering unsteady flows, we have observed that the migration does not occur if the timescale of flow variation is short compared to the time required for the parabolic flow to develop across the gap. In this case, there is no transverse velocity gradient and the PIV algorithm provides the fluid velocity. Altogether, these results allow us to propose guidelines for the interpretation of PIV measurements in confined flow, which are based on the theoretical predictions of the tracer migration derived by Asmolov [1]

Homogeneous swarm of high-Reynolds-number bubbles rising within a thin gap. Part 1: Bubble dynamics

The spatial distribution, the velocity statistics and the dispersion of the gas phase have been investigated experimentally in a homogeneous swarm of bubbles confined within a thin gap. In the considered flow regime, the bubbles rise on oscillatory paths while keeping a constant shape. They are followed by unstable wakes which are strongly attenuated due to wall friction. According to the direction that is considered, the physical mechanisms are totally different. In the vertical direction, the entrainment by the wake controls the bubble agitation, causing the velocity variance and the dispersion coefficient to increase almost linearly with the gas volume fraction. In the horizontal direction, path oscillations are the major cause of bubble agitation, leading to a constant velocity variance. The horizontal dispersion, which is lower than that in the vertical direction, is again observed to increase almost linearly with the gas volume fraction. It is however not directly due to regular path oscillations, which are unable to generate a neat deviation over a whole period, but results from bubble interactions which cause a loss of the bubble velocity time correlation