Hydrodynamic and mass transfer in inertial gas–liquid flow regimes through straight and meandering millimetric square channels

Heat-exchanger reactors are an important part of process intensification technology. For plate geometries, one solution for intensifying transfer and increasing residence times is to construct two-dimensional meandering channels. Supported by this scientific context, the present work aims at characterising gas–liquid mass transfer in the same square millimetric meandering channel, as in Anxionnaz (2009), this constituted the preliminary step required for performing exothermic gas–liquid reactions. Firstly, the gas–liquid hydrodynamics were characterised for a water/air system. When compared to a straight channel of identical compactness and sectional-area (2×2 mm2), the meandering channel induced (i) a delay in the transition from Taylor to annular-slug regimes, (ii) a rise of 10–20% in bubble lengths while conserving almost identical slug lengths, (iii) higher deformations of bubble nose and rear due to centrifugal forces (bends). Secondly, an original method for verifying the relevancy of the plug flow model and accurately determining kla was used (measurements of concentrations in dissolved oxygen along the channel length). For the Taylor flow regime, kla increased coherently when increasing jg, and the meandering geometry had a small influence. On the contrary, this effect was found no more negligible for the slug-annular flow regime. Whatever the channels, the NTUl remained low, thus showing that, even if millimetric channels allowed to intensify kla, a special attention should be paid for generating sufficient residence times. At identical compactness, the meandering channel was found to be the most competitive. Finally, results on gas–liquid interfacial areas and mass transfer coefficients were confronted and discussed with respect to the predictions issued from the model developed by Van Baten and Krishna (2004)

Corrigendum to "Hydrodynamic and mass transfer in inertial gas-liquid flow regimes through straight and meandering millimetric square channels" [Chem. Eng. Sci. 66 (2011) 2974-2990]

Heat-exchanger reactors are an important part of process intensification technology. For plate geometries, one solution for intensifying transfer and increasing residence times is to construct two-dimensional meandering channels. Supported by this scientific context, the present work aims at characterising gas-liquid mass transfer in the same square millimetric meandering channel, as in Anxionnaz (2009), this constituted the preliminary step required for performing exothermic gas-liquid reactions. Firstly, the gas-liquid hydrodynamics were characterised for a water/air system. When compared to a straight channel of identical compactness and sectional-area (2×2 mm²), the meandering channel induced (i) a delay in the transition from Taylor to annular-slug regimes, (ii) a rise of 10-20% in bubble lengths while conserving almost identical slug lengths, (iii) higher deformations of bubble nose and rear due to centrifugal forces (bends). Secondly, an original method for verifying the relevancy of the plug flow model and accurately determining kla was used (measurements of concentrations in dissolved oxygen along the channel length). For the Taylor flow regime, kla increased coherently when increasing jg, and the meandering geometry had a small influence. On the contrary, this effect was found no more negligible for the slug-annular flow regime. Whatever the channels, the NTUl remained low, thus showing that, even if millimetric channels allowed to intensify kla, a special attention should be paid for generating sufficient residence times. At identical compactness, the meandering channel was found to be the most competitive. Finally, results on gas-liquid interfacial areas and mass transfer coefficients were confronted and discussed with respect to the predictions issued from the model developed by Van Baten and Krishna (2004)

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]

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

Hydrodynamique et transfert de masse autour d'une bulle confinée entre deux plaques

Ce travail de recherche est consacré à l'étude expérimentale de la dynamique d'une bulle isolée "bidimensionnelle", ainsi qu'à celle du transfert de masse de cette bulle vers la phase liquide pour le couple de fluides eau-oxygène. L'aspect bidimensionnel des bulles vient du fait qu'elles sont confinées entre 2 plaques. La gamme de nombres a dimensionnels que nous avons balayée dans notre étude a été peu étudiée pour ce genre de dispositif. Dans cette étude, les effets inertiels lies a la perturbation de vitesse provoquée par le passage de la bulle ne sont pas négligeables devant le frottement aux parois. L’étude de l'hydrodynamique de la bulle est réalisée grâce a l'utilisation de techniques de mesure optique : ombroscopie et vélocimétrie par image laser (PIV). En raison de la géométrie 2D de la cellule, un éclairage en volume du champ d'investigation est réalisé pour les mesures PIV. Il est montre que la PIV mesure la vitesse moyenne du liquide dans l’épaisseur entre les plaques, lors du passage d'une bulle. Les mesures par ombroscopie permettent d’étudier la trajectoire et la forme de la bulle au cours du temps pour une large gamme de nombres de Reynolds (50-6000). On montre que la vitesse moyenne des bulles vérifie la loi d’échelle U = 0.5√gd compatible avec une interface non contaminée. D'autre part, l’étude de l’instabilité du mouvement et de la forme permet d'identifier différents régimes d’écoulement et notamment de montrer que la forme moyenne des bulles et la structure du sillage sont essentiels pour comprendre l’évolution des oscillations. De plus, les champs de vitesse issus de la PIV donnent l’évolution du sillage a l’arrière de la bulle en fonction du nombre d’Archimède et mettent en évidence deux structures clairement différentes, suivant que les bulles sont des calottes avec une trajectoire rectiligne, ou des ellipsoïdes qui oscillent. Ils montrent également que, quelque soit le nombre de Reynolds, le sillage décroit rapidement a cause du confinement. L’étude du transfert de l’oxygène, contenu dans la bulle, vers le liquide, est réalisée au moyen de la technique de fluorescence induite par nappe laser (PLIF), avec inhibition de la fluorescence par l’oxygène dissous, et avec un éclairage en volume de la cellule. L'analyse des images de fluorescence permet de distinguer le transfert de matière issu de deux régions de l'interface de la bulle : la surface en contact avec les films liquides entre la bulle et les plaques, et la surface périphérique de la bulle. Le transfert de masse par les films se faisant dans un espace mince et proche des parois, l’oxygène n'est pas reparti de manière uniforme dans l’épaisseur aux temps courts après le passage de la bulle. La non linéarité du signal de fluorescence avec la concentration en oxygène implique de prendre en compte la répartition de l’oxygène entre les plaques. Un modèle de répartition de l’oxygène entre les plaques est propose, et confronte a des mesures de concentration entre les plaques après diffusion ; cette démarche permet de déterminer la contribution relative des deux surfaces au transfert. Les flux de matière ainsi que les densités de flux issus de chacune des deux régions de la surface de la bulle sont calcules et discutes

Dust emission by powder handling: Comparison between numerical analysis and experimental results

The dust generation occurring during the handling of bulk materials in free falls or at the impact on a stockpile can be a source of danger for the operators health. Proper design of control systems of fugitive dust requires knowledge of the behavior of the free falling powder, the air it entrains, and the concentration of dust liberated. This paper presents first a simple model for a free falling column of bulk solids and compares it with relevant previous research. This two phase model predicts the particle and air velocities, and especially the volumetric flow of induced air in the column without dependence on any empirical constant like the entrainment constant used in the plume model. For small drop heights, the predictions of the theory appear to be in qualitative agreement with the available data for the quantity of air entrained, but the theory needs to be extended in the case of large drop heights, when the expansion of the jet of particles is large. In a second part, the description of an expanded jet of particles is experimentally studied with PIV measurements. The data obtained are well fitted by the model by Liu, when the entrainment constant is taken as the angle of expansion of the jet obtained from the velocity field

Experimental Investigation of Interfacial Mass Transfer Mechanisms for a Confined High-Reynolds-Number Bubble Rising in a Thin Gap

Interfacial mass transfer is known to be enhanced for confined bubbles due to the efficiency of the transfer in the thin liquid films between them and the wall. In the present experimental investigation, the mechanisms of gas–liquid mass transfer are studied for isolated bubbles rising at high Reynolds number in a thin gap. A planar laser induced fluores- cence (PLIF) technique is applied with a dye the fluorescence of which is quenched by dissolved oxygen. The aim is to measure the interfacial mass fluxes for pure oxygen bubbles of various shapes and paths rising in water at rest. In the wakes of the bubbles, patterns due to the presence of dissolved oxygen are observed on PLIF images. They reveal the contrasted contributions to mass transfer of two different regions of the interface. The flow around a bubble consists of both two thin liquid films between the bubble and the walls of the cell and an external high-Reynolds-number in-plane flow surrounding the bubble. Mass transfer mechanisms associated to both regions are discussed. Measurement of the concentration of dissolved oxygen is a difficult task due to the nonlinear relation between the fluorescence intensity and the concentration in the gap. It is however possible to accurately measure the global mass flux transferred through the bubble interface. It is determined from the fluorescence intensity recorded in the wakes when the oxygen distribution has been made homogeneous through the gap by diffusion. Assuming a reasonable distribution of oxygen concentration through the gap at short time also allows a measurement of the mass fluxes due to the liquid films. A discussion of the results points out the specific physics of mass transfer for bubbles confined between two plates as compared to bubbles free to move in unconfined flows. VC 2016 American Institute of Chemical Engineers AIChE J, 63: 2394–2408, 201

Hydrodynamique et transfert de masse autour d'une bulle confinée entre deux plaques

Ce travail de recherche est consacré à l'étude expérimentale de la dynamique d'une bulle isolée "bidimensionnelle", ainsi qu'à celle du transfert de masse de cette bulle vers la phase liquide pour le couple de fluides eau-oxygène. L'aspect bidimensionnel des bulles vient du fait qu'elles sont confinées entre 2 plaques. La gamme de nombres adimensionnels que nous avons balayée dans notre étude a été peu étudiée pour ce genre de dispositif. Dans cette étude, les effets inertiels liés à la perturbation de vitesse provoquée par le passage de la bulle ne sont pas négligeables devant le frottement aux parois. L'étude de l'hydrodynamique de la bulle est réalisée grâce à l'utilisation de techniques de mesure optique : ombroscopie et vélocimétrie par image laser (PIV). En raison de la géométrie 2D de la cellule, un éclairage en volume du champ d'investigation est réalisé pour les mesures PIV. Il est montré que la PIV mesure la vitesse moyenne du liquide dans l'épaisseur entre les plaques, lors du passage d'une bulle. Les mesures par ombroscopie permettent d'étudier la trajectoire et la forme de la bulle au cours du temps pour une large gamme de nombres de Reynolds (50-6000). On montre que la vitesse moyenne des bulles vérifie la loi d'échelle U = 0.5 gd compatible avec une interface non contaminée. D'autre part, l'étude de l'instabilité du mouvement et de la forme permet d'identifier différents régimes d'écoulement et notamment de montrer que la forme moyenne des bulles et la structure du sillage sont essentiels pour comprendre l'évolution des oscillations. De plus, les champs de vitesse issus de la PIV donnent l'évolution du sillage à l'arrière de la bulle en fonction du nombre d'Archimède et mettent en évidence deux structures clairement différentes, suivant que les bulles sont des calottes avec une trajectoire rectiligne, ou des ellipsoïdes qui oscillent. Ils montrent également que, quelque soit le nombre de Reynolds, le sillage décroît rapidement à cause du confinement. L'étude du transfert de l'oxygène, contenu dans la bulle, vers le liquide, est réalisée au moyen de la technique de fluorescence induite par nappe laser (PLIF), avec inhibition de la fluorescence par l'oxygène dissous, et avec un éclairage en volume de la cellule. L'analyse des images de fluorescence permet de distinguer le transfert de matière issu de deux régions de l'interface de la bulle : la surface en contact avec les films liquides entre la bulle et les plaques, et la surface périphérique de la bulle. Le transfert de masse par les films se faisant dans un espace mince et proche des parois, l'oxygène n'est pas réparti de manière uniforme dans l'épaisseur aux temps courts après le passage de la bulle. La non linéarité du signal de fluorescence avec la concentration en oxygène implique de prendre en compte la répartition de l'oxygène entre les plaques. Un modèle de répartition de l'oxygène entre les plaques est proposé, et confronté à des mesures de concentration entre les plaques après diffusion ; cette démarche permet de déterminer la contribution relative des deux surfaces au transfert. Les flux de matière ainsi que les densités de flux issus de chacune des deux régions de la surface de la bulle sont calculés et discutés.This work reports an experimental investigation dedicated to hydrodynamics and mass transfer for an isolated bubble confined between two plates separated by a distance smaller than bubble diameter. In a large range of bubble Reynolds numbers (50-6000), the path and shape oscillations have been studied by means of the shadowgraph technique, and the fluid velocity has been measured by Particle Image Velocimetry. Different flow regimes have been identified, and it appears that the mean bubble shape and the wake structure are key parameters of the shape and path oscillation phenomena. The oxygen transfer from an oxygen bubble to the liquid was studied by means of the Planar Laser Induced Fluorescence technique. The results emphasize the contribution to the oxygen transfer of two interface areas : the surface in contact with the liquid films laying between bubble and plates, and the lateral surface of the bubble.TOULOUSE-ENSIACET (315552325) / SudocSudocFranceF

Modelisation for particles column during free falling process

National audienc

Photobioreactor Modeling and Radiative Transfer Analysis for Engineering Purposes

International audienceThe present chapter introduces the theoretical framework for constructing predictive knowledge-models leading to the calculation of the volumetric rate of biomass production, the surface rate of biomass production and the thermodynamic efficiency of photobioreactors. Here, the main assumption is that photosynthesis reaction is limited by radiative transfer only. First, the predictive determination of the scattering and absorption properties of photosynthetic microorganisms of various types is addressed. Then, these radiative properties are used to calculate the radiation field within the reaction volume by solving the radiative transfer equation. Both the development of approximate solutions appropriated with typical photobioreactor configurations (intermediate scattering optical-thickness) and the rigorous solution of the radiative transfer equation by the Monte Carlo method are addressed, including the treatment of complex geometric structures. Finally, the thermokinetic coupling between the radiation field, the photosynthesis reaction rates and thermodynamic efficiency are investigated. For the special case of the cyanobacterium Arthrospira platensis, a complete stoichiometric, kinetic and thermodynamic model is constructed using the linear thermody-namics of irreversible processes to analyze the primary events of photosynthesis (Z-scheme). Comparison between the theoretical calculations presented in this chapter and experimental results confirms the ability of the proposed predictive approach, after parameters reification, to quantify performances of many kinds of photobioreactors (geometry, size) functioning under different operating conditions. An extension of the proposed coupling approach for the more complicated case of eukaryotic (microalgae) microorganisms is then proposed as further perspective of this work