Mécanisme de mélange par convection intermittente dans un nuage de bulles confinées

Ce travail s'intéresse au mélange d'un traceur passif peu diffusif dans un nuage homogène de bulles en ascension dans une cellule Hele-Shaw. Le nombre de Reynolds du mouvement relatif des bulles est élevé. L'écoulement peut être considéré comme bidimensionnel et possède une agitation du liquide très particulière, liée principalement aux sillages des bulles ([1], [2]). Nous avons réalisé des expériences de mélange en injectant un traceur fluorescent au sein du nuage de bulles pendant un temps fini. Une technique de mesure LIF originale et adaptée à cet écoulement à bulles a été développée. Elle consiste à éclairer avec un laser un volume de 0,5 mm³, à différentes distances de l'injecteur, et à observer avec une fibre optique la lumière de ce volume qui est transmise à un spectromètre permettant d'analyser la lumière fluorescée. Il est donc possible de remonter à la concentration locale à une fréquence de 250 Hz (résolution temporelle de la mesure). La figure 1.a montre une évolution typique de la concentration du colorant en un point situé à 150 mm au dessus de l'injection. Dans un premier temps, le traceur arrivant dans le volume de mesure, la concentration augmente, la diminution de concentration qui suit, se fait de manière exponentielle ce qui montre que le mélange en cellule Hele-Shaw n'est pas un processus diffusif de type Fickien [3]. De plus, nous observons à une échelle temporelle beaucoup plus courte des fluctuations de concentration très marquées. Cette intermittence est principalement due aux mécanismes intrinsèques au mélange en cellule de Hele-Shaw. Ce mélange se fait principalement par séquences de capture - transport - largage de colorant par les sillages des bulles (figure 1.b). Un modèle de mélange convectif intermittent a été développé et reproduit bien les expériences

Dynamic Behaviour of a Flexible Yacht Sail Plan

• Dynamic fluid structure interaction of a sail plan is modeled in harmonic pitching • Aerodynamic forces oscillations show hysteresis phenomena • Neglecting the structural deformation underestimates the forces oscillations • Both aerodynamic and structure inertia affect loads in the rig.A numerical investigation of the dynamic Fluid Structure Interaction (FSI) of a yacht sail plan submitted to harmonic pitching is presented to address both issues of aerodynamic unsteadiness and structural deformation. The FSI model | Vortex Lattice Method uid model and Finite Element structure model | has been validated with full-scale measurements. It is shown that the dynamic behaviour of a sail plan subject to yacht motion clearly deviates from the quasi-steady theory. The aerodynamic forces presented as a function of the instantaneous apparent wind angle show hysteresis loops, suggesting that some energy is exchanged by the system. The area included in the hysteresis loop increases with the motion reduced frequency and amplitude. Comparison of rigid versus soft structures shows that FSI increases the energy exchanged by the system and that the oscillations of aerodynamic forces are underestimated when the structure deformation is not considered. Dynamic loads in the fore and aft rigging wires are dominated by structural and inertial effects. This FSI model and the obtained results may be useful firstly for yacht design, and also in the field of auxiliary wind assisted ship propulsion, or to investigate other marine soft structures.This work was supported by the French Naval Academy

A suitable parametrization to simulate slug flows with the Volume-Of-Fluid method

Diffuse–interface methods, such as the Volume-Of-Fluid method, are often used to simulate complex multiphase flows even if they require significant computation time. Moreover, it can be difficult to simulate some particular two-phase flows such as slug flows with thin liquid films. Suitable parametrization is necessary to provide accuracy and computation speed. Based on a numerical study of slug flows in capillary tubes, we show that it is not trivial to optimize the parametrization of these methods. Some simulation problems described in the literature are directly related to a poor model parametrization, such as an unsuitable discretization scheme or too large time steps. The weak influence of the mesh irregularity is also highlighted. It is shown how to capture accurately thin liquid films with reasonably low computation times

On the reliability of an optical fibre probe in bubble column under industrial relevant operating conditions

When bubble columns are operated under industrial relevant conditions (high gas and liquid flow rates, large bubbles and vortices,. . .), local data, and especially bubble size values, are difficult to obtain. However, such data are essential for the comprehension of two-phase flow phenomena in order to design or to improve industrial installations. When high gas flow rates and organic liquids are used, intrusive optic probes are considered. This work investigates different ways to derive reliable local information on gas phase from double optic probe raw data. As far as possible, these results have been compared with global data, easier to measure in such conditions. Local gas hold-up, eG, and bubble frequency, fB, are easily obtained, but bubble velocity and bubble diameter determination is not obvious. For a better reliability, the final treatment that is proposed for velocity and size estimation is based on mean values only: the bubble velocity is considered as the most probable velocity ~v issued from raw signals inter-correlation function and the mean Sauter diameter is calculated through dSM ¼ 3~veG 2f B

Mass transfer in bubble column for industrial conditions—effects of organic medium, gas and liquid flowrates and column design

Most of available gas–liquid mass transfer data in bubble column have been obtained in aqueous media and in liquid batch conditions, contrary to industrial chemical reactor conditions. This work provides new data more relevant for industrial conditions, including comparison of water and organic media, effects of large liquid and gas velocities, perforated plates and sparger hole diameter. The usual dynamic O2 methods for mass transfer investigation were not convenient in this work (cyclohexane, liquid circulation). Steadystate mass transfer of CO2 in an absorption–desorption loop has been quantified by IR spectrometry. Using a simple RTD characterization, mass transfer efficiency and kLa have been calculated in a wide range of experimental conditions. Due to large column height and gas velocity, mass transfer efficiency is high, ranging between 40% and 90%. kLa values stand between 0.015 and 0.050 s−1 and depend mainly on superficial gas velocity. No significant effects of column design and media have been shown. At last, using both global and local hydrodynamics data, mass transfer connection with hydrodynamics has been investigated through kLa/G and kLa/a

Application of the double optic probe technique to distorted tumbling bubbles in aqueous or organic liquid

The optic probe technique is widely used to investigate bubble reactors. To derive values of bubble local velocities and bubble local sizes, a specific signal treatment is usually applied under severe assumptions for bubble path and shape. However, in most industrial reactors, bubble motion is chaotic and no common shape can be assumed. In this work, the reliability of the signal treatment associated with the optic probe technique is examined for distorted and tumbling bubbles. A double-tip optic probe is settled in a glass tank and the rise of bubbles is filmed simultaneously. Several trains of bubbles are studied, interactions between bubbles being gradually increased. Referring to image analysis, several ways to derive mean bubble velocities from optic probe data have been compared. Crenels from front tipand rear tipra w signals are associated and individual bubble velocities are derived. Nevertheless, complete velocity distributions are difficult to obtain, as they depend on the choice of the time within which the bubble is searched on the second tip. Using a simpler approach it is shown that the most probable velocity, calculated through the raw signals inter-correlation, is a correct estimation of the average bubble velocity. Concerning bubble size, bubble chord distributions show too high values due to bubble distortion and deviation. A simplified estimation of bubble mean Sauter diameter, using the most reliable measurements only (i.e., local gas hold-up, local mean bubbling frequency, and most probable bubble velocity), was tested for highly distorted bubbles; this method was validated both in water and cyclohexane

Piv study of mixing characteristics in a stirred vessel with a non-Newtonian fluid

PIV is used to analyze the flow induced by a Rushton turbine in a shear-thinning fluid, at constant input power, constant impeller velocity but different concentrations. The rheology of each shear-thinning fluid is first addressed. The mean velocity fields are compared. POD methodology is applied to estimate coherent structures and turbulence levels. Finally, the heterogeneity of shear rate is estimated and the spatial distribution of dissipation rate of total kinetic energy is addressed

Mélange dans un écoulement à bulles : mesures expérimentales et modélisation

Nous étudions le mélange d’un traceur passif peu diffusif dans un essaim de bulles homogène en ascension dans une cellule de Hele-Shaw remplie d’eau à grand nombre de Reynolds particulaire. Cet écoulement peut être considéré comme bidimensionnel. L’agitation du liquide, principalement due aux sillages des bulles, possède des propriétés originales (Bouche et al., 2012 ; Roig et al., 2012). Pour effectuer des mesures de concentration résolues en temps, nous avons adapté la technique de Fluorescence Induite par Laser (LIF) à l’investigation des écoulements à bulles. Les mesures de LIF sont faites en un point et couplée à de la spectrométrie. D’un côté de la cellule de Hele-Shaw, un laser éclaire un volume de mesure et permet d’exciter le colorant. De l'autre côté, une fibre optique collecte la lumière et la transmet à un spectromètre qui permet d’isoler la lumière fluorescée et d’en déduire la concentration locale à une cadence de 250 Hz. A une certaine distance de l’injecteur, le signal de concentration mesuré présente d’intenses et rapides fluctuations qui sont la trace du passage de paquets de colorant transportés par les bulles individuellement. Une évolution de la concentration bien plus lente se superpose à ces fluctuations et montre deux phases d’évolution. Dans un premier temps, le niveau de concentration augmente globalement car les bulles transportent du colorant jusqu’au volume de mesure. Puis, comme l’injection se fait pendant un temps fini, une phase de décroissance de la concentration est observée. Cette décroissance est exponentielle et ne peut être compatible avec un modèle diffusif. Néanmoins, elle est compatible avec un modèle de transport de colorant par les sillages moyens des bulles, sillages qui sont fortement écrantés et atténués par le frottement aux parois. De ce fait, le mélange dans un écoulement à bulles en cellule de Hele-Shaw est convectif et intermittent

Liquid spreading in trickle-bed reactors: Experiments and numerical simulations using Eulerian--Eulerian two-fluid approach

Liquid spreading in gas-liquid concurrent trickle-bed reactors is simulated using an Eulerian twofluid CFD approach. In order to propose a model that describes exhaustively all interaction forces acting on each fluid phase with an emphasis on dispersion mechanisms, a discussion of closure laws available in the literature is proposed. Liquid dispersion is recognized to result from two main mechanisms: capillary and mechanical (Attou and Ferschneider, 2000; Lappalainen et al., 2009- The proposed model is then implemented in two trickle-bed configurations matching with two experimental set ups: In the first configuration, simulations on a 2D axisymmetric geometry are considered and the model is validated upon a new set of experimental data. Overall pressure drop and liquid distribution obtained from $\gamma$-ray tomography are provided for different geometrical and operating conditions. In the second configuration, a 3D simulation is considered and the model is compared to experimental liquid flux patterns at the bed outlet. A sensitivity analysis of liquid spreading to bed geometrical characteristics (void-fraction and particles diameter) as well as to gas and liquid flow rates is proposed. The model is shown to achieve very good agreement with experimental data and to predict, accurately, tendencies of liquid spreading for various geometrical bed characteristics and/or phases flow-rates

Mixing mechanism in a two-dimensional bubble column

The present contribution investigates the mixing of a passive scalar by a homogeneous bidimensional bubble swarm rising at high Reynolds number in a liquid initially at rest. Mixing experiments are performed in a Hele-Shaw cell for gas volume fractions α ranging from 3.0% to 14.0%. A weakly diffusive passive dye is injected within the swarm, and the temporal evolution of the spatial distribution of concentration is measured. The vertical distribution of concentration shows an upward propagation and a spreading due to the mixing induced by the unsteady open wakes of the bubbles. A one-dimensional large-scale model involving an intermittent and convective mechanism has been developed to describe the global evolution of the concentration distribution in the vertical direction. Based on experimental observations, it assumes that each bubble catches a given volume of fluid V t in its wake and transports it over a certain length L before releasing it. A good agreement is found between the experimental concentration profiles measured in the vertical direction and the model prediction. The comparison between the model and the experiments allows the determination of the transported volume V t and the transport length L as a function of the gas volume fraction. It appears that the transported volume is related to the characteristic length of the velocity deficit at the rear of the bubble. The transport length, which is related to the correlation length of the dye patches, shows two regimes. At low gas volume fraction, it is controlled by the viscous length related to the flow damping at the walls, whereas, at higher gas volume fraction, it is limited by the distance between two successive bubbles. The mixing properties are finally characterized from the first three-order moments of the dye distribution, which are determined by means of the model. The upward propagation of the dye is shown to scale as αV , where V is the bubble rise velocity. The spreading of the concentration distribution is characterized by an effective diffusivity, which presents strong similarities with the diffusion coefficient measured in a three-dimensional bubble column [Alméras et al. J. Fluid Mech. 776, 458 (2015)]. At low gas volume fraction, it √ increases as α, whereas it saturates at high gas volume fraction. The dye distribution also is asymmetric with a significant skewness coefficient which slowly decreases in time. Therefore, the dye transport cannot be described as a purely diffusive process over the time required for the dye to spread over the cell