Diagnostic optique simultané de champ de vitesse et de scalaire par technique de marquage moléculaire: application au transfert de masse dans un écoulement diphasique (S23)

Depuis la fin des années 1990 se développe une nouvelle méthode de diagnostic optique appelée marquage moléculaire ou MTV (Molecular Tagging Velocimetry). Le principe de cette technique repose sur l'utilisation de supramolécules qui une fois excitées par impulsion lumineuse dans l'ultraviolet vont phosphorescer dans le visible sur une durée bien plus importante que les temps caractéristiques de la fluorescence (jusqu'à quelques millisecondes). Cette propriété permet de les utiliser comme marqueurs lagrangiens de l'écoulement pour ainsi déterminer le champ de vitesse en tout point de la zone marquée. La dépendance de la phosphorescence à différents scalaires passifs du fluide tels que la température, le pH, la concentration permettent également de déduire le champ de ce scalaire à partir de l'intensité locale de la réponse lumineuse de l'écoulement. L'objectif de cette étude est d'appliquer la technique de MTV à l'observation du transfert de masse en écoulement diphasique gaz-liquide

Mesure de pH et de concentration de gaz dissout par PLIF inhibée ratiométrique

International audienc

La turbulence de grille oscillante et son influence sur le transfert de masse gaz-liquide et le mélange en milieu non newtonien

The study of turbulence induced mass transfer at the interface between a gas and a liquid is of great interest in many environmental phenomena and industrial processes. Even though this issue has already been studied for several decades, its understanding is still not good enough to create realistic models (RANS or sub-grid LES), especially when considering a liquid phase with a complex rheology. This experimental work aims at studying fundamental aspects of turbulent mass transfer at a flat interface between carbon dioxide and a Newtonian or non-Newtonian liquid, stirred by homogeneous and quasi isotropic turbulence. Non-Newtonian fluids studied are aqueous solutions of a model polymer, Xanthan gum (XG), at various concentrations, showing viscoelastic and shear-thinning properties. Optical techniques for the acquisition of the liquid phase velocity field (Stereoscopic Particle Image Velocimetry, SPIV) and dissolved gas concentration field (Inhibited Planar Laser Induced Fluorescence, I-PLIF) are for the first time coupled, keeping a high spatial resolution, to access velocity and concentration statistics in the first few millimetres under the interface. A new version of I-PLIF is developed. It is designed to be more efficient for near surface measurements, but its use can be generalized to other single or multiphase mass transfer situations. Bottom shear turbulence in the liquid phase is generated by an oscillating grid apparatus. The mechanisms of turbulence production and the characteristics of oscillating grid turbulence (OGT) are studied. The importance of the oscillatory component of turbulence is discussed. A mean flow enhancement effect upon polymer addition is evidenced. The mechanisms of turbulent mass transfer at a flat interface are finally observed in water and low concentration polymer solutions. A conditional analysis of turbulent mass fluxes allows to distinguish the type of events contributing to mass transfer and discuss their respective impact in water and polymer solutions.L’étude du transfert de masse turbulent aux interfaces gaz-liquide est d’un grand intérêt dans de nombreuses applications environnementales et industrielles. Bien que ce problème soit étudié depuis de nombreuses années, sa compréhension n’est pas encore suffisante pour la création de modèles de transfert de masse réalistes (de type RANS ou LES sous maille), en particulier en présence d’une phase liquide à rhéologie complexe. Ce travail expérimental a pour but l’étude des aspects fondamentaux du transfert de masse turbulent à une interface plane horizontale entre du dioxyde de carbone gazeux et une phase liquide newtonienne ou non, agitée par une turbulence homogène quasi isotrope. Les milieux liquides non newtoniens étudiés sont des solutions aqueuses d’un polymère dilué à des concentrations variables et aux propriétés viscoélastiques et rhéofluidifiantes. Deux méthodes de mesure optiques permettant l’obtention du champ de vitesse de la phase liquide (SPIV) et de concentration du gaz dissout (I-PLIF) sont couplées tout en maintenant une haute résolution spatiale, afin de déduire les statistiques de vitesse et de concentration couplées dans les premiers millimètres sous la surface. Une nouvelle version de I-PLIF est développée pour les mesures en proche surface. Elle peut également s’appliquer dans différentes études de transfert de masse. La turbulence de fond est générée par un dispositif de grille oscillante. Les mécanismes de production et les caractéristiques de la turbulence sont étudiés. L’importance de la composante oscillante de la turbulence est discutée, et un phénomène d’amplification de l’écoulement moyen est mis en évidence. Les mécanismes du transfert de masse turbulent à l’interface sont finalement observés pour l’eau et une solution de polymère dilué à faible concentration. L’analyse conditionnelle des flux de masse turbulent permet de mettre en évidence les évènements contribuant au transfert de masse et de discuter de leur impact relatif sur le transfert total

Ratiometric, single-dye, pH-sensitive inhibited laser-induced fluorescence for the characterization of mixing and mass transfer

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Experimental Study of a Gas–Liquid Flow in Vacuum Air-Lift Column Using an Optical Bi-Probe

The vacuum airlift column process was patented in 2007 and is under development. The experimental study of its hydrodynamics is one of the axes explored to optimize its design and operation. The object of the study presented in this paper is to determine the functions of phase indicator (gas holdup, superficial gas velocity and bubble size) of the gas–liquid flow. The experimental analysis is carried out using a two-phase instrumentation consisting of an optical fiber bi-probe. The use of experimental techniques has made it possible to better understand the hydrodynamics of a two-phase flow. The optical bi-probe placed between two column flanges made it possible to have a complete mapping of the flow of the dispersed phase. The use of a mass flow meter and an ultrasonic flowmeter, in different flow configurations, provided data on the column operation

Study of Gas Liquid Mass Transfer in a Grid Stirred Tank

Dissolution and mass transfer at gas liquid interfaces are central to many industrial and environmental issues. All these applications aim at predicting the transfer velocities between the two phases, and the dependency of these velocities on several factors such as pressure, temperature, flow regime and so on. The goal of the present study is to understand the physical phenomena acting in the liquid phase during the dissolution and transfer of a gas at a flat two-phase interface; and to measure the influence of these phenomena on mass exchanges. To that end, an oscillating grid apparatus is used to generate a controlled liquid side turbulence. The dissolution of an atmospheric sparingly soluble gas, carbon dioxide, is considered. Optical measurement techniques are used simultaneously in order to gain further insight on the hydrodynamics influence on dissolved gas mixing. The phenomena responsible for mass transfer acceleration are found to happen in thin characteristic depth scales under the interface. In those regions, a complex combination of dissolved gas injection and diffusion layers renewing events is observed. The analysis of velocity fields highlights their strongly three dimensional aspects, and simultaneous measurements, lead to the conclusion that these three-dimensional effects have an impact on dissolved scalar concentration structures, and consequently on mass transfer

Study of Gas Liquid Mass Transfer in a Grid Stirred Tank

Dissolution and mass transfer at gas liquid interfaces are central to many industrial and environmental issues. All these applications aim at predicting the transfer velocities between the two phases, and the dependency of these velocities on several factors such as pressure, temperature, flow regime and so on. The goal of the present study is to understand the physical phenomena acting in the liquid phase during the dissolution and transfer of a gas at a flat two-phase interface; and to measure the influence of these phenomena on mass exchanges. To that end, an oscillating grid apparatus is used to generate a controlled liquid side turbulence. The dissolution of an atmospheric sparingly soluble gas, carbon dioxide, is considered. Optical measurement techniques are used simultaneously in order to gain further insight on the hydrodynamics influence on dissolved gas mixing. The phenomena responsible for mass transfer acceleration are found to happen in thin characteristic depth scales under the interface. In those regions, a complex combination of dissolved gas injection and diffusion layers renewing events is observed. The analysis of velocity fields highlights their strongly three dimensional aspects, and simultaneous measurements, lead to the conclusion that these three-dimensional effects have an impact on dissolved scalar concentration structures, and consequently on mass transfer

Flow around an oscillating grid in water and shear-thinning polymer solution at low Reynolds number

International audienc

Mesure de pH et de concentration de gaz dissout par PLIF inhibée ratiométrique

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