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International audience; Highlights  N.t m versus Re* allows identification of stirrer or added liquid regimes  Re* does not allow N.t m to be replicated across scales  Impeller tip speed or specific power replicate N.t m better across scales

Colloque avec actes et comité de lecture. Internationale.; International audience; Cette étude, comportant un volet expérimental et un volet numérique, vise à mieux comprendre l'hydrodynamique des écoulements de Taylor gaz-liquide en microcanaux rectangulaires (dh ~ 500 µm). L'étape de génération des bulles lors de la mise en contact des fluides est étudiée à travers la taille des poches de gaz et bouchons liquides ainsi que la fréquence de formation. La dynamique de l'écoulement établi est également analysée (forme des bulles, épaisseur de film, vitesse).

Vortex Structure and Mixing in Meandering Microchannels

L’agitation d’un fluide à seuil rhéofluidifiant et thixotrope est investigué expérimentalement et numériquement. Les champs d’écoulements sont mesurés à trois vitesses de rotations par la vélocimétrie par images de particules dans une cuve agitée standard munie d’une hélice. A la vitesse de rotation la plus élevée, les caractéristiques de l’écoulement sont semblables à celles obtenues en régime turbulent. A mesure que la vitesse de rotation diminue, une caverne se forme, l’écoulement généré au voisinage de l’agitateur se réorganise (jusqu’à devenir essentiellement radial) et la capacité de pompage de celui-ci chute. La simulation numérique des écoulements nécessite la détermination préalable de la loi de comportement rhéologique du fluide modèle, puis l’implémentation de celle-ci via plusieurs modèles (Hershel-Bulkley, Coussot) dans le code de calcul de mécanique des fluides numérique (Ansys CFX 10.0). Une bonne adéquation entre expérience et simulation est constatée à vitesse de rotation élevée.

Whilst the use of CFD to study mixing vessels is now commonplace,there are still many specialised aplications that are yet to be adressed. Here we present CDF and PIV results for a partially baffled vessel with a free surface. we show that there are significant transient effects that mean many of the "rules of thumb" that have been developed for fully-baffled vessels must be revisited. In particular such flows have central vortices that are unsteady and complex transient flow structures generated by the impeller-baffle interactions.

International audience

The performance of a down- and up-pumping pitched blade turbine and A315 for gas-liquid dispersion and mass transfer was evaluated and then compared with that of Rushton and Scaba turbines in a small laboratory scale vessel. The results show that when the axial flow impellers are operated in the up-pumping mode, the overall performance is largely improved compared with the down-pumping configuration. Compared with the radial turbines, the up-pumping A315 has a high gas handling capacity, equivalent to the Scaba turbine and is economically much more efficient in terms of mass transfer than both turbines. On the other hand, the uppumping pitched blade turbine is not as well adapted to such applications. Finally, the axial flow impellers in the down-pumping mode have the lowest performance of all the impellers studied, although the A315 is preferred of the pitched blade turbine.

The performance of three sparger diameters (DS = 0.6D, DS = D, DS = 1.6D) in combination with three positions (below, above or level with the impeller) for gas-liquid dispersion and mass transfer were evaluated in the case of the Rushton turbine and the A315 propeller in up- or down-pumping mode. The results show that the best results in terms of gas handling and mass transfer capacities are obtained for all impellers with the sparger placed below it and with a diameter at least equal to the impeller diameter. For the sparger position below the agitator, the kLa values of the Rushton turbine are greater than those of the A315 propeller, whatever the pumping mode. The A315 propeller in up-pumping mode is, however, more economically efficient in terms of mass transfer. In all cases, the up-pumping mode gives better results than the down-pumping one.

Liquid phase hydrodynamics in an aerated tank stirred by a down- and an up-pumping pitched blade turbine have been investigated using Particle Image Velocimetry. The effect of agitator configuration and the gas phase on the mean velocity fields and turbulent quantities in the vessel have been investigated. The global mean gas holdup has also been evaluated for the two pumping conditions. For the gas flow rate used, the presence of gas only slightly alters the liquid flow patterns produced by both the down- and up-pumping configurations and causes a general decrease in the mean liquid velocities. The turbulent kinetic energy in the impeller discharge region was not affected by the presence of gas, but in the bulk of the tank, aeration caused a decrease in this value. Global gas holdup was found to be ~36% greater for the up-pumping impeller and a large amount of gas was found to be entrained by the primary circulation loop.

Axial flow impellers, like pitched blade impellers, are being increasingly used for gas-liquid systems in stirred vessels. In this work we have used particle image velocimetry (PIV) and computational fluid dynamics (CFD) models to investigate gas-liquid flow generated by a down-flow pitched blade turbine. PIV measurements were carried out in a fully baffled stirred vessel (of 0.19 m diameter) with a dished bottom. Angle resolved measurements of the flow field with and without gas dispersion were carried out. An attempt was made to capture key details of the trailing vortex, the accumulation of gas and the flow around the impeller blades. A two-fluid model along with the standard k-e turbulence model was used to simulate dispersed gas-liquid flow in stirred vessel. The computational snapshot approach was used to simulate impeller rotation and was implemented in the commercial CFD code, FLUENT4.5 (of Fluent. Inc., USA). The model predictions were verified by comparison with the PIV measurements and other available experimental data. The computational model and results discussed in this work are useful for better understanding and simulating of gas-liquid flow generated by axial impellers in stirred vessels.