Étude expérimentale et numérique du mélange et de la dispersion axiale dans une colonne à effet Taylor-Couette

Les contacteurs centrifuges, basés sur les écoulements de Taylor-Couette, sont bien adaptés pour la mise en œuvre de réactions chimiques ou biochimiques, y compris en milieu polyphasique. Ils possèdent particulièrement plusieurs propriétés favorables à la mise en œuvre des opérations d'extraction liquide-liquide. Un dispositif expérimental a été conçu avec cette idée en tête. Il est constitué de deux cylindres concentriques avec le cylindre intérieur entraîné en rotation et l'externe fixe. L’écoulement de Taylor-Couette se produit dans l’espace annulaire entre eux. Il présente la particularité d’évoluer vers la turbulence par apparition successive d’instabilités. La dispersion axiale ainsi que le mélange, sont extrêmement sensibles à ces structures d’écoulement, ce qui rend difficile la modélisation du couplage entre l’hydrodynamique et le transfert de matière. Ce point particulier a été étudié expérimentalement et numériquement. L’écoulement et le mélange ont été caractérisés par des mesures simultanées de PIV (Vélocimétrie par Imagerie de Particules) et PLIF (Fluorescence Induite par Laser). Les champs de concentration PLIF ont permis d’identifier les différents mécanismes de transport intra et inter-vortex. Pour les régimes ondulatoires (WVF et MWVF), le mélange intra-vortex est contrôlé par l’advection chaotique, directement lié aux caractéristiques du champ de vitesse, qui confère aux vortex une capacité plus importante à convecter et à étirer les filets de fluide. En revanche, l’apparition des vagues brisent les frontières qui séparent les vortex ce qui favorise le transport inter-vortex. La combinaison de ces deux mécanismes contrôle principalement la dispersion axiale. Nous avons également mis en évidence le comportement non monotone des propriétés de mélange en fonction de l’histoire de l’écoulement. Notamment l’état d’onde (la longueur d’onde axiale et l’amplitude de la vague). Nous avons calculé le coefficient de dispersion axiale Dx à l’aide des mesures de distribution de temps de séjour (DTS) et de suivi Lagrangien de particules (DNS). Les deux résultats numériques et expérimentaux ont confirmé l’effet significatif des structures de l’écoulement et de l’histoire sur la dispersion axiale. ABSTRACT : Taylor-Couette flows between two concentric cylinders have great potential applications in chemical engineering. They are particularly convenient for two-phase small scale devices enabling solvent extraction operations. An experimental device was designed with this idea in mind. It consists of two concentric cylinders with the inner one rotating and the outer one fixed. Taylor-Couette flows take place in the annular gap between them, and are known to evolve towards turbulence through a sequence of successive instabilities. Macroscopic quantities, such as axial dispersion and mixing index, are extremely sensitive to these flow structures, which may lead to flawed modelling of the coupling between hydrodynamics and mass transfer. This particular point has been studied both experimentally and numerically. The flow and mixing have been characterized by means of flow visualization and simultaneous PIV (Particle Imaging Velocimetry) and PLIF (Planar Laser Induced Fluorescence) measurements. PLIF visualizations showed clear evidences of different transport mechanisms including « intravortex mixing » and « inter-vortex mixing ». Under WVF and MWVF regimes, intra-vortex mixing is controlled by chaotic advection, due to the 3D nature of the flow, while inter-vortex transport occurs due to the presence of waves between neighbouring vortices. The combination of these two mechanisms results in enhanced axial dispersion. We showed that hysteresis may occur between consecutive regimes depending on flow history and this may have a significant effect on mixing for a given Reynolds number. The axial dispersion coefficient Dx evolution along the successive flow states was investigated thanks to dye Residence Time Distribution measurements (RTD) and particle tracking (DNS). Both experimental and numerical results have confirmed the significant effect of the flow structure and history on axial dispersion. Our study confirmed that the commonly used 1-parameter chemical engineering models (e.g. the « well-mixed stirred tanks in serie » model) are not valid for Taylor-Couette reactors modelling : two parameters are at least required for an efficient description of mixing in Taylor-Couette flows

Mixing and axial dispersion in Taylor–Couette flows: the effect of the flow regime

The paper focuses on mixing properties of different Taylor–Couette flow regimes and their consequence on axial dispersion of a passive tracer. A joint approach, relying both on targeted experiments and numerical simulations, has been used to investigate the interaction between the flow characteristics and local or global properties of mixing. Hence, the flow and mixing have been characterized by means of flow visualization and simultaneous PIV (Particle Imaging Velocimetry) and PLIF (Planar Laser Induced Fluorescence) measurements, whereas the axial dispersion coefficient evolving along the successive flow states was investigated thanks to dye Residence Time Distribution measurements (RTD). The experimental results were complemented, for each flow pattern, by Direct Numerical Simulations (DNS), allowing access to 3D information. Both experimental and numerical results have been compared and confirmed the significant effect of the flow structure (axial wavelength of Taylor vortices and azimuthal wavenumber) on axial dispersion

Experimental investigation of mixing and axial dispersion in Taylor–Couette flow patterns

The flow and mixing in a Taylor–Couette device have been characterized by means of simultaneous particle image velocimetry and planar laser-induced fluorescence (PLIF) measurements. Concentration of a passive tracer measurements was used to investigate mixing efficiency for different flow patterns (from steady Taylor vortex flow to modulated wavy vortex flow, MWVF). Taylor–Couette flow is known to evolve toward turbulence through a sequence of flow instabilities. Macroscopic quantities, such as axial dispersion and mixing index, are extremely sensitive to internal flow structures. PLIF measurements show clear evidences of different transport mechanisms including intravortex mixing and tracer fluxes through neighboring vortices. Under WVF and MWVF regimes, intravortex mixing is controlled by chaotic advection, due to the 3D nature of the flow, while intervortex transport occurs due to the presence of waves between neighboring vortices. The combination of these two mechanisms results in enhanced axial dispersion. We show that hysteresis may occur between consecutive regimes depending on flow history, and this may have a significant effect on mixing for a given Reynolds numbe

Étude expérimentale et numérique du mélange et de la dispersion axiale dans une colonne à effet Taylor-Couette

Les contacteurs centrifuges, basés sur les écoulements de Taylor-Couette, sont bien adaptés pour la mise en œuvre de réactions chimiques ou biochimiques, y compris en milieu polyphasique. Ils possèdent particulièrement plusieurs propriétés favorables à la mise en œuvre des opérations d'extraction liquide-liquide. Un dispositif expérimental a été conçu avec cette idée en tête. Il est constitué de deux cylindres concentriques avec le cylindre intérieur entraîné en rotation et l'externe fixe. L écoulement de Taylor-Couette se produit dans l espace annulaire entre eux. Il présente la particularité d évoluer vers la turbulence par apparition successive d instabilités. La dispersion axiale ainsi que le mélange, sont extrêmement sensibles à ces structures d écoulement, ce qui rend difficile la modélisation du couplage entre l hydrodynamique et le transfert de matière. Ce point particulier a été étudié expérimentalement et numériquement. L écoulement et le mélange ont été caractérisés par des mesures simultanées de PIV (Vélocimétrie par Imagerie de Particules) et PLIF (Fluorescence Induite par Laser). Les champs de concentration PLIF ont permis d identifier les différents mécanismes de transport intra et inter-vortex. Pour les régimes ondulatoires (WVF et MWVF), le mélange intra-vortex est contrôlé par l advection chaotique, directement lié aux caractéristiques du champ de vitesse, qui confère aux vortex une capacité plus importante à convecter et à étirer les filets de fluide. En revanche, l apparition des vagues brisent les frontières qui séparent les vortex ce qui favorise le transport inter-vortex. La combinaison de ces deux mécanismes contrôle principalement la dispersion axiale. Nous avons également mis en évidence le comportement non monotone des propriétés de mélange en fonction de l histoire de l écoulement. Notamment l état d onde (la longueur d onde axiale et l amplitude de la vague). Nous avons calculé le coefficient de dispersion axiale Dx à l aide des mesures de distribution de temps de séjour (DTS) et de suivi Lagrangien de particules (DNS). Les deux résultats numériques et expérimentaux ont confirmé l effet significatif des structures de l écoulement et de l histoire sur la dispersion axiale.Taylor-Couette flows between two concentric cylinders have great potential applications in chemical engineering. They are particularly convenient for two-phase small scale devices enabling solvent extraction operations. An experimental device was designed with this idea in mind. It consists of two concentric cylinders with the inner one rotating and the outer one fixed. Taylor-Couette flows take place in the annular gap between them, and are known to evolve towards turbulence through a sequence of successive instabilities. Macroscopic quantities, such as axial dispersion and mixing index, are extremely sensitive to these flow structures, which may lead to flawed modelling of the coupling between hydrodynamics and mass transfer. This particular point has been studied both experimentally and numerically. The flow and mixing have been characterized by means of flow visualization and simultaneous PIV (Particle Imaging Velocimetry) and PLIF (Planar Laser Induced Fluorescence) measurements. PLIF visualizations showed clear evidences of different transport mechanisms including intravortex mixing and inter-vortex mixing . Under WVF and MWVF regimes, intra-vortex mixing is controlled by chaotic advection, due to the 3D nature of the flow, while inter-vortex transport occurs due to the presence of waves between neighbouring vortices. The combination of these two mechanisms results in enhanced axial dispersion. We showed that hysteresis may occur between consecutive regimes depending on flow history and this may have a significant effect on mixing for a given Reynolds number. The axial dispersion coefficient Dx evolution along the successive flow states was investigated thanks to dye Residence Time Distribution measurements (RTD) and particle tracking (DNS). Both experimental and numerical results have confirmed the significant effect of the flow structure and history on axial dispersion. Our study confirmed that the commonly used 1-parameter chemical engineering models (e.g. the well-mixed stirred tanks in serie model) are not valid for Taylor-Couette reactors modelling : two parameters are at least required for an efficient description of mixing in Taylor-Couette flows.TOULOUSE-INP (315552154) / SudocSudocFranceF

From passive tracer to bubbles dispersion in Taylor-Couette flows

We investigate dispersion of passive tracer and bubble preferential accumulation in the flow between two concentric cylinders. The dispersive characteristics are analysed for Taylor Vortex Flow, Wavy Vortex Flows and fully Turbulent Taylor-Couette flows. Experiments based on flow visualization, PIV and PLIF measurements are compared to direct numerical simulations of Navier-Stokes equations coupled to Lagrangian tracking of fluid elements and bubbles. In vortical flows, bubble accumulation is driven by a competition between added-mass effect, lift and buoyancy forces. At low to moderate Reynolds numbers, the flow is strongly coherent and bubble accumulation patterns can be predicted theoretically (stability analysis of fixed points). When turbulence sets in, small scale structures enhance dispersion. This complex situation where large-scale coherent structures interact with fine scale turbulence leads to bubble mixing which have been analyzed by numerical simulations. Several distributions of bubbles are observed depending on the respective magnitude of turbulence and buoyancy force

Experimental and numerical investigation on mixing and axial dispersion in Taylor-Couette flow patterns

Taylor-Couette flows between two concentric cylinders have great potential applications in chemical engineering. They are particularly convenient for two-phase small scale devices enabling solvent extraction operations. An experimental device was designed with this idea in mind. It consists of two concentric cylinders with the inner one rotating and the outer one fixed. Moreover, a pressure driven axial flow can be superimposed. Taylor-Couette flow is known to evolve towards turbulence through a sequence of successive hydrodynamic instabilities. Mixing characterized by an axial dispersion coefficient is extremely sensitive to these flow bifurcations, which may lead to flawed modelling of the coupling between flow and mass transfer. This particular point has been studied using experimental and numerical approaches. Direct numerical simulations (DNS) of the flow have been carried out. The effective diffusion coefficient was estimated using particles tracking in the different Taylor-Couette regimes. Simulation results have been compared with literature data and also with our own experimental results. The experimental study first consists in visualizing the vortices with a small amount of particles (Kalliroscope) added to the fluid. Tracer residence time distribution (RTD) is used to determine dispersion coefficients. Both numerical and experimental results show a significant effect of the flow structure on the axial dispersion

Experimental and numerical study of mixing and axial dispersion in a Taylor-Couette device

Les contacteurs centrifuges, basés sur les écoulements de Taylor-Couette, sont bien adaptés pour la mise en œuvre de réactions chimiques ou biochimiques, y compris en milieu polyphasique. Ils possèdent particulièrement plusieurs propriétés favorables à la mise en œuvre des opérations d'extraction liquide-liquide. Un dispositif expérimental a été conçu avec cette idée en tête. Il est constitué de deux cylindres concentriques avec le cylindre intérieur entraîné en rotation et l'externe fixe. L’écoulement de Taylor-Couette se produit dans l’espace annulaire entre eux. Il présente la particularité d’évoluer vers la turbulence par apparition successive d’instabilités. La dispersion axiale ainsi que le mélange, sont extrêmement sensibles à ces structures d’écoulement, ce qui rend difficile la modélisation du couplage entre l’hydrodynamique et le transfert de matière. Ce point particulier a été étudié expérimentalement et numériquement. L’écoulement et le mélange ont été caractérisés par des mesures simultanées de PIV (Vélocimétrie par Imagerie de Particules) et PLIF (Fluorescence Induite par Laser). Les champs de concentration PLIF ont permis d’identifier les différents mécanismes de transport intra et inter-vortex. Pour les régimes ondulatoires (WVF et MWVF), le mélange intra-vortex est contrôlé par l’advection chaotique, directement lié aux caractéristiques du champ de vitesse, qui confère aux vortex une capacité plus importante à convecter et à étirer les filets de fluide. En revanche, l’apparition des vagues brisent les frontières qui séparent les vortex ce qui favorise le transport inter-vortex. La combinaison de ces deux mécanismes contrôle principalement la dispersion axiale. Nous avons également mis en évidence le comportement non monotone des propriétés de mélange en fonction de l’histoire de l’écoulement. Notamment l’état d’onde (la longueur d’onde axiale et l’amplitude de la vague). Nous avons calculé le coefficient de dispersion axiale Dx à l’aide des mesures de distribution de temps de séjour (DTS) et de suivi Lagrangien de particules (DNS). Les deux résultats numériques et expérimentaux ont confirmé l’effet significatif des structures de l’écoulement et de l’histoire sur la dispersion axiale.Taylor-Couette flows between two concentric cylinders have great potential applications in chemical engineering. They are particularly convenient for two-phase small scale devices enabling solvent extraction operations. An experimental device was designed with this idea in mind. It consists of two concentric cylinders with the inner one rotating and the outer one fixed. Taylor-Couette flows take place in the annular gap between them, and are known to evolve towards turbulence through a sequence of successive instabilities. Macroscopic quantities, such as axial dispersion and mixing index, are extremely sensitive to these flow structures, which may lead to flawed modelling of the coupling between hydrodynamics and mass transfer. This particular point has been studied both experimentally and numerically. The flow and mixing have been characterized by means of flow visualization and simultaneous PIV (Particle Imaging Velocimetry) and PLIF (Planar Laser Induced Fluorescence) measurements. PLIF visualizations showed clear evidences of different transport mechanisms including « intravortex mixing » and « inter-vortex mixing ». Under WVF and MWVF regimes, intra-vortex mixing is controlled by chaotic advection, due to the 3D nature of the flow, while inter-vortex transport occurs due to the presence of waves between neighbouring vortices. The combination of these two mechanisms results in enhanced axial dispersion. We showed that hysteresis may occur between consecutive regimes depending on flow history and this may have a significant effect on mixing for a given Reynolds number. The axial dispersion coefficient Dx evolution along the successive flow states was investigated thanks to dye Residence Time Distribution measurements (RTD) and particle tracking (DNS). Both experimental and numerical results have confirmed the significant effect of the flow structure and history on axial dispersion. Our study confirmed that the commonly used 1-parameter chemical engineering models (e.g. the « well-mixed stirred tanks in serie » model) are not valid for Taylor-Couette reactors modelling : two parameters are at least required for an efficient description of mixing in Taylor-Couette flows

Mixing and axial dispersion in Taylor-Couette flows: a multi-scale study

Liquid-liquid extraction in an industrial apparatus is a complex process involving chemistry, interface dynamics, mass transfer and fluid mechanics. Among these phenomena, transport processes, which are size-dependent, deserve a particular attention in the scope of R and D studies (where size-reduction is encouraged, as in the nuclear industry) and for scale-up purpose. Hence, flow patterns and properties in extraction devices are the subject of increasing interest, involving numerical as well as experimental studies. In this aim, we choose to take advantage of Taylor-Couette flows, already used to perform small-scale solvent extraction studies, in order to investigate the specific hydrodynamic issues: mainly mixing and axial dispersion

Experimental and Numerical investigations of mixing and axial diffusion in Taylor-Couette flow patterns

Two-phase Taylor-Couette flows between two concentric cylinders have great potential applications in nuclear engineering. They are particularly convenient for small scale devices enabling solvent extraction operations. The flow has been characterized by means of simultaneous PIV (particle image velocimetry) and PLIF (planar laser induced fluorescence) measurements. Moreover, direct numerical simulations have been used to investigate mixing properties of different flow patterns (from steady TVF Taylor vortex flow towards MWVF modulated wavy vortex flow)

Experimental investigation of mixing and axial dispersion in Taylor–Couette flow patterns

International audienceThe flow and mixing in a Taylor–Couette device have been characterized by means of simultaneous particle image velocimetry and planar laser-induced fluorescence (PLIF) measurements. Concentration of a passive tracer measurements was used to investigate mixing efficiency for different flow patterns (from steady Taylor vortex flow to modulated wavy vortex flow, MWVF). Taylor–Couette flow is known to evolve toward turbulence through a sequence of flow instabilities. Macroscopic quantities, such as axial dispersion and mixing index, are extremely sensitive to internal flow structures. PLIF measurements show clear evidences of different transport mechanisms including intravortex mixing and tracer fluxes through neighboring vortices. Under WVF and MWVF regimes, intravortex mixing is controlled by chaotic advection, due to the 3D nature of the flow, while intervortex transport occurs due to the presence of waves between neighboring vortices. The combination of these two mechanisms results in enhanced axial dispersion. We show that hysteresis may occur between consecutive regimes depending on flow history, and this may have a significant effect on mixing for a given Reynolds numbe