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Experimental Study of Bubble-Drag Interaction in a Taylor-Couette Flow Georges Fokoua, Céline Gabillet, Catherine Colin

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Experimental Study of Bubble-Drag Interaction in a Taylor-Couette Flow Georges Fokoua, Céline Gabillet, Catherine Colin Experimental study of bubble-drag interaction in a Taylor-Couette flow Georges Fokoua, Céline Gabillet, Catherine Colin To cite this version: Georges Fokoua, Céline Gabillet, Catherine Colin. Experimental study of bubble-drag interaction in a Taylor-Couette flow. 8th International Conference on Multiphase Flow ICMF 2013, May 2013, Jeju Island, South Korea. hal-01071582v2 HAL Id: hal-01071582 https://hal.archives-ouvertes.fr/hal-01071582v2 Submitted on 4 Apr 2014 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Open Archive TOULOUSE Archive Ouverte (OATAO) OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 11187 To cite this version : Fokoua, Georges and Gabillet, Céline and Colin, Catherine Experimental study of bubble-drag interaction in a Taylor-Couette flow. In: 8th International Conference on Multiphase Flow ICMF 2013, 26 May 2013 - 31 May 2013 (Jeju Island, Korea, Republic Of). Any correspondance concerning this service should be sent to the repository administrator: [email protected] Experimental study of bubble-drag interaction in a Taylor-Couette flow G. Fokoua1, C. Gabillet1, C. Colin2 1 IRENav, Research Institute of the French Naval Academy, Lanveoc Poulmic, France 2IMFT, Institute of Fluid Mechanics, Toulouse, France Keywords: bubble dispersion, viscous torque, Taylor vortices Abstract This study is an experimental investigation of the interactions between the bubbles, the coherent motion and the viscous drag in a Taylor Couette flow, for the outer cylinder at rest. The cylinder radius ratio η is 0.9. Bubbles are injected through a needle at the bottom of the apparatus inside the gap. Different bubble sizes are investigated (ratio between the bubble size and the gap width 0.05 and 0.12) for very small void fraction (α≤0.012). Different flow regimes are studied corresponding to Reynolds number Re based on the gap width and the velocity of the inner cylinder ranging from 400 to 20000. For these Re values, Taylor vortices are persistent leading to an axial periodicity of the flow. PIV measurements of the liquid flow features, bubble tracking in a meridian plane and viscous torque of the inner cylinder measurements are performed. This study provides a first evidence of the link between the bubble localisation, the Taylor vortices and viscous torque modifications. Bubbles are attracted towards the inner cylinder, due to the rotation of the cylinder. For small buoyancy effect, bubbles are trapped and induce a decrease in the outflow intensity, thus leading to an increase of the viscous torque. When buoyancy induced bubble motion, by comparison to the coherent motion of the liquid is increased, a decrease in the viscous torque is suspected. Introduction associated inflow/outflow jets regions) and small scale turbulent structures. Thus injecting bubble in this kind of The Taylor Couette device can be considered as an flow is interesting because bubbles can interact with these academic one to study bubble induced modifications of the different kinds of motions and can lead, for very specific viscous drag. Indeed, it is a closed system and conditions, to viscous torque reduction. In the literature, characterizing the viscous drag consists in characterizing the bubbly drag reduction in the Taylor Couette flow was the viscous torque applied on the inner cylinder. Moreover, studied for the outer cylinder at rest. When it is observed, for high Reynolds numbers, the Taylor Couette flow has the torque reduction is likely to be associated either with a many similarities with the boundary layer flow developing destructuration of the Taylor vortices by the bubble over a flat plate. upward motion in the case of weak turbulent and turbulent In the context of naval hydrodynamics, bubble injection Taylor vortex flow (Murai, 2008, Sugiyama et al., 2008) or can be intended to reduce the ship hulls’ viscous resistance. associated with the contribution of the deformation of the Actually, the physical mechanisms implied into the bubbly bubbles in the case of the high Reynolds numbers (Re>8 drag reduction expected for ship hulls applications are not 105), (Van den Berg et al., 2005, 2007, Van Gils et al., completely understood. As a consequence, it is difficult to 2011a, 2011b). According to Murai et al. (2008), there is extrapolate results obtained for small scale models to large a Reynolds range, for which the relative contribution of the scale ship’s hull model and a bubble injection system that Taylor vortices to the global flow and the bubble is appropriate for a typical ship hull and a specific velocity deformation are too small to bring about torque reduction, range can be no more fitted when it is carried out for a thus leading on the contrary to a torque augmentation. different ship hull and/or other velocity ranges. Nevertheless, a common point when viscous torque Therefore, injecting bubbles inside a Taylor Couette flow reduction is observed is the existence of a void fraction can make it possible to study the interactions between peak near the inner cylinder wall. It is then worth trying to bubbles and the wall shear stress. In a turbulent Taylor characterize the bubble dispersion in this typical flow and Couette flow, there are 3 main contributions: a mean trying to understand the link between the bubble dispersion azimuthal flow, large scale Taylor vortices (with and the viscous drag increase or decrease at the inner cylinder. 0.05) are tested. In order to enable the characterization of Dispersion of bubbles in a Taylor Couette flow has been the bubble dispersion by visualisation, the global void studied experimentally for different geometries (different fraction was willingly limited to a small value (α<0.012), radius ratios) and different Reynolds numbers range. For smaller than in Mehel et al. the weak turbulent flow and turbulent Taylor vortex flow, Section 1 is a description of the experimental set up, The it is highlighted that bubbles have preferential viscous torque measurements are shown and discussed in accumulation regions, depending on the bubble size to the section 2. Section 3 deals with the description of the gap width ratio (db/d) and the Reynolds number: either in bubble arrangement. Void fraction distributions and the the Taylor vortices (Mehel, 2007) or in the outflow region eulerian velocity fields of the bubbles measured in a (induced by the vortices) near the inner cylinder (Mehel, meridian plane are presented in section 4. Section 5 shows 2007, Murai, 2008). For the turbulent flow, there is a some PIV /velocity maps of the liquid flow. Section 7 is preferential accumulation near the inner cylinder with a dedicated to the discussion. homogeneous axial distribution (Van Gils, 2011a). The strong accumulation of the bubbles near the wall is Nomenclature linked to the contribution of the added mass force induced by the mean azimuthal flow. This force plays the same role d Gap between outer and inner cylinders (m) as the buoyancy force when bubbles are injected under a db Bubble diameter (m) ships hull. g gravitational constant (ms-2) Climent et al. (2007) and Chouippe et al. (2012) have G dimensional torque performed numerical analysis of the bubble dispersion for L Cylinder length (m) the first instabilities and for the turbulent Taylor vortex Lc Needle’s length (m) flow respectively. Preferential accumulation of the bubbles P pressure (Nm-2) in the axial and radial directions is discussed as a function Qg Air injection rate (m3/s) of respective dimensionless parameters. For very small R Cylinder radius (m) bubbles, the size of which is of the order of the viscous length scale, it is evidenced that the small scale turbulence Re Reynolds number can play a role, by trapping bubbles inside the low shear T Viscous Torque (Nm) stress streaks near the inner cylinder (Chouippe, 2012). Ta Taylor number In the case of experimental study of bubble dispersion U Radial velocity (m/s) inside Taylor Couette flow, as the void fraction was u* Friction velocity (m/s) characterized by intrusive method (optical probes), very Vb Upward rising velocity of the bubble (m/s) few profiles of void fraction are provided in the literature W Axial velocity (m/s) (Mehel, 2006, 2007, Murai, 2008, Van Gils, 2011a). Measurements can evidence the radial distribution of the Greek letters void fraction at an axial position but they do not enable to α void fraction characterize the axial distribution when it is subjected to η ratio of inner over outer cylinder radii the Taylor Vortices. In the numerical studies dedicated to λ axial wavelength (m) 2 -1 bubble dispersion in Taylor Couette flow, the calculation ν kinematic viscosity of the fluid (m .s ) carried out by bubble lagrangian tracking in the case of Φ needle’s diameter (m) bubble passive dispersion (one way coupling, Chouippe, c 2 τ wall shear stress (N/m ) 2012) or bubble active dispersion (two way coupling, Ω rotational angular velocity (rad/s) Sugiyama et al., 2008, Chouippe, 2012) provides the void fraction distribution. Nevertheless, the numerical models Subsripts do not take into account the interactions between the bubbles and the results are very sensitive to the i relative to inner cylinder formulation of the lift coefficient (Sugiyama et al., o relative to outer cylinder 2008).
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