Gas-liquid mass transfer around Taylor bubbles moving in a meandering millimetric square channel was locally visualized and characterized in the present study. For that, the colorimetric technique proposed by Dietrich et al. (2013) was implemented. With this technique, the evolution of equivalent oxygen concentration fields in the liquid slugs passing through one and several bends was firstly described. In particular, it was observed how the flow structure (recirculation zones) inside the liquid slugs were twisted and split by the periodic bends (centrifugal effect), until reaching, after several bends, a uniform O 2 concentration inside the liquid slugs. The influence of the ''turning point", joining two ''straight" sections of meandering channel was also highlighted: a slowing down of the gas-liquid mass transfer was clearly shown. Volumetric mass transfer coefficients were determined at last by fitting the experimental axial profiles of averaged oxygen concentrations in the liquid slugs (before the turning point) with the ones predicted by a classical plug-flow model.
Local hydrodynamics within a complex and confined dynamic filtration module named RVF (Rotating and Vibrating Filtration) was scrutinized under laminar flow with viscous Newtonian fluid. This module was designed to treat complex fluids such as fermentation broth or liquid food. Previous results reported global hydrodynamics (friction and power consumption curves) and RVF efficiency during filtration of fermentation broths. However, the access to local variables (shear rate, velocity) appeared as challenging in a confined volume. By associating Particle Image Velocimetry (PIV) and an optical trigger strategy, the instantaneous velocity fields were measured and analyzed. PIV experiments were carried out at 6 different planes in the 3 mm narrow gap between the membrane and the impeller, considering two feeding flowrates (Q f =25 and 45 L/h) under mixing rates 2 Hz. In addition, a simulation approach by Computational Fluid Dynamics (CFD) was developed, confronted to PIV (Q f =45L/h, N=2Hz) and compared to global parameters.In this work, we showed the ability to measure local velocity in a complex, confined and rotating device. As expected, velocity in filtration cell was observed to be independent of flowrate. Magnitude of mean velocity fluctuation was quantified and discussed as a function of radial and axial position (constant impeller position). Moreover, velocity and shear rate profiles were introduced by simulation, they proved that simulations were in a good agreement with PIV measurements, and streamlines was also discussed combining Residence Time Distribution (RTD). Finally, local and global core velocity coefficients (issued from PIV and CFD), as an indicator of mechanical efficiency were calculated and compared to similar devices and alternative methodologies which were reported in the literature.
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