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This study deals with the development of a self-sustained passive thermal regulation technique in Algofilm © solar photobioreactor. The reported experimental investigation includes, the evaluation of an average equivalent condensate film thickness, in indoor operating conditions, as function of the flowing liquid Tl and ambient Tamb temperatures, inclination angle ϕ as well as air flowrate. For instance, an average condensate film thickness of 2.2 ± 0.2 mm at Tl = 50 °C, for liquid flow rate of 0.08 l/s and ϕ = 4° in steady state conditions was recorded. Furthermore, a correlation estimating the condensation rate as function of the flowing liquid and ambient temperatures, was developed. A theoretical, heat and mass transfer model was proposed and validated with the indoor experimental results. The numerical predictions of the model adapted to a simulated sunny day were in good agreement with the experimental recordings. This validation evidenced the model applicability to real solar conditions, where an accurate evaluation of the condensed water is required for predicting the infrared filtration.

International audience; The transition from stratified to intermittent air-water two-phase flow was investigated experimentally, by flow visualization and pressure drop signals analyses, in a 30 mm ID pipe. The intermittent flow’s onset was found to be mainly dependent on the liquid superficial velocity and the pipe diameter. Plug flow, Less Aerated Slug (LAS) or Highly Aerated Slug (HAS) flows could be obtained on the gas superficial velocity grounds. The available models, compared to experiments, could not predict adequately the intermittent flow onset. The appearance of liquid slugs was revealed by peaks in the pressure drop signal. Furthermore, it was shown that the available slug frequency correlations were not valid in the zone of the onset of intermittent flow.

Among the intrinsic parameters identified in intermittent gas-liquid flows; slug frequency is known as the most complicated one to model. The present work concerns the investigation of slug frequency for various sub-regimes which might be found in intermittent flow. Experiments, near atmospheric pressure conditions, were carried out in a 13-meter long horizontal pipe with a 30 mm inside diameter. The study involved plug regime, less aerated slug and highly aerated slug flows. Comparison between different methods of slug frequency quantification, from pressure drop signal, was performed with counting, Wilkens & Thomas as well as the Power Spectral Density (PSD) methods. PSD was found to be the best method for slug frequency measurements. A new flow map, using dimensionless numbers as coordinates for the sub-regimes distinction, was established. After performing substantial comparisons between the present results and models available in the literature a new correlation is proposed including the sub-regime types.

The purpose of this work is to analyze the influence of a sudden expansion on the upstream behavior of the horizontal gas-liquid slug flow. Measurements were made on a 40 mm ID pipe with and without a sudden enlargement of aspect ratio σ = 0.444. The experiments were carried out with two-phase air-water mixture. The slug lengths and frequencies were measured using a non-intrusive video technique. Upstream the sudden enlargement, it was observed that the mixture velocity has no influence on slug length. The variation of slug frequency is found proportional to the liquid superficial velocity for the two cases within this study. It was also observed that the behavior of the slug length and frequency was affected by the presence of the sudden enlargement. The comparison of the results obtained with various empirical correlations available in the literature showed that the latter are not worthwhile in the case where singularity is installed.

The influence of sub-regimes of intermittent flow on the pressure drop has been investigated. Two-phase, air-water, flow experiments were conducted on 30 mm ID pipe. The superficial velocities of the working fluids were chosen to cover three sub-regimes: Plug flow, Less Aerated Slug flow (LAS flow) and Highly Aerated Slug flow (HAS flow). The analysis of the experimental data, including the data drawn from the literature showed that the pressure drop depends on the flow sub-regime. A new empirical correlation, based on the Lockhart-Martinelli approach, taking into account the nature of sub-regime was proposed. The present correlations, in comparison with the existing correlations, give the best results.

Sub-regimes of intermittent two-phase flows through a sudden expansion have been investigated experimentally. Two pipes of 30 and 40 mm ID connected by a sudden expansion with aspect ratio σ = 0.562 were used. The study was concerned by Plug, Less Aerated Slug (LAS) and Highly Aerated Slug (HAS) flows. The flow visualization showed that the liquid slugs, for Plug and LAS flows, were broken after crossing the singularity which was not the case for HAS flow. However, in general the sub-regime observed on either side of the expansion was the same. Qualitative and quantitative analysis, by means of flow visualizations and standard deviation of pressure drop signals, showed that the flow presents less fluctuations downstream the singularity. Furthermore, the persistence of slug frequency depends on the nature of the sub-regime.

This work is devoted to a non-intrusive experimental approach, based on Laser Multi-reflection technique, in the investigation of thickness distribution variations and wave’s dynamics of a liquid film flowing over an inclined plane. This investigation was first founded on the needs of quantifying the liquid film thickness and on minimizing, as much as possible, some drawbacks pointed out, in the literature, throughout the experimental techniques available. Moreover, the technique could be applied to transparent, opaque as well as particle laden liquid films. The technique is validated and evaluated using two approaches according to the flow case: stable or instable. In case of stable flow, the comparison was made using Spectroscopic Ellipsometry and theoretical prediction established by the Nusselt model. For a wavy interface a setup, especially devoted to that purpose, was used to validate the accuracy of the measurements. In both cases the uncertainties were within 5%. The experiments are discussed hereafter including the accuracy of the results. Some experimental data, for plane inclination ranging from 1° to 10°, are reported. The data takes into account the film thickness at various positions. The instability threshold is also reported.

An efficient numerical approach is proposed on the crust formation of an isothermal drying spherical colloidal droplet in a stagnant gas, where the heat and mass transfer were governed by the Stefan flow. A purely diffusive mechanism with a moving boundary was considered. The droplet shrinking induced an accumulation of particles, in the vicinity of the droplet interface, up to the onset of a crust formation known as the locking point. The latter was used as a control parameter of the numerical procedure global behaviour. The performances of the present numerical scheme compared to classical methods, showed a rapid convergence with a drastic reduction in the number of nodes, at least two-hundred fold less. Furthermore, the accuracy and speed of the proposed method allowed to carry out an extensive study with respect to the Peclet number, thus enabling to point out three drying regimes.

Experimental investigations were conducted to study the evolution of air/water slug flow characteristics through a horizontal sudden expansion having a ratio of $$\sigma_{A}$$ = 0.444. A series of acquisition of differential pressure upstream and downstream were carried out leading to statistical as well as spectral analyses. The influences of both liquid and gas superficial velocities on flow behavior as well as standard deviation were studied. Substantial modifications of two-phase flow distribution were reported downstream the singularity with a quantitative reduction in standard deviation. Besides the fact that the slug flow regime didn’t persist downstream the singularity for low values of superficial phasic-velocities, the latter ( $$J_{\ell }$$ and $$J_{g}$$ ) seem to have no noticeable effects downstream the expansion. Reduction of slug frequency between upstream and downstream the singularity was also observed.

International audience; The Taylor-Couette flow is an axisymmetric, sheared, and azimuthal flow. It holds a primary site in the history of fluid dynamics. Several works have been devoted to study this problem. Despite the fact that it is very old, the problem of Taylor-Couette flow has received renewed interests in the past years.This renaissance is due to its high instability and the fact that it is particularly favorable to the rigorous mathematical analysis due to infinitesimal disturbances.The goal of this work is to provide an experimental study characterizing the Taylor-Couette instabilities in terms of interactions between vortices and wall.An experimental setup with a rotating inner cylinder and a fixed external cylinder was performed to study the flow at the gap between these two latters. Different types of movements imposed on mobile inner cylinder are available: cylinder oscillating, abrupt, or gradual disturbance. An axial flow was superimposed on the main flow of the Couette-Taylor.A qualitative study is firstly presented consisting of the visualization of the flow with the aim to draw the topology of the developed instabilities. Thus the different flow regimes conditioned by the dimensionless Taylor number are defined. The effect of superposition of an axial flow is shown.Experimental PIV measurements are then devoted to characterize the unsteady dynamics of the Taylor-Couette flow. Velocity and vorticity are investigated, and Γ2 criterion is used to locate the vortex.In the other hand, electrochemical method is used and synchronized with PIV technique to determine the velocity parietal gradient. This technique allowed studying the vortex-wall interaction using response of single and triple probe and PIV.

This paper presents an experimental investigation of Couette-Taylor system (CTS) characterized by a radius ratio η = 0.97 and an aspect ratio Γ = 150 to study the spatial distribution of instantaneous mass transfer and wall shear rates. Many electrochemical probes were used to measure the local and instantaneous diffusion limited current at different radial positions of the inner wall of the outer fixed cylinder. Two neighboring probes are spaced apart by an angle of 7° on the circumference of the inner wall of the outer cylinder of the CTS. The electrochemical solution used is a Ferri-Ferrocyanure of potassium with 2% of K2SO4 as supporting electrolyte, 40% of glycerin which delays the instabilities apparition, and 2% of Kalliroscope for vortices visualization. The local and instantaneous mass transfer was determined from the diffusion limited current using the Faraday law. The radial distribution of the azimuthal and instantaneous wall shear rate was determined using different approaches for different Taylor numbers. The results of these approaches are illustrated and discussed.

On the basis of previous experimental results in a torus reactor, micromixing time is determined using the incorporation model. Obtained results allowed the characterisation of the performances of this new configuration of reactor in comparison to other reactors, such as the stirred tank reactor. In addition, a correlation is proposed for each incorporation law, in order to determine the micromixing time from the experimental micromixedness ratio (α). Finally, in terms of Kolmogorov's turbulence theory, a relationship between micromixing time and the local energy dissipation rate is obtained and compared to those previously published.

This paper presents a quantitative visualization study and a theoretical analysis of two-phase flow relevant to polymer electrolyte membrane fuel cells (PEMFCs) in which liquid water management is critical to performance. Experiments were conducted in an air-flow microchannel with a hydrophobic surface and a side pore through which water was injected to mimic the cathode of a PEMFC. Four distinct flow patterns were identified: liquid bridge (plug), slug/plug, film flow, and water droplet flow under small Weber number conditions. Liquid bridges first evolve with quasi-static properties while remaining pinned; after reaching a critical volume, bridges depart from axisymmetry, block the flow channel, and exhibit lateral oscillations. A model that accounts for capillarity at low Bond number is proposed and shown to successfully predict the morphology, critical liquid volume and evolution of the liquid bridge, including deformation and complete blockage under specific conditions. The generality of the model is also illustrated for flow conditions encountered in the manipulation of polymeric materials and formation of liquid bridges between patterned surfaces. The experiments provide a database for validation of theoretical and computational methods.

Rehimi et al. [1] numerically studied the frequency response of an electrochemical probe. They applied an inverse sequential algorithm of the convection diffusion equation to simulated periodic mass transfer signals in order to determine the wall shear rate. This paper presents an experimental validation of the inverse method in mass transfer and a critical analysis of its advantages and limits of application. An electrochemical sensor (probe) was used to determine the mass transfer from the current delivered by the probe. The mass transfer is related to the wall shear rate via the convection diffusion equation. The database, obtained by electrochemical method also known as “Electro-Diffusion Method”, was exploited to validate the inverse method experimentally. The inverse method was checked over a specific range of Peclet numbers varied from 4.58 × 103 to 1.06 × 105 with a well-controlled shear flow with known wall shear rate. An optimized computational algorithm using Matlab® was developed for the post-processing, the filtering of the electrochemical results and for programming available models, mostly used in polarography for the determination of the wall shear rate (Leveque [2]; Sobolik et al. [3]; Deslouis et al. [4]). The comparison of these methods with the inverse method and the experimental wall shear rate allows a critical analysis of the restrictions of the different approaches. We demonstrated experimentally, that the difference between the real wall shear rate and the quasi-steady one of Leveque [2], can reach 9% for dimensionless frequencies f∗=fl2D⩽205 and oscillation amplitudes β ⩾ 0.3. For low frequencies (f∗ ⩽ 205), Sobolik et al. [3] solution and Deslouis et al. [4] transfer function are in agreement with the experimental wall shear rate. The inverse method is validated for high frequencies of oscillations, for which the linear approaches lead to amplitude attenuation and phase shift of wall shear stress fluctuations with respect to the experimental one.

Numerical methods for solving convection-diffusion-reaction (CDR) scalar transport equation in three-dimensional flow are used in the present investigation. The flow is confined between two concentric cylinders both the inner cylinder and the outer one are allowed to rotate. Direct numerical simulations (DNS) have been achieved to study the effects of the gravitational and the centrifugal potentials on the stability of incompressible Taylor-Couette flow. The Navier-Stokes equations and the uncoupled convection–diffusion–reaction equation are solved using a spectral development in one direction combined together with a finite element discretization in the two remaining directions. The complexity of the patterns is highlighted. Since, it increases as the rotation rates of the cylinders increase. In addition, the effect of the counter-rotation of the cylinders on the mass transfer is pointed out.

International audience; Couette-Taylor-Poiseuille flow CTPF consists on the superposition of Couette-Taylor flow to an axial flow. The CTPF flow hydrodynamics studies remain rather qualitative or numerical or are restricted to relatively low Taylor and/or axial Reynolds numbers. For more comprehensive and control of CTPF, especially for relatively high Taylor numbers and high axial Reynolds numbers, we investigated experimentally CTF with and without an axial flow, using the electro-diffusion ED method. This technique requires the use of Electro-Diffusion ED probe which allows the determination of the local mass transfer rate from the Limiting Diffusion current measurement delivered by the ED probe while it is polarized by a polarization voltage. From the local mass transfer (the Sherwood number), we determined the wall shear rate using different approaches. The results illustrate that low axial flow can generate a stabilizing effect on the CT flow. The time-evolutions of the local mass transfer and the wall shear rate are periodic. These evolutions characterize the waviness or the stretching of the vortices. However, Taylor Wavy Vortex Flow TWVF is destabilized under the effect of relatively important axial flow. The time-evolutions of wall shear rate are no longer periodic. Indeed, Taylor vortices are overlapped or completely destructed.

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Dispersed two-phase flow was experimentally investigated, by pulsed ultrasonic velocimetry (PUV) technique, in Sulzer SMX static mixers for turbulent flow regimes (2280

Mass transfer measurements were conducted on a Parallel Plate Rheometer (PPR) using the limiting diffusion current method, i.e. the polarography technique. The database constructed was exploited for the validation of the inverse method under a well-controlled unsteady shear flow. This method is based on a numerical sequential estimation. It has been applied to determine of the wall shear rate in the upper disk of the PPR. It requires the numerical inversion of the convection diffusion equation in order to apply it to instantaneous mass transfer measurements. This requires the use of electrochemical probe, which allows the determination of the local mass transfer rate for known flow kinematics. A multi-segment electrochemical probe has been used. The directional characteristics of the three-segment probe were highlighted. The directional angle effect and the frequencies of oscillations effect were studied in order to test the robustness of the inverse method within the presence of such factors. The obtained results demonstrate that the inverse method allows a good determination of the shear rate, which follows the experimental one for different cases tested in a satisfactory way.

The paper presents the results of numerical simulations of the fluid flow in milli torus reactor operated as airlift without agitation. CFD simulations were performed in 3D using FLUENT 6.2.16 numerical software. Unstructured mesh for prediction of gas hold up inside the milli torus reactor was investigated. An Eulerian–Eulerian multi-fluid approach was applied with a k − ɛ turbulence model. The inter-phase drag forces were used. For simplification of the solution, the bubbles inside the reactor were assumed to have the same size. The computed gas hold up was compared with the experimental data, a good agreement was obtained for unstructured meshing. For such mesh, simulations were validated with experimental residence time distribution (RTD) determination.

The present paper contributes to improve the understanding of two-phase flow hydrodynamics characteristics in milli torus reactor. To reach this goal, experiments were performed in a vertical milli torus reactor, having a capacity of 0.140 l. The effect of the aeration numbers and the impeller rotation speeds has been studied. It was found that the homogenization of the tracer inside the gas–liquid milli torus reactor was nearly achieved after more recirculations than those obtained in one-phase flow. Under aerated conditions, the flow pattern depended on both impeller rotation speeds and aeration numbers. Two flow regimes have been distinguished: not-dispersed and dispersed flow regimes. The flow induced by a rotating marine impeller reduced bubble size and prevented bubbles coalescence for aeration numbers Fl > 0.0021, at which the dispersed flow regime can be maintained in the milli torus reactor. The flow behaviour inside the milli torus reactor was modelled by the dispersed plug flow model. Correlations have been proposed to predict gas hold up and axial dispersion.

International audience; The effects of pulsating turbulent flows on wall shear stress components were investigated in a straight pipe using the non-intrusive electrochemical method. Experiments were made using a new pulsation generator system which allows high amplitude pulsations in addition to a perfect stability of the installation. Maximum pulsation frequency equal to 2.86 Hz is used, above which fluid inertia dominates over most part of the flow field. Analysis showed that pulsating flows induce an increase of local velocity gradient at the wall pipe. This result is explained by the periodic renewal of the boundary layers. Spectral analysis showed high increasing rates of the fluctuation energy for the different pulsating conditions in comparison with a steady flow. The tested condition involving a recirculation flow induced a modification in the energy dissipation cascade, which can be explained by the redistribution of eddies size near the wall.

Torus reactors are characterized by a homogeneous fluid circulation without dead zones. Torus reactors were used for applications in biotechnology, food processing, polymerization and liquid waste treatments. The relatively simple extrapolation of performances, due to the absence of dead volume, is one of the main advantages of this reactor, with low shear stresses and an effective radial mixing allowing efficient heat dissipation. This study is based on the mixing in order to analyse the fluid circulation, mainly in turbulent flow regime, and to characterize the torus reactor with the axial dispersion plug flow model. The objective of this study is to characterize the flow and the mixing in the torus reactors in batch and continuous modes. The mixing analysis was made according to the flow parameters and to the geometrical characteristics of the reactor and impeller. The mixing in the torus reactor can be characterized by the Peclet number, Pe D , defined with torus diameter. A representative model based on plug flow with axial dispersion and partial recirculation was proposed.

When processing highly viscous fluids in scraped surface heat exchangers, it is often difficult to follow the real heat flux received by each part of the product. Only average temperatures are measured, and the process cannot always guarantee a correct thermal treatment. During a pasteurisation process, the average temperature is around 80 °C, but some parts of the product may not stay long enough at this temperature compared to the average residence time. The aim of this study is to develop a tracer in order to follow the thermal treatment of Newtonian or non Newtonian viscous fluids in an experimental scraped surface heat exchanger in conditions similar to a pasteurisation treatment. A natural coloured tracer (Betanin, from red beets) which thermal degradation is well known is used as time temperature integrator. Thermal degradation of the tracer is determined experimentally and compared with a dispersion RTD model. First results show that Betanin degradation diluted in viscous fluids follows a classical first order reaction in a batch system. However, this degradation is enhanced when mechanical treatments occur in a well identified shearing channel as well as in the SSHE, for shear rates higher than 20 000 s − 1 . The major concern was then to identify the origin of this enhancement. Temperature increase due to viscous friction was measured in the SSHE and compared with the prediction conversion factors. It was shown that thermal treatment received by the Betanin was highly dependent on the mechanical treatment delivered during the thermal treatment. Industrial relevance When processing highly viscous fluids in scraped surface heat exchangers, it is often difficult to follow the real heat flux received by each part of the product, especially in industrial apparatus. Only average temperatures are measured, and the process cannot always guarantee a correct thermal treatment. During a pasteurisation process, the average temperature is around 80 °C, but some part of the product may not stay long enough at this temperature compared to the average residence time. The aim of this study is to develop a tracer in order to follow the thermal treatment in an experimental scraped surface heat exchanger in conditions similar to a pasteurisation treatment. When this tracer is found to be appropriate, it will be used directly in industrial equipments, in order to help industrials in: first, understanding their processes; verifying that their actual operating conditions are feasible for a correct thermal treatment; if not, in optimising these operating conditions. Betanin, representative of sensitive food products treated in a scraped surface heat exchanger was found to be, not only sensitive to temperature but also to mechanical treatment. This rather unexpected result has to be taken into account very carefully by industrials in order to insure a content product quality, whereas the mechanical treatment undergone by the product can lead to an accentuated degradation of thermally sensitive products.

Knowledge of local hydrodynamics inside miniaturized systems is necessary for industrial applications. In this work two techniques are used in order to characterize the flow in a network of crossing minichannels. These techniques are respectively based on the particle image velocimetry and on electrochemical microsensors used in the local diagnostic of the flow in order to set-up multi-scale analysis of the hydrodynamics inside a tested system. PIV measurements are applied in front of electrochemical sensors in order to obtain mean velocity fields along the cell. The comparison of velocity profiles with that obtained in fully developed channel flow emphasize the developing behaviour of the flow between two successive crossings. Vortices in the near wall region have been detected using the electrochemical technique at different positions corresponding to the location of the electrodes. The calculation of statistic parameters such as flatness and skewness factors of the instantaneous values of wall shear rate shows that the flow exhibits an intermittent regime behaviour. Finally, a multi-scale hydrodynamics pattern is proposed to describe the turbulence properties in the near wall region at the crossing of two channels.

An experimental investigation, by pulsed ultrasonic velocimetry technique has been conducted for flow analysis in Sulzer SMX static mixers in the transitional and turbulent flow regimes (1030 Re Z = 0.7 D . A circumferential dependence is observed and affects all the hydrodynamic parameters studied.

Improvement of mixing can lead to an enhancement of photobioreactors productivity, provided that the adverse effect of shear forces is kept below the fragility- threshold of cells. Implementation of a swirling decaying flow induced by a tangential inlet in an annular photobioreactor was investigated. The study focused on a compromise solution between the mixing and the resulting shear stress, and various hydrodynamical characteristics were measured for five types of tangential inlets and three Reynolds number values. Comparison with a pseudo-axial flow induced by a radial inlet indicates that swirling flows are suitable in the particular application of microalgal cultures. L'amelioration du melange peut permettre une meilleure productivite des photobioreacteurs, dans la mesure ou l'effet indesirable des contraintes de cisaillement est garde en-dessous du seuil de fragilite des cellules. On a etudie l'effet d'un ecoulement declinant tourbillonnaire induit par une entree tangentielle dans un photobioreacteur annulaire. L'accent a ete mis sur un compromis entre le melange et la contrainte de cisaillement qui en resulte, et diverses caracteristiques hydrodynamiques ont ete mesurees pour cinq types d'entrees tangentielles et trois valeurs du nombre de Reynolds. La comparaison avec un ecoulement pseudo-axial induit par une entree radiale indique que les ecoulements tourbillonnaires conviennent a l'application particuliere des cultures de microalgues.

The convective diffusion problem near small electrodes is solved numerically using finite volume method (direct problem). The results obtained are first used to define the limits of the quasi-steady state Leveque solution for a rectangular and circular probe. After, the algorithm is integrated to an inverse sequential method in order to obtain the wall shear rate from the measurements of the limiting diffusion currents. The results are compared with the classical techniques of calculation of wall shear rate as the quasi-steady solution, transfer function methods and the Leveque solution with transient correction.

Taylor–Couette flows are frequently reencountered on engineering processes, such as for cooling the rotors of electric motors by admitting cold air and improving the mixing in the combustion area, for the injection of fuel into the cylinders of gasoline engines, for the injection of just-prepared emulsions on diesel engines, etc. On its simplest configurations, Taylor–Couette systems (TCS) are widely studied when one or both cylinders steadily rotate. Limited studies are available as concerned on the acceleration-deceleration of one or both cylinders. This paper deals with the effect of acceleration-deceleration imposed to the inner cylinder of the CTS on flow instabilities. Imposing progressive periodic transitional motions through the cylindrical CTS walls alters the stability of the flow. The objective of this work is to study the flow dynamics and interactions between wall and vortices while imposing progressive periodic motions. We used the electrodiffusion (ED) method, known also as the polarography technique. The apparition and disappearance of the regime transitions, due to the acceleration-deceleration of the inner cylinder speed, on the frequency response of the ED sensor were illustrated and discussed. The regimes’ instabilities and the corresponding Taylor numbers of the first and the second transitions of TC flow were determined. The results show coherence between the temporal wall velocity gradient (determined from the mass transfer rates) and the flow turbulent characteristics, particularly at the transitions between the Couette flow (CF) regime, the wavy vortex flow (WVF), and the modulated wavy vortex flow (MWVF). Instantaneous mass transfer rate and velocity gradient have been compared for the CF, the WVF, and the MWVF regimes. The apparition of small then large vortices, when the Taylor number increases, i.e., from laminar regime to turbulent one, affects the mass transfer and the wall shear rate. Sensor inertia effect and a slight hysteresis effect, due to sudden periodic flow velocity decrease, were detected. The ED results allowed testing the robustness of the inverse method, based on the inversion of the convection-diffusion equation, on mass transfer to determine the wall shear rate.

The effect of a liquid dispersed on the wall flow structure in static mixer is analyzed by using an electrochemical method. Both laminar and turbulent flows have been investigated. The axial wall velocity gradient and turbulent intensity have been studied along the static mixer in both flow regimes and for different dispersed phase concentrations. The spectral analysis of the wall velocity gradient fluctuations was analyzed in the turbulent regime. For volume fraction higher than 5%, the effect of the dispersed liquid phase is very important for all the studied parameters. The turbulence associated to the dispersed phase leads to an increase of the energy dissipation in the static mixer and also to a modification of energy dissipation mechanism.

The electrochemical technique is commonly used to measure the instantaneous mass transfer between a liquid consisting of an electrochemical solution and a small probe mounted flush in an inert wall. The local mass transfer allows the calculation of the instantaneous wall shear rate averaged on the probe surface. In practice, there are different models used for the calculation of the wall shear rate (Leveque solution, transfer function and Sobolik methods). The probe inertia is closely dependent on its length; a small probe allows to enlarge the band-width of the probe response. For small probes, the axial diffusion can become important in contradiction with the assumptions used in the different models. In this paper, an inverse sequential algorithm of the convection diffusion equation is applied to simulated periodic signals in order to obtain the “true” wall shear rate. A comparison between the different methods is made for different amplitudes and frequencies of wall shear rate oscillations. The influence of noise on the different methods for wall shear rate calculation is also investigated.

An experimental investigation of a scaled-down model of an industrial exchanger, using an electrochemical technique, was undertaken in order to show the presence of hydrodynamic heterogeneities at low axial Reynolds number. Heterogeneities were revealed in the bowls with respect to the generalised Taylor number as the result of the perturbations added to the flow by blade rotation at both ends of the exchanger. Shear heterogeneities associated to flow visualisations were correlated to temperature heterogeneities observed in the bowls. Shear fluctuations were revealed in the scraped part describing two distinctive zones at low rotation speed caused by varying viscosity in the flow field. A complex spiral flow was observed by flow visualisation characterising a mass transfer evolution comprised between these two distinctive zones at low Taylor number.

Viscous products treated in a scraped surface heat exchanger receive a thermal treatment as well as a mechanical treatment. The new method developed by Mabit et al. [J. Food Engrg. 57 (2) (2003) 165] using starch as a marker for the shearing treatment was performed in order to show the influence of blades movement and flow pattern on shearing conditions. Two different flow regimes with respect to the generalised Taylor Number Ta g were investigated: a laminar regime with Taylor vortices and a weakly turbulent one. The blade rotation without scraping in the annulus space can be sufficient to apply a shear treatment above 20,000 s −1 . The product is submitted to the highest shear treatment for high rotational speed with a laminar regime with Taylor vortices. Blades scraping creates high wall shear rates, Taylor vortices improve radial mixing and renew product to be treated at the wall surface. The quantity of stressed product is directly influenced by the flow behaviour and the product residence time in the exchanger.

Calculation of the 6ow pattern in a new small homogenising valve design (Stansted, U.K.), able to reach operating pressure as high as 350 MPa, was investigated in the :rst part of this paper using a Computational Fluid Dynamics method. Numerical simulation results are used in the present paper to better understand the emulsi:cation process in the Stansted high-pressure homogeniser. Deformation of drops is supposed to occur in the intense elongational 6ow caused bythe restriction between the piston and the seat of the valve. Deformation maybe mainlyfollowed bydrop disruption in the narrow valve gap. Break-up probablyalso occurred in the highly turbulent region, located just at the exit of the gap, and underlined bythe numerical investigation. Cavitation and the rate of recoalescence, :rst assumed from numerical results, are determined thank to experimental methods. Intensities of both phenomena stronglyincrease with homogenising pressure. Final droplet size of model oil-in-water emulsions is then the result of equilibrium between droplet break-up and recoalescence, which stronglydepends on operating pressure. ? 2004 Elsevier Ltd. All rights reserved.

The electrodiffusion technique was performed in order to investigate the shear rate on a scraped surface heat exchanger. Microelectrodes were placed inside: the walls of the outer cylinder; the inlet and outlet bowls; the rotor and the blades. Highly viscous Newtonian fluid (Emkarox HV45 solutions) and non-Newtonian model fluid (aqueous solutions of CMC) were used. The electrodiffusion method allowed us to measure wall shear rates. Maximum shear rate was observed at the scraping surface and caused by blades scraping, high shear rate was also measured on the leading edge of the blades. In the other parts of the exchanger, shear rate remained low but the development of Taylor vortices completely modified the scraped surface heat exchangers behaviour inside the surface of the bowls. A dimensionless representation of the friction factor was established for the inner and outer wall surface of the exchanger.

Light is the main limiting factor in photoautotrophic-intensive production of microorganisms, and improvement of its use is an important concern for photobioreactor design and operation. Swirling flows, which are known to improve mass and photon transfers, were applied to annular light chambers of a photobioreactor and studied by simulation and microalgal culture. Two hydrodynamic conditions were compared: axial flow generating poor radial mixing, and tangential flow generating three-dimensional swirling motion. Batch and continuous cultures of the Rhodophyte Porphyridium cruentum were performed in a 100-L, 1.5-m 2 , fully controlled photobioreactor with eight light chambers. The inlet design of these chambers was modified to create the hydrodynamic conditions for comparison. Various intensities of swirling motion were used, characterized by the velocity factor (VF), defined as the ratio between annular chamber flow and inlet aperture sections. Experiments were performed within the range of photon flux densities (PFD) optimizing the yield of light energy transformation into living substance for the species and the temperature used. Culture kinetics with swirling flows generated by apertures of VF = 2, 4, and 9 were compared with pseudoaxial VF = 2 chosen as reference. Batch cultures with VF = 4 swirling flow showed no significant difference, whereas continuous cultures proved more discriminating. Although no significant difference was obtained for VF = 2, a 7% increase of steady-state productivity and a 26% decrease in time required to reach this steady state were obtained with VF = 4 swirling flow. This beneficial effect of swirling flow could have accounted for increased mixing. Conversely, VF = 9 swirling flow resulted in a 9% decrease of steady-state productivity and a 9% increase in the time required to reach this steady state, a negative effect that could have accounted for increased shear stress. CO 2 bioconversion yield at steady state showed a 34% increase for VF = 4. These results suggest that swirling motion makes microalgal cultures more efficient, provided that the resulting adverse effects remain acceptable. Experimental investigation was completed by a theoretical approach in which simulation of continuous cultures of P. cruentum was based on the hydrodynamic conditions achieved in the photobioreactor. Although the results obtained with pseudoaxial flow were correctly predicted, simulations with swirling flow showed a marked enhancement of productivity not observed experimentally. The influence of side effects induced by increased mixing (particularly hydrodynamic shear stress) was considered with respect to modeling assumptions. Comparison of experimental results with theoretical simulation provided a better understanding of the mixing effect, a key factor in improving the efficiency of such bioprocesses.

Centrifugal partition chromatography (CPC) is a support-free liquid-liquid separation process that depends for efficiency on the behavior of the two liquid phases. Hydrodynamics of phases was studied according to flow rate and centrifugal acceleration, using a transparent column and a stroboscopic video system. For the heptane-methanol two-phase system, three main flow regimes - stuck film, oscillating sheet, and atomization - are observed, highlighting the coriolis acceleration effect as well as the influence of the column shape. Mass transport in the CPC column is modeled by a plug flow with axial dispersion and mass transfer with a stagnant volume. Model parameters (residence time, Peclet number, partition ratio, and mass-transfer coefficient) are fitted on solute residence-time distribution. Off-column dispersion is an important source of peak broadening in CPC, whereas its irregular geometry provides a plug flow for mobile phase. Importance of flow pattern on mass transfer is demonstrated. CPC interest for preparative applications is confirmed.

An experimental investigation was done by the use of visual observations and the electrochemical technique in order to study the appearance of hydrodynamic instabilities at low Re axg in the gap between two coaxial cylinders, with radius ratio R r / R s =0.615 and aspect ratio L / e =24. A motor drove the inner cylinder and the outer cylinder was fixed. A Newtonian fluid (Emkarox HV45) and two non-Newtonian fluids (aqueous solutions of guar and CMC) have been used. The analysis of the evolution of the size of the Taylor vortices is carried out for Re axg Re axg Ta g ≈80, and a stretching phase for Ta g >80. For higher Re axg , the contraction phase vanished and the vortices progressively stretched with increasing Ta g . A dimensionless representation was also proposed for wall friction generalized to non-Newtonian fluids following the Ostwald law.

The investigation of wakes of bluff bodies in a channel is still relevant despite the large number of works devoted on it, in both experimental and numerical studies. This attractiveness is mainly due to its related applications and practical interest in varied engineering fields. The understanding of dynamic flow behavior and the topology of the instability structures occurring in the wake is essential in order to optimize the obstacle shape according to the desired objectives. A confined laminar flow around a square and a circle, placed in a channel is numerically investigated in this work using Lattice Boltzmann method. The study is then extended to 3D computations with horizontal cylinder within a square then a circular cross-section mounted inside a rectangular duct. The Reynolds number (Re), based on the maximum velocity and the cross-section height varies between 50 and 120 and the blockage ratio is r=1/3. This geometry is representative of a passive method to enhance mixing in the laminar channel flow. LBM was built up on the D2Q9 and D3Q19 model for respectively 2D and 3D computations. The single relaxation time approach called the lattice-BGK method was adopted. The topology of the vortex-shedding phenomena and wake behavior according the Reynolds numbers, for both geometries of the obstacle are focused. The effect of wall confinement on the flow transition to the vortex shedding regime is discussed. Velocity profiles and integral parameters such as recirculation length and Strouhal number were investigated. The numerical results are supported by literatures works results for the same configuration showing the performance of LBM as numerical tools simulation for such kind of flows.

International audience; In the simplest and original case of study of the Taylor Couette TC problems, the fluid is contained between a fixed outer cylinder and a concentric inner cylinder which rotates at constant angular velocity. Much of the works done has been concerned on steady rotating cylinder(s) i.e. rotating cylinders with constant velocity and the various transitions that take place as the cylinder(s) velocity (ies) is (are) steadily increased. On this work, we concentrated our attention in the case in which the inner cylinder velocity is not constant, but oscillates harmonically (in time) clockwise and counter-clockwise while the outer cylinder is maintained fixed. Our aim is to attempt to answer the question if the modulation makes the flow more or less stable with respect to the vortices apparition than in the steady case. If the modulation amplitude is large enough to destabilise the circular Couette flow, two classes of axisymmetric Taylor vortex flow are possible: reversing Taylor Vortex Flow (RTVF) and Non-Reversing Taylor Vortex Flow (NRTVF) (Youd et al., 2003; Lopez and Marques, 2002). Our work presents an experimental investigation of the effect of oscillatory Couefte-Taylor flow, i.e. both the oscillation frequency and amplitude on the apparition of RTVF and NRTVF by analysing the instantaneous and local mass transfer and wall shear rates evolutions, i.e. the impact of vortices at wall. The vortices may manifest themselves by the presence of time-oscillations of mass transfer and wall shear rates, this generally corresponds to an instability apparition even for steady rotating cylinder. On laminar CT flow, the time-evolution of wall shear rate is linear. It may be presented as a linear function of the angular velocity, i.e. the evolution is steady even if the angular velocity is not steady. At a ``critical'' frequency and amplitude, the laminar CT flow is disturbed and Taylor vortices appear. Comparing to a steady velocity case, oscillatory flow accelerate the instability apparition, i.e. the critical Taylor number corresponds to the transition is smaller than that of the steady case. For high oscillation amplitudes of the inner cylinder rotation, the mass transfer time-evolution has a sinusoidal evolution with non equal oscillation amplitudes. If the oscillation amplitude is large enough, it can destabilize the laminar Couefte flow, Taylor vortices appears. The vortices direction can be deduced from the sign of the instantaneous wall shear rate time evolution.

The electrodiffusion technique has been used to investigate reattaching and recirculating flows behind a backward-facing step. The instantaneous wall shear rate vectors were determined using the current signals provided by a three-segment electrodiffusion probe. The near-wall extents of two counter-rotating recirculation zones located behind the step were determined under turbulent flow conditions in a water channel. The near-wall flow inside these recirculation zones was found to be very unsteady, with strong low-frequency fluctuations. The streamwise profiles of the wall shear stress were measured at several values of the Reynolds number and a high level of skin friction was obtained in the reverse-flow region. The strong dependence of the peak value of skin friction on the Reynolds number confirms the viscous-dominated character of the reverse flow appearing inside the recirculation zone.

This work is devoted to the numerical modeling of mass transfer on both cylinders of an annular cell subjected to swirling laminar decaying flow induced by means of a single tangential inlet greater in diameter, φ e =12.5 mm , than the annular gap thickness, e=7 mm . The complete diffusion–convection equation has been solved in a three-dimensional frame by means of the FIDAP finite element package using the velocity fields previously predicted by the same numerical technique. As for the flow-field, we have shown that a correct prediction of mass transfer coefficients in such a flow is strongly linked to an adequate mesh generation, especially when mass transfer occurs on the inner wall of the annulus, near which recirculating eddies exist. Numerical results have been found to be in good agreement with experimental data previously obtained on both cylinders of the annulus. A simulation study of the influence of the Schmidt number on the overall mass transfer on the outer cylinder has shown that the mean Sherwood number varies as Sc 0.44 . Even for forced convection, the numerical simulation of mass transfer is not directly obtained from the simulation of the dynamics of the flow. Due to the difference in mass and momentum boundary layer thicknesses, the mesh refinement has to be adapted to Schmidt number.

Mixing in an axisymmetric concentrated KCl solution jet emerging in a less salty coflowing water has been experimentally investigated. This has been achieved by coupling laser Doppler velocimetry to a microconductimetry technique in view of measuring the turbulent fluxes cu and cv and deducing the turbulent diffusivity. The cross-correlation coefficient between radial velocity and concentration R cv reaches a maximum value of 0.77 when the one relative to axial velocity R cu gets smaller and does not exceed 0.37 under the same conditions. The total turbulence intensity, global microscale, kinetic energy dissipation rate, and turbulent diffusion coefficient in the radial direction have also been determined. In a jet section, the turbulent diffusion coefficient was found to vary from 180 to 2000 times the KCl molecular diffusivity D m .

The influence of different fluid inlet types, slits or tubes, on mass transfer in a rectangular reactor was studied. Measurements of mass transfer coefficients were made using the limiting diffusion current technique based on ferricyanide ion reduction at a large nickel electrode located downstream of abrupt expansions. The overall mass transfer coefficients obtained were 3 to 13 times greater than those obtained in fully developed flows. Overall mass transfer coefficients were correlated for Reynolds numbers ranging from 400 to 3500 by a unique equation by introducing a nondimensional expansion factor defined by the ratio of the fluid inlet cross-section to that of the reactor. The correlation equation obtained was compared with literature data.

International audience; Particle image velocimetry (PIV) has been used in order to measure the three mean components and turbulence intensities of the velocity vector in a swirling decaying flow induced by a tangential inlet in an annulus. This kind of flow motion is found to be very complex, exhibiting three-dimensional and non-axisymmetric characteristics coupled with a free decay of the swirling intensity along the flow path, thereby making it difficult to study. A method allowing the measurement of the three components of the velocity flow-field with a standard PIV system with two-dimensional acquisitions, is presented. The evolution of each velocity component between the inlet and the outlet of the annulus is obtained. Furthermore, the PIV technique is extended to the measurement of turbulent characteristics such as turbulent intensities and dimensionless turbulent energy. The main characteristics of the swirling flow are discussed and the swirl number is estimated as a function of the axial distance from the tangential inlet.

An experimental investigation of a scraped surface heat exchanger (SSHE) was undertaken using visual observations and the electrochemical technique in order to, firstly, study the transition between laminar and vortex flows and, secondly, evaluate the wall shear rates. Visual observations and the electrochemical technique had undergone preliminary testing in a SSHE without blades, a well-known reference annulus. Then, visual observations and wall shear rates was compared in both geometries. It was established that flow patterns in a SSHE are noticeably different from those observed in an annular space in the same conditions. In a SSHE, the formation of the vortices is thwarted by the rotation of the blades (and by the clips of the blades) and the transition between laminar and vortex flows occurs at Ta gc ≈80 ( Ta gc ≈45 in the annular space without blades). Local measurements of the shear rate at the tube wall of the SSHE showed that it is fully controlled by the rotation of the blades. Vortices have a negligible influence in comparison with that of blade scrapings. Wall shear rates due to blade scraping can reach up to 40 000 s −1 . Under these conditions, the lowest value of the clearance between the edge of the blades and the stator is about 50 × 10 −6 m.

This work is dedicated to an experimental study of the wall shear stress in a turbulent swirling decaying flow induced by means of a tangential inlet in an annular cell. The flow pattern near both cylinders of the annulus is analyzed by means of measurements of wall velocity gradients using an electrochemical method involving micro-electrodes working in inert wall. The analysis of the axial and circumferential evolutions of the mean wall velocity gradient and of its power spectral density emphasizes the complexity and highly three-dimensional nature of the flow pattern of the annular swirling decaying flow induced by means of a single tangential inlet. Near the inner wall of the annulus, a recirculating zone, consisting of several pairs of contra-rotating cells, develops following the main helical motion of the fluid along the flow path. Near the external wall of the annular cell, high local velocities and energetic turbulent structures are detected in the vicinity of the tangential inlet. These eddies rapidly decay along the flow path because of the decrease of the swirl intensity. The setup of a recirculation bubble near the inner wall induces a quasi-axisymmetric flow behavior at the outer cylinder of the annulus. Near both walls, the power spectral densities of the velocity gradient reveal a superposition of small turbulent eddies having a low energetic level in the high-frequency domain.

An electrochemical technique was used to measure local and instantaneous wall shear rates in a scraped surface heat exchanger (SSHE). The inner cylinder, equipped with two blades symmetrically opposed, was driven by a stepping motor and the outer cylinder was fixed. Highly viscous Newtonian fluids (Emkarox HV45 solutions) and non-Newtonian model fluids (aqueous solutions of guar and CMC) were used. The reliability of the electrochemical technique was validated for these fluids in an annular space (SSHE without blades). The evolution of the limiting diffusion current was given by the probes in a scraping situation. In the general case of scraping, the flow was unsteady, and very fast large fluctuations of the limiting diffusion current were measured. The shear rate at the wall of the stator of the SSHE appeared fully controlled by the rotation of the blades. Then, electrodiffusion measurements were interpreted using three different wall shear rate calculating procedures. Wall shear rate in SSHE appeared ten to 100 times more important than in annulus, and the lowest values of the clearance between the edge of the blades and the stator was about 50 × 10−6 m.

In this paper, the hydrodynamics in the FM01-LC electrolyzer is studied by flow visualization and residence time distribution (RTD). Divided and undivided modes are studied by direct visualization, with two injection points. The use of foams instead of turbulence promoters as well as that of electrodes without turbulence promoters is tested. In the membrane-partitioned-mode configurations, radial and axial dispersions are observed. Dispersion is drastically extended in case of empty channel flow. A high-flow asymmetry is generally induced by the flow distributors. RTD is implemented in the whole cell and in the reaction area. The reactor working with a plane plate alone gives a rather constant, low Peclet number, Pe, whatever the tested flow rates. The low Pe values denote an important axial dispersion. The use of turbulence promoters or foams induces a significant increase of Pe, giving plug flow character.

Local and simultaneous velocity and concentration fluctuations were analysed in an axisymmetric jet of an aqueous electrolyte solution moving in a codirectional water stream. The determination of the convection velocity of the turbulent eddies as well as correlation between axial velocity and concentration fluctuations was made possible by coupling L.D.A. to an electrochemical technique. The influence of the ratio of the coflowing stream velocity to the jet exit velocity was investigated. The mean velocity and concentration profiles, the turbulence intensity, the integral length scales relative to axial velocity and concentration were also determined.