Distribution des temps de séjour en écoulement tourbillonnaire annulaire induit par une entrée tangentielle du liquide

In this work, a thorough model for the flow in annular cell provided with various feed systems, based on an experimental determination of the residence time distribution, was presented for values of the Reynolds number in the range 100 to 5000. Using the plug flow model with axial dispersion, followed by a series of perfectly stirred plug-reactors, it was shown that the non-maintained swirling flow induced by a tangential inlet exhibits a very pronounced piston behavior in conjunction with a stirred zone due to the flow inlet/outlet system in the cell. When an axial inlet or an inlet perpendicular to the cell was used, the percentage of the overall reactor volume having a plug flow with respect to the one with a tangential inlet was decreased.     

Contrôle hydrodynamique de l'evaporation d'un film liquide ruisselant sur une paroi isotherme à symétrie de révolution dans un courant d'air humide

A thin-film evaporation from a surface of a axisymmetrical body in a humid air stream was studied using a finite difference method associated with Thomas' algorithm. The surface of the body is supposed isothermal. The variability of physical properties is taken into account, and a steady-state and laminar regime is supposed. The influence of the main parameters of the system on the evaporation is determined, especially the wall profile. It is shown that, for certain conditions of calculation, it is possible to find a flat ellipsoid of revolution offering a uniformly accessible wall between the pole and the equator.     

Comparaison de deux types d'écoulements (tangentiel plan et tourbillonnaire) dans des modules d'ultrafiltration. Influence de la contrainte pariétale

This work consists of evaluating the performances of a new membrance separation process: an annular, swirling, decaying flow induced by a tangential inlet. For this purpose, we have compared the permeation flux obtained in swirling flow with that of a crossflow. We ultrafiltered a model suspension of bentonite in order to compare the performances of the two modules for different operating conditions. Parameters such as flow rate, Reynolds number and wall velocity gradient were studied, and this last one appears to be a significant hydrodynamic parameter to compare the performances of the two configurations. Indeed, this parameter takes into account the flow field nature and, more precisely, the shear stress at the membrane surface. Improvement of the permeation flux induced by the swirling, decaying flow in comparison with that observed in cross‐flow can reach more than 30% under specific conditions. The study of the energy consumption of these two cells allows us to confirm the interest of the swirling, decaying flow in order to design new types of membrane separation modules.     

Déformation subie par une nappe de mercure soumise à l'impact d'un jet liquide bidimensionnel immergé

The paper reports experimental results on the deformation suffered by a mercury surface impinged perpendicularly by a stationary 2-dimensional liquid submerged jet of small thickness. The deformation is visually observed and measurements on it are made using a cathetometer or directly on photographs. The jet width, Reynolds number and height above the mercury surface are the experimental parameters. The values taken by the maximum height and the diameter of the cavity are analysed; they are compared with theoretical expressions from the literature. It is concluded that, in spite of the small scale of the systems studied, they behave accordingly to models existing in the literature and verified for large scale systems.     

Transfert photonique dans un photoréacteur à écoulement tourbillonnaire

Photonic transfer has been analyzed for two hydrodynamic conditions in an annular photoreactor: a swirling decay- ing flow generated by a tangential inlet, and an axial flow generated by a radial inlet. The photonic transfer coefficient was determined by carrying out the homogeneous photoreduction of potassium ferrioxalate. A comparison of the two hydrodynamic conditions is made in aqueous solution and in a suspension of black carbon in order to reduce the light penetration depth. The last condition emphasizes the interest in using tridimensional flow to design a photoreactor.     

Study on the Flow Field inside the Microfiltration Separator with Rotary Tubular Membrane

Abstract

By using a new type of laser surveying instrument named particle image velocimetry (PIV), the flow field inside a rotary tubular membrane separator with a rotating inner tubular microfiltration membrane and a transparent outer cylinder was measured experimentally, and from which some new observations were resulted. Pairs of stable Taylor vortices with similar dimensions and opposite flowing directions were directly visualized by the measured streamlines and vorticity of flow field in the annular gap of the membrane separator. No matter how the axial Reynolds number, radial Reynolds number and Taylor number changed, the dimensions of the Taylor vortices and the distances between the centers of adjacent Taylor vortices were almost the same, but the shapes of the Taylor vortices at lower Taylor numbers were more regular than those at higher Taylor numbers. The Taylor vortices disappeared because of the turbulence when the Taylor number was too high. The maximum axial velocity near the membrane surface was about 20 times larger than the mean velocity of axial flow inside the annular gap, and the maximum outward radial velocity near the membrane surface was even about 3000 times larger than the average velocity of the radial permeating flow through the tubular microfiltration membrane. The large velocities near the membrane surface, which were due to the Taylor vortices, could prevent solid fine particles from depositing onto the membrane surface and/or entering into the membrane pores and therefore result in reduced concentration polarization and reduced membrane fouling. The results in this study provided some valuable guidelines on the hydrodynamic way to reduce membrane fouling.     

Convection naturelle transitoire dans les liquides à viscosité variable

The role of a linearly dependent viscosity on temperature in laminar free convection is studied for a vertical flat plate with an imposed uniform heat flux. The analysis is based on the numerical solution of the transient state boundary-layer equations using an implicit finite difference procedure. The characteristic transient and steady state velocities, the temperature and the local Nusselt numbers are presented. The transient velocity and temperature profiles go through maximum values before decreasing to their respective steady state values. This overshoot phenomenon is amplified or attenuated depending on the values selected for the dimensionless parameters characterizing the viscosity variation.     

Amelioration du transfert convectif de chaleur par l'écoulement pulse dans un fluide visqueux

Pulsed perturbation effects on convective heat transfer in the laminar flow of a viscous fluid were studied in a co-axial cylindrical tube heat exchanger over the experimental temperature range 30 to 45°C. The fluid studied here, whose viscosity is twelve times larger than water at room temperature, is an aqueous solution of glycerol (60 wt %), and the Reynolds numbers of the steady flow are between 150 and 1000. The pulses significantly increase the thermal transfer at constant dissipated mechanical power. Indeed, enhancement of the heat transfer coefficient by more than 300% was obtained with strong pulsed perturbations. Therefore, pulses superimposed to a steady flow are a simple and efficient manner to improve thermal exchanges in viscous fluids that are usually pumped in the laminar regime. A correlation based on our experimental data on pulsatile flow is used to evaluate the Nusselt number with an average error of 10%.     

Électrodes poreuses percolées constituées de métal fritté: Hydrodynamique et transfert de matière

Percolated porous electrodes made of commercial sintered nickel were stuided for different configurations (percolated sheet; radially percolated tube wall; porous tube with simultaneous axial and percolated radial flows). The materials were characterized using permeametry experiments, and the volumetric mass transfer coefficients were electrochemically measured. Empirical correlations of the results were obtained for the case of one-dimensional electrodes (axial and radial electrodes); they were compared with other correlatinos from the literature. The results obtained for the case of combined axial and aspiration flows are discussed.     

Réacteur électrochimique à lit granulaire transporté

This work deals with an electrochemical reactor composed of a solid granular matrix located in a conical vessel. The solution to be treated flows through the reactor by forced convection. This mode of operation allows the solid matrix to move downwards because of gravity, assures good cohesion, hence good conductivity. The numerical solution of the differential equations obtained from the mass and charge balances permits the determination of the concentration, current and potential distributions in the case where the granular bed is kept fixed; the moving bed configuration is also discussed. The concentration development (plug flow reactor) and the influence of liquid and solid flow rates are discussed and illustrated in the case of selective copper recovery from dilute solutions (< 500 ppm) in a sulfuric acid medium. Finally a reactor design diagram for such devices is proposed.     

Velocity profiles and frictional pressure drop for shear thinning materials in lid-driven cavities with fully developed axial flow

A finite element numerical study has been carried out on the isothermal flow of power law fluids in lid-driven cavities with axial throughflow. The effects of the tangential flow Reynolds number (Re_U), axial flow Reynolds number (Re_W), cavity aspect ratio and shear thinning property of the fluids on tangential and axial velocity distributions and the frictional pressure drop are studied. Where comparison is possible, very good agreement is found between current numerical results and published asymptotic and numerical results. For shear thinning materials in long thin cavities in the tangential flow dominated flow regime, the numerical results show that the frictional pressure drop lies between two extreme conditions, namely the results for duct flow and analytical results from lubrication theory. For shear thinning materials in a lid-driven cavity, the interaction between the tangential flow and axial flow is very complex because the flow is dependent on the flow Reynolds numbers and the ratio of the average axial velocity and the lid velocity. For both Newtonian and shear thinning fluids, the axial velocity peak is shifted and the frictional pressure drop is increased with increasing tangential flow Reynolds number. The results are highly relevant to industrial devices such as screw extruders and scraped surface heat exchangers.     

Transfert de matière et chute de pression lors d'un écoulement tourbillonnaire annulaire non-entretenu induit par une entrée tangentielle

This work is devoted to an experimental study of overall mass transfer and pressure in decaying swirlng annular flow induced by means of a tangential inlet. Overall mass transfer coefficients, obtained using an electrochemcal method, are found to be greater on the outer cylinder that on the inner core of the annulus, and remains largely greater than that obtained in fully developed axial flow on both cylinders. This enhancement in mass transfer rate is coupled with an increase in pressure drop, mainly due to the tangential inlet of the fluid. Nevertheless, an energetic correlation of overall mass transfer on both cylinders of the annular cell shows that decaying swirling flow induced by a single tangential inlet allows an important increase in mass transfer coefficients for a given energy consumption, compared with a fully developed annular axial flow.     

γ射线透射技术测量气液鼓泡塔内气含率的轴向分布

引　言鼓泡床反应器以其具有良好的传热、传质、相间充分接触和高效的可连续操作等特点在许多领域得到了广泛的应用 ,如氧化、加氢、烷基化、污水处理等工业过程 .在鼓泡反应器中气体以分散相的形式存在 ,轴向气含率分布是反映气泡在鼓泡塔反应器内运动行为的重要参数之一 .虽然采用了各种测量方法对流体力学行为进行了大量研究[1～ 3] ,取得了一定的研究成果 ,但工业过程往往是在高温高压的条件下操作 ,大部分的测量方法难以适应这种条件 ,所以探求新的测试技术尤为重要 .　　γ射线透射技术不仅用于塔设备的故障检测 ,而且用于其他的石油…     

Étude d'un procédé de pyrolyse rapide du charbon dans un réacteur cyclone

Experimental results on fast pyrolysis of coal in a steady state pilot reactor at a flowrate of 0.1 kg/h are examined in this article. The pyrolysis reactor is a cyclone. Using such a system it was possible to achieve simultaneously the reaction and the separation of semi-cokes from volatiles produced by pyrolysis. A pump enabled gas recycling on the experimental setup and allowed a good hydrodynamic regime in the reactor. Only gases were recycled, liquids and tars being condensed and recovered. Experiments were carried out on a Lorraine coal (Houve well). From 100 grams of coal, 60 grams of semi-coke, 35 grams of liquids, and 5 grams of gas containing 66% hydrogen and 30% methane were obtained. For the interpretation of results, an experimental kinetics study on coal grain pyrolysis is presented for grains in the range of 2.5 to 20 mm and temperatures between 700 C and 1050 C.     

Fours rotatifs: Modèle dynamique du mouvement du lit

The motions of the granular bed inside a rotary furnace are governed by complex dynamics consisting of two modes. The axial velocity of the active layer on top of the bed represents the fast mode, while the overall velocity in the bed follows a slower mode. The proposed nonlinear model focuses on the dynamics of the slow mode. The difficulties of an analytic solution are avoided by a numerical method that employs several correlations. The model can reproduce the experimental transient responses. It is concluded that rotary furnaces respond better to small amplitude excitations than to large ones; consequently, closed-loop control is well suited for this type of furnace.     

旋转流场流态预测模型验证及其速度分量选择的差异性

鉴于目前高速旋流场中的流体流态判定准则不统一、预测模型契合度不高的问题,依据流体力学基本原理及管道、缝隙流场的判定方法,本文对经典一维雷诺数及二维流量因子预测模型进行了理论重构,并尝试提出了适用于旋转流场中流体流态判定和预测的椭球模型。文章首先根据经典雷诺数模型和流动因子模型,对仿真计算和椭球模型进行了理论验证;然后对不同介质和工况参数下的速度场进行了分析计算,并与相关文献进行对比研究;最后结合对旋转流场中拐点的理论剖析,对椭球模型的合理性和科学性进行了论证,并对模型中速度分量的选择及差异性进行了讨论。结果表明：椭球模型对管道流动的预测结果与经典雷诺数模型完全一致,新模型对旋转流场中转折点的预测值较传统模型明显偏低,与实际工况更加贴近;根据椭球模型进行旋转流场的流态判定时,应选择平均直径处的线速度为剪切平均速度、进出口径向速度平均值为径向平均速度及最大轴向速度为模型输入因子。椭球模型的提出,为旋转流场在理论计算时如何科学判定流体流态提供了新的思路和判定方法。     

Analyse des signaux temporels de pression dans un lit fluidisé turbulent de FCC

The fluidized bed turbulent regime is an intermediate regime between bubbling fluidization and pneumatic transport, which can be characterized by the study of wall pressure fluctuations with time. Our experimental results confirm that the statistical analysis of the signal enables to determine the regime boundaries. An original frequency analysis shows that there is no systematic change in dominant frequencies in the turbulent fluidization regime. However, an enrichment of frequency spectra occurs. The signal stochastic analysis by Hurst's method leads to the same conclusion.     

Précipitations sélectives des métaux solubilisés biologiquement de boues aérobies d'épuration

The selective and successive precipitation resulting from the biological solubilization (Thiobacillus ferrooxidans) of sewage sludge metals was studied. At the same time, theoretical diagrams representing the solubility of hydroxides and metal carbonates (Ca, Mg, Fe, Al, Cu, Zn, Ni, Cd, Mn) and their complexes are compared to the experimental values. The pH was progressively increased by means of Ca(OH)₂- The iron hydroxide precipitate, which was the least contaminated, was obtained at pH of 3.5. The theoretical pH's for acceptable metal precipitation were identified as: Fe⁺³ at pH 3.2, Al⁺³ at pH 4.7, Cu⁺² at pH 6.9, Zn⁺² at pH 8.15, Ni⁺² at pH 8.4 and Mn⁺² at pH 9.1, and were closed to theoretical values. The hydroxide ion controls Ca, Mg and Cu solubilities. A precipitate with about 20% of Cu can be obtained at a pH of 7 after iron hydroxide was precipitated. Recovery of other metals than iron is thus possible.     

Characterization of Taylor vortex flow in a short liquid column

We present a study on Taylor vortex flow in the annulus between a rotating inner cylinder and a stationary outer cylinder, featured with a wide gap (radius ratio is 0.613) and a short column (aspect ratio is 5.17). A particle image velocimetry (PIV) system was used to determine the position, shape, and velocity distribution of the vortices, by which the flow was also confirmed to lie in the nonwavy Taylor vortex regime for all operating conditions explored in this study. Our results suggest that end boundary effects are important, in which the vortex number decreases with decreasing column length. For a system with an aspect ratio of 5.17, six vortices appear in the gap with their position, size, and shape varying at different Reynolds numbers. The fluid velocities show an asymmetric feature with respect to the vortex centers, while the maximum axial and radial velocities increase almost linearly with the increasing reduced Reynolds number (Re ? Re_c). In addition, computational fluid dynamics study was employed under the same conditions, and its results agree well with the PIV measurements. Overall, this study provides a quantitative understanding of the formation of Taylor vortices in a constrained space. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Contrǒle hydrodynamique en convection mixte de l'épaisseur de dépǒts en phase gazeuse de semi-conducteurs sur des corps à symétrie de révolution

The authors solve using an finite difference method the equations of steady-state laminar boundary layer around a body of revolution (sphere, extended and flattened ellipsoids). Hydrogen gas at 25°C flows upward around the body. Its nose faces downward, the temperature of its surface is 500°C and it rotates about its vertical axe at a constant velocity. The variability of the physical properties of the fluid as a function of temperature is considered as well as the resulting natural convection. It is revealed that flow, heat and mass transfers depend of the physical properties of the gas. It is possible to control the development of thermal and diffusional boundary layers through the operational conditions, specially the contour of the body.