International Scientific Committee Comité Scientifique

chairmen Joël BERTRAND, FranceLionel CHOPLIN, France

members Edmundo BRITO DE LA FUENTE, Mexico

Arthur W. ETCHELLS, USADavid FLETCHER, Australia

Ivan FORT, Czeck Republic

Yushi HIRATA, JapanFrank JEURISSEN, The Netherlands

Franco MAGELLI, ItalyAlvin NIENOW, UK

Vivek RANADE, IndiaAnders RASMUSON, Sweeden

Jean-Louis SALAGER, Venezuela

PhilippeTANGUY, CanadaCatherine XUEREB, France

Organizing Committee Comité d'Organisation

Marion ALLIET, Toulouse, France

Jack LEGRAND, St Nazaire, France

Nathalie LE SAUZE, Toulouse, FranceH.Z. LI, Nancy, France

Paul MAVROS, Thessalonique, GrèceMartine POUX, Toulouse, France

Coordinators Coordination

Joël BERTRAND, LGC, Toulouse, Francee-mail : Joel.Bertrand@ensiacet.fr

Lionel CHOPIN, GEMICO, Nancy, Francee-mail : Lionel.Choplin@ensic.u-nancy.fr

Contact - Information

PROGEPFlorence FOUCAUD

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TABLE OF CONTENTSSommaire

PLENARY CONFERENCE

Mixing of floating solids 2ETCHELLS A.W.Du Pont de Nemours, WILMINGTON DE USA

INDUSTRIAL MIXINGAdvantages and applications for efficient, intermediate power number, 3axial flow impellersFASANO1 J.B., REEDER1 M.F., MYERS2 K. J.1 Chemineer Inc., DAYTON USA2 Univ. of Dayton Dept. Chemical & Engineering, DAYTON USA

Welling flow and heat transfer characteristics of a new mixing-type evaporator 4YAMAJI1 H., NODA1 H., KATAOKA2 K.1 Kansai Chem. Eng. Co Ltd., AMAGASAKI-CITY Japan2 Kobe Univ Dpt. of Chem. Science & Eng., KOBE Japan

BROGIM - a new three-phase mixing system.testwork and scale-up 5BOUQUET1 F., MORIN2 D.H., AUDIDIERE1 P., OLLIVIER1 P.1 Robin Industries, AVON France2 BRGM, ORLEANS France

Effects of agitation and aeration on the productivity of xylanolytic enzymes in batch 6fermentors – Design and scale-up criteriaGASPAR1 A., BRUXELMANE2 M., THOMAS2 D., THONART1 P.1 Faculte des Sciences Agronomiques Unité des Bioindustries,GEMBLOUX Belgium2 Fac Polytechnique Mons Dpt. Chimie, MONS Belgium

Design and scale-up of static mixers in laminar flow 7FLEISCHLI1 M., SCHLEGEL2 R., WEHRLI2 M.1 Sulzer Chemtech AG., WINTERTHUR Switzerland2 Sulzer Markets & Technol. Ltd., WINTERTHUR Switzerland

Application of static mixers in drinking water treatment - The ozonation process 8DE TRAVERSAY1 CH., BONNARD2 R., ADRIEN1 C., LUCK1 F.1 Anjou Recherche Vivendi Water, MAISONS LAFFITTE France2 Trailigaz Vivendi Water, GARGES LES GONESSE France

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MICROMIXING and CHEMICAL REACTIONMixing effects in precipitation - aggregation problem 9BALDYGA1 J., JASINSKA1 M., KUBICKI1 D., FRANKE2 D., GOSELE2 W.,ZAUNER2 R., WELKER2 S.1 Warsaw Univ. of Technol. Dept. of. Chem. Eng., WARSAW Poland2 BASF AG., LUDWIGSHAFEN Germany

A simple method for producing micron-sized, highly monodisperse polystyrene 10particles in aqueous media - Effects of impeller speed on particle size distributionINUKAI S.T., TANMA T., ORIHARA S., KONNO M.Tohoku Univ. Dept. of Chemical Engineering, SENDAI Japan

Effects of mixing on parallel chemical reactions in a continuous-flow 11stirred-tank reactorBALDYGA J., HENCZKA M., MAKOWSKI L.Warsaw Univ. of Technol. Dept. of. Process & Chem. Eng.,WARSAW Poland

Experimental comparison of the iodide-iodate and the diazo coupling micromixing test 12reactions in stirred reactorsGUICHARDON1 P., FALK2 L., ANDRIEU2 M.1 Univ. de St. Jerome Lab. Etudes & Applic. Procédés Séparatifs, MARSEILLE France2 ENSIC LSGC, NANCY France

HYDRODYNAMICSIN SINGLE PHASE SYSTEMS

Power consumption and mixing time in an agitated vessel with double impellers 13HIRAOKA S., KATO H., TADA Y., OZAKI N., MURAKAMI Y., LEE Y.S. Nagoya Inst. of Technol. Dept of Applied Chem., NAGOYA Japan

Cooperative test : experimental standards for measurements of various parameters 14in stirred tanksKRAUME1 M., ZEHNER2 P.1 TU Berlin Inst. für Verfahrenstechnik, BERLIN Germany2 BASF AG ZAV/A - M300, LUDWIGSHAFEN Germany

Effect of axial agitator configuration (up-pumping, down-pumping, reverse rotation) 15on flow patterns generated in stirred vesselsAUBIN1 J., MAVROS2 P., FLETCHER3 D.F., XUEREB1 C., BERTRAND1 J.1 ENSIACET-INPT LGC UMR CNRS 5503, TOULOUSE France2 Univ. Aristote Dpt. Chemistry, THESSALONIQUE Greece3 Univ. of Sidney Dpt. of Chem. Engineering, SYDNEY Australia

Distribution of dynamic pressure along a radial baffle in an agitated system 16with standard Rushton turbine impellerKRATENA1 J., FORT1 I., BRUHA2 O., PAVEL1 J.1 Czech Tech. Univ. Dept. Chemical & Food Process, PRAHA Czech Rep.2 Czech Tech. Univ. FAC of Mech. Engineering PRAHA, Czech Rep.

Flow and mixing patterns induced by reciprocating a disk in a cylindrical vessel 17KOMADA Y., INOUE Y., HIRATA Y.Osaka Univ. Dept of Chem Science & Eng., TOYONAKA Japan

Numerical study of jet-to-cross flow mixing phenomena 18GOURARA1 A., ROGER1 F., WANG1 H. Y., MOST1 J. M., LOUEDIN2 O., ILLY2 F.1 ENSMA Lab. Combustion et Détonique - UPR 9028 CNRS, Futuroscope POITIERS France2 Air Liquide Direction Rech. & Dévelop., JOUY EN JOSAS France

Experimental visualisation and CFD simulation of flow patterns induced by a novel 19energy-saving dual-configuration impeller in stirred vesselsMAVROS1 P., MANN2 R., VLAEV3 S.D, BERTRAND4 J.1 Univ. Aristote Dpt. Chemistry, THESSALONIQUE Greece2 UMIST Dept. Chem. Eng., MANCHESTER U.K.3 Bulgarian Acad. of Sciences Instit. Chemical Engineering, SOFIA Bulgaria4 ENSIACET-INPT CNRS - LGC UMR 5503, TOULOUSE France

Assessment of large eddy simulations for agitated flows 20DERKSEN J.J.TU Delft Kramers Labo. voor Fysische T., DELFT The Netherlands

Mixing time studies in mechanically agitated contactors 21PATIL1 S.M., GOGATE1 P.R., PATWARDHAN1 A.W., PANDIT2 A.B.1 Univ. of Bombay Dept. Chemical Engineering, BOMBAY India2 Univ. of Mumbay Dept. Chemical Engineering, MUMBAY India

Measurement of the concentration and velocity distribution in miscible liquid mixing 22using electrical resistance tomagraphyWANG M.Univ. of Leeds Dpt. of Mixing @ Mineral Eng., LEEDS U.K.

Characterization of mixing in a stirred tank by planar laser induced fluorescence 24(P.L.I.F.)FALL A., LECOQ O., DAVID R.Ecole Mines Albi-Carmaux, ALBI France

The effect of size, location and pumping direction of pitched blade turbine impellers 25on flow patterns : LDA measurements and CFD predictionsJAWORSKI1 Z., DYSTER2 K.N., NIENOW2 A.W.1 Technical Univ. of Szczecin Chem. Eng. Dpt., SZCZECIN Poland2 Univ. of Birmingham School of Chem. Eng., BIRMINGHAM U.K.

Turbulent shear stress effects on plant cell suspension cultures 26SOWANA1 D.D., WILLIAMS1 D.R.G., DUNLOP1 E.H., DALLY3 B., O'NEILL1 B.K.,FLETCHER4 D.F.1 Univ. of Adelaide Dpt. of Chem. Eng., ADELAIDE Australia2 Univ. of New Mexico Chemical & Nuclear Eng. Dept.,ALBUQUERQUE USA3 Univ. of Adelaide Dpt. of Mechanical Eng., ADELAIDE Australia4 Univ. of Sidney Dpt. of Chem. Engineering, SYDNEY Australia

CFD simulation of mass transfer in the mixing head of a RIM machine 27SANTOS R.J., TEIXEIRA A.M., LOPES J.C.Univ. do Porto LSRE, PORTO Portugal

CFD simulation and LDV validation of flow patterns in continuous-flow 28stirred vessels. Effect of liquid flow rateALLIET-GAUBERT1 M., MAVROS2 P., XUEREB1 C., BERTRAND1 J.1 ENSIACET-INPT LGC-CNRS UMR 5503, TOULOUSE France2 Univ. Aristote Dpt. Chemistry, THESSALONIQUE Greece

Characterization of single phase flows in stirred tanks via computer automated 29radioactive particle tracking (CARPT)RAMMOHAN1 A.R., KEMOUN2 A., AL-DAHHAN1 M.H., DUDUKOVIC3 M.P.1 Washington Univ. Chem. Reaction Eng. Lab., ST-LOUIS USA2 LEMTA-CNRS URA 875, VANDOEUVRE-LES-NANCY France3 Washington Univ. Inst. Louis Dpt. of Chem. Eng., ST-LOUIS USA

LIQUID-LIQUID SYSTEMSLiquid-liquid dispersion in a SMX sulzer static mixer 30LEGRAND1 J., MORANCAIS2 P., CARNELLE1 G.1 GEPEA CRTT - IUT St. Nazaire, ST NAZAIRE France2 Univ. de Nantes CREA – GP, ST NAZAIRE France

A modeling of emulsification in a colloid mill: forecast of asphalt emulsion 31droplet size distributionsDICHARRY CH., MENDIBOURE B., LACHAISE J.Univ. Pau et Pays de l’Adour Lab. des Fluides Complexes, PAU France

Emulsification efficiency related to the combination of mechanical energy input 32and system formulation and composition variablesSALAGER1 J.-L., MARQUEZ1 L., MIRA1 I., PEREZ1 M., RAMIREZ1 M., TYRODE1 E.,ZAMBRANO1 N., CHOPLIN2 L.1 Universidad de Los Andes Lab. FIRP, MERIDA Venezuela2 GEMICO – ENSIC, NANCY France

Influence of the dispersed phase viscosity on the mixing of concentrated oil-in-water 33emulsions in the transition flow regimeBRICENO1 M.I., SALAGER1 J.-L., BERTRAND2 J.1 Univ. de los Andes Lab. FIRP, MERIDA Venezuela2 ENSIACET-INPT LGC-CNRS UMR 5503, TOULOUSE France

HIGH VISCOUS FLUIDSIn-line rheological follow-up of concentrated dispersions preparation 34FURLING1 O., CHOPLIN2 L., TANGUY3 PH.1 Ecole Polytechnique Chem Eng Dept, MONTREAL Canada2 GEMICO – ENSIC, NANCY France3 Atochem Centre Application Levallois, LEVALLOIS-PERRET France

The influence of mixing on the molecular weight distribution during controlled 35polymer degradation in a static mixer reactorHOEFSLOOT1 H. C. J., FOURCADE1 E., VAN VLIET2 G., BUNGE2 W., MUTSERS1 S., IEDEMA1 P. D.1 Univ. Amsterdam Dept. of Chem. Eng., AMSTERDAM The Netherlands2 DSM Research BV, GELEEN The Netherlands

Experimental and CFD simulation of heat transfer to highly viscous fluids 36in an agitated vessel equipped with a non standard helical ribbon impellerDELAPLACE1 G., TORREZ2 C., LEULIET1 J.-C., BELAUBRE1 N., ANDRE2 CH.1 INRA Lab. Génie des Procédés et Technol. Alim., VILLENEUVE D'ASCQ France2 HEI Lab. Génie des Procédés, LILLE France

Effect of operating parameters and screw geometry on micromixing in a co-rotating 37twin-screw extruderROZEN1 A., BAKKER2 R.A., BALDYGA1 J.1 Warsaw Univ. of Technol. Dept. of. Chem. Eng., WARSAW Poland2 DSM Research, GELEEN The Netherlands

Power characteristics of a screw agitator in a tube 39RIEGER F.Czech Technical Univ. Dpt. of Process Eng., PRAGUE Czech Rep.

Optimization of designing a high-viscosity mixing field by constructive approach 40using chaotic mixing and information entropy theoriesTAKIGAWA1 T., OHMURA2 N., YAGYUU1 K., KATAOKA3 A.1 Kobe Univ Div. of Resource & Energy Science, KOBE Japan2 Kobe Univ Dpt. of Chem. Science & Eng., KOBE Japan3 Toho Gas Co. Ltd., TOKAI-CITY Japan

GAS-LIQUID SYSTEMSAeration of agitated non-Newtonian liquids 41WICHTERLE K., WICHTERLOVA J.Technical Univ. of Ostrava, OSTRAVA Czech Rep.

Influence of gas flow rate on structure of trailing vortices of Rushton turbine : 42PIV measurements and CFD simulationsRANADE1 V.V., PERRARD2 M., LE SAUZE2 N., XUEREB2 C., BERTRAND2 J.1 National Chemical Lab., PUNE India2 ENSIACET-INPT CNRS - LGC UMR 5503, TOULOUSE France

Numerical modelling of a ventilated cavity using a volume of fluid method 43GASTON1 M.J., REIZES1 J.A., EVANS2 G.M., RIGBY2 G.D.1 Univ. of Technol. of Sydney Fac. of Engineering, SYDNEY Australia2 Univ. of Newcastle Dpt. Chem. Eng., CALLAGHAN Australia

Mass transfer and hold-up characteristics in a gassed, stirred vessel at intensified 44operating conditionsGEZORK1 K., BUJALSKI1 W., COOKE2 M., NIENOW1 A.W.1 The Univ. of Birmingham Centre for Biochemical Eng., BIRMINGHAM U.K.2 UMIST Dept. of Chem. Eng., MANCHESTER U.K.

Residence-time distribution in continuous three phase stirred reactor 45MACHON V., JAHODA M., KUDRNA V., CERMAKOVA J.Prague Inst. of Chem. Technol. Dpt. of Chemical Engineering,PRAGUE Czech Rep.

A comparison of the efficiency of self-aspirating and flat-blade impellers 46STELMACH J., KUNCEWICZ C.Technical Univ. of Lodz Dpt. of Process Equipment, LODZ Poland

Gas dispersion in sparged and boiling reactors 47GAO Z., SMITH J.M., MULLER-STEINHAGEN H.Univ of Surrey Dept of Chem & Process Eng., GUILDFORD U.K.

Cavity formation, growth and dispersion behind rotating impeller blades 48EVANS G.M., MANNING S.A., JAMESON G.J.Univ. of Newcastle Dpt. Chem. Eng., CALLAGHNA Australia

Validation of the use of the fluent software for the simulation of gas-liquid 49agitated vesselsDIMIER1 A., SEIDLITZ2 F.1 Rhone-Poulenc CRIT, DECINES CHARPIEU France2 Fac. De Sciences de St. Etienne, ST. ETIENNE France

Simulation of gas-liquid flows in stirred vessels agitated by dual rushton impellers : 50influence of impeller spacing and clearanceDESHPANDE1 V.R., RANADE2 V.V.1 Swiss Center for Scientific Computing, MANNO Switerland2 National Chem. Lab., PUNE India

SOLID-LIQUID SYSTEMSEffect of process variables on single-feed semibatch precipitation of barium sulfate 51UEHARA-NAGAMINE E., ARMENANTE P.M.New Jersey Inst. of Technol. Dpt. of Chem. Eng., NEWARK USA

Effect of vessel to impeller diameter ratio onto particle suspension 52RIEGER F., DITL P.Czech Tech. Univ. Dpt. of Process Eng., PRAGUE Czech Rep.

Impeller geometry effect on velocity and solids suspension 53WU J., ZHU Y., PULLUM L.CSIRO, MELBOURNE Australia

Experiments and CFD predictions of particle distribution in a vessel agitated 54with four pitched blade turbinesMONTANTE1 G., MICALE1 G., PINELLI2 D., MAGELLI2 F., BRUCATO1 A.1 Univ. di Palermo Dpt. Ingegn.Chimic.dei Procesi e dei Materiali, PALERMO Italy2 Univ. di Bologna Dpt. Chem., Mining & Env. Eng., BOLOGNA Italy

On the simulation of turbulent precipitation in a tubular reactor via 55computational fluid dynamicsMARCHISIO1 D. L., FOX2 R. O., BARRESI1 A., GARBERO1 M., BALDI1 G.1 Politecnico di Torino Dept. Scienza Materiali e Ing. Chimica, TORINO Italy2 Iowa State Univ. Dept. of Chem. Eng., AMES USA

DNS validation of a fully predictive model for turbulent particle deposition 56in vertical pipe flowCERBELLI S., GIUSTI A., SOLDATI A.Univ. di Udine Dip. di Scienze e Technogie Chimiche, UDINE Italy

Local heat transfer in a solid-liquid system agitated by concave disc turbine 57in a jacketed vesselKARCZ J., ABRAGIMOWICZ A.Techn. Univ. of Szczecin Dpt. of Chem. Eng., SZCZECIN Poland

Effect of particle size on simulation of three-dimensional solid dispersion 58in stirred tankALTWAY A., WINARDI S., SETYAWAN H.Inst. of Tech Sepuluh Nopember Dept of Chemical Eng., SURABAYA Idonesia

Experimental analysis of floc population balance for different mixing rates 59BOUYER1 D., LINE1 A., COCKX2 A., DO-QUANG2 Z.1 INSA Dpt. GPI, TOULOUSE France2 Lyonnaise des Eaux CIRSEE, LE PECQ France

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International Symposium on Mixing in Industrial Processes – ISMIP4 14-16 May 2001 – Toulouse (France) EFFECT OF AXIAL AGITATOR CONFIGURATION (UP-PUMPING, DOWN-PUMPING,

REVERSE ROTATION) ON FLOW PATTERNS GENERATED IN STIRRED VESSELS J. AUBIN†,‡, P. MAVROS*, D. F. FLETCHER‡, C. XUEREB† AND J. BERTRAND†

♦ †Laboratoire de Génie Chimique UMR CNRS 5503, 31078 Toulouse Cedex 4, France Joelle.Aubin@ensigct.fr ; Catherine.Xuereb@ensigct.fr; Joel.Bertrand@ensigct.fr

♦ ‡Department of Chemical Engineering, University of Sydney, NSW 2006, AUSTRALIA

davidf@chem.eng.usyd.edu.au ♦ *Department of Chemistry, Aristotle University, GR-54006, Thessaloniki, GREECE

pmavros@eng.auth.gr

Abstract. Single phase turbulent flow in a tank stirred with two different axial impellers - a pitched blade turbine (PBT) and a Mixel TT (MTT)- has been studied using Laser Doppler Velocimetry. The effect of the agitator configuration, i.e. up-pumping, down-pumping and reverse rotation, on the turbulent flow field, as well as power, circulation and pumping numbers has been investigated. An agitation index for each configuration was also determined. In the down-pumping mode, the impellers induced one circulation loop and the upper part of the tank was poorly mixed. When up-pumping, two circulation loops are formed, the second in the upper vessel. The PBT pumping upwards was observed to have a lower flow number and consume more power than when down-pumping, however the agitation index and circulation efficiencies were notably higher. The MTT has been shown to circulate liquid more efficiently in the up-pumping configuration than in the other two modes. Only small effects of the MTT configuration on the power number, flow number and pumping effectiveness have been observed. Résumé. Des écoulements turbulents générés dans une cuve agitée par deux différents agitateurs axiaux (une turbine à pâles inclinées (PBT) et un Mixel TT (MTT)) ont été étudiés en utilisant la Vélocimétrie Doppler à Laser. Différents configurations du mobile d’agitation (pompage vers le bas, pompage vers le haut ou rotation en sens inverse) et leurs influence sur les champs de vitesse, le nombre de puissance, le nombre de pompage et le nombre circulation, ont été examinés. De plus, un indice d’agitation a été déterminé pour chaque configuration. Lorsque l’agitateur pompe vers le bas, une boucle de circulation se forme et la partie supérieure de la cuve est mal agitée. Lorsque le mobile pompe vers le haut, deux boucles de circulation se sont crées, la deuxième étant dans la partie supérieure de la cuve. En pompant vers le haut, la PBT a un nombre de pompage plus faible que celle qui pompe vers le bas, de plus elle a un nombre de puissance plus élevé. Cependant, l’indice d’agitation et l’efficacité de circulation sont particulièrement plus élevés lors d’un pompage vers le haut. Lors de l’utilisation du mobile MTT en pompage vers le haut, l’efficacité de circulation est optimale, alors que les trois configurations testées ont peu d’influence sur le nombre de puissance, le nombre de pompage et l’efficacité de pompage.

INTRODUCTION

Mechanically agitated vessels are widely used in the chemical, mineral and wastewater processing industries for simple fluids mixing or even complex multiphase processes including gas-liquid and gas-liquid-solid mixing. In such complex applications, the agitator has goals of circulation and homogenisation, holding the gas within the vessel for as long as possible, and /or suspending solids, promoting contact between gas, liquid and solid phases. The study and understanding of the flow produced by agitators in such stirred vessels is very important as it allows the performance of an agitator to be quantified and then, if necessary, optimised in order to achieve the desired process results. In the early 1980s, it was proposed that inverting axial flow impellers and changing their rotational direction giving an ‘up-pumping’ effect could be interesting for gas-liquid applications1,2. Recently, it has been shown that up-pumping axial hydrofoil impellers provide considerable advantages over the down-pumping configuration3-5 and this has brought on the manufacture of axial hydrofoils specifically designed for up-pumping applications6-8. Most of the recent studies on the up-pumping concept have concentrated mainly on power characteristics and mixing times, and have shown that by applying up-pumping impellers to gas-liquid systems torque and flow instabilities are reduced, flooding is less likely to occur and there is no loss in power upon gassing9-11. Laser Doppler Velocimetry (LDV) is a successful experimental technique used for the study of flow fields, having the advantage of being non-intrusive. By investigating the flow patterns produced by an impeller and relating them to various global indices, such as the power number, pumping and circulation efficiencies, the performance of the mechanically agitated vessel can be quantified and optimised. With respect to the study of axial flow impellers, there have been a limited number of investigations using LDV to determine local

109

characteristics of velocity fields and turbulence, especially when the impellers are operated in the up-pumping mode. Ranade and Joshi12 carried out a detailed LDV study on down-pumping pitched blade turbines, investigating the influence of the impeller size and geometry on the flow patterns in the vessel. Mavros et al.13 used LDV to compare the flow fields of a radial flow turbine and two different down-pumping axial flow impellers in both water and a non-Newtonian fluid. Evaluation of the performance of two commercial down-flow impellers, the Chemineer HE3 (CHE3) and the Prochem Maxflo T (PMT), using LDV to detail the mean and r.m.s. velocities was carried out by Jaworski et al.14. A comparison of flow fields, as well as flow and circulation characteristics, produced by an APV-B2 impeller in the down- and up-pumping modes were reported by Mishra et al.15. In the down-pumping configuration, the flow fields were similar to those reported by the other workers mentioned above and that liquid in upper part of the vessel was poorly circulated. It was shown that by operating the impeller in the up-pumping mode, two distinct circulation loops were produced, one in the upper and one in the lower part of the vessel, and as a consequence the agitation in the upper part of the tank was improved. In the present work, single-phase flow patterns in a stirred vessel have been investigated using LDV in a turbulent flow regime using two different axial impellers, a 6-bladed 45° pitched bladed turbine (PBT) and a Mixel TT (MTT) (Mixel, France). Comparisons have been made of the flow fields produced when the impellers are operated in the down- and up-pumping modes, and the reverse mode for the MTT. These velocity fields have been assessed by examining the power consumption, circulation and pumping capacities of the impeller, the agitation index, as well as spatial distributions of turbulent kinetic energy and turbulence intensity. This investigation has been carried out, not only to characterize impellers operating in the up-pumping mode, being different than what has been previously published, but also to show the effects of the impeller configuration on flow fields and mixing efficiency which can easily be altered by a simple retrofitting operation. The results presented here will be used as a reference for future experimental gas-liquid mixing studies and for the validation of CFD models in stirred tanks.

EXPERIMENTAL APPARATUS AND PROCEDURES LDV measurements of instantaneous radial, axial and tangential velocity components were performed in a dished-bottom cylindrical vessel made of Perspex (T = 0.19 m). An aspect ratio of 1 was used, i.e. the liquid height (H) in the vessel is equal to the tank diameter (T). The tank was equipped with four transparent baffles made of Perspex (b = T/10), which were placed 90º from one another, flush against the vessel wall. The impeller clearance was C = T/3, where C is defined as the distance from the vessel bottom to the lowest horizontal plane swept by the impeller. The cylindrical vessel was placed inside a square tank, whose front panel was made of Altuglas™ to allow distortion-free measurements; this tank was filled with water in order to minimize refraction at the cylindrical surface of the inner vessel.

Figure 1. Experimental set-up : (1) Laser source, (2,3) Fiber-optical module, (4) Photomultipliers, (5) Flow velo-city analyser, (6) Oscilloscope, (7) Computer. Two impellers were studied: a 6-blade 45° pitched blade turbine and a Mixel TT axial agitator (Mixel, France). The impeller diameter was equal to D = T/2 in all cases and the agitator shaft (s = 0.008 m) extended to the bottom of the vessel, where it fitted into a Teflon hub to avoid ‘wobbling’ of the impeller. The vessel was filled with plain tap water and was seeded using a small amount of Iriodin 111 Rutile Fine Satin (Merck) particles (dp=15 µm). The rotational speed of the agitator was 5 Hz (= 300 rpm), which corresponded to a Reynolds number of 45000. The description of the different operational modes of the impellers are as follows:

110

(a) Down-pumping mode: The impeller is operated in the classical manner, i.e. the impeller is rotated so that blades push the fluid downwards.

(b) Reverse mode: The rotational direction of the impeller is reversed with the impeller remaining in a downwards position.

(c) Up-pumping mode: The impeller is turned upside down and the direction of rotation is the reverse of the down-pumping mode.

The LDV measurements were taken with a two-beam Dantec Fiberflow system, operating in back-scattering mode (Fig. 1). The light source is a 4 W Argon ion laser (Stabilite 2017 by Spectra Physics) with a focal length of 600 mm and beam wavelengths of 514.5 nm (green) and 488 nm (blue). The Doppler signal was transmitted to a 58N20 Flow Velocity Analyzer (Dantec) for signal processing and the results were then collected on a computer.

Measurements were taken on planes midway between two baffles. 5000 validated samples were taken at each measuring point, with a maximum sampling time of 400 seconds. A seed-particle ‘residence time’ weighting was applied when calculating the mean velocity from the distribution in order to eliminate the bias towards higher values caused by high velocity particles and their consequent short residence time within the measuring volume.

The instantaneous velocity data attained by LDV is used to determine the 2D radial-axial velocity vector fields, and flow maps of the radial, axial and tangential velocities, as well as turbulent kinetic energy and turbulence intensity. Global quantities are also calculated including the power number, the flow number and the circulation number and time, as well as performance indices such as the circulation efficiency, the pumping efficiency and the agitation index.

Power Measurements The power consumption, P, of each agitator configuration was determined by measuring the torque using the integrated torque meter of a Labmaster mixer (Lightnin). Torque measurements were corrected for friction losses using measurements taken in an empty vessel.

Pumping and Circulation Flow Rates The pumping flow rate or pumping capacity, QFl, corresponds to the fluid flow rate that passes through the impeller swept volume16. For reasons of practicality, this calculation is performed on a control volume, which just encompasses the impeller swept volume, as shown by relation (1):

( ) ( ) ( )0 0

2 2z r r

o o oFl r z z

r r z z z zz

Q D v dz r v dr r v drπ π π++ ++ ++

++ ++ −−−−

= = == + +∫ ∫ ∫ (1)

where z++, z-- and r++ are the boundaries of the impeller control volume and the superscript ‘o’ refers to the fluid moving out of this volume. This expression takes into account both radial and axial pumping flow rates.

The discharge flow rate of the impeller may then be expressed as the dimensionless pumping number by normalizing QFl by ND3:

3

FlQFl

ND= (2)

The circulation flow rate, Qc, in the tank is the total volumetric turnover rate occurring in the tank. It is

the sum of the discharged fluid flow and the flow entrained by the impeller16. Assuming axial symmetry, the absolute maximum values of the radial and axial flow rates, Qr and Qz, in the vessel are equal to the total circulation flow rate14. This circulation flow rate may be evaluated by calculating the flow rate through a plane that passes through the centre of the circulation loop with radial-axial coordinates (r*, z*)17. In the case where two circulation loops are produced, the overall circulation flow rate is simply the sum of the circulation rate of the two loops. The volumetric flow rates passing through the surfaces established by the loop centres can be calculated using Equations (3) and (4) using the surface area of the plane section and the velocity component normal to the surface14.

( ) *

*

2R

z z z zr

Q r v drπ == ∫ (3)

111

( )*1

*

*2

z

r r r rz

Q D v dzπ == ∫ (4)

where r* and z* are the coordinates of the loop centre. Normalizing the circulation flow rate by ND3, results in the dimensionless circulation flow rate number, Flc:

3

cc

QFl

ND= (5)

Using Fl, Flc and Po, two indices characterizing the effectiveness of an agitator can be described. The pumping effectiveness, ηE, gives the pumping rate of the impeller per unit power consumption and is given by relation (6):

EFl

Poη = (6)

The second index is the circulation efficiency number, Ec, which indicates the impellers capacity to circulate the liquid with respect to its power consumption. This relationship, defined by Jaworski et al.14 enables different types and sizes of agitators to be compared in a turbulent flow regime, independent of impeller speed.

4

cc

Fl DE

Po T =

(7)

Turbulent Kinetic Energy

Accurate estimation of the turbulent kinetic energy, kT, induced in a stirred tank requires the root mean square velocities (velocity fluctuations) of the three velocity components:

( )2 2 21

2T r zk v v vθ′ ′ ′= + + (8)

This quantity is made dimensionless by dividing by the square of the impeller tip speed, 2tipV .

Turbulence Intensity

The turbulence intensity, Tu, i.e., the ratio of the r.m.s. to the mean velocity, gives an indication of the randomness of the flow. It may be calculated using the geometric sum of the three velocity components:

( )( )

1/ 22 2 2

1/ 22 2 2

r z

r z

v v vTu

v v v

θ

θ

′ ′ ′+ +=

+ + (9)

Agitation Index

Mavros and Baudou18 defined an agitation index, Ig, in order to determine the effectiveness of an impeller to induce flow in a tank. Ig is calculated with the resultant mean velocity, vij, of the three instantaneous velocity components at each measuring point in the vessel. It is supposed that the each resultant velocity corresponds to a liquid volume Vij, surrounding the measuring point. The liquid volumes in the vessel are summed and a volume weighted average velocity for the entire tank is obtained:

v

ˆV

ij iji j

iji j

Vv =

∑ ∑∑ ∑

(10)

The volume-weighted velocity is then normalized by Vtip to give the agitation index, Ig:

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[ ]ˆ100 %g

tip

vI

V= (11)

For further details on the definition and calculation of the agitation index, readers are referred to Mavros and Baudou18.

(a) PBT-downwards (b) MTT-downwards

(c) PBT-upwards (d) MTT-upwards (e) MTT-reverse Figure 2. 2D flow maps of axial-flow impellers: (a) PBTD; (b) MTTD; (c) PBTU; (d) MTTU; (e) MTTR.

RESULTS AND DISSCUSION

Mean Velocity Fields And Flow Patterns

Down-pumping Vector plots of mean velocity fields in the r-z plane at 45° to the baffle plane are shown in Fig. 2 for the PBTD and the MTTD. These plots show that a circulation loop is formed in the lower part of the vessel; in the upper part of the tank, liquid velocities are low, circulation is minimal and therefore mixing is rather poor. These

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observations are characteristic of down-pumping axial agitators. In the outflow of the impeller, the fluid is discharged axially for the most part – for the PBTD, a noticeable radial component exists in this region which confirms the observations made be Ranade and Joshi12. Once the down-flowing high speed jet encounters the vessel bottom, the fluid moves along the bottom and then up the vessel wall. Due to low pressure regions behind the impeller blades, most of the fluid is taken in at the top of the impeller-swept volume, with some being drawn into the side. This causes the formation of a primary circulation loop. Little liquid movement is observed in the upper part of the vessel for both impellers, although circulation for the PBTD-stirred tank appears to more significant. A third fluid zone exists in the vessel stirred by the PBTD, which is a conical region of up-flowing liquid below the centre of the impeller. This observation has already been made for pitched-blade turbines and also other axial impellers including the Chemineer HE3 (CHE3), the Prochem Maxflo T (PMT) and the APV-B212, 14, 15.

PB

TD

(a-i) Vz/Vtip (a-ii) Vr/Vtip (a-iii) Vθ/Vtip

MT

TD

(b-i) Vz/Vtip (b-ii) Vr/Vtip (b-iii) Vθ/Vtip Figure 3. Dimensionless velocity distribution maps for the PBTD and the MTTD. Radial velocities are positive RIGHT (→), axial velocities are positive UP (↑) and tangential velocities are positive LEFT (←).

Figure 3 shows velocity contour maps of axial, radial and tangential components normalized by Vtip for the two down-pumping impellers investigated. Positive values are defined as upwards for the axial component,

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away from the impeller shaft for the radial component and in the impeller rotational direction for the tangential component. Negative values are downwards, towards the impeller shaft and in the direction opposite of agitator rotation for the axial, radial and tangential components respectively.

The axial velocity maps are similar for both impellers, with the maximum down-flowing velocities occurring in the impeller discharge region and the maximum positive values of axial velocities found close to the vessel wall. The axial down-flow induced by the PBTD is approximately 10% greater than that induced by the MTTD being 44.1% and 40.0% of Vtip respectively. For both agitators, the maximum up-flowing velocity is approximately 22% less than the discharge flow at 35.8% and 31.1% of Vtip for the PBTD and MTTD. In the upper part of the tank, axial velocities are small, with a value of about 2% of Vtip, flowing generally downwards. Such low velocities in this region of the tank indicate poor circulation and mixing. The cone-shaped up-flow zone observed in the tank agitated by the PBTD has a maximum velocity of 7% of Vtip. This result compares well with the work of Ranade and Joshi12, who found a maximum velocity of 5% of Vtip in this region for a PBTD.

Contours of the radial velocity component show high velocity zones in the planes above and below the impeller. The zone of liquid being drawn in by the impeller (with a maximum radial velocity) is larger for the PBTD than for the MTTD although at a slightly lower speed (13.6% Vtip compared to 15.9% Vtip). However, the liquid discharged below the PBTD has a greater maximum velocity, 22% of Vtip, than the MTTD at 15.2% Vtip. Similar maximal discharge velocities have already been reported for a PBTD (25% Vtip)

12, a PMT and a CHE3 (22% and 20% of Vtip respectively)14.

For both impellers, maximum tangential velocities are very localized and in the rest of the vessel only small tangential velocities exist. The PBTD induces a maximum tangential velocity at the tip of the impeller which is 63.3% of Vtip and the MTTD produces the maximal tangential velocity below the centre of the agitator with a significantly lower value of 33.0% Vtip. The latter is evidence of a small region of swirling liquid with minimal radial /axial movement. In the upper part of the tank, a zone of liquid is observed to rotate in the opposite direction to the agitator rotation. Although this effect is slight (1.7% % of Vtip for both the PBTD and the MTTD), it has already been reported by Mavros et al.13.

Up-pumping Velocity vectors in the r-z plane for the impellers in the up-pumping configuration are shown in Fig. 2. Operation of the impellers in this mode induces two circulation loops, one in the upper part of the tank and one in the lower part. These observations confirm previous works on up-pumping hydrofoil impellers and pitched blade turbines15, 19. The agitator generates a strong discharge flow into the upper part of the vessel which is projected at approximately 45° towards the vessel wall. Due to this strong radial component, the liquid is propelled towards the vessel wall where it either moves axially up to the liquid surface or down to the tank bottom. In the lower half of the tank, the liquid is forced down the vessel wall and upon encountering the tank base, its movement is redirected in an upwards direction and is drawn into the underside of the impeller swept region. This circulation path forms the primary circulation loop. For the MTTU, a short circulation loop of the liquid is observed and it is not only taken up at the underside of the impeller but at the vertical side of the impeller as well. Axial movement of the liquid close to the vessel in the upper region of the tank experiences a change in direction due to the liquid surface. This gives rise to the secondary circulation loop with an ensuing downward flow and induces fluid velocities in the upper region of the tank higher than those observed in the down-pumping mode, and therefore the degree of circulation and mixing appears to be better. A third observation has been made for the flow produced by the PBTU – a region of down-flowing liquid into the centre of the upper side of the impeller as seen also in the down-pumping configuration. Mishra et al.15 have reported similar flow patterns for an up-pumping APV-B2 including the presence of two distinct circulation loops, as well as a small down flowing region in the centre of the upper side of the impeller. It should be noted that the velocities induced by the PBTU in the upper region of the vessel and at the bottom vessel corners appear greater than in the case of the MTTU, indicating a stronger overall circulation for the PBTU.

Figure 4 presents contour maps for the axial, radial and tangential velocity components. In Figs. 4a-i and 4b-i the maps are generally alike for the two impellers with a strong flow of liquid moving upwards, into and out of the impeller-swept volume. Axial velocities are shown moving down the vessel wall in the lower half of the tank. The vessel agitated by the PBTU also exhibits high up-flowing velocities close to the vessel wall in the upper part of the vessel, which are not observed with the MTTU. The PBTU induces considerably higher axial velocities than the MTTU – maximums of 40.6% Vtip compared with 29.2% Vtip for up-flowing velocities and 50.7% Vtip compared with 31.9% Vtip in the downwards direction. It is also observed that the downward flowing velocities are stronger than those moving upwards in the case of both agitators. For the PBTU, this effect is relatively important, as the maximum down-flowing axial velocity is ~25% greater than the maximum velocity flow upwards.

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PB

TU

(a-i) Vz/Vtip (a-ii) Vr/Vtip (a-iii) Vθ/Vtip

MT

TU

(b-i) Vz/Vtip (b-ii) Vr/Vtip (b-iii) Vθ/Vtip Figure 4. Dimensionless velocity distribution maps for the upwards-pumping impellers: (a) PBTU and (b) MTTU.

Three general regions of high radial velocities have been observed for the up-pumping impeller configurations as shown in Figs. 4a-ii and 4b-ii. An important out-flowing radial movement is observed above the impeller in the discharge zone, as well as two zones of high inward-flowing velocities, which are situated close to the tank bottom and near the free liquid surface. In the impeller discharge, the maximum velocity zone for the PBTU is small but at 36.8% of Vtip. For the MTTU, this zone is larger but has a lower maximum velocity of 23.9% of Vtip. Mishra et al.15 reported a maximum radial velocity in the discharge region of the APV-B2 agitator of 33% of Vtip. The zones of highest in-flowing velocity are found in the lower part of the vessel, the PBTU induces a maximum radial velocity of 22.9% of Vtip, whereas for the MTTU this value is 15.5% of Vtip. These velocities below the impeller are greater than the ones found for the APV-B2, which were reported to be less than 10% Vtip

15. In the upper vessel, these radial velocities are high, but lower than those observed below the impeller, especially for the MTTU. The maximum tangential velocity distributions in the up-pumping impeller configurations – Figs. 4a-iii and 4b-iii - differ considerably from those of the down-pumping configurations. Here, the highest velocities are, for the most part, localized in the impeller discharge, and in the case of the PBTU a strong tangential flow is also

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observed below the centre of the agitator. It should be noted, however, that these velocity magnitudes are substantially lower for the up-pumping configuration, being 22.2% and 18.5% of Vtip for the PBTU and MTTU respectively, when compared with the values measured for the down-pumping arrangement. Close to the impeller shaft (in the upper part of the vessel for the PBTU and in the lower part for the MTTU) the liquid moves in the direction of rotation at ~ 10% of Vtip. Like for the down-pumping mode, liquid is observed to rotate in the reverse direction in the upper part of the vessel and also below the MTTU. For the PBTU, this reverse rotation effect is of a similar magnitude to the down-pumping configuration (1.5% of Vtip). For the MTTU however, the maximum reverse velocity is found to be 4.2% of Vtip in both the upper and lower regions of the tank.

Reverse mode The r-z plane vector plots are shown in Fig. 2. The flow pattern observed for the MTTR is similar to that produced by the MTTU. Two circulation loops are formed in the vessel, the primary in the lower half of the tank and the secondary in the upper part. As the impeller discharge is not as strong as in the up-pumping mode, the upper circulation loop is slightly less significant.

(a-i) Vz/Vtip (a-ii) Vr/Vtip (a-iii) Vθ/Vtip Figure 5. Dimensionless velocity distribution maps for the reverse-rotating MTT impeller. Contour diagrams of axial, radial and tangential velocity components produced by the MTTR are shown in Fig. 5. Both the axial and radial flow patterns are very similar to those induced by the MTTU. Axially, the maximum velocities occur below the impeller where the liquid is drawn and in the discharge above the impeller (22.7% of Vtip), as well as maximum down-flowing velocities close to the vessel wall in lower half of the tank (25.2% of Vtip). These values are approximately 20% lower than those induced by the MTTU. Concerning the radial velocity component, three high velocity zones are observed as previously noted for the up-pumping configurations. The MTTR produces maximum radial velocities above and below the impeller with a zone of relatively high velocity close to the liquid surface. For in-flowing (negative) radial velocities, the maximum is 15.1% of Vtip which is not much different to that found for the MTTU. The out-flowing radial velocities for the MTTR are however substantially smaller (16.6% of Vtip) in comparison to those of the MTTU (23.9% of Vtip). Maximum tangential flow is observed at the impeller tip and in the centre below the impeller with a value of 15.1% Vtip. Again, liquid in rotating in the counter direction is observed in the upper part of the vessel reaching a maximum of 3.3% Vtip.

R.m.s. Velocities, Turbulent Kinetic Energy And Turbulence Intensity Figures 6-8 show contour maps of the r.m.s. velocities for each of the velocity components in the down, up and reverse modes. In all three pumping configurations, the r.m.s. velocities are considerably higher in the discharge stream of the impeller than in the bulk of the tank. In the up-pumping and reverse modes, the high r.m.s. velocity

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region around the impeller is more spread out than in the down-pumping mode. Even though the r.m.s. velocities observed in the bulk of the tank are significantly smaller than those found in the discharge flow, the difference in the component values varies noticeably. Therefore, the flow induced by these two impellers in the three different configurations is in general not isotropic. Mishra et al.15 have also observed that the flows induced by an APV-B2 impeller in both the up- and down-pumping configurations are not isotropic.

PB

TD

(a-i) V'z/Vtip (a-ii) V'r/Vtip (a-iii) V'θ/Vtip

MT

TD

(b-i) V'z/Vtip (b-ii) V'r/Vtip (b-iii) V'θ/Vtip Figure 6. RMS velocity distribution maps for the downwards-pumping impellers: (a) PBTD; (b) MTTD. Figure 9 presents spatial distributions of dimensionless turbulent kinetic energy (k* = kT/V2

tip), determined using the three r.m.s. components, are shown in. For the three pumping configurations, the maximum values of k* are generally found in the impeller discharge region. For the PBTD, the highest k* values are found at the impeller tip and just below with a maximum of 13.7% of V2

tip. This value is somewhat higher than 9% of V2tip for a 6-

bladed pitched blade turbine (D =T/3)20. The spatial distribution of k* for the MTTD is similar to that reported by Mavros et al. (1998) and its maximum value of 3.4% V2

tip is comparable to 3.3% of V2tip for a T/3 propeller20

and 2.7% of V2tip for an down-pumping APV-B215.

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PB

TU

(a-i) V'z/Vtip (a-ii) V'r/Vtip (a-iii) V'θ/Vtip

MT

TU

(b-i) V'z/Vtip (b-ii) V'r/Vtip (b-iii) V'θ/Vtip

Figure 7. RMS velocity distribution maps for the upwards-pumping impellers: (a) PBTU; (b) MTTU.

(a-i) V'z/Vtip (a-ii) V'r/Vtip (a-iii) V'θ/Vtip

Figure 8. RMS velocity distribution maps for the reverse-rotating MTT impeller.

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(a) PBTD (b) MTTD

(c) PBTU (d) MTTU (e) MTTR Figure 9. Maps of dimensionless turbulent kinetic energy, k* = kT/V2

tip.

In the up-pumping and reverse modes, the maximum turbulent kinetic energy values are smaller than for the down-pumping configuration, although the high energy regions are larger in size. The largest difference is for the PBTU with a maximum value of only 5.0% V2

tip, for the MTTU and the MTTR the highest values are only slightly less than that found in the down-pumping mode being 2.7% and 2.5% of V2

tip respectively. Spatial distributions of turbulence intensity, Tu, (Fig. 10) illustrate the variability of the flow. For both

agitators in the down-pumping mode, the highest values of Tu were found to be overhead the impeller, close to the liquid surface. The MTTD induces a turbulent intensity that is double the intensity produced by the PBTD. These results are similar to those reported by Mavros et al.21 for a Rushton turbine, a Mixel TT impeller and a Lightnin A310. In the up-pumping and reverse configurations, the highest turbulence intensity is located close to the vessel wall in proximity to the upper circulation loop. The up-pumping turbulence intensities are 15-30% smaller than those of the corresponding down-pumping modes and in the case of the MTTU, the intensity is approximately double that of the MTTR.

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(a) PBTD (b) MTTD

(c) PBTU (d) MTTU (e) MTTR Figure 10. Maps of turbulence intensity, Tu.

Power, Pumping and Circulation Power and flow characteristics of the different impellers and configurations are presented in Table 1. In the down-pumping mode, the PBTD exhibits a power number which is in the same range as previously reported values12, 20, 22, 23 and is approximately 2.5 times greater than that of the MTTD. The value for this latter impeller is comparable to other studies of the MTTD13, 24. In the up-pumping mode, the power number for the PBTU is 33% greater than in the down-pumping configuration. Not such a great difference is shown for the power numbers of the down-, up-pumping and reverse modes for the MTT. When up-pumping and in reverse rotation, power consumption decreases only slightly in a progressive manner. The flow numbers for both impellers in the up-pumping mode are slightly inferior to those for the down-pumping mode which is in good agreement with flow number data found for a down- and up-pumping APV-B215. The flow number for the MTTR however is approximately 33% smaller than the down- and up- pumping modes.

Assessment of the circulation numbers shows that in the up-pumping configuration, circulation is much greater than in the down-pumping mode. This is in coherence with the second circulation loop formed in the upper part of the vessel when the agitator pumps upwards. Circulation produced by the MTTR is poorer than

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both the down- and up- pumping configurations. The pumping effectiveness for the three configurations of the MTT is greater than both pumping modes of the PBT. The PBTD uses power more effectively than the PBTU, although there is no difference between the down- and up-pumping modes of the MTT. As expected, the pumping effectiveness of the MTTR is less than in the other configurations. The last index shows clearly that the up-pumping impellers circulate liquid more efficiently in the vessel. The comparatively low circulation efficiencies for the down-pumping mode reflect the poor agitation in the upper tank as already shown with LDV data.

Table 1. Power and flow characteristics of the impellers used in this work. Impeller Po Fl Flc ηηE Ec

PBTD 2.21a, 2.1b, 1.70c,

1.55d, 1.93g 0.8a, 0.91b, 0.73c,

0.81d, 0.75g 1.73a, 1.38c,

0.91g 0.43b, 0.39g 0.029b, 0.024g

PBTU 2.58g 0.68g 1.82g 0.26g 0.146g

MTTD 0.65e, 0.60f, 0.74g 0.73e, 0.63f, 0.67g 0.91e, 0.79f, 1.13g 1.12e, 1.05f, 0.91g 0.122g

MTTU 0.67g 0.61g 1.35g 0.91g 0.230g

MTTR 0.62g 0.44g 1.08g 0.71g 0.127g

a Ranade and Joshi12; b Mishra20; c Jaworski et al.22; d Bakker23; e Mavros et al.13; f Baudou24; g This work.

Agitation Index The agitation indices, Ig, for all impeller configurations have been calculated and the results are shown in Table 2. Considering the down-pumping configurations, the global mean velocity produced by the PBTD is 16.0% of Vtip which is greater than the index calculated for the MTTD of 14.1%. Comparing these results with those of the up-pumping mode shows some differences. For the PBTU the agitation index is increased notably to 22.5% of Vtip. This is coherent with observations made from analysis of the LDV velocity data: the PBT in the up-pumping configuration induces the development of a relatively strong secondary circulation loop in the upper part of the vessel which increases the overall liquid velocity. The agitation index of the MTTU, however, is more or less the same as the down-pumping mode, even slightly inferior at 13.7%. This suggests that although the MTTU may appear to circulate liquid better than the MTTD from the vector diagrams, the agitation index shows that the global liquid velocity of the MTTU is slightly less.

Table 2. Agitation index for the various impellers used in the present work. Pumping mode

Impeller Down Up Reverse PBT 16.0% 22.5% - MTT 14.1% 13.7% 11.9%

In the reverse mode, the MTT is even less effective than the MTTU, giving an agitation index of only 11.9%. Mavros and Baudou18 have presented similar results for a Rushton turbine, a down-pumping MTT and a down-pumping A310. Their agitation index for the Rushton turbine is 17.8%, which is slightly greater than the PBTD value found in this work but considerably inferior to the value found for the PBTU. This indicates a potential effectiveness for some impellers to operate in the up-pumping mode. The distributions of liquid volumes, which are associated with a particular velocity range, have been determined for each agitator configuration and are presented using histograms in Figure 11. For the down-pumping configurations, the liquid volume distribution is very close to a Gaussian distribution that is centred around a very low velocity range – 70% of the liquid volume has a velocity of less than 20% of Vtip. In the up-pumping mode, the distributions still have a Gaussian form though their centres are shifted to higher velocity ranges, the PBTU having a more important shift than the MTTU. In the PBTU configuration, only 45% of the liquid volume has a velocity of less than 20% Vtip whereas for the MTTU 60% of the liquid is moving at less than 20% Vtip. The reverse mode produces an uneven distribution of the liquid volumes and approximately 95% of the liquid volume has velocity of less than 25% of Vtip.

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(a) PBTD (b) MTTD

(c) PBTU (d) MTTU

(e) MTTR Figure 11. Distribution of liquid volume fractions in the vessel with respect to dimensionless composite velocity intervals.

CONCLUSION LDV measurements have been carried out in a vessel stirred by two different axial impellers operating in either the down-, up-pumping or reverse modes. 2-D velocity vector fields and velocity component maps demonstrate the differences in flow fields produced by impellers in different operating modes. In the down-pumping mode, one circulation loop is produced and the velocities in the upper part of the tank are very weak, resulting in poor mixing. The impellers pumping upwards induce two circulation loops, one in the lower and one in the upper part of the tank. This results in a better circulation in the upper part of the vessel.

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Operating in the reverse mode, the impeller produces a flow pattern similar to that of the up-pumping configuration with two circulation loops, however the velocities are much smaller. R.m.s velocities have found to be considerably higher in the impeller discharge stream than in the bulk of the tank for the three configurations and the flow is generally non-isotropic which is coherent with previous up- and down-pumping mixing studies15. Maximum values of turbulent kinetic energy are found in the impeller discharge region for the different pumping modes. The values in the up-pumping and reverse modes are however slightly smaller than for the down-pumping agitator.

Turbulence intensity induced by the impellers in the down-pumping configuration is highest above the agitator, close to the liquid surface which corresponds well with previously reported results21. When up-pumping, the maximum turbulence intensity is found close to the vessel wall, near the upper circulation loop and is approximately 15-30% smaller than the maximum value in the down-pumping configuration. Comparing pumping effectiveness of up- and down-pumping configurations reveals that the PBTU is less effective than in the down-pumping mode, although the MTTD and MTTU are equivalent. The circulation efficiency has been shown clearly to be superior for up-pumping impellers (especially for the PBT) than for the down-pumping or reverse mode operations. In conclusion, it has been shown that by simply changing the pumping direction of axial impellers to operate in the up-pumping mode, some interesting flow features are produced. Even though power consumption and pumping effectiveness may appear to be less favourable for some up-pumping impellers in single-phase systems, circulation is considerably improved and the formation of two circulation loops may be promising for gas-liquid dispersions10,25. The liquid flow would interfere with the flow of the bubbles, hence retaining more gas within the vessel. This would result in higher gas holdup and interfacial areas which would possibly have positive effects on gas-liquid reactions and other gas-liquid or gas-liquid-solid contacting processes.

NOTATION b baffle width (m) C impeller clearance (m) dp seeding particle diameter (µm) D impeller diameter (m) Dsh shaft diameter (m) Fl flow number (-) Flc circulation number (-) H liquid height in tank (m) Ig agitation index (dimensionless) kT turbulent kinetic energy (m2s-1) k* dimensionless kinetic energy (-) N impeller rotational speed (s-1) P power (W) Po power number (-) Qc circulation flow rate (m3s-1) QFl pumping flow rate (m3s-1) Qr, Qz radial and axial flow rates (m3s-1) r position in the radial direction r* radial coordinate of circulation loop center r.m.s. root mean square R radius of tank (m) T tank diameter (m) Tu turbulence intensity (-) Vij liquid volume surrounding measuring point (m3) vr, vz, vθ velocity components (ms-1)

, ,r zv v vθ′ ′ ′ r.m.s. velocity components (ms-1)

v̂ volume weighted average velocity (ms-1) Vtip impeller tip speed (ms-1) z position in the axial direction z* axial coordinate of circulation loop center

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Impeller types CHE3 Chemineer HE3 impeller MTTD, U, R Mixel TT impeller, down-pumping, up-pumping, reverse rotation PBTD, U pitched blade turbine, down-pumping, up-pumping PMT Prochem Maxflo T impeller

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11. Hari-Prajitno D., 1999, An Experimental Study of Unaerated and Aerated Single, Dual and Triple Hydrofoil Impellers, PhD Thesis, The University of Birmingham, U.K.

12. Ranade V.V. and Joshi J.B., 1989, ‘Flow Generated by Pitched Blade Turbines I : Measurements Using Laser Doppler Anemometer’, Chem. Eng. Comm., 81, 197-224.

13. Mavros P., Xuereb C., and Bertrand J., 1996, ‘Determination of 3-D Flow Fields in Agitated Vessels by Laser Doppler Velocimetry : Effect of Impeller Type and Liquid Viscosity on Liquid Flow Patterns’, Trans IChemE, 74 A, 658-668.

14. Jaworski Z., Nienow A.W. and Dyster K.N., 1996, ‘An LDA Study of the Turbulent Flow Field in a Baffled Vessel Agitated by an Axial, Down-pumping Hydrofoil Impeller’, Can. J. Chem. Eng., 74, 3-15.

15. Mishra V.P., Dyster K.N., Jaworski Z., Nienow A.W., and McKemmie J.,1998, ‘A Study of an Up- and a Down-Pumping Wide Blade Hydrofoil Impeller : Part I. LDA Measurements’, Can. J. Chem. Eng., 76, 577-588.

16. Tatterson G.B., 1991, ‘Circulation Times and Pumping Capacities’, Fluid Mixing and Gas Dispersion in Agitated Tanks, McGraw-Hill Inc., New York, 208-222.

17. Costes J., 1986, Structure des Ecoulements Générés par une Turbine de Rushton dans une Cuve Chicanée, PhD Thesis, INP, Toulouse, France.

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19. Meyers K.J. and Bakker A., 1998, ‘Solids Suspension with Up-Pumping Pitched-Blade and High-Efficiency Impellers’, Can. J. Chem. Eng., 76, 3, 433-440.

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