Joël Bertrand

Effect of Axial Agitator Configuration (Up-Pumping, Down-Pumping, Reverse Rotation) on Flow Patterns Generated in Stirred Vessels

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 to consume more power than 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.

Scale-up in laminar and transient regimes of a multi-stage stirrer, a CFD approach

A multi-stage industrial agitator system adapted to the mixing of a mixture whose viscosity varies during the process has been characterized by using CFD. In the entire study the mixture is supposed to have a Newtonian behavior even though it is rarely the case. It is shown that the well-adapted propeller is able to e7ciently blend high viscous media provided the Reynolds number is not too low. A scale-up study of the agitated system has also been carried out by respecting the classical scale-up rules such as the geometrical similarity and the conservation of the power per volume in the particular case of viscous media. Using an Eulerian approach, the hydrodynamics of three di9erent scales with geometrical similarity have been numerically characterized by the energy curve (power number versus Reynolds number) and by the Metzner and Otto constant in which both are required for scale-up procedure. Experimental power measurements have been carried out at the smaller scale so that simulations have been partially validated. New hydrodynamic criteria have also been introduced in order to quantify the =ows in the case of a multi-stage stirrer running at low Reynolds number. It has been shown how this hydrodynamic di9ers dramatically from one scale to another when scale-up at constant energy per volume is applied. From the CFD results, recommendations about the widely used scale-up rules have been suggested and modi>cations of stirring geometry have been proposed in order to reduce the =ow pattern variations during scale-up. ? 2002 Elsevier Science Ltd. All rights reserved.

Power consumption, pumping capacity and turbulence intensity in baffled stirred tanks: Comparison between several turbines

Abstract Power consumption, pumping capacity, mean velocities and velocity fluctuations in the discharge flow of five open or closed turbines in a baffled vessel have been measured. For a given power consumption, the open turbines develop a higher pumping capacity and a higher turbulence intensity.

Local measurements of fluid and particle velocities in a stirred suspension

Abstract An experimental approach is proposed for the measurement of the local behaviour of continuous and dispersed phases in a stirred suspension at low particle concentrations (0.5 vol.%). The basic principle involves the simultaneous measurement of the local velocity and particle size with a phase Doppler velocimeter and the separation of the data obtained from large particles (representing the dispersed phase) and very small particles (representing the motion of the continuous liquid phase). This technique was applied to the investigation of a fully baffled cylindrical vessel, with a flat bottom, stirred by an industrial axial propeller. The diameters of the vessel and the propeller and the vertical clearance were 0.3 m, 0.14 m and 0. l m respectively. The liquid was water and spherical glass particles were used as the dispersed phase. In the absence of particles, the circulation pattern created by the propeller in the middle plane between two baffles revealed a major circulation loop in the lower part of the vessel and a minor contra-rotative loop in the wall surface corner. The calculated pumping coefficient, circulation flow number, non-dimensional time of renewal and circulation time were 0.62, 0.89, 11.76 and 8.15 respectively. The tangential velocities remained at values of less than 15% of the impeller tip velocity in the plane of the measurements. Axial, radial and tangential mean and root-mean-square velocities for the carrier liquid phase and particles were measured in the two-phase flow, using particles with a mean diameter of 253 μm. These measurements showed that the particles lagged behind the liquid phase in the upward parts of the flow field, but were ahead in some downward parts. The root-mean-square axial velocities obtained for the particles were always greater than these obtained for the continuous phase. Non-homogeneities in the suspension were observed by the local mean diameter field. The effects of the stirring rate and size of the particles were also checked in the upward part of the flow in the vessel.

Simulation of flow generated by an axial-flow impeller: Batch and continuous operation

It is important to extend and to validate computational flow models to simulate continuous operation of stirred vessels and to capture possible interaction of feed inlet/outlet with the flow generated by impellers. In the present work, we have developed and used a computational model to understand the flow generated by an axial flow impeller in a batch and a continuously operated baffled vessel. A multiple reference frames approach was used to simulate flow generated by the Mixel TT impeller in stirred vessel. The predicted velocity results show reasonably good agreement (qualitative as well as quantitative) with the experimental data. Characteristics of flow around blades of Mixel TT were studied using the computational model. The computational model was extended to simulate flow and mixing in a continuous operation. Simulations were carried out to understand the interaction of the jet emanating from the feed pipe and the flow generated by the impeller. Model predictions were compared with published experimental data, obtained by laser Doppler velocimetry. The differences and similarities between batch and continuous operation are highlighted. Mixing simulations were carried out to examine possible short-circuiting and non-ideal behaviour of the continuous operation of the stirred vessel. Influence of the impeller speed, feed rate and location of inlet/outlet on mixing and on the extent of non-ideality of flow was studied. The computational model and results discussed in this work will be useful for understanding the mixing process in continuous-flow stirred vessels.

Blending of newtonian and shear-thinning fluids in a tank stirred with a helical screw agitator

Newtonian and non-Newtonian laminar fluid flow has been simulated using Computational Fluid Dynamics for a cylindrical vessel stirred by a helical screw agitator. Simulations have been performed for a vessel geometry with and without a draft tube. Simulated flow patterns in the vessel have been examined and compared with the experimental work of previous authors. The power number and the circulation number have been evaluated, and interpreted in a similar manner to other works. The PO.Re constant, A, has been determined to be 295 for the geometry with the draft tube and 150 for that without the draft tube. These results are in the same range as previously reported values. The Metzner and Otto constant, k, has been evaluated to be 16.23 which is in excellent agreement with experimental results reported in the literature.

Laser Doppler Velocimetry in Agitated Vessels: Effect of Continuous Liquid Stream On Flow Patterns

Liquid velocities were measured by laser Doppler velocimetry for a standard Rushton turbine and an axial-flow Mixel TT agitator, with a stream of liquid, having a volumetric flowrate such that the residence time was approximately 1/10 of the mixing time, being fed continuously in the agitated vessel. Measurements taken in the feeding-tube plane showed a major disruption of the flow pattern, especially of axial velocities, due to the continuous liquid stream. The incoming liquid jet combined with the stream discharged by the impeller in a complex 3-D way; however, the disruption of the flow gradually attenuated, and measurements taken in the plane rotated 90° clockwise indicated a flow pattern, with characteristics like velocity magnitudes, turbulence intensity and energy dissipation, similar to the batch case (with no through flow).

Investigation by laser Doppler velocimetry of the effects of liquid flow rates and feed positions on the flow patterns induced in a stirred tank by an axial-flow impeller

The (ow patterns established in a continuously-fed stirred tank, equipped with a Mixel TT axial-(ow impeller, have been investigated bylaser Doppler velocimetry , for a high and a low value of mean residence time—mixing time ratio. The pseudo-two-dimensional axial– radial-velocityvector plots, as well as the spatial distributions of the tangential velocitycomponent and the velocitypro;les around the impeller, show that the interaction between the incoming liquid and the liquid entrained bythe agitator rotation cause the (ow pattern in the vessel to become stronglythree-dimensional, especiallyin the region between the plane, where the feeding tube lies, and the 180 ◦ -downstream plane. The increase in the liquid (ow rate and the location of the feed entryboth a

Influence of gas flow rate on the structure of trailing vortices of a rushton turbine: PIV measurements and CFD simulations

ect the (ow pattern, with the latter having a more pronounced e

Determination of 3-D Flow Fields in Agitated Vessels by Laser-Doppler Velocimetry: Use and Interpretation of RMS Velocities

ect. The overall process, in this mode of operation, depends upon the appropriate con;guration and choice of parameters: for conditions corresponding to high liquid (ow rates, the (ow patterns indicate the possibilityof short-circuiting, when the liquid is fed into the stream being drawn bythe agitator and when the outlet is located at the bottom of the vessel. ? 2002 Elsevier Science Ltd. All rights reserved.