Mixing Of Shear-thinning Fluids Research Articles

Some modifications in the Scaba 6SRGT impeller to enhance the mixing characteristics of Hershel–Bulkley fluids

Mixing in unbaffled cylindrical tanks by open turbines rotating at low Reynolds numbers requires usually a longer mixing time, especially for shear thinning fluids possessing yields stress, where the mixing operation is characterized by the formation of caverns. To overcome this issue, some modifications in the impeller design are introduced. We are interested in mixing of shear thinning fluids with yield stress by curved bladed impellers (known as Scaba 6SRGT) in the laminar and transitional flow regimes. Two types of cuts have been introduced in the impeller blade: one at the middle of blade (referred as Design 2) and the other from the middle until the tip of blade (referred as Design 3). Lengths of cuts in Design 2 and 3 are: l/d2=0.3 and 0.6, respectively, where d2 is the length of blade for the classical impeller. The cuts introduced in the blades allowed a reduction in power consumption by 17.5% and 33% for Design 2 and 3 compared to the classical blade. Also, other modifications are introduced in order to enhance the mixing characteristics. It concerns the impeller eccentricity (e/D=0.12 and 0.25) and blade number (nb=2, 3 and 6) for the case which has given the most reduced power input (i.e. Design 3). The off-centred impeller (case e/D=0.25) with 6-blades has yielded the widest cavern size.

Mixing of Complex Fluids in a Cylindrical Tank by a Modified Anchor Impeller

AbstractThe mixing of shear thinning fluids in a cylindrical vessel has been investigated in this paper via numerical simulation. The agitation is ensured by a modified anchor impeller (MAI): it consists of curved blade turbine (CBT) mounted on a classical anchor impeller (CAI). These agitators were mounted in single and combined configurations (co‐axial). The effects of agitation rate, design of the stirred system and curvature of the bottomed arm of anchor on the fluid circulation, well‐stirred region size and power consumption have been studied. The obtained results revealed that the modified anchor impeller (i. e. the combination of CBT with CAI) performs better than the classical anchor impeller.

Mixing of Shear Thinning Fluids in Cylindrical Tanks: Effect of the Impeller Blade Design and Operating Conditions

Abstract The 3D flow fields and power consumption within a cylindrical vessel stirred by a rotating turbine are numerically studied. Simulations are performed to determine the impact of changes in operating parameters on the mixing characteristics. Investigations are focused on effects of the impeller blade curvature, shaft speed and impeller rotational direction. The fluid simulated has a shear thinning behavior. Designing the blade in retreat shape seems very promising in term of power consumption since a reduction of Np is obtained with increasing blade curvature. In the positive rotational direction, the retreat bladed impeller yields highly radial flows with less power consumption than the straight bladed impeller. The 45° retreat blade gave an increase in the radial velocity by 39 %, compared with the straight blade. But, a better axial circulation is obtained with the straight blade. The comparison between the positive rotational direction (+w) and the negative rotational direction (–w) cases revealed that, a reduced mixing time can be obtained with a retreat bladed impeller operating in the negative rotational direction (–w), but with further power consumption.

Effect of mixer geometry and operating conditions on flow mixing of shear thinning fluids with yield stress

Flow mixing of a non‐Newtonian fluid in a stirred tank equipped with a side‐entry impeller was observed using particle image velocimetry (PIV). The effects of some geometrical parameters including the mixer shape and impeller type and position on the flow pattern were studied on velocity fields obtained at different locations inside the mixing domain. The different flow structures revealed that the ratio of inertial and viscous forces largely defines the flow pattern. Dead zones were observed inside the tank due to the rheological properties of the fluid. The size of the dynamic regions and the average velocity near the impeller were enhanced by increasing the suction area. Likewise, large pitch ratios were found to improve the active mixing zone and the axial discharge. Curves for the power and pumping numbers are reported for different axial flow impellers. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1156–1167, 2014

Cavern Development of Pseudoplastic Fluids Stirred by Impeller of Perturbed Six-bent-bladed Turbine

The prediction of the cavern size is important in the mixing of shear-thinning fluids with yield stress.The cavern development of xanthan gum solutions,a pseudoplastic fluid with yield stress,stirred by a impeller of perturbed six-bent-bladed turbine was numerical researched using computational fluid dynamics(CFD).The cavern boundary velocity is properly determined and the cavern dimensions of shape,diameter and height to diameter ratio are obtained versus apparent Reynolds number,Re*.In order to determine the capability of CFD to forecast the flow process,the agitational experiment is used to measure the power consumption.The results show that the power data predicted by the simulation are in good agreement with those measured in the experiment,which validate CFD model developed in laminar flow.The cylindrical cavern development is found as Re* increased,which shows a good agreement with the representation of the cylindrical model(Elson’s model).The variations of dimensionless cavern diameter(Dc/D) verse dimensionless NpRey,as well as cavern height to diameter ratio,have nothing to do with the rheological index of fluids all together.CFD simulation predicts a linear slope of 0.35 from a log-log plot of Dc/D verse NpReyat DcT,which agrees well with the theoretical value,0.33,of Elson’model.The height to diameter ratio is 0.68 until the cavern reaches the tank wall,while the rate constant of axial fluid expansion describing cavern height once the cavern touched the vessel wall,β is 0.79.The research results promote the understanding of the mixing performance of the impeller of perturbed six-bent-bladed turbine and provide valuable instructions and references for the design,application and development of new type impeller in pseudoplastic fluids.

Mixing of Shear-Thinning Fluids with Dual Off-Centred Impellers

Mixing times for inelastic shear-thinning fluids in stirred tanks have been experimentally investigated using a combination of two off-centred impellers operating in both co- and counter-rotating modes. A colour-discolouration technique based on fast acid-base reaction was used for the determination of the mixing times as well as to reveal the possible presence of caverns and dead regions. A statistical plan of experiments allowed determining the effects of the impeller position, the rotational speed, the flow behaviour index, the impeller type and their mutual interactions. A stronger influence of the impeller position on mixing times was observed for both rotating modes with fluids exhibiting pronounced shear-thinning. It was also found that segregated regions could be readily destroyed by dual off-centred impellers as compared with the single centred impeller configuration. Mixed flow impellers were shown to be less efficient in terms of mixing times than radial flow impellers. Results obtained under the best operating conditions were compared to steady stirring experiments showing the potential and drawbacks of the proposed scenarios. On a étudié expérimentalement les temps de mélange pour des fluides rhéofluidifiants inélastiques à l'aide d'une combinaison de deux turbines décentrées en mode rotatif à co-courant et à contre-courant. On a employé une technique de décoloration reposant sur une réaction rapide acide-base pour déterminer les temps de mélange et révéler la présence possible de cavernes et de régions mortes. Un plan d'expérience statistique a permis de déterminer les effets de la position des turbines, de la vitesse de rotation, de l'indice de comportement des fluides, du type de turbines et de leurs interactions réciproques. Une plus forte influence de la position des turbines sur les temps de mélange a été observée pour les deux modes rotatifs avec des fluides montrant une rhéofluidisance prononcée. On a également trouvé que les régions ségréguées pouvaient être facilement détruites par des turbines décentrées doubles comparativement à une turbine centrée unique. Les turbines à écoulements mixtes s'avèrent moins efficaces en termes de temps de mélange que les turbines à écoulements radiaux. Les résultats obtenus dans les meilleures conditions opératoires ont été comparés aux expériences d'agitation stationnaire, ce qui permet de montrer le potentiel et les inconvénients des scénarios proposés.

Laminar mixing of shear thinning fluids in a SMX static mixer

Flow and mixing of power-law fluids in a standard SMX static mixer were simulated using computational fluid dynamics (CFD). Results showed that shear thinning reduces the ratio of pressure drop in the static mixer to pressure drop in empty tube as compared to Newtonian fluids. The correlations for pressure drop and friction factor were obtained at Re MR ⩽ 100 . The friction factor is a function of both Reynolds number and power-law index. A proper apparent strain rate, area-weighted average strain rate on the solid surface in mixing section, was proposed to calculate pressure drop for a non-Newtonian fluid. Particle tracking showed that shear thinning fluids exhibit better mixing quality, lower pressure drop and higher mixing efficiency as compared to a Newtonian fluid in the SMX static mixer.