Performance Evaluation of Two High‐Shear Impellers in an Unbaffled Stirred Tank

Abstract

High-shear impellers (HSIs) are mixers used in industrial stirred tanks to incorporate powders into liquids and break down particle agglomerates. A detailed numerical study of two commercial ring-style HSIs of laboratory scale was carried out and their performance was compared with the Rushton turbine (RT). It was found that power and pumping numbers or their ratio cannot be simply connected for properly selecting an impeller in applications where highly localized viscous dissipation is desirable. The ratio of the average viscous dissipation in the impeller swept volume to the mean in the entire volume at two constant values of power input turned out to be lower for HSIs compared to that evaluated for RT. However, at higher power input, the dimensionless average viscous dissipation in the blade swept volume was found to be similar for the HSI of two rings and the RT, corroborating the high local viscous dissipation of this HSI when operated at higher speeds.

Computational Fluid Dynamics Study of Flow Induced by a Grooved High‐Shear Impeller in an Unbaffled Tank

“…In a later work, Ramírez‐Muñoz et al performed experiments to investigate the influence of the viscous dissipation on the amount of fines generated from mineral powders. In agreement with Ramírez‐Gómez et al , they found that, based on the effects on the breakage of particle agglomerates, the volume swept by the blades is the region of maximum effective viscous dissipation.…”

“…The study was carried out in the transitional flow regime (20 ≤ Re ≤ 133). In previous works , , it has been reported that in this flow regime the laminar model practically delivers the same predictions as a transitional flow model, but with a lower computational cost. Therefore, in this study only the laminar model was used in all simulations.…”

“…In the absence of baffles, two whirlpools (known as a doughnut flow structure) are formed, one between the impeller and the vessel bottom and the other above the impeller. The second one becomes visible in the gas‐liquid interface around the shaft and is useful for floating pigments to be quickly entrained in the fluid by the vortex action toward the volume swept by the impeller and incorporated into the liquid , . The energy transferred from the impeller toward the mixture induces high‐velocity gradients on the fluid passing through this volume.…”

“…Ramírez‐Gómez et al studied the hydrodynamics induced by two Hockmeyer Type D HSIs at the lab scale in the transitional flow regime (50 ≤ Re ≤ 75), with two (HockD‐2R) and four rings (HockD‐4R), respectively, by applying the computational fluid dynamics (CFD) approach. Their simulation results agree very well with what was mentioned above regarding the flow structure induced by these impellers.…”

“…Ramírez‐Gómez et al suggested, without experimental evidence, that the volume swept by the blades of the Hockmeyer Type D impeller might be considered as the maximum effective dissipation region for solid‐liquid dispersions. They pointed out that it is likely that the ultimate particle size due to the breakage of particle agglomerates is mainly correlated with the energy dissipation in this region.…”