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.

A detailed hydrodynamic characterization in the transitional flow regime of two variants of the Norstone Polyblade high-shear impeller (HSI), of industrial relevance, is presented. The study was carried out on a simulated Newtonian fluid using computational fluid dynamics (CFD). Measurements of power number were carried out to validate the simulation results. Hydrodynamic parameters considered of key importance in powder dispersion processes (i.e., viscous dissipation and effective circulation) were used to assess the impellers' performance. Furthermore, their performances were compared with reported data for two ring-style HSIs of two and four rings, and power number measurements of a sawtooth (Cowles-type) impeller.

The understanding of the effect of impeller-sparger configurations on gas dispersion and mass transfer is very important to improve the performance of gas/liquid contactor systems. The influence of the impeller positions, the upper turbine diameter, the sparger ring diameter and its location in regard to the lower impeller on the power consumption, the volumetric masstransfer coefficient and the overall oxygen transfer efficiency were studied in a nonstandard curved bottomed reactor with an agitated system with dual disk style turbines. In the range of the gas flow rates studied, the most efficient impeller-sparger arrangement for the oxygen transfer is the impeller system with turbines of different diameters located at C = 0.25 and IC = 0.5, and with the sparger of smaller diameter than the lower impeller settled below the impeller. A new model to estimate the k L a with an average relative error of 8 %, which takes the reactor operation conditions and the influence of the impellersparger geometry into account, was also proposed.

Flow induced by a dual turbine stirred tank was characterized measuring local velocities with a LDV and drawing the main velocity fields and the maps of turbulence intensities. The hydrodynamic regime studied in all the experiments was the so-called merging flow regime. Two impeller configurations were studied. In the first one, two disk style turbine of the same dimensions (configuration A) were used, while in the second one, the dimensions of the upper turbine were 20 % proportionally smaller than those of the lower turbine (configuration B). The agitation and turbulence indices were used to evaluate, as a first order approximation, the power consumption distribution between convective and turbulent flows. The comparison of the two-phase agitation systems studied showed that configuration B seems to be more efficient than configuration A, since both induce a similar global convective flow, but the first one assures a significant reduction of power consumption. The distribution of power consumption between convective and turbulent flows was evaluated using the agitation index and a new global parameter: turbulence index.

ResumenSe realizó una investigación numérica a fin de capturar la formación del vórtice y los parámetros hidrodinámicos (potencia, bombeo, torque, campo de velocidad y disipación viscosa) inducidos por un impulsor de alto corte tipo Hockmeyer ® usando un fluido newtoniano (disolución de glucosa). El estudio se realizó en un tanque cilíndrico sin deflectores a números de Reynolds menores a 138, usando FLUENT ® . La formación del vórtice se simuló empleando el modelo multifásico volumen de fluido (VOF) con un enfoque de marcos de referencia múltiples. En el marco de referencia móvil (región que encierra el impulsor) se utilizó un modelo de transición a turbulento y en el volumen restante un modelo laminar. Se usaron mediciones de potencia para validar las simulaciones. Se encontró que el modelo VOF reproduce en forma aceptable el vórtice alrededor del eje. El campo de velocidad obtenido mostró dos lazos de circulación, uno por arriba y otro por debajo del impulsor, lo cual es característico de impulsores de flujo radial.Palabras clave: parámetros hidrodinámicos; sistema de dispersión de pigmentos; impulsores de alto corte; tanque sin deflectores; disipación viscosa Abstract A numerical study has been done to capture the vortex formation and the hydrodynamic parameters (power draw, pumping, torque, field velocity and viscous dissipation) produced by a High Shear Impeller of two rings Hockmeyer ® type by using a Newtonian fluid (glucose solution). The study was carried out at low Reynolds numbers (Re<138) in an unbaffled cylindrical tank by using FLUENT ® . The multiphase Volume of Fluid model (VOF) was used to capture the vortex formation together with the multiple reference frame approach. In this way, a transition model from laminar to turbulent regimen can be used in the rotation reference frame (the region enclosing the impeller) and in the remaining volume the laminar model can be used. Experimental measurements of power draw were used to validate the computational results. It was found that the VOF model acceptably predicts the vortex formation in the region near the shaft. The velocity field shows two circulation loops above and below the impeller center, which is characteristic of radial flow impellers.

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