In 1957 Metzner and Otto proposed a simplified way of calculating the power consumption in impeller-driven systems for power-law type of fluids. Assuming a proportionality between the shear rate and rotational speed of the impeller, they reported that the proportionality constant was independent of the geometric and fluid properties of the mixing system. This was later contested by a number of researchers, and different correlations have been given to evaluate the proportionality constant. An in-depth investigation of the Metzner-Otto concept is discussed with the help of computational fluid dynamics (CFD) techniques to calculate the flow field created by an anchor impeller. Fundamentally, both of the assumptions are broadly valid for typical operating parameters used in anchor-driven impeller systems. A new correlation, based on the Metzner-Otto concept and making use of the present CFD simulations, was developed to calculate power consumption in anchor-driven mixing systems.

Sreenivas Jayanti

Department Of Chemical Engineering

Computational fluid dynamics

Computer simulation

Correlation methods

Impellers

Mixing

Rotational flow

shear rate

Flow of fluids

impeller

LIQUID-LIQUID MIXING

Power law fluid

article

calculation

flow rate

Fluid flow

geometry

mathematical analysis

measurement

Rotation

Shear stress

Simulation

velocity

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IIT Madras

AIChE Journal

Experiments on dispersion of floating solids into continuous liquid medium were carried out with low density microspheres with particle density of 680kg/m3 and size 100, 230 and 325μm in different liquids having density varying from 778 to 1830kg/m3 and kinematic viscosity varying from 0.3 to 19cP. A four-bladed radial impeller and axial paddle impellers with upward and downward flow directions were employed. The critical impeller speed (Ncrit) required for uniform dispersion of particles throughout the continuous medium was experimentally determined for various conditions. Experimental results indicate that the radial impeller with impeller location near the surface required minimum impeller speed for uniform dispersion. Strong effect of the density difference between the particles and the liquid and submergence of the impeller was noted. Based on the experimental results, a correlation in terms of relevant dimensionless groups was proposed for calculating Ncrit for the three impellers. The overall correlation indicates that the critical impeller speed is not significantly affected by liquid viscosity and particle diameter but that it is strongly influenced by the impeller diameter and the density difference. These results are in agreement with trends reported in the literature. © 2015 The Institution of Chemical Engineers.

Chemical Engineering Research and Design

Effect of impeller type and density difference on the draw down of low density microspheres

Helical ribbon impellers are widely used in chemical and process industries for the mixing of pseudoplastic fluids of high viscosity. The design of such impellers is based on an assumed linear relation between shear rate and the rotation speed of the impeller. A number of computational fluid dynamics (CFD) simulations of the flow field have been carried to verify this hypothesis. It is shown that while the shear rate varies greatly within the mixing vessel, there does exist a linear relationship between the impeller speed and the local shear rate near the tip of the impeller. The proportionality constant K s associated with this linear relation is found to be dependent on the geometric parameters of the system, but is largely independent of the flow behavior index. Based on these results, a new correlation, applicable to both Newtonian and power-law fluids for power consumption, is proposed.

Journal	AIChE Journal
ISSN	00011541
Open Access	No