Power Consumption in Mixing and Aerating of Shear Thinning Fluid in a Stirred Vessel

A. Bombač, M. Žumer and I. Žun

Abstract
The paper presents an analysis of power consumption in mixing and dispersing of air into a shear thinning fluid in a standard cylindrical vessel stirred with a Rushton turbine. Water-CMC solutions at w = 0.5 % and w = 1 % mass fraction and compressed air were used. The apparent viscosity enabling the power vs. Reynolds number relation was calculated using the Metzner-Otto method. A comparison of the power-consumption measurement results in single-phase mixing is in good agreement with the results found in the literature. When air is dispersed into a shear thinning fluid, an obvious reduction of power consumption can be noticed especially at higher impeller speeds and gas flow rates, which is more noticeable in an w = 0.5 % than in an 1.0 % CMC solution. In fact, the dissipated mixing power depended on gas-filled cavity structures behind the blades, which affected the impeller pumping efficiency. All experimental flow regimes were analyzed and depicted in a generalized flow regime map as Froude vs. Flow number dependence. Developing of structures was shown from high effective to low effective operation, e.g. complete dispersion and bubble column operation, respectively. Mixing power was found to be independent of airflow rate at low Reynolds numbers. A comparison of the measured values in air-dispersion and the results from correlations found in the literature confirms a general trend of impeller power reduction.

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Keywords
Mixing, aerating, power consumption, cavity structures, rheology, carboxymethyl cellulose, Rushton turbine