Abstract
Results of numerical simulations of momentum transfer for a highly shear-thinning fluid (0.2% Carbopol) in a stirred tank equipped with a Prochem Maxflo T type impeller are presented. The simulation results were validated using LDA data and both tangential and axial force measurements in the laminar and early transitional flow range. A good agreement between the predicted and experimental results of the local fluid velocity components was found. From the predicted and experimental values of both tangential and axial forces, the power number, Po, and thrust number, Th, were also calculated. Values of the absolute relative deviations were below 4.0 and 10.5%, respectively, for Po and Th, which confirms a satisfactory agreement with experiments. An intensive mixing zone, known as cavern, was observed near the impeller. In this zone, the local values of fluid velocity, strain rate, Metzner–Otto coefficient, shear stress and intensity of energy dissipation were all characterized by strong variability. Based on the results of experimental study a new model using non-dimensional impeller force number was proposed to predict the cavern diameter. Comparative numerical simulations were also carried out for a Newtonian fluid (water) and their results were similarly well verified using LDA measurements, as well as experimental power number values.
Highlights
Shear-thinning phenomenon is the most common deviation from the Newtonian behavior of fluids
Results of numerical simulations of momentum transfer for a highly shear-thinning fluid (0.2% Carbopol) in a stirred tank equipped with a Prochem Maxflo T type impeller are presented
Computational fluid dynamics numerical simulations were successfully performed for a highly shear-thinning fluid flow in a stirred tank equipped with the Prochem Maxflo T (PMT) type impeller, rotating with a constant speed in the range of N = 2–12 s-1
Summary
Shear-thinning phenomenon is the most common deviation from the Newtonian behavior of fluids. Shear-thinning fluids, e.g. polymer solutions and melts, suspensions, emulsions and even food products are found in almost all industries. High shear forces occur near an impeller during stirring. With highly shear-thinning fluids in that region an intensive mixing zone called cavern (Wichterle and Wein 1975; Moore et al 1995; Wilkens et al 2005) is generated. The fluid velocity is close to zero. When stirring highly shear-thinning fluids, their rheological parameters have decisive influence on the intensive mixing zone size. Complex rheological properties of stirred fluids require proper selection of the agitator, system geometry and impeller rotational speed because the impeller parameters substantially affect efficiency of the mixing process
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