A new model of cavern diameter based on a validated CFD study on stirring of a highly shear-thinning fluid

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.

Chemical and Process Engineering

“…Nevertheless, typical devices used to process yield-stress materials are stirred tanks and the avoidance of cavern formation is crucial in successful mixing (Simmons et al, 2009). The size and shape of a cavern depends on the stirrer-tank geometry, the impeller rotational speed and the rheological flow curve of the material (Story and Jaworski, 2017).…”

“…The measurement methods of the two forces have also been applied to stirring of yield-stress fluids. The tangential force, usually expressed in the form of power draw in YSF, was measured in a range of stirrer-tank configurations (Amanullah et al, 1997;Amanullah et al, 1998;Bhole and Bennington, 2010;Bhole et al, 2011;Galindo et al, 1996;Galindo and Nienow, 1992;Nienow and Elson, 1988;Story and Jaworski, 2017). The stirrer axial force has been less frequently investigated since the force is associated with inclined stirrer blades and the first advanced report about yield-stress fluids appeared in 1998 (Amanullah et al, 1998).…”

“…A similar approach to build a model and validate CFD simulations was also applied in standard stirred tanks with a PMT hydrofoil impeller (Story and Jaworski, 2017;Story et al, 2018).…”

Hydrodynamic analysis of an axial impeller in a non‐Newtonian fluid through particle image velocimetry

“…Most of the work done with hydrofoil impellers tends to experimentally determine global quantities and local mean velocity profiles in water using different measurement techniques such as laser Doppler anemometry and particle image velocimetry (PIV) . Note that some work has been devoted to study the A310 mixing capability for other non‐Newtonian fluids, or to study the mixing of Carbopol with other impellers . Since shear rate is usually a controlling factor in processes such as flocculation or precipitation, numerous studies have focused on the determination of local shear rate and its relation with process performance in water .…”

Cavern Boundary Determination with Pseudoplastic Fluid Based on the Apparent Viscosity Method

“…The determination of cavern boundaries is essential to describe the cavern size and shape accurately. The velocity magnitude contours of the flow field are widely used to estimate the cavern dimensions (height and diameter), and the cavern boundary velocity is defined as 0.01v tip [20][21][22][23]. It is clear that the boundary velocity method applied to the prediction of caverns is closely connected to the velocity of the blade tip, while the tip velocity changes with the different speeds.…”