Determination method of the cavern boundary viscosity in a stirred tank with pseudoplastic fluid

Abstract

It is essential to predict the cavern boundary accurately for investigating the cavern characteristics and achieving efficient mixing of the pseudoplastic fluids. The distribution characteristics of apparent viscosity of xanthan gum solutions at different mass concentrations stirred by the impeller of perturbed six-bent-bladed turbine (6PBT impeller) were numerically investigated through the computational fluid dynamics (CFD) simulation based on laminar flow model. The determination method of the cavern boundary viscosity was presented, and the predicted results were compared with those of traditional velocity method. Results showed that the data of the power consumption predicted using CFD simulation agreed well with those measured by torque experiment, which validated the laminar flow model. The η a − γ curve changes from nearly horizontal straight line to the power function curve along the direction of center axis toward tank wall. As a result, the location of the transitional point can be determined as the cavern boundary, and the corresponding viscosity is the cavern boundary viscosity. The ratio of the cavern boundary viscosity and the fluid yield viscosity (η a /η y ) always remains approximately constant, that is, η a = 0:25η y at different speeds. Compared with the traditional velocity method, the boundary viscosity method can maintain its high accuracy for exhibiting the cavern boundary, and the predicted cavern size and development are always in good agreement with the CFD results free from the speed and the impeller type. So this work can provide a new way for accurately characterizing the cavern boundary. K E Y W O R D S boundary viscosity, cavern, numerical simulation, pseudoplastic fluid

Mixing Characteristics of Shear-Thinning Fluids in a Fractal Perforating Impeller Stirred Reactor

“…The average shear rate can be gained in accordance with Metzner and Otto′s correlation − γ̇ avg = k normals N …”

“…For radial flow impeller, k s = 11.5. 26,28,29 The apparent viscosity (η) of the xanthan gum solution can be expressed as 24−26…”

“…The xanthan gum solution was used in the experiment and numerical simulation as a shear-thinning fluid, and the rheology properties of the xanthan gum solution can be estimated by the Herschel–Bulkley model − τ = τ normaly + K γ̇ n where τ y is the yield stress, K is the consistency index, n is the flow behavior index, and γ̇ is the shear rate.…”

Study of hydrodynamics and flow characteristics in a twin‐blade planetary mixer with non‐Newtonian fluids

“…Hence, many investigations have focused on the mixing characteristics of different non-Newtonian fluids. [27][28][29][30][31][32] Jegatheeswaran and Ein-Mozaffari 33 found that it is possible to cut the energy consumption while enhancing the aeration efficiency at low Reynolds numbers in a Scaba-anchor coaxial mixer involving power-law fluids. Ramsay et al 34 characterized the mixing of Newtonian and viscoelastic fluids in batch tanks with "butterfly" impellers and found that the laminar constant was equal to 122 and the Metzner-Otto constant was equal to 16.0, and the two constants are closely related to the correlation of power consumption.…”

“…Since the viscosity of non‐Newtonian fluids depends on the shear rate, some problems such as complex flow, 20,21 formation of dead zones, 22–24 low mixing efficiency, and considerable energy consumption 25,26 arise in the mixing process. Hence, many investigations have focused on the mixing characteristics of different non‐Newtonian fluids 27–32 . Jegatheeswaran and Ein‐Mozaffari 33 found that it is possible to cut the energy consumption while enhancing the aeration efficiency at low Reynolds numbers in a Scaba‐anchor coaxial mixer involving power‐law fluids.…”