Abstract
Mixing of a viscoplastic fluid using Scaba 6SRGT impellers in a cylindrical vessel has been studied. Cuts have been introduced in each blade of the impeller in order to reduce the power requirements. Effects of the cut-height (h2/D = 0, 0.015, 0.04, 0.065 and 0.09, respectively), cut-length (l/D = 0, 0.06, 0.12 and 0.18, respectively), and of the number of cuts (nb = 1, 2, 3, 4, and 5) on the hydrodynamics and power consumption were explored. The suggested designs, compared to the classical Scaba 6SRGT impeller, display a reduction in power number by about 20%, 19.9% and 66.6% when the cut-height, cut-length and number of cuts are changed from h2/D = 0 to 0.09, l/D = 0 to 0.18 and nb = 1–5, respectively. However, the increased surface area of cuts is accompanied by a reduction in the size of radial jet of fluid, resulting thus in a weakened axial flow and a decrease in cavern size. As a result, the best trade-off between the reduced power consumption and enlarged cavern size correspond to the case nb = 3, l/D = 0.12. The reduction in power number for this case is estimated to be as about 15%.
Highlights
Mechanical agitation of fluids in cylindrical tanks is a fundamental operation in to achieve a wide variety of tasks, such as gas dispersion into liquid to form foams or for mass transfer, powder dispersion or solid blending, dispersion of immiscible liquids for emulsification, preparation of ingredients, etc
Effect of the height of cut in blades (h2) In the first part of our investigation, we explored the effect of the height (h2) of cut in blade son the flow patterns and power consumption
As observed in this figure, the tangential velocity increases first gradually from the impeller shaft and along the blade length until reaching its maximum value at the blade tip. It decreases continuously until becoming negligible at the tank wall. This figure reveals that and the tangential flow is maximized by the conventional impeller design (h2/D = 0), as expected, and that it decreases with increasing the height of cut in blade
Summary
Mechanical agitation of fluids in cylindrical tanks is a fundamental operation in to achieve a wide variety of tasks, such as gas dispersion into liquid to form foams or for mass transfer, powder dispersion or solid blending, dispersion of immiscible liquids for emulsification, preparation of ingredients, etc. Ameur (2016a) and Zhao et al (2011) reported that trailing vortices generated behind flat-bladed impellers are the main source of energy dissipation In aerated agitation, these vortices (filled with gas) are responsible for the formation of cavities, which yields a reduction in mass transfer and unstabilities in power consumption (Zheng et al, 2017). These vortices (filled with gas) are responsible for the formation of cavities, which yields a reduction in mass transfer and unstabilities in power consumption (Zheng et al, 2017) This issue has been overcome by the development of impellers exhibiting semi-circular tube blades (CD), and the evolution of the flow field between the CD-6 impeller and Rushton turbine (RT) has been deeply analyzed by Devi and Kumar (2013). For shear-thinning fluids with yield stress, research have been focused on the Scaba impellers under different operating and geometrical conditions, including the coaxial, multistaged, and the multiple eccentric configurations (Pakzad et al 2013a, 2013b, 2013c; Kazemzadeh et al, 2016a, 2016b, 2017; Ameur, 2016b, Ameur and Ghenaim, 2018)
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