Using CFD and Ultrasonic velocimetry to Study the Mixing of Pseudoplastic Fluids with a Helical Ribbon Impeller

Abstract

A commercial CFD package was used to simulate the 3D flow field generated in a cylindrical tank by a helical ribbon impeller. The study was carried out using a pseudoplastic fluid with yield stress in the laminar mixing region. Ultrasonic Doppler velocimetry (UDV), a noninvasive fluid flow measurement technique for opaque systems, was used to measure xanthan gum velocity. From flow field calculations and tracer homogenization simulations, power consumption and mixing time results were obtained. The torque and power characteristics remain the same for upward and downward pumping of the impeller, but the mixing times are considerably longer for the downward pumping mode. Overall, the numerical results showed good agreement with experimental results and correlations developed by other researchers. From the power and mixing time results, two efficiency criteria were utilized to determine the best pumping mode of the impeller.

CFD Investigation of the Mixing of Yield‐Pseudoplastic Fluids with Anchor Impellers

“…As expected, the mixing time decreases with increasing power consumption [5,43]. A similar trend was also observed by Ihejirika and EinMozaffari [37] for the close-clearance impeller. It can be seen that the mixing time achieved by the four-blade impeller is less than that achieved for the two-blade impeller for a given specific power consumption.…”

“…This approximation has been used and tested by other researchers who confirmed the accuracy of the modified Herschel-Bulkley equation in the CFD modeling of the mixing of pseudoplastic fluids with yield stress. For instance, Pakzad et al [31], Ford et al [35], Saeed et al [36], Ihejirika and Ein-Mozaffari [37], and Saeed et al [38] employed this approximation for the CFD modeling of the mixing of pseudoplastic fluids with yield stress.…”

Measuring Mixing Time in the Agitation of Non‐Newtonian Fluids through Electrical Resistance Tomography

“…Pseudoplastic fluids with yield stress are an important class of non-Newtonian fluids, commonly encountered in industrial mixing operations. Pulp suspension, food substances like ketchup and mayonnaise, paints, cement, pigment slurries, certain polymer and biopolymer solutions, and wastewater sludge are pseudoplastic fluids possessing yield stress [19,20]. Non-Newtonian fluids are homogenized more slowly than Newtonian fluids in laminar and transition ranges due to the shear rate field existing in the mixing vessel [21].…”

Investigation of Mixing in Shear Thinning Fluids Using Computational Fluid Dynamics

“…Γ has negligible effect on the tracer distribution possibly because the insignificant contribution of molecular diffusion to the overall tracer dispersion (Montante et al, 2005). Some researchers also find that the effect of the molecular diffusivity is negligible in the mixing of non-Newtonian fluids in the laminar regime Ihejirika & Ein-Mozaffari, 2007;Ein-Mozaffari & Upreti, 2009). Pakzad (2007) used CFD to predict the mixing time for the agitation of the shear-thinning fluid possessing yield stress with a Scaba 6SRGT impeller.…”