Hydrodynamic and Energetic Investigations of a Laminar Flow of a Non-Newtonian Fluid Inside a Tank Equipped With a Zigzag-Bladed Anchor Impeller

Abstract

Most fluids in industries such as chemical, food, and nuclear industries exhibit complex rheological behaviors. Specifically, high-viscosity fluids are typically agitated by impellers that generate predominantly circular flow, such as anchors. However, modifying other flow directions is important to improve mobility in areas of stagnation. In this context, this research presents a conceptual numerical contribution by designing a new anchor-type impeller geometry. The design features a zigzag-bladed anchor impeller symmetrically aligned with its shaft, aimed at enhancing axial mobility while almost retaining effective wall-scraping performance. The study focuses on analyzing the flow behavior and energy consumption inside a cylindrical tank equipped with the studied impeller, while considering the effects of various control parameters. These include the non-Newtonian behavior index of the fluid (0.6 ≤ n ≤ 1.4), ranging from a shear-thinning fluid to a shear-thickening fluid, the Reynolds number (1 ≤ Re ≤ 50, anchor-type agitators operate at low Reynolds number in stirred tank, which is common in mixing high-viscosity fluids), which highlights the rotational speed of the impeller, and finally, the angle of inclination of the zigzag blades (0 ≤ α 30°); beyond the 30° angle (for α > 30°) there are dead zones in the upper part of the tank, which have a negative influence on mixing. The theoretical study is governed by the Navier–Stokes equations in a laminar flow regime, which are solved using the finite element method with the Galerkin approach. The numerical results have demonstrated a significant enhancement in both hydrodynamic and energetic performance. In terms of hydrodynamic performance, the axial velocity has been notably enhanced, which helped break up stagnant zones and increased axial mobility within the agitated tank. Regarding energy consumption, the new anchor design showed its capability to reduce energy usage by up to almost 47%, with the most significant reductions occurring at moderate Reynolds numbers, particularly in industries handling shear-thickening fluids.