CFD Investigation Of Mixing Of Yield-Pseudoplastic Fluid With Anchor Impeller

Poonam Prajapati

doi:10.32920/ryerson.14665215.v1

Abstract

The Anchor impeller, which is a close clearance impeller, produces high shear near the vessel wall and is recommended for mixing of highly viscous fluids. A thorough search of the literature suggests that few publications have beeen devoted to the computational fluid dynamics (CFD) modeling of mixing of non-Newtonian fluids with the anchor impeller. Thus the objectives of this study are (i)to generate a 3-D flow field for mixing of yield-pseudoplastic fluid in a flat bottom cylindrical tank equipped with two-and four-blade anchor impellers using CFD modeling technique, (ii) to evaluate the effects of fluid rheology agitator speed, number of blades, vessel clearance and impeller blade width on power consumption, mixing time and flow patterns, and (iii) to determine the optimum value of clearance to diameter ratio and impeller blade width to diameter ratio on the basis of minimum mixing time. The study was carried out for a yield-stress pseudoplastic fluid, using a CFD package (Fluent), to simulate the 3-D flow domain generated in a cylindrical tank equipped with two-and four-blade anchor impellers. The multiple reference frame (MRF) technique was employed to model the rotation of impellers. The rheology of the fluid was approximated using the Herschel-Bulkley model. To validate the model, CFD results for the power were compared to experimental data. After the flow fields were calculated, the simulations for tracer homogenization was performed to simulate the mixing time. The effect of impeller speed, fluid rheology, and number of impellers on power consumption, mixing time, and flow pattern were explored. The optimum values of c/D (clearance to diameter) and w/D (impeller blade width to diameter) ratios were determined on the basis of minimum mixing time.

Highlights

Close clearance impellers are highly recommended for mixing of high viscosity fluids in laminar regime, especially pseudoplastic fluids (Nomura et al, 1996), due to their ability to keep the entire vessel contents circulating (Tatterson, 1986). ln polymerization reactors it is desirable to ensure efficient mixing to prevent phenomena like hot spots, to control the molecular weight distribution of the final product and to avoid dead zones
Chapter four is organized in two sections: first section reviews the general information about computational fluid dynamics (CFD) such as governing equations, numerical methods, discretization methods, grid generation and other relevant information, and the second section of the chapter is devoted to the current CFD model development
In this study the performance of an anchor impeller was evaluated on the basis of power consumption and mixing time

Summary

Introduction

Yield-stress fluids, which can be found frequently in food, paint, cosmetic, waste water treatment, pulp & paper and pharmaceutical industry, are a common class of non-Newtonian fluids These fluids start flowing when the imposed shear stress is more than a particular threshold value due to structured networks that build up at low shear rates and break down at high shear rates (Lobe and White, 1979). Flow patterns, mixing time, optimum vessel clearance and blade width are discussed in this chapter. The partial differential transport equation (Equation 4.8) explains the continuous movement of fluid in space and time To solve these transport equations numerically, computational fluid domain is discretized; i.e. changed from continuous to discontinuous domain by series of connected control volume which is known as computational cells. They can result in a very large number of meshes for smalldimension elements such as baffle (Fluent, 2006)

Objectives

Methods

Results