Investigation of Mixing in Shear Thinning Fluids （调查剪切稀化流体的混合）.pdf

4 Investigation of Mixing in Shear Thinning Fluids Using Computational Fluid Dynamics Farhad Ein-Mozaffari and Simant R. Upreti Ryerson University, Toronto Canada 1. Introduction Mixing is an important unit operation employed in several industries such as chemical, biochemical, pharmaceutical, cosmetic, polymer, mineral, petrochemical, food, wastewater treatment, and pulp and paper (Zlokarnik, 2001). Crucial for industrial scale-up, understanding mixing is still difficult for non-Newtonian fluids (Zlokarnik, 2006), especially for the ubiquitous shear-thinning fluids possessing yield stress. Yield-stress fluids start to flow when the imposed shear stress exceeds a particular threshold yield stress. This threshold is due to the structured networks, which form at low shear rates but break down at high shear rates (Macosko, 1994). Many slurries of fine particles, certain polymer and biopolymer solutions, wastewater sludge, pulp suspension, and food substances like margarine and ketchup exhibit yield stress (Elson, 1988). Mixing of such fluids result in the formation of a well mixed region called cavern around the impeller, and essentially stagnant or slow moving fluids elsewhere in the vessel. Thus, the prediction of the cavern size becomes very important in evaluating the extent and quality of mixing. When the cavern size is small, stagnant zones prevail causing poor heat and mass transfer, high temperature gradients, and oxygen deficiency for exa