The effects of rheology on the mixing mechanisms in laminar flows in stirred tanks

A number of industries rely heavily upon efficient fluid mixing in order to obtain consistent, high quality products. Examples include paper & pulp, polymer, food, biotechnology, and pharmaceutical processes. In many of these operations, laminar regime is required due to working fluids high viscosity values and/or materials that are shear-sensitive. Despite its obvious practical importance, laminar mixing in realistic, industrially relevant geometries, such as the stirred tank reactor, has not been properly addressed in the literature until recently. The poor understanding of the fundamental mechanisms of mixing in laminar regime is a chief limiting factor in the effective design of mixing equipment, scale-up, and mixing operations at low agitation speeds or under viscous conditions.; Mixing in stirred tanks is extensively studied to understand the underlying physics of mass, momentum and energy transport and their mutual interactions, fundamental studies range from purely experimental investigations of mixing phenomena to their analytical, or most often numerical solution, but a unified understanding of mixing in stirred tanks is still lacking despite these efforts. One of the main issues in the study of mixing has been how to characterize it. The first part of this dissertation aims to establish robust methods to experimentally characterize mixing based on image analysis techniques. The properties of chaotic flows are analyzed using statistical and scaling methods developed initially for theoretical 2D time-periodic flows. Spectral analysis is also implemented to investigate structural length scale distribution.; The effects of rheology on mixing in stirred tanks are investigated experimentally and numerically. The role of different rheological behavior, such as shear-thinning, yield-stress, and viscoelasticity on fluid mixing will be investigated. We seek the understanding of the effects of the interplay among inertial, viscous and elastic forces on the mixing process. Two main types of non-Newtonian fluids are investigated: viscoelastic and viscoelastic. Viscoplastic fluids investigated in this dissertation possess marked yield-stress and shear-thinning viscosity and are investigated numerically using CFD coupled with a bounded-stress model. Viscoelastic fluids are investigated experimentally with constant and shear-thinning viscosity. A map based simulation is used to examine the mechanisms of mixing of viscoelastic fluid with shear-thinning viscosity.