Chapter 1 - Non-Newtonian Fluid Behaviour

Abstract

This chapter focuses on the flow characteristics of single-phase liquids, solutions, and pseudo-homogeneous mixtures such as slurries, emulsions, and gas–liquid dispersions, which are considered as a continuum when they are stable in the absence of turbulent eddies, depending upon their response to externally imposed shearing action. A non-Newtonian fluid is one whose flow curve is nonlinear or does not pass through the origin, i.e., where the apparent viscosity, shear stress divided by shear rate, is not constant at a given temperature and pressure but is dependent on flow conditions such as flow geometry, shear rate, etc., and sometimes even on the kinematic history of the fluid element under consideration. The most common type of time-independent non-Newtonian fluid behavior observed is pseudoplasticity or shear-thinning which is characterized by an apparent viscosity that decreases with increasing shear rate. Many mathematical expressions of varying complexity and form have been proposed in the literature to model shear-thinning characteristics and some of them are straightforward attempts at curve fitting, giving empirical relationships for the shear stress or apparent viscosity while others are theoretically based on statistical mechanics, as an extension of the application of the kinetic theory to the liquid state or the theory of rate processes, etc. Viscoplastic fluid behavior is characterized by the existence of a yield stress (τ0) which has to be exceeded before the fluid deforms or flows and such a material deforms elastically or flows en masse like a rigid body when the externally applied stress is smaller than the yield stress. Three commonly used models used for viscoplastic fluids include Bingham plastic model, Herschel-Bulkley fluid model, and Casson fluid model.