An experimental study of the settling velocities of single particles in non-Newtonian fluids

Abstract

Therminal settling velocities of single particles, both spherical and non-spherical, were measured in various non-Newtonian fluids. The shear rheology of each fluid was determined using a Weissenberg rheogoniometer. The spherical particles varied in radius and density, and the non-spherical particles were marble chippings selected such that their volume was within the range of sphere volumes. Terminal settling velocities were analysed using a method for determining the drag force on a sphere falling in a power law fluid. A particle falling through a fluid generates a localised shear rate in the fluid surrounding it. The analysis for the drag force was limited to the region bounded by slow particle settling, generating a shear rate within the first Newtonian region, and a rapid particle settling giving rise to inertial affects with Reynolds numbers greater than 0.1. Within the bounds of these constraints it was found to be difficult to obtain a polymer fluid which behaved as a power law fluid at the shear rates generated by the falling spheres. The sensitivity of a falling particle to a fluids shear history, and thus to a viscosity regime governed by an “effective shear rate” which itself generates, was demonstrated using a thixotropic fluid. The results indicate that the settling velocities of particles of irregular shape could be approximated by that of a sphere of equivalent volume and density.