The combined effect of irregular shape particles and fluid rheology on settling velocity measurement

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

The drag coefficient of a solid particle depends mainly on the particle Reynolds number, particle sphericity, and fluid rheology. The sphericity of a solid particle is the degree to which the shape of a solid particle approaches that of a sphere. Non-Newtonian fluids are those fluids which do not show linear relationship between shear stress and shear rate. Practically, the apparent viscosity for shear-thinning fluids is decreasing with increasing shear rate. Settlement of solid particles in shear-thinning fluids is of great importance and has many applications in drilling operations, chemical industry, and mining processes. In this study, the combined effects of particle sphericity and fluid rheology on settling velocity measurement have been studied experimentally. Fifty irregular-shape solid particles with different sphericities (ranged from 0.575 to 0.875), and four shear-thinning fluids with flow behavior indices (ranged from 0.60 to 0.92) were used. A new drag coefficient charts have been developed for irregular-shape solid particles when they settled down through various shear-thinning fluids, which cover laminar to transient flow regimes around the particles. These charts show linear relationships between the drag coefficient and particle Reynolds number for all fluids, which have the same slope but with different intercepts. These charts show that the drag coefficient at a given particle Reynolds number is increased as the flow behavior index is decreased (i.e. as the fluid becomes more non- Newtonian), which means higher resistance to particle movement. And, for a given fluid rheology, the drag coefficient is decreased as the particle Reynolds number is increased, which means less resistance to particle movement (i.e. faster slip velocity). Finally a general equation has been developed for irregularshape particles when they settle down in various shear-thinning fluids, which can be used to calculate easily and directly the settling velocity and the particle Reynolds number of these particles. This equation can also be used for spherical and disk particles.