THE FLOW STRUCTURE OF SLURRIES IN HORIZONTAL PIPES

Abstract

An experimental investigation was performed to study the flow structure of slurries of concentrated settling suspensions in horizontal pipes. Visual observation showed that the saltation of solid particles was caused by a transverse vortex which is generated and grows randomly in space and time; and that large-scale motion existed in the whole range of heterogeneously suspended flow.
To interpret the flow structure, this study utilized the stability concept of stratified flow, presuming that the solid phase behaves like a fluid in the early part of the process. The results showed that the formation of a transverse vortex was predictable from the Kelvin-Helmholtz instability of the stratified flow. The densimetric Froude number based on the depth and net velocity of the water phase classifies three flow regimes: viz., (1) stable, (2) neutral and (3) unstable flows. These correspond to those based on the observed distinct modes of the flow; i.e., (1) sliding on a stationary bed, (2) saltation with a deposit and (3) saltation without a deposit.
The pressure gradient has no effect on the stability criterion but affects the resulting dynamic process by changing the frequency of generation in the transverse vortex. Measured intensity and the time scale of the wall pressure fluctuations were representative of the vortical motion, which is far larger and stronger than those found in wall turbulence.