The Flow of Fluidised Solids

Abstract

The design and construction of a large test rig comprising an open sloping channel along which fluidised particulate materials may flow is described. A large number of experiments have been performed in this rig, aimed at establishing the rheological properties of a bed of flowing sand under a wide range of conditions, the parameters varied being channel width and slope, bed depth and fluidising velocity. The results of these experiments were analysed in terms of a comparison between the fluidised bed and a power-law non-Newtonian fluid flowing under similar conditions. Over a large range of shear rates the power law model is adequate, and it shows the bed to be substantially Newtonian at high fluidising velocities. At lower fluidising velocities, the bed becomes increasingly pseudo plastic in nature. At low shear rates, however, the behaviour of the bed is much more difficult to describe; tentative explanations for the various phenomena evident in this region are advanced, these being in terms of the interaction of two mechanisms, styled "inviscid layer dissipation" and "bubble suppression and segregated layer dissipation", by the author. The existence of these mechanisms has been established both by experimental methods and visual observations. Correlations of friction factor and modified Reynolds number show that the fluidised bed follows the liquid laminar flow correlation up to Reynolds numbers in excess of 4000; again, tentative explanations are advanced for the absence of any transition between flow regimes. Despite the limitations of the theory used throughout, the study has established the best range of operating conditions for the flow of fluidised sand, and the test rig built as part of it should be useful for a further considerable range of experiments.