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Hindered settling Chong, Yu Sen

Abstract

Natural and commercially available particles of uniform shape and size were used to study the effect of particle shape on hindered settling in creeping flow (Re₀< 0.1), where fluid flow behaviour is independent of particle Reynolds number and the effect of shape is most prominent. Particles of different shapes used were spherical glass beads, cubic salt (NaCl) crystals and ABS plastic pellets, flaky sugar crystals and angular mineral (silicate) crystals. They were carefully sized by sieving and liquid elutriation to avoid other effects like size segregation. Constant settling data were processed in the form of uѵ rather than u to eliminate the effect of temperature variation on viscosity. The effect of the wall on hindered settling rate was found to be small in most cases. The method proposed by Beranek and Klumpar for correlating fluidization data on different shaped particles was found to be only moderately successful in correlating the present settling data for different shapes. Results were plotted as loguѵ versus log €, and the index n of the equation uѵ/(uѵ)ext = €ⁿ was calculated by least squares. It varied from an average value of 4.8 for the smooth spheres to 5.4 for the cubes to 5.8 for the flaky or angular particles. In contrast to the corresponding term proposed by Richardson and Zaki, the term (uѵ)ext was measurably lower than uѵ for free settling of the spherically isotropic particles. More significantly, the index n was graphically found to display a definite trend with the random loose fixed bed porosity, which is shape dependent and easily measured, and may therefore turn out to be a simple and useful parameter for taking account of shape variation.

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