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https://hdl.handle.net/2142/69779
Description
Title
The Stability of Multiphase Suspensions
Author(s)
Thiokas, John
Issue Date
1987
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Abstract
When sufficient quantities of buoyant particles are mixed with a sedimenting suspension of heavy particles, the two types of solids undergo rapid lateral segregation from each other. Streams containing the less populous species form and flow through a concentrated continuum suspension of the more populous species. This convection may result in greatly enhanced settling rates.
A model is developed to determine the circumstances under which a bidisperse suspension is unstable, to predict the observed concentration patterns and to identify the mechanism responsible for the appearance of the streams. Averaged equations are used to describe the motion of the fluid and particles as though they were interpenetrating media. The stability of the system subject to infinitesimal disturbances is evaluated. Linear and non-linear theories are presented.
The linear theory leads to the instability criterion, which is a function of the initial particle concentrations and the physical properties of the system. The critical buoyant/solid ratio needed for instability is in good agreement with that given by published experimental results. Solid and buoyant concentration waves propagate with a specific phase difference resulting in segregation of the particles. This relative phase is a property of the system. At small times the formation of periodic concentration patterns gives rise to a buoyancy driven flow field characterized by circulation centers.
Non-linear interactions of concentration patterns and flow field of both equal and varying wavelengths are presented. The vertical accumulation and separation of particles results either from self interactions under the influence of the side walls of the suspension container or from interactions of different wavelengths in the absence of walls.
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