Targeted Unmixing of Particles in Suspension

Abstract

Solid-liquid separation in residential and industrial effluents generally describes the separation and removal of specific particulates from the effluents. This process is often difficult to achieve when dealing with toxic or valuable particulates that must be extracted from the working liquid. Despite the availability of numerous separation methods, situations exist where particles are too fine to settle at an appreciable rate or to be extracted economically using existing separation techniques. The work presented in this thesis involves the examination of a novel approach to particle separation using fluid turbulence to induce directed motion in particles to accumulate in specific regions of the fluid flow field; a mechanism known as preferential concentration. Hence, an extended objective of the presented research is to determine the feasibility of using the preferential concentration mechanism for the purpose of selective particle un-mixing. Experiments investigated the capabilities of the preferential concentration mechanism at causing directed motion in non-inertial particles within a fluid suspension. By tracking non-inertial particles within a vortex ring flow field, the influences of particle inertia were effectively suppressed to allow the examination of other factors that may influence the preferential concentration mechanism; factors such as particle-fluid surface interaction. Particles within the vicinity of vortex structures were observed to move in a directed manner; a behaviour that cannot be fully explained using existing preferential concentration theories based solely on particle mass and inertia. Further investigations examined the possibility that particle- fluid surface interactions contribute to directed motion in particles and/or influence the preferential concentration mechanism. The results obtained supported previous indications that particle-fluid surface interactions are able to influence particle and fluid behaviour, which in turn contributed to directed particle motion. However, the relationship between these interactions and the preferential concentration mechanism could not be validated within the experimental work undertaken. Consideration was therefore given to the possibility that the observed directed motion, although associated with particle-fluid surface interactions, may not be due to the preferential concentration mechanism. Collectively, the outcomes of the current research established that particle-fluid surface interactions are able to influence particle and fluid behaviour, thus contributing to directed particle motion. However, the occurrence of the preferential concentration mechanism could not be confirmed and as such the influence of particle-fluid surface interactions on the mechanism itself could not be substantiated experimentally. The current work supports the conclusion that experiments involving particle-laden fluids are highly dynamic and complex, and particle and fluid behaviour can be influenced by numerous factors to varying degrees. Further understanding of particle-laden fluids as well as the preferential concentration mechanism will require a means of examining individual factors contributing to particle and fluid behaviour either in isolation or with minimal effects from other factors. It is anticipated that through future work that integrates experiments with computational methods, a further understanding of directed particle motion and the preferential concentration will lead to the development of a successful preferential-concentration-based selective particle un-mixing technique suitable for industrial applications.