Lift Induced Particle Migration in Dilute Suspensions
AffiliationChemical and Biomolecular Engineering
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2023-05-24. This item is currently available to University of Melbourne staff and students only, login required.
© 2020 Nilanka Indunil Kumari Ekanayake
Small particles moving near a wall experience particle-scale inertial lift forces in a direction normal to the wall and hence, migrate away from the wall. In addition, particles in suspensions experience hydrodynamic collision forces and migrate away from or towards the wall. These forces are critical in the biological context as they contribute to the separation between platelets and red blood cells that ensure the repair and integrity of blood vessels. These migration mechanisms have also been utilised to design and optimise micro-scale cell-sorting microfluidics for (e.g.) novel health detection systems and `smarter' industrial shear enhanced membrane filtration devices. Despite these important applications, a comprehensive model that can predict the lift and drag forces acting on a small particle moving near a wall is not available. Models that have previously been developed are limited to specific wall separation distances, fluid shear rates, and particle slip velocities, which do not cover practically relevant parameter ranges. Further, particle-scale lift models have not been previously employed in the context of suspension modelling to predict the averaged motion of many particles, as opposed to the motion of a single particle. Such suspension modelling is necessary to predict the performance of real biological and industrial multiphase flows. Hence, this thesis aims to develop a comprehensive model of particle lift applicable to the parameter ranges found in typical biological and industrial flows and apply this model to predict the behaviour of particle suspensions in these flows. The work is conducted in two parts. Firstly, the hydrodynamic forces acting on a small spherical particle moving with a finite particle Reynolds numbers in single wall-bounded flows are investigated via direct numerical simulation. Based on these results, new lift and drag models are proposed for rigid spherical particles moving in quiescent and simple shear flows, valid for any wall separation distance, and shear and slip particle Reynolds numbers of order 0.1 or less. The models are used to examine the behaviour of single buoyant and neutrally-buoyant particles moving near walls, and the results are validated against existing experimental and numerical data. Secondly, a two-fluid model, which includes the developed wall-bounded forces, is implemented to predict particle migration in mono-disperse, dilute suspensions at low particle Reynolds numbers. Different implementation methods related to solid phase velocity boundary conditions, the force application phase, and secondary wall effects are discussed. Using this model, the transient solid concentration profiles in Taylor-Couette flows are examined and compared against available experimental results.
KeywordsLift forces; Drag forces; Wall bounded forces; Dilute suspensions; Suspension modelling; Particle migration; Two fluid model
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