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|Title:||Modeling particle-particle and particle-wall interactions in liquid-particle flows in complex geometries|
|Advisor:||Hrymak, Andrew N.|
Michael Thompson, Maryline Lightstone
|Keywords:||coupled particle fluid flows;numerical simulation;discrete element method;particle-fluid interactions;complex geometries;free surface flows;Other Chemical Engineering;Other Chemical Engineering|
|Abstract:||<p>Many practical fluid flows involve liquid-particle systems and so there is a need to better understand the mechanism of particle deposition, adhesion, and agglomeration in suspensions, especially in complex geometries with moving boundaries and free surfaces. In this thesis, the nature of the particle-solid interactions and particle-fluid interactions is studied where the above complexities are present, taking into account particle collision, colloidal, and hydrodynamic forces, and two way coupling between the fluid flow and particles. The research is motivated by the industrial examples of: flow of dross particles near the sink roll surface in a galvanizing bath (moving surface), and the flow of particles in slot coating dies (free and moving surfaces). Particle motion and agglomeration play important roles in the example systems chosen for this fundamental 3-D study. Numerical studies of flow of dispersed suspensions makes it possible to understand the effects of flow conditions, particle characteristics, and flow geometry specifications that lead to agglomeration of particles in complex systems, especially where experimental studies are difficult to perform. Often the effects of these conditions are discovered due to process or product failures, rather than through insight into the processing steps.</p> <p>The modeling methodology used in this work is that micron sized spherical particles are tracked in the fluid phase by solution of Newton`s second law of motion for each particle. Fluid phase applies hydrodynamic forces on particles (drag, lift). Body forces, (soft sphere) particle-particle collisions and particle-wall collisions are considered. Particle concentrations are in the dilute regime between 0.01-5%vol. Flow of particles with the fluid phase is a fully coupled formulation in systems with particle concentrations > 1%vol.</p> <p>The thesis is organized around three example problems taken from industry that pose challenging modeling issues. The first involves particle collisions with a moving wall (dross particles in a zinc bath). The second problem includes particle-particle and particle-wall collisions in a turning flow geometry. The third problem, particle dispersion flows in a slot coating die, has the most complexity and includes particle-particle, particle-wall and free surfaces.</p> <p>Dross particle build up on the sink roll inside the zinc bath is an industrial problem that causes significant down time, and where an experimental study of the molten zinc in a bath running at C is difficult to perform. With the aid of computational fluid dynamics, turbulent flow of molten zinc in galvanizing bath is simulated, compared with previous cold model experiments, and coupled with the motion of dross particles around the sink roll. The presence of fixed position hardware and moving sink roll and guide rolls in a bath with dimensions in the orders of meters, and micron sized (20-100 ) dross particles makes this case a complex study. Drag, buoyancy, lift force and soft sphere nonlinear collision is considered in solution of Newton`s law of motion for each particle. Turbulent flow is simulated using a standard model. Simulations show regions on the sink roll where particles are dragged toward the surface of sink roll and have long residence times. These regions have been reported to experience large particle build-ups in the hot-dip galvanizing process.</p> <p>In another study, formation and breakage of agglomerates in a turning flow is studied. Neutrally buoyant particles with concentration of 5%vol are tracked in a fully coupled flow. Particles form agglomerates at the corner, where drag and lift force from the fluid breaks a number of agglomerates. The presence of a moving wall in the turning flow shifts the suspended particle formations toward the inside of channel. Location of particles agglomerates shifts toward the free surface with the presence of free surface at the turning flow.</p> <p>Motion of micron sized spherical particles with 1-4%vol through a slot die coating system is elucidated in a separate study. The system is complex with presence of moving web and free surface. Discrete element method (DEM) for motion of dispersed phase and volume of fluid (VOF) method for solution of continuous phase are integrated in a simulation study. Particles are 2-4 and the flow dimensions of the system are in the order of 100 . Particles experience collision, colloidal and hydrodynamic forces. Coupling between flow of particles and fluid phase is conducted. The results of this study show particle positions on the coating film can be predefined and depends on their initial positions within the feed slot. Particles agglomerate in recirculating regions of the coating gap and follow the streamlines of flow on the moving web. Regions in the coating gap where particles have high residence times (inside the die and near the feed slot edges) have particle agglomerations in the slot die coating system.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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