Kinematics and Segregation in Granular Flows: Modeling Density Difference, Shape Effects, and Unsteady Flow Fields

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Segregation of granular materials with different particle properties (e.g.,~size and density) is a ubiquitous phenomenon in nature and industry. In this thesis, different segregation mechanisms are studied in steady granular heap flows. Flow kinematics and segregation in transient granular flows are also explored. ', '\t', 'First, this thesis studies segregation of flowing density bidisperse materials. The flow kinematics are extracted from Discrete Element Method (DEM) simulations, and a relation between the density segregation velocity and the particle density ratio, the local shear rate, and the local concentration is found. A continuum segregation model is adapted to include the density segregation velocity and the model is capable of quantitatively predicting density segregation in bounded heap flows. Segregation of rod-shaped particles with different lengths is also investigated. DEM simulations of cylindrical rod particles reveal a relation between the segregation velocity and the local shear rate, the local species concentration, and the rod length ratio, similar to the relation for spherical particles. This relation is again incorporated into the continuum segregation model which accurately predicts segregation of rod-shaped particles in bounded heap flows.', 'By modulating the feed rate in size bidisperse bounded heap flows, unsteady flows are generated leading to stratification of small and large particles, which is different from the streamwise segregation formed in steady flows. The stratification pattern formed in the modulated flow can be controlled by modulation parameters and demonstrates better mixing than the streamwise segregation. The stratification mechanism is related to non-uniform deposition during transient heap flows. When the feed rate suddenly changes, a growing (or vanishing) wedge of materials originates on the free surface near the feed zone and propagates downstream, indicating non-uniform deposition or erosion occurring during the transient process. The transient flux and surface height profile can be modeled by a local relationship between the local flux and the local slope angle in combination of a continuity equation. Modulating the feed rate is not the only method to generate unsteady heap flows. By increasing the water content in damp granular materials, a transition from steady to unsteady flow can occur. The unsteady flow is a time-periodic flow with each period consisting of a non-depositing downslope avalanche and an upslope propagating granular jump. The transition occurs when the surface angle of the heap is increased (as a result of increasing cohesion) beyond the neutral angle of deposition. The unsteady flow results in inhomogeneous packing density in the deposited heap. In addition, hopper discharge segregation is studied, which is a transient industrial process which occurs after heap formation (hopper filling). DEM simulations reveal that segregation mainly occurs in a surface layer where particles are transported from the sidewall to the hopper center. The continuum segregation model is applied to modeling the particle concentration distribution in the surface layer and the bulk region using velocity profiles developed based on a kinematic model and DEM observations.

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  • 10/08/2019
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