Quantitative Study of Granular Flow, Mixing, and Segregation Using the Discrete Element Method (DEM)

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Granular materials exhibit a wide variety of fascinating physical properties that can be observed both in nature and industry. The goal of this work is to better understand the flow, mixing, and segregation of granular materials in a prototypical system, the rotating tumbler, using numerical modeling, specifically, the Discrete Element Method (DEM). Three topics are considered. >First, we study the subsurface flow field of mono-disperse particles in half-full cylindrical tumblers with different axial lengths: quasi-2D, as well as short and long tumblers. Our results show that the flow field is significantly altered near endwall regions. Simulations with longer tumblers or tumblers with one frictionless endwall clearly indicate that this phenomenon is a direct result of endwall friction and that it extends less than one tumbler radius (R) from the endwall. When frictional endwalls are closer than 2R, these regions merge partially (for the short tumbler) or completely (for the quasi-2D tumbler). The onset mechanism of axial segregation in long cylindrical tumblers forms the second aim of study. Particular attention is focused on the axial flow field of two species during the development of axial segregation. Results show a small axial flow between segregation bands, a result which was not reported before. The mechanism for axial segregation is identified as arising from the non-uniform distribution of axial velocity and species concentration on transverse planes. The third topic is axial segregation in spherical tumblers which is examined both experimentally and numerically. Two different patterns are observed: for low fill levels, band of large particles appear at the equator; for high fill levels, the opposite occurs. The fill level of the tumbler at which the transition occurs varies depending on particle size and rotational speed of the tumbler. DEM simulations produce identical results and particle trajectories indicates different flow performances of two species of particles in the two reversed segregation patterns.

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  • 09/20/2018
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