Advanced Simulation of Particle Processing: The Roles of Cohesion, Mass and Heat Transfer in Gas-Solid Flows

摘要

This dissertation addresses the simulation of several important unit operations in the field of granular processing, which includes particle mixing and segregation, cohesive gas-solid flows, liquid transfer between particles, heat transfer in gas-solid flows and the drying process in gas-solid flows. Particle dynamics (PD) is employed to probe the solid flows and computational fluid dynamics (CFD) is used to simulate the gas phase.Achieving good mixing of free-flowing particulate solids with different properties is not a trivial exercise. By introducing periodic flow inversions, we show both experimentally and computationally that forcing with a value above a critical frequency can effectively eliminate both density and size segregation.The mechanics of cohesive flowing gas-particle systems is still poorly understood. Toward that end, we introduce a discrete characterization tool for gas-solid flow of wet (cohesive) granular material- the Granular Capillary Number. The utility of this tool is computationally tested over a range of cohesive strengths in two prototypical applications of gas-solid flows.While slow granular flows have been an area of active research in recent years, heat transfer in flowing particulate systems has received relatively little attention. We employ a computational technique that couples the PD, CFD, and heat transfer calculations to simulate realistic heat transfer in a rotary kiln. Our results suggest a transition in heat transfer regime as the conductivity of the particles changes.Liquid transfer between particles plays a central role in the operation of a variety of particle processing equipment. We introduce a dynamic liquid transfer model for use in PD of heterogeneous particle systems. As a test of this new model we present results from the simulation of a rotary drum spray-coating system.The drying process in gas-solid flows involves complex mass, momentum and heat transfer. By incorporating mass transfer modeling into our existing gas-solid PD-based heat transfer code, the drying process is successfully simulated. Results are reported for both mono-disperse and bi-disperse cases.Finally, we outline how to simulate amphiphilic particles, which are spheres comprised of two parts. We use the quaternion method to track the particle rotation, such that we can study the issues relating to anisotropic particles.