Particle Dynamic Simulation of Free Surface Granular Flows

A large collection of particles can behave collectively like a fluid. In this work, we examine the fluid behavior of a large collection of particles in the absence of a background gas phase. The use of granular fluids, as opposed to liquids, has several advantages for free-surface plasma-facing coolants: low vapor pressure, absence of electromagnetic interactions, and chemical inertness, to name a few. Overall, granular fluids are expected to benefit from substantially reduced plasma interaction as compared with liquids. However, one of the primary concerns is the lack of fluid cohesion which, in an unconfined geometry, creates a difficult problem with flow control. Some of the concerns include maintenance of adequate packing fraction, particle ejection into the plasma and flow around obstructions. Numerical particle dynamic simulations have been performed in order to improve our understanding of flow control issues and to suggest design solutions which would allow this class of coolant and plasma-facing material to be used in a commercial fusion power plant. Simulations were performed using the “distinct element” method, which directly simulates the momentum equation and force-displacement relations for a large collection of interacting circular particles. The method allows for static and sliding friction between the particles (and with the walls), finite normal and shear stiffness, and bonding forces. Initial results highlight the difficulty in maintaining dense uniform packing in a fully free-falling region. Internal contact forces must be avoided in the inlet zone to prevent the bed from dissociating and creating excessive porosity. Alternatively, the geometry of the substrate can be tailored with baffles or curvature. The basic flow behavior for these geometric elements are presented and design strategies proposed.