Particle Tracking Using Molecular Dynamics Simulation For Pebble Bed Reactors.

摘要

Characteristics of the pebble tracks generated through Molecular Dynamics (MD) Simulations were investigated for a Pebble Bed Reactor (PBR). Through monitoring the mass flow rate and its resultant local volume fraction as well as the local coordination numbers (the local means that were defined by the regime within the separate parts of the reactor; the coordination number is defined as the number of contacting its neighbors), the pebble piling up and subsequent discharge were investigated. Simulations were then conducted through the implementation of the non-cohesive Hertz-Mindlin theory. The governing equation can be derived from Lagrangian dynamics that are given by the kinetic and potential energies. The kinetic energy consists of the translational and the rotational kinetic energies and the potential energy consists of the gravitational and the Hertz potentials. MD simulation builds off of the virial theorem (relating to the system of forces and their positions) which states that the stress energy tensor is in relation to the gravitational potential and the Hertz potential. The output of the simulation data gives the force, the velocity, and the position of the particles. Calculation from the interactions between the particles resides within the Hertz-Mindlin potential. These interactions resulted in a pilling effect from the deposited particles being simply dropped into the hopper of the PBR. At which point, the discharge in the local region was observed and from this it was then determined if the jamming phenomenon was occurring. The phase diagram of the volume fraction and the coordination number was found to yield the transition of the jamming or flowing condition. Regardless of the situation experienced, the mass flow rate and the phase diagram agreed on the flow established. Specific analysis on the flowing condition requires a heterogeneous medium that can influence the wall applied shear and normal stresses which are then used to determine the pebble flow and to evaluate their subsequent energies. When implementing the MD simulation, it was found that a random walk process within Monte Carlo simulation was the most accurate approach to the solution.