Convective flow speed of particles in a vibrated powder bed

When a powder bed is vibrated, the particles may move like a fluid, and various flow patterns and surface shapes can exist, depending on the vibration conditions — particularly the frequency and amplitude. A two-dimensional (2D) bed is often used so that the flow pattern in a vibrated powder bed can be directly visualized. Because three-dimensional (3D) fluidization phenomena are complicated, 2D observation is not sufficient. However, it is difficult to observe the internal flow of a 3D fluidized bed. A unique method for doing this is positron emission particle tracking (PEPT), a technique derived from the same physical phenomena as in positron emission tomography (PET). In the most common version of the PEPT technique, the flow in a bed is analyzed by following the behavior of a single radioactively labeled particle. We adopted the PEPT technique to analyze flow patterns in a cylindrical vibrated powder bed. From the particle trajectory data obtained with PEPT, some characteristic internal flow structures that cannot be understood from observing surface particle motion were successfully obtained. However, it is difficult to clarify the detailed mechanisms driving such flow patterns from experimental data alone. Therefore, we also carried out numerical simulations based on the discrete element method (DEM). Because the DEM is a Lagrangian method, its computational time depends on the number of particles, so that simulations were restricted to 2D. In order to compare the simulated results to the experimental ones, the average speeds of particles were obtained. Consequently, it was found that the simulated average speed of particles depends only on the amplitude of the applied vibration. In general, good agreement between simulated and experimental results was obtained, but agreement became poorer under conditions of high frequency and low vibration strength.