关键词: 离散单元法, 移动床, 卸料流型, 速度脉动

Abstract: Moving beds are ubiquitous in various process industries. Thoroughly understanding and accurately characterizing the complex flow behavior of granular materials from the component particle scale is obviously of great significance for the design, scale-up, and optimization of moving beds. In this work, the flow behavior of granular assemblies in moving beds under both the funnel flow and semi-mass flow discharge regimes are investigated by performing three-dimensional discrete element method (DEM) simulations, with a focus on the possible similarities and differences between the fluctuating characteristics and also the corresponding underlying mechanisms of the transient motions of particles under these two discharging conditions. The reliability of the DEM simulation is verified by comparing the predicted evolutions of the boundary of the flowing zone and also its characteristic width with experimental results. The simulation results demonstrate that under both discharge conditions, the temporal variations of the spatially averaged axial velocity of particles in the upper part of the flowing zone present notable non-random fluctuating characteristics, manifested by the appearance of a clear peak in the Fourier spectrum of the time series of spatially averaged particle axial velocity. The spatial correlation analysis results show that the temporal fluctuations of the spatially averaged axial velocity of particles in different axial regions of the upper part of the flowing zone are closely correlated, suggesting the occurrences of resonance under both discharging types. The delayed correlation analyses of the time series of the spatially averaged axial velocities of particles in different axial zones indicate that such resonant behavior originates from a bottom zone right above the outlet. The delayed correlation analyses of the time series of the spatially averaged particle axial velocity and the spatially averaged particle contact force demonstrate that there exists a strong correlation between the temporal fluctuations of these two parameters, and the latter precedes the former, which hints that the observed resonance could be ascribed to the free-fall arch mechanism. In brief, the presented simulation results clearly demonstrate that resonance can occur during both funnel and semi-mass flow discharges and there is no intrinsic difference between the resonant features of particles under these two discharging conditions.

Key words: discrete element method, moving-bed, flow pattern, velocity fluctuation