Numerical Investigation Of The Flow And Mechanical Behavior Of Granular Material In Moving Bed

As a typical reactor type and commonly used transport equipment,moving bed has been widely adopted in various process industries.particles inside moving bed are in close-packing and slow-moving state.Due to the its discrete nature and the frictional interaction between its component particles,inside granular assembly the spatial distribution of internal stress is generally non-uniform and anisotropic.Such characteristics might lead to the non-uniform or unstable flow of granular material during the discharge process,and subsequently the abnormities of transfer,reaction and mechanical properties.Thus,it is valuable and useful to investigate the complex flow behavior of granular materials inside moving bed.This research work is triggered by the requirement of design and scaleup of moving bed system for coal pyrolysis.Aiming at the fundamental issues related to granular discharge from moving bed,such as the transition of flow pattern,velocity fluctuation,et al.,discrete element method(DEM)simulations combined with physical experiments were conducted.The main findings of this work are as flow:(1)Flow pattern.Both experimental and simulation results demonstrate that there is a critical outlet size,larger than which the characteristic width of flow zone sharply increases with the outlet size.With the increase of bed height,the flow pattern will shift from funnel flow to semi-mass flow.The value of the critical height of flow pattern transition depends on the measurement method.The critical height obtained based on the flow pattern at the initial discharge state is clearly larger than that identified through monitoring the flow pattern transition during the whole discharge process.Such result indicate that the discharge behavior of granular assembly from moving bed is closely related to the discharge history.(2)Particle velocity fluctuation.For both funnel flow pattern and semi-mass flow pattern,particles inside the flowing zone present notable velocity fluctuation.The analysis results of discrete Fourier transform show that the particle velocity fluctuation has a characteristic frequency.Such result suggest that the fluctuation of particle velocity is not random.Two parameter correlation analyses indicate that velocity fluctuation is closely tied to the fluctuation of force.And there exist time delay between them.The analyses of delayed correlation between velocities indicate that velocity fluctuation origins from the bed bottom.Referring to results published in the literature,we infer that velocity fluctuation comes from the frequent buildup and breakage of force chain right above the outlet.(3)The influence of internal.Simulation results of moving bed installed with splayed-shape internal demonstrate that the installation of internal has significant influence on the flow rate of particles and the characteristic width of flow zone.There exist an optimal installation position and the distance between the fins,under which conditions the flow rate is clearly larger that of moving bed without internal.For moving bed with internal,the characteristic width of flow zone is approximately equal to the sum of the characteristic flow zone with of moving bed without internal and the projected size of internal along the flow direction.