Study On Discharging Flow Characteristics Of Silos Based On DEM

During the silo discharging,the complex flow characteristics of bulk materials often cause the vibration,wear and even collapse of the silo in production practice.Thus,a deep understanding about the discharging flow characteristics of bulk materials in the silo is helpful to the safety and stability of the discharging production.In the study of particle flow characteristics,mass flow and funnel flow have been widely concerned as two main discharging flows.In mass flow,the entire material is in motion and moves down with a certain velocity of the same order,but the discharge dynamic pressure is large,and the wear of the silo wall is more serious.While in funnel flow,the particles near the wall flow slower than that in the silo center,and the silo wall is less wear,but can easily cause unstable discharge flow.There exists the transition from mass flow to funnel flow during the discharging.And it is affected by both the silo geometry parameters and particles characteristics.During the discharging,the position of transition and the effect of parameters on the flow pattern transition are still not clear.Therefore,to clarify the flow pattern transition of discharging is helpful to optimize the design of conical silos and control the discharging performance.In this paper,the discharging of ellipsoidal particles in conical silos was simulated.The macroscopic flow pattern and the microscopic flow characteristics including velocity,porosity and orientation of ellipsoidal particles were investigated which aiming at clarifying the relationship between them.Meanwhile,the effects of silo geometry parameters and friction coefficients on the discharging flow characteristics were also discussed.The main conclusions are as follows:(1)Macroscopic discharging flow of ellipsoid particles in conical silos can be divided into two stages: at the initial of discharging,the upper part of the silo is mass flow,and the lower part is funnel flow.With the discharging,when the top layer of material begins to form a “funnel-shaped” type,the discharging flow enters the full funnel flow stage.(2)The distribution of the velocity,porosity,and orientation of ellipsoidal particles during discharging has significant differences,and the critical height of the flow pattern transition corresponds to the initial position of the difference in the distribution of microscopic flow characteristics.(3)The silo geometry parameters have a significant effect on the discharge rate and flow pattern.Increasing the outlet size or decreasing the hopper half angle leads to the increase in the discharge rate and its standard deviation which reduce the stability of discharging.The silo diameter has no significant effect on both the discharge rate and the stability of discharging.Increasing the hopper half angle,silo diameter and reducing the outlet size will cause the increase in the porosity and the difference of axial velocity distribution,and the final result is an increase in the critical height of flow pattern transition.(4)Different friction characteristics have different effects on the discharge rate and flow pattern.The particle-wall friction coefficient has no significant influence on the discharge rate,increasing the particle-wall friction coefficient,the discharge rate remains unchanged after a slight increase.The particle-particle friction coefficient has a significant influence on the discharge rate,increasing the particle-particle friction coefficient,the discharge rate increases significantly.However,both of them have no significant effect on the stability of discharging.The increase in the particle-wall friction coefficient or the particle-particle friction coefficient will result in an increase in the porosity among particles,which further affect the distribution of axial velocity,and the final result is an increase in the critical height of flow pattern transition.(5)Based on the effects of various parameters on the critical height,the critical height as a function of silo geometry parameters or friction coefficients can be estimated using the method of polynomial fitting,which can provide guidelines to the appropriate design of conical silos.