Analysis Of Dynamic Lateral Pressure And Influencing Factors During Unloading Of Grain Bulk In Vertical Silos

Food security is related to national security,social stability and economic development,and the granary is an important facility to ensure the security of national grain reserves.Grain vertical silo is a special cylindrical structure,which has many advantages such as small footprint,large silo capacity,and can meet the needs of transit and storage.The dynamic side pressure of the grain vertical silo during unloading is an important factor to be considered in the design of the silo body.The value of the dynamic side pressure is directly related to the safety of the vertical silo during unloading.In the actual unloading engineering,there will be different flow velocity unloading conditions and layered unloading conditions.The dynamic arch formed in the unloading process has the characteristics of continuous formation and collapse,which is closely related to the formation of dynamic lateral pressure and the flow state of storage materials.Because the density,internal friction angle and friction coefficient of different storage materials are different,they all have an impact on the lateral pressure of the silo wall,and a unified conclusion has not been formed at present.Therefore,it is necessary to further study the influence factors of dynamic lateral pressure of silo discharge under different types of storage materials.This paper takes the grain vertical silo as the research object,uses the geometric scale of 1:25 to design the vertical silo model,conducts the discharge test of the vertical silo model,and combines the discrete element simulation software EDEM to numerically simulate the discharge of the grain vertical silo.The research contents and conclusions are as follows:(1)Considering the actual unloading conditions,two conditions of one-time unloading and stratified unloading were carried out respectively.The dynamic side pressure distribution law and the corresponding overpressure coefficient during the material discharge,as well as the flow state of the stored material during the discharge process.The maximum value of static side pressure appears in the range of the connection between the bin wall and the funnel,and its distribution value is greater than the value calculated by the standard formula;During one-time discharge,under different discharge flow rates,the overpressure coefficients in the lower 2/3h_nrange of the silo body are obviously different,and the overpressure coefficients of fast flow rate and slow flow rate are increased respectively;One-time unloading and layered unloading When unloading for the first time,the overpressure coefficient in the lower 1/3h_nrange of the silo body is the largest and exceeds2.0.The current standard horizontal side pressure correction coefficient C_h=2.0 is unsafe;One-time discharge and layered discharge When the first discharge,the dynamic side pressure at the funnel suddenly drops;after stabilization,the static side pressure at the funnel during layered discharge is about 1/2 of the standard calculation value.(2)Based on the discrete element simulation software EDEM,numerical simulation of the unloading of the grain silo was carried out.The size of the silo and the particle model,the particle material properties and the contact parameter settings were consistent with the experimental design.The static side pressure value between the simulation and the test is close,and the dynamic side pressure distribution law is consistent.The simulated discharge a at the depth of 0.8m measuring point is compared with the one-time discharge a of the test,and the simulated discharge b is compared with the test.The overpressure coefficient error of one-time discharge b is 3.4%and 7.6%,the difference is small and within a reasonable error range,The feasibility and rationality of the EDEM discrete element unloading simulation model can be verified by experiments.(3)When the grain silo is unloading,the overall flow index MFI and the visual observation of unloading are used to analyze the flow state of the stored material in the silo;combined with the contact force between particles,the relationship between dynamic lateral pressure and arching mechanism was analyzed.The central velocity of the stored material is greater than the velocity of the silo walls on both sides,and with the progress of unloading,the vertical average velocity distribution at different heights from top to bottom gradually forms a"funnel shape";The flow state exhibited by the storage material is related to the position of the silo,and the development process is overall flow-tubular flow-funnel flow;When the silo is unloaded,the arching mechanism is related to the instantaneous increase of the pressure on the side of the silo wall.By observing the distribution of the contact force between the particles,a tubular flow state appears in the lower 1/3h_nrange of the silo wall,and an arching area appears.The impact on the wall of the silo is large,which causes the instantaneous oscillation of the lateral pressure to increase.(4)On the basis of the feasibility and rationality of the EDEM discrete element unloading simulation model,the EDEM discrete element numerical simulation of the dynamic side pressure when the vertical silo is unloaded under different storage materials is carried out considering the physical characteristics and contact properties of different materials.Analyze the static side pressure of the silo wall,the dynamic side pressure and the corresponding overpressure coefficient generated by the stored material particles on the silo silo wall during the discharge process,and analyze the flow state and velocity distribution of the stored material.Based on the grey correlation analysis method,it is concluded that when different types of storage materials are discharged,the influencing factors affecting the static and dynamic side pressure at the same time have a correlation degree of more than 0.8,such as density,storage material and silo structure static friction coefficient,storage material and silo Silo structure collision recovery coefficient;The current silo specification lacks the mention and analysis of the collision recovery coefficient of the storage material and the silo structure.When designing and calculating the silo,the impact of the collision recovery characteristics of the storage material and the structure can be considered.