Research On Arching Mechanism Of Silo Unloading Grain Based On Grain Friction Effect

As a convenient granular storage building,silos have been widely used in many industries such as grain,coal and petroleum,chemical and chemical materials and building materials.With the rapid development of the economy,the demand for raw materials of granular materials has increased greatly,and the oversized circular silos with a large diameter and load have appeared.The mechanical properties of the large-sized silos have been enlarged due to various factors,so in recent years,accidents related to unloading grain from silos have occurred frequently.However,there are few studies on the friction and velocity during the unloading grain from silos.The mechanical behavior,intragranular friction and arching mechanism of the silo unloading grain under gravity load are still unclear.The design of silos lacks a reliable and complete theoretical system and solution,using only the design criteria of general silos.Therefore,the design of the silo is flawed,resulting in arching during the unloading of grain from a few silos and causing economic losses and even casualties.Based on this status quo,this paper relies on the National Natural Science Foundation project(51708182)"Study on the dynamic evolution mechanism of silo unloading grain and its effect on overpressure of silo wall" and the basic research project of Henan Provincial Department of Education(16B560002)" multi-factor coupling The study on the mechanism of grain arching and collapse in the warehouse " has carried out a comprehensive and in-depth study on the relationship between the friction characteristics and the arching mechanism during the unloading of the silo.The specific research work consists of the following three parts:(1)The internal friction angle between grain,the friction coefficient of grain grain and wall material are the most basic mechanical parameters for silo arching during unloading grain,and are the main basis for designing and manufacturing technical parameters of grain processing machinery and barn.In this paper,the internal friction angle of wheat with different dust-containing rates and the friction coefficient of wheat and different wall materials at different speeds were determined by an improved instrument.The results show that when the stress is 200 kPa,the internal friction angle of the wheat pile(with dust-containing rate of 1.63%,2.81%,3.68%,4.37%,5.13%)at different speeds are 47.19°~29.39°,58.19°~36.99°and 46.81°~21.21°.Under the same dust-containing rate,the internal friction angle of wheat under normal pressure decreases with the increase of normal pressure.Under the same normal compressive stress,the effect of dust-containing rate on the internal friction angle of wheat is not significant.Under the same dust-containing rate,the friction coefficient between the wall and the wheat from the largest to the smallest are: concrete silo,wooden silo,steel silo.Under the same normal compressive stress,the dust-containing rate is linearly related to the wall friction coefficient.(2)According to the silo in the accident of the unloading grain arching of the Lanxi County Hongding Grain Purchase and Sale Co.,Ltd.of Heilongjiang Province,scale down to make a silo model(made of plexiglass)and simulate the silos of different materials by attaching sandpaper with different friction coefficients on the wall.The dynamic friction values of the three model silos during the unloading process were recorded,photographs were taken,and the flow state was observed.Firstly,the dynamic friction values of the three silos during the unloading process are compared and analyzed,and then the frictional force values of the various measuring points on the silo wall are calculated and analyzed.The results show that the condition that causes the maximum probability of silo arching during unloading the grain is in the process of unloading with the dust rate of 4.37% in the concrete silo.(3)In this paper,based on the existing PFC discrete element program,the micro particle simulation dust with larger viscosity coefficient was added to create an improved particle for the defect that the internal friction force of the circular particle in the discrete element simulation was smaller than that in the real grain.The structural mechanics model uses geometric methods to determine the contact between the circular particles and derives the force-displacement relationship between the basic units.Based on typical accident cases and laboratory test results,the established particle structure mechanics model was used to simulate the force-displacement relationship between circular large particles and between small round particles and walls in the dynamic process of unloading grain and arching.The displacement map of shear stress and its stress characteristic curve are basically consistent with the experimental results.Using this model,the dynamic meso-mechanism of unloading and arching silos can be objectively reflected.The paper obtains the internal and external friction parameters of the grain in the dynamic process,establishes the dynamic discrete element model,improves the particle mechanics model,deduces the force-displacement relationship between the basic elements,and analyzed the mesoscopic evolution mechanism and arching conditions of particles during the process of unloading grain arching.