Research On The Key Factors Of Grain Particle Breakage In Pneumatic Conveying

As a transportation method with high efficiency,flexible technological arrangement,airtight,environmental protection,safety and reliability,pneumatic conveying is very beneficial to the processing of grain and other agricultural granular materials.At present,it plays a key connecting role and is widely used in all aspects of grain storage,logistics,production and processing.Besides its unique advantages,if it wants to achieve high conveying capacity,Often accompanied by high particle breakage rate and power consumption and other inevitable problems.Therefore,this paper takes corn grains with high breakage rate in the pneumatic conveying process as the research object,combines the dynamic finite element analysis of inclined collision process,computational fluid dynamics,discrete element coupled numerical calculation(CFD-DEM)and the pneumatic conveying experiment of corn grains,and aims at reducing the breakage rate of corn grains in the pneumatic conveying process and reducing energy consumption.The motion distribution of corn grains in the conveying pipe,especially at the bend,and the system performance of the conveying flow field were studied to provide theoretical and experimental basis for preventing crushing and reducing energy consumption in the pneumatic conveying process of grain grains.Firstly,the density and three-dimensional size of corn grains were measured,and corn grains with different water content were prepared to study the changes of static friction coefficient,compression characteristics and collision recovery coefficient of corn grains with different water content.The static friction coefficient test showed that the static friction coefficient between corn grains and wall and between corn grains increased with the increase of water content,and the static friction coefficient between corn grains was greater than the static friction coefficient between corn grains and wall.It was found in the study of compression characteristics that the apparent contact elastic modulus,crushing force,crushing energy,compressive strength,yield force and yield limit of corn grains decreased with the increase of water content in the range of 10.0%-14.0%,while the crushing strain increased with the increase of water content.It is found that the impact Angle,initial velocity and water content all have great influence on the normal recovery coefficient,but little influence or no correlation on the tangential recovery coefficient.Based on the hypothetical wall method,the modified motion equation of particle and wall collision process was obtained.The simplified contact dynamics equation of particle and wall oblique collision was obtained by Hertz contact theory and soft ball contact model.Based on the above model,the finite element method was used to study the effects of particle size,normal velocity,tangential velocity and water content on the collision characteristics of corn grains.The results showed that the increase of the above four factors would lead to the increase of the maximum local equivalent plastic strain and maximum impact internal energy,while the tangential velocity had little effect on the two collision characteristics.It is found that the two collision characteristics change in the same trend,energy as a collision characteristic has better applicability to discrete element simulation with a large number of particles.At the same time,the distribution and law of the impact response characteristics under different impact directions and attitudes are analyzed.It is found that the local equivalent plastic strain distribution is different under different impact directions and attitudes.The discrete element polyhedron model of corn grains was reconstructed,and the particle motion process and the bidirectional influence law of flow field distribution of corn grains in the combined curved pipe were analyzed by CFD-DEM coupled numerical simulation method.The influence law of the two conveying performance indexes of corn grain breakage rate and pipeline pressure drop in the conveying process were studied by the material-to-gas ratio,conveying wind speed,water content,bend-to-diameter ratio,particle size.The results show that the local turbulence is affected by the particle motion distribution,and the local turbulence will lead to irregular changes of pipeline pressure drop and wind speed.It is found that the breakage rate is positively correlated with particle size and conveying wind speed,and negatively correlated with bend-to-diameter ratio,water content and material-to-gas ratio.Among all the factors,wind speed,bend-to-diameter ratio and water content have a greater impact on the breakage rate.It is found that the pipeline pressure drop is positively correlated with particle size,conveying wind speed,material-to-gas ratio and bend-to-diameter ratio,but it first decreases and then increases with the increase of water content in a quadratic function relationship.Among all the factors,wind speed and material-to-gas ratio have a greater impact on the breakage rate.This paper designs and develops an air conveying test platform for grain particles,and takes wind speed,solid-to-gas ratio and water content,which are the most important factors influencing conveying performance,obtained from simulation test and takes as the influencing factors by the simulation test,and the breakage rate and pipeline pressure drop were taken as the conveying performance indexes.The regression equation model between each test index and each factor is established respectively.Through multi-objective optimization,the optimal parameter combination is obtained as follows: the corresponding conveying wind speed is 25m/s,the ratio of fuel to gas is 7.68,the water content is 12.136%,and the corresponding pneumatic conveying index is the breakage rate of 1.126%,the pressure drop of 6.024 k Pa.At this time,the conveying capacity is 0.313kg/s.The reliability of the multi-objective optimization model is verified by experiments on the optimal parameter combination.