Design And Experiment Of Curved Screen For Cleaning Maize In Harvester

The cleaning device is the “digestive system” of the maize grain harvester.After the maize mixture is entered into the cleaning device,the maize mixtures are collected by the airflow and the vibrating screen,the maize grains are collected into the grain collecting device,and the other impurities are discharged through the outlet,thereby achieving the purpose of cleaning.In this paper,Relying on the National Natural Science Foundation of China(5147509 0),Heilongjiang Science Foundation(E2017004),the National Key Research and Development Program(2018YFD0300103)and the Key Laboratory of Heilongjiang Province for Modern Agricultural Equipment in Northern Cold Areas,based on the drive mechanism of the existing traditional planar reciprocating vibrating screen in the maize air-screen cleaning device,computational fluid dynamics and discrete element method(CFD-DEM)are coupled to compare and analyze the airflow in the cleaning device and the characteristics of flat,convex and concave screening.A sinusoidal braided screen with better screening of different materials in different regions of the screen surface is proposed.The main contents of this paper include: model simplification and numerical simula tion of the cleaning device,establishment of the maize extract model,establishment of the mathematical model of the force on the screen surface,Design of cleaning surface screen,numerical simulation of the force on the screen and optimization of the pa rameters of the cleaning system.The content and conclusions are as follows:(1)Based on the structure of the air-screen cleaning device of maize grain harvester,a simplified model of the cleaning device is established,the mesh is divided according to t he characteristics of the model,the dynamic and static regions in the device are defined.The UDF function is compiled to realize the movement of the screen surface and the update calculation of the moving grid during the numerical simulation.(2)Considering that the existing screen surface is mainly plane screen,and the vertical profile of the screen surface is single,the screen surface can not achieve better screening of different materials in different regions.This paper establishes a mathematical model of the force exerted on the particles on the screen,and determines that the screen surface equation f(x)affects the movement state of the particles on the screen.Therefore,the braided screens with higher screening efficiency and lower grain loss rate are selected as the research objects,and two kinds of braided screens,convex and concave,are proposed.(3)In addition,in order to analyze the screening situation of materials at different positions on the screen surface more carefully,the three screen surfaces,flat,convex and concave,are divided into four regions along the longitudinal direction of the screen surface(labeled I,II,III,IV respectively).Using CFD-DEM coupling technology,the movement of maize stripper in plane,convex and concave braiding screen cleaning devices is simulated respectively.The simulation results show that: For the region I,concave braided screen has the strongest material moving ability,which is higher than plane and convex braided screen.For the region II,convex braided screen has the weakest pushing capacity,which is lower than plane and concave braided screen.For the region III,concave braided screen has the weakest pushing capacity,which is conducive to separation of components of the stripped materi al and continuous grain penetration.For the region IV,convex braiding screen has the strongest pushing capacity,corresponding to the average speed of maize ejecta is 2.82 m/s,which is 20% higher than plane and concave braiding screen,which is conducive to the throwing of surplus and the improvement of grain cleanliness.(4)In order to improve the cleaning performance of the screen surface,improve the moving ability of the screen face in region I,and make the material move quickly and evenly before the screen;take region II and III as the main grain passing region,reduce the moving ability of the screen face in region II and III,increase the residence time of the stripped material in region II and III,and reduce the rate of grain loss;and improve the moving ability of the screen face in region IV.The shape of curve screen is determined with the aim of rapidly discharging the residue from the screen tail and improving the cleanliness rate of the grain.(5)In order to obtain the optimal parameter combination of the cleaning device,the coefficient of screen-shape and the inlet velocity of airflow and the direction angle of airflow were factors,the loss and cleanliness of maize grains collected were indexe s.The regression mathematical models between factors and indexes were multi-objective optimized by using Design-Expert software.The optimum combination of parameters is obtained as follows: the screen coefficient is 32.35 mm,the inlet air velocity is 13.73 m/s,and the flow direction angle is 23.86 degrees.(6)The sinusoidal screen working mechanism test and performance comparison test were carried out on the indoor gantry.The experimental results show that: while the total feeding quantity was 5kg/s and the vibration frequency of the screen was 5.15 Hz.The cleaning rate of maize grains was increased and the rate of maize loss was reduced,which meet the requirements of technical specification for quality evaluation of screening in our country.Considering that the main screening regions of sinusoidal curve screen grains are II and III in the simulation test,this paper uses high-speed camera to track the maize exudates in the cleaning device during the cleaning process.The working mechanism of sinusoidal curve screen has been verified.The experimental results showed that,the state of movement of maize mixture on different screen was more favorable to the performance of the cleaning device.