| Maize is the largest food crop and the key raw materials for animal husbandry,chemical and energy industries in China.It plays an extremely important role in ensuring national food security and reserving strategic resource.Maize harvesting is a key link in maize production,and the maize grain harvester is an important equipment to realize the whole process mechanization of maize at present.As the"digestive system"of maize grain combine harvester,the final harvest quality of maize is directly affected its performance.The development of cleaning device can effectively improve its screening performance,and the research of screening theory and the development of cleaning device complement each other.It is great significance for new screening mode being explored and new screen structure being designed by enriching and perfecting the screening theory of particle.With the increase of the yield of maize per unit area and the new requirements of high-speed operation of combine harvester,the cleaning device with the high-quality and efficient treatment capacity for maize mixture with large feeding mass(8 kg/s)should be possessed.In the previous research,a bionic screen cleaning device in which an earthworm as the bionic prototype was designed by our research team,which the rapid pushing of maize mixture on the screen can be realized,and the cleaning of maize mixture with large feeding mass can be realized.Based on the bionic screen cleaning device,the kinetic characteristics of each plate of the bionic screen are clarified through theoretical analysis and numerical simulation.The motion law of maize grains on each screen plate and the behavior of maize grains penetrating screen holes were explored,respectively.The kinematic model of the particles on the screen and the model of the maize grains penetrating the screen holes were founded.The separation mechanism of maize mixture and the mechanism of maize grains penetrating the screen holes in the screening process of bionic screen were revealed.The screening theory of maize grains was improved.The main contents and conclusions were as follows:(1)Measurement of the physical and mechanical characteristic parameters of maize mixture.The maize variety Demeiya NO.1 with a wide planting area was selected as the research object by a large number of field investigations.The proportion and geometric size of each component material in maize mixture were determined based on the cleaning device of maize grain combine harvester(John Deere S660).The moisture content and density of maize grain,stalk and cob were determined by drying method and Archimedes’law.The static and rolling friction coefficient between the components of maize mixture and the screen plate(Q235)was measured based on the static and dynamic friction coefficient measurement disc.The coefficients of restitution of the components of maize mixture between the maize mixture and screen plate were measured based on the test platform of collision recovery coefficient of unbound particles.Poisson’s ratio and elastic modulus of maize mixture were measured based on universal testing machine and high-speed photography technology.The basic data for subsequent theoretical calculation and simulation can be provided by the above research results.(2)Analysis of kinetic characteristic of bionic screenBased on the analytical method and theoretical analysis,the motion equations of the bionic screen are obtained.The variation laws of the inclination,displacement,velocity and acceleration of each screen plate with time were obtained using MATLAB to simulate the motion of the bionic screen.The results show that the motion of each screen plate is nonlinear,and the amplitude of each screen plate is different.The velocity that the bionic screen move concavely downward is higher than that flatten upward.The average motion velocity of the front screen plate is higher than that of the rear screen plate.The acceleration of the front screen plate is relatively large,and there is an obvious sudden mutation in velocity.The acceleration variation of the rear screen plate is stable within the range of 0.36 m/s~2～0.53m/s~2 in a motion period.The research results can lay a foundation for the follow-up study of the motions of particles on screen and the particles penetrating the screen holes.(3)Theoretical analysis of particle motion on bionic sieveBased on the kinetic characteristics of the bionic screen,the dynamics model for particles on each screen plate was founded.Based on this model,the influence of the motion of each screen plate on the motion of particles on the screen is qualitatively analyzed.When the screen plate moves downward and is inclined downward,and the screen plate moves upward and is inclined upward,the force on the particles on the screen is gradually increased along the horizontal direction of the screen,which is helpful for the materials on the screen being pushed.When the screen plate moves downward and is inclined upward,and the screen plate flattens upward and is inclined downward,the component of the forces on the particles on the screen along the horizontal backward direction of the screen is gradually decreased,which the migration velocity of materials on the screen can be appropriately reduced.The resultant force of the particles on the front bionic screen is greatly changed in the normal direction,and the resultant force of the particles on the rear bionic screen is slightly changed in the normal direction.(4)Study on the model for particles penetrating the bionic screenThe kinematic model in the process of particles penetrating the screen holes was founded.The relative motion between particles and screen holes was analyzed.The relative motion conditions between the particles and the screen plates during the particles penetrating the screen were clarified,which the particle should move vertically downward relative to the screen plate,and the relative displacement between the particle and the screen plate in the horizontal direction should not exceed the diameter of screen hole.The model of particles penetrating the screen holes was founded,and the behavior of particles penetrating the screen holes was divided into three forms with jumping,rolling,and sliding.A probability model for particles with different behavior penetrating the bionic sieve was founded based on the probability statistics.The influence of the motion of bionic screen on the probability of particles with different behavior penetrating the screen holes was analyzed.(5)Numerical simulation of gas-solid two-phase motion in cleaning device of the bionic screenThe motion equations of bionic screen were imported into EDEM through API interface program to realize the non-harmonic motion of bionic screen(two translations and one rotation).And the numerical simulation of gas-solid two-phase motion in the cleaning device of the bionic screen was realized by coupling with FLUENT.The materials on the screen can be effectively layered through the continuous concave of the bionic screen.In this process,the falling velocity of maize grains is higher than that of impurities.The contact forces between maize grains and impurities are one order of magnitude smaller than that between maize grains and maize grains,and impurities and impurities.It indicates that there is a good stratification between the maize grains and the impurities,and the materials on the screen from the screen plate 3 to the tail of the screen maintain a good stratification and migrate backward.The difference of the kinetic characteristics of particles on plane screen and bionic screen was analyzed.The results show that the maize grains penetrate the bionic sieve was mainly in sliding and rolling states,and the maize grains were easier to penetrate the bionic screen,and its effective probability of maize grains penetrating the screen holes was higher.the motion of maize grains on the middle of the bionic screen is mainly in sliding and rolling states,and the rate of maize grains penetrating the screen holes in this area is higher.It is conducive to the maize grains penetrating the screen holes when the bionic screen plates flatten upward.(6)Single factor and multi factor tests and their results analysesThe single factor test was carried out by taking the concave depth of the bionic screen surface,the installation angle of the bionic screen,the rotation velocity of the cam,the airflow velocity at the inlet of the cleaning device of bionic screen,and the direction angle of airflow as the test factors.The effects of these factors on the stratification coefficient of materials on the bionic screen cleaning device and the rate of particles penetrating the screen holes were investigated.The results show that the effects of the direction angle of airflow on the stratification coefficient of materials on the bionic screen cleaning device and the rate of particles penetrating the screen holes were not significant,and the effects of the other four factors on the stratification coefficient of materials on the bionic screen cleaning device and the rate of particles penetrating the screen holes were significant.With the increase of the concave depth of the bionic screen surface,the stratification coefficient of the materials on the screen is gradually increased,while the rate of maize grains penetrating the screen is increased first and then decreased.With the increase of the installation angle of the bionic screen surface,the stratification coefficient of the materials on the screen is gradually decreased,while the rate of particles penetrating the screen is increased first and then decreased.With the increase of the rotation velocity of the cam,the stratification coefficient of the materials on the screen and the rate of the particles penetrating the screen is increased first and then decreased gradually.With the increase of the airflow velocity at the inlet of the cleaning device of bionic screen,the stratification coefficient of the materials on the screen is gradually increased,while the rate of the particles penetrating the screen is increased first and then decreased.Based on the results of the single-factor tests,the quadratic orthogonal rotation combination test with four factors and five levels was designed with the concave depth of the bionic screen surface,the installation angle of the bionic screen,the rotation velocity of the cam,and the airflow velocity at the inlet of the cleaning device of bionic screen as the test factors after the insignificant factor was eliminated.The variance,regression and response surface of model and various factors were analyzed,respectively.The optimal combination of parameters was obtained as follows:the concave depth of the bionic screen surface was 44.8 mm,the installation angle of the bionic screen was-3.5°,the rotation velocity of the cam was 108.6 r/min,and the airflow velocity at the inlet of the cleaning device of bionic screen was 12.1 m/s,and under this condition,the stratification coefficient of materials on the bionic screen cleaning device was 3.04 and the rate of particles penetrating the screen holes was97.48%,respectively.At the same time,the simulation test was carried out based on the above parameters.Through the analysis of the test results,it was found that under the conditions of this parameter,the maize grains cleaning rate was 98.96%and the maize grains loss rate was 1.41%,which met the requirements of national standards.(7)Bench test of the cleaning device of bionic screenThe motions of the centroid of the screen plates and the trajectory of maize grains penetrating the screen were tracked and extracted based on high-speed photography.It is found that the bench test results are basically consistent with the theoretical analysis results by comparing with the theoretical analysis results of the motion of bionic screen,and the correctness of the theoretical analysis is verified.The bench test results show that the proportion of particles penetrate the screen in bouncing,rolling,and sliding states is 22.5%,36.3%,and 41.2%,respectively,and the deviation from the simulation test results is within the range of 0～0.7%,which the accuracy of the simulation results is verified.The probability of maize grains penetrating the different screen plates was counted and the behavior of maize grains penetrating the screen holes was analyzed.The probability of particles penetrating the middle screen is 52.8%higher than that the front screen.The particles that penetrate the middle screen are mainly in sliding or rolling states.And the screening theory of bionic screen is revealed.Under the conditions that the feeding mass of maize mixture is 8 kg/s,and the concave depth of bionic screen surface,the installation angle of bionic screen,the rotating velocity of cam,the airflow velocity and the direction angle of airflow are 45 mm,-3.5°,110 r/min,12 m/s and 25°,respectively,the maize grains loss rate and the cleaning rate obtained from the bench test are 1.43%and 98.79%,respectively,which met the requirements of national standards. |
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