| The mechanized harvest of maize grain is one of the main processes of the comprehensive industry.Maize grain combine harvester is now widely used in maize grain harvesting.Cleaning device is the “digestive system” of maize grain harvester.Many indices include the percentage of loss grains,the percentage of cleaning grains are affected by the performance of cleaning device.To meet the harvesting requirements of high-yield maize and ensure the quality of high-speed operation of maize harvester,the cleaning capacity of the cleaning device for high mass of maize fed should be improved.There would be also a series of problems,include a high percentage of loss grains and a low percentage of cleaning grains.In this paper,the researches of the motion of particles in cleaning device and the design of the sieve at home and abroad were investigated.The interaction mechanism of airflow and motion of sieve on motion of particle in cleaning would be clarified.The mechanism of particle penetrating sieve hole after impact ing on sieve hole would be explored.The traditional layout of single type of sieve hole and structure constraint of the planar sieve would be broken.In this paper,the theoretical analysis,numerical simulation and bench test were combined.The mechanism of cleaning device working and structural optimization of sieve were studied.The main contents of this paper were as follows:(1)Measurement of propertise parameters of maize mixtureDemeiya No.1 maize that was widely planted in farm,which was selected as research object to investigate its physical properties.The geometrical dimensions of maize mixture was measured and the proportion of each component of maize mixture was counted.The area and contour characteristics of maize grain were measured based on the three-dimensional scanning instrument and computer technology.The static friction factor of grains,which ranges from 0.43 to 0.51,was obtained based on a revolving test platform.The coefficient of restitution of grain-wall,which ranges from 0.51 to 0.59,was obtained based on a high-speed camera test.The coefficient of restitution of grain-grain,which ranges from 0.27 to 0.36,was obtained based on a measurement platform of coefficient of restitution of unbounded grain-grain.The results would provide basic data for subsequent theoretical analysis and numerical simulation.(2)Theoretical analysis of motion of particle in cleaning deviceThe kinetic model of particle on the sieve was established based on an air-and-screen cleaning device.The effects of buoyancy on the non-ideal sphere particle and the motion of the sieve on the motion of the particle were analyzed based on Matlab.The define for the centroid line of the particle was proposed.The initial postures of the particle that be t hrown upwards by the sieve were classified based on the angle between the centroid line of the particle and the sieve.The effect of the torque of the particle with different postures on its rotation was investigated.The kinetic model of materials in the airflow after being thrown upwards.The effect of the upwind area of the maize mixture on its vertical and horizontal displacements was discussed.There would be a impact between the particle and sieve hole before it penetrating the sieve hole.The percent age of the particle penetrating the sieve hole after impact on the sieve hole was investigated based on the relative position of particle centroid and impacting point and the motion of the particle after impacting.The forces of particle on the upper of the shell sieve were analyzed.The particles would be diverged as the special space shape of the shell sieve.The kinetic model of particle on the plane sieve was mapped to that of the inclined plane of shell sieve.The kinetic model of particle on inclined plane of shell sieve was established based on the relative motion of the particle and the sieve.The area that the particle penetrating the sieve hole was obtained by numerical simulation,when the acceleration of the sieve were positive and negative.(3)Analysis on numerical simulationof gas-solid two-phase flow in cleaning deviceThe effect of the periodical motion of the sieve on the motion of particle was analyzed.When the sieve undergoes upwards decelerated motion,downwards accelerated motion,downwards decelerated motion,upwards accelerated motion,the corresponding motion form of most particles is sliding,being thrown,falling,sliding.The rotating behavior of the particle after being thrown was explored based on the particles with the classified postures.The motion the particle after impacting on the sieve hole was tracked based on the relative positions between the particle centroid and impacting point.It was beneficial to penetrate the sieve hole,when the particle centroid on the left of the impact point.The correctness of the theoretical model was verified.The percentage of the particle penetrating the sieve hole under different impacting forms between the particle and the sieve hole was counted.The distribution of airflow above the sie ve and in the sieve hole were obtained.The change of airflow in the sieve hole along the longitudinal direction of the sieve was shown a stepped down.The change of airflow above the sieve along the longitudinal direction of the sieve was shown a arched shape.The horizontal and vertical displacement of the maize stalk after being thrown is the largest among materials,followed by the maize cob and the maize grain.The front-middle of the sieve was the suitable region for the particles to penetrate the sieve hole.(4)Design and simulation test of combined and stepped sievesThe divergent effect on the particles in cleaning could be produced by special space shape of the shell sieve.There is a high percentage of particles penetrating sieve hole in round sieve hole.To achieve distributed particle to penetrate sieve hole,the round sieve hole was distributed around shell sieve hole to design the shell-round combined sieve.The performance of the combined sieve,round hole sieve and shell sieve were investigated based on the single factor test.It was proved that the combined hole sieve not only retained the strong pushing ability of the shell sieve but also had the strong penetration ability of the round hole sieve.The percentage of loss grains of th e combined sieve were less 4.79% and 7.95% than that of round hole sieve and shell sieve,respectively,when the mass of maize fed was 5 kg/s.The percentage of loss grains of the combined sieve were less 5.79% and 7.84% than that of round hole sieve and shell sieve,respectively,when the mass of maize fed was 6 kg/s.Traditional plane vibrating sieve was considered.The motion characteristics of different materials in the airflow were discussed.The theory of motion characteristics of different materials in the airflow were applied.Effects of different collision angles between particle and wall on normal and tangential coefficient of restitution of particle were applied to design unique local structure of sieve to achieve temporary stay of particle and successful passage of materials other than particle.The non-planar stepped sieve with stepped buffer zone was designed.Effects of the velocity and angle of airflow,the high of step and vibrating frequency of the sieve on the percentage of loss grains and cleaning grains of the stepped sieve were analyzed based on the single factor tests.The combination of operational parameters of stepped sieve was obtained based on multi-factor test.The percentage of loss grains and cleaning grains of the stepped sieve were 1.69% and 98.8% when the velocity and angle of airflow,the high of step and vibrating frequency of the sieve were 16 m/s,25°,8.36 mm and 4.45 Hz,respectively,which met the performance requirements of maize grain combine harvesters.(5)Bench test of performance of sievesThe trajectories of the particles at different sieving stages were tracked based on the high-speed camera technology.The effect of the motion of the sieve on the motion of particle on the sieve was analyzed.The motion forms of the particle,including sliding,being thrown upwards and falling,were obtained in one cycle of motion of the sieve.The accuracy of theory and numerical simulation analysis were verified.The motion of materials in the stepped buffer zone was tracked based on high-speed camera technology.The stepped buffer zone could be used to temporarily “stagnate” the maize grain and materials other than grain could directly be thrown over the step,which proved the correctness of the design of the step buffer zone.The tests of the performance of combined sieve and stepped sieve were conducted.The percentage of loss grains of the combined sieve was 0.77%,which was less 4.68% and 7.70% than that of round hole sieve and shell sieve,respectively,when the mass of maize fed was 5 kg/s.The changes of the percentage of loss grains and cleaning grains increased and decreased with the increase of mass of maize fed.The percentage of loss grains of the combined sieve was 1.51% that could met the requirements of harvesting maize,when the mass of maize fed was 7 kg/s.The percentage of loss grains and cleaning grains of the stepped sieve were 2.12% and 98.8% when the velocity and angle of airflow,the high of step and vibrating frequency of the sieve were 16 m/s,25°,8.4 mm and 4.45 Hz,respectively.The results of test were consistent with that of simulation. |
PDF Full Text Request