Experimental Study On Cleaning Kinetic Characteristics Of Buckwheat Crops Harvested By Mechinery

Buckwheat is rich in nutrition,which is an important grain variety to improve the dietary structure,to promote nutrition and health.China is one of the main producing countries of buckwheat.Its planting area and yield are ranked second in the world.Because buckwheat is an infinite inflorescence crop,the maturity of its grains is very inconsistent,the mature grains are easy to fall off and to be damaged and broken during harvest.The characteristics of high moisture content of stems makes the mechanization of buckwheat harvest more difficult.Especially in the cleaning link of combined harvest,the stems with higher moisture content become one of the main components of the mixture of mechanical threshing material.The differences of aerodynamic characteristics,size characteristics between stems and grains are not obvious,leading to the problem of high impurity rate and large loss rate in mechanical harvesting and cleaning of buckwheat.This topic starts from the perspective of mechanical cleaning of buckwheat.The research trends of the cleaning principle and method both at home and abroad,the design and test on cleaning device,the material properties of threshing material were analyzed.Theoretical analys is,structural design and optimization,simulation and bench test were adopted.The theoretical and experimental study on the mechanical threshing material of buckwheat was carried out.Aim to provide theoretical basis for the design and optimization of existing cleaning device and combined harvester for buckwheat,and provide reference and inspiration for the development of new cleaning device.The main research contents of this topic are as follows:1.Study on characteristics of buckwheat mechanical threshing materialThe buckwheat threshing material of ‘Heifeng No.1’ and ‘Hongshan’ harvested by combine harvester(XG788S)was collected.The composition of two kinds of buckwheat mechanical threshing material was analyzed.Measuring devices for dynamic repos angle and collision recovery coefficient of agricultural materials were developed.The characteristic parameters of each component in the threshing mixture were measured and analyzed by means of direct measurement with different instruments and devices.The three dimensional range of grain,branch,stem and light impurity in the threshing mixture of buckwheat was obtained.The data intervals of thousand-grain weight,density,water content,suspension coefficient,dynamic repose angle,elastic modulus and shear modulus of the main components were obtained.The data range of the contact parameters between grains,stems,and their interaction with steel plate,such as recovery coefficient,friction coefficient,was also obtained.The results showed that the main components of buckwheat mechanical threshing material were grains and stems,and the proportion of impurities was up to about 32%.It is difficult to separate grain from stem effectively by air separation a lone.The dynamic repose angle of buckwheat grains was affected by the rotation speed of drum,filling rate and water content of buckwheat grains.The upper,lower repose angles and dynamic repose angle of buckwheat grains had a signif icant linear relationship with water content.The dynamic repose angle is approximately equal to the average of the upper repose angle and the lower repose angle.There is a signif icant linear relationship between the upper repose angle and the lower repose angle.The measured characteristic parameters and conclusions can provide data support and basis for the establishment of relatively accurate discrete element model of buckwheat grain and stem,the numerical simulation of the cleaning process of buckwheat threshing material,and the design of the cleaning device of strong adaptability.2.Construction of discrete element model and virtual calibration of simulation parameters for main components of buckwheat harvested by machineryThe discrete element models of ‘Heifeng No.1’ buckwheat grain/stem were established by 3D model reconstruction.A measuring device based on stacking test for static repose angle of buckwheat grain/stem was designed and trial-manufactured.Modern experimental design methods were used to calibrate disc rete element simulation parameters of buckwheat grain/stem.The cylindrical lifting and rotating drum simulation tests were used to verify the calibration.The results showed that the static friction coefficient of grain-grain,grain-steel plate had significant effects on the static repose angle of buckwheat grains,while the rolling friction coefficient and collision recovery coefficient of stem-stem had significant effect on static repose angle of buckwheat stem.The discrete element simulation parameters of buckwheat grains were as follows: Poisson’s ratio 0.29,shear modulus 34.325 MPa,static friction coefficient 0.482,rolling friction coefficient 0.290.The recovery coefficient is 0.260 between buckwheat and buckwheat.The recovery coefficient,static friction coefficient,rolling friction coefficient between buckwheat and steel plate are 0.500,0.446,0.235,respectively.The static friction coefficient,rolling friction coefficient and the recovery coefficient of buckwheat stem-stem were 0.600,0.460,0.160,respectively,and that of buckwheat stem-plate were 0.500,0.300,0.360,respectively.There is no significant difference of the results between simulation verification test and the physical test for repose angle of buckwheat grain/stem,which indicates that the response surface method is feasible to calibrate the discrete element simulation parameters of buckwheat grains/stems,and the results are reliable.The obtained discrete element model and simulation parameters of buckwheat grain/stem can lay a foundation for the subsequent simulation experiment of cleaning of buckwheat mechanical threshing mixture3.Experimental study on the cleaning of buckwheat mechanical threshing mixtureThe buckwheat mechanical threshing material of ’Hongshan’ was the test material.Single factor tests of crank length,crank speed,inclination angle of upper screen surface and lower screen surface,swing angle of screen surface,wind direction angle of fan and fan speed were carried out on the air-screen grain cleaning test stand.According to the results of single factor test,orthogonal test was carried out with 3levels of these seven factors.The range,variance and regression analysis of the results were carried out.The regression models of impurity rate,loss rate and cleaning time were established,and the working parameters of buckwheat cleaning process were optimized by multi-objective genetic algorithm.The results showed that the effects of various factors on cleaning loss rate,grain impurity rate and cleanin g time were different.The optimal solution set of buckwheat cleaning operation parameters was as follows.The crank length is 30 mm,the crank speed and the fan speed are 231 r/min,600 r/min.The wind direction angle of the fan,the inclination angle of the upper and lower screen surface,and the vibration direction angle of the screen surface were 30°,-3.8°,-1°,5°,respectively.The loss rate,impurity rate and cleaning time were2.80%,3.09% and 7.98 s,respectively.The results of verification test conducted under the optimal working parameters showed that the values of each evaluation index were 2.89%,3.21%,8.09 s,The relative error with the theoretical value was less than 4%,and the optimization results were highly consistent with the results of range variance analysis,indicating that it is feasible to use genetic algorithm to optimize the cleaning regression model,and the optimization results are reliable.The conclusion can provide scientif ic basis for the selection of working parameters,either than the design and optimization of traditional buckwheat cleaning equipment.4.Experimental study on improvement of grain cleaning test bed and optimization of cleaning performanceAccording to the above cleaning test,the feeding system of grain cleaning test bed was improved.Under the conditions of feeding amount 0.5 kg/s,inclination angle of upper screen surface-3.8° and inclination angle of lower screen surface-1°,the material distribution test under different conditions of fan speed,wind angle of fan,crank speed and crank length was carried out with buckwheat mechanical threshing material of ’Hongshan’.The distribution rules of grain,residual weight,total weight of materials and impurity rate of grain under sieve were obtained.According to the distribution law,a third layer of screen surface and a cross-flow fan were added to the grain cleaning test bench,and the verification test was carried out under the optimal parameters obtained from the cleaning test.The results showed that the feeding system consisting of double feeding roller and step shaking plate with forced tiling function could feed buckwheat material along the full screen width,improving the utilization rate of sieve surface and the cleaning operation environment.Adding the third layer of sieve surface and cross flow fan can effectively improve the uniformity of material distribution under the sieve and the cleanliness rate of grain under the sieve.The conclusion can provide reference for the improvement design of the air-screen buckwheat cleaning device.5.Study on design and cleaning test of non-planar sieve surfaceIn view of the characteristics of easy accumulation and blockage of the buckwheat mechanical threshing mixture in the cleaning process,based on the theory of loose and stratified material on the sieve,the non-planar convex-column sieve,concave pit sieve and wave sieve were innovatively designed.Discrete element method(DEM)and Box-Behnken exper imental design pr inciple were used to carry out the screening simulation test and the bench cleaning test of ‘Heifeng No.1’ buckwheat mechanical threshing material.Combined with the experimental phenomenon and results,the operation mechanism of non-planar convex-column sieve surface was analyzed.The mathematical models of convex-column ratio,convex-column height,vibrating screen amplitude,frequency and cleaning quality index were established,and the factors and their interaction were analyzed.The regression equation of performance index was optimized with mu ltiple objectives,and the optimal combination of working parameters for cleaning effect of convex-column screen surface was obtained and verif ied by exper iments.The results show that,the screening effect of non-planar sieve surface(except wave screen)was obviously better than that of planar sieve under the same motion parameters,and the screening effect of convex-column sieve was better than that of concave screen.The convex-column ratio,convex-column height of convex-column sieve had signif icant influence on screening performance.When the convex-column height was 13~33mm and the convex column ratio was 1.0%~2.5%,the material on the sieve was loose and stratified obviously,and the screening effect was better.The results of response surface test s howed that the effects of different factors on the loss rate,impurity rate and cleaning time were different.The optimal working parameters of non-planar convex-column sieve surface are as follows: convex-column ratio 1.616%,convex-column height 21.199 mm,crank length 23.980 mm,and crank speed 210.581 r/min,and the cleaning loss rate,grain impurity rate,cleaning time were 1.974%,3.099%,7.262 s,respectively.The results showed that the non-planar convex-column sieve surface could effectively alleviate the accumulation and pasting of the buckwheat mechanical threshing mixture in the process of cleaning,improving the grain sieve environment and the cleaning quality.The conclusion can provide the basis for the optimization of traditional buckwheat cleaning device and the development of new buckwheat cleaning device.