| Flax,also called oil seed flax,is one of the important oil crops and cash crops in Northwest and North China.Gansu province is one of the main producing areas of flax in China.In 2019,the cultivated area of flax accounted for about 30% of the total cultivated area in China.Currently,flax is generally harvested in stages,and mechanized combine harvesting technology of flax and its equipment are still in testing design and development.The separation and cleaning of flax threshing material is one of the key links in its harvesting process.Flax threshing materials have the characteristics of complex composition and small differences,which make them have the common problems of high impurity content,difficult separation and multiple losses in the separation and cleaning process,which are the key core factors that directly affect the separation impurity rate and clearing loss rate of supporting agricultural machinery and equipment,and seriously restricted the level and quality of mechanized stages harvesting of flax in China.It has seriously restricted the level and quality of mechanized stages harvesting of flax in our country.Therefore,it is urgent to study and clarify the mechanism of separating and cleaning the flax threshing materials,and to create the supporting equipment of high performance mechanized separating and cleaning.To this end,the physical and mechanical characteristics of flax and its threshing materials were studied,and the suspension velocity of each group of flax threshing materials was determined.Aimed at the flax threshing materials of different types,the Air-flow flax threshing material separation and cleaning machine and double duct system of air-screen flax threshing material separating and cleaning machine were developed respectively.The performance test and parameter optimization of the prototype were carried out,and the numerical simulation was carried out on the different separating-cleaning process of flax threshing material based on CFD-DEM method.The mechanism of separating-cleaning of flax threshing material by air-flow type and by air-screen type was revealed.To explore the core technology more suitable for mechanized separation-cleaning of flax,it lays a theoretical foundation for improving the performance of mechanized stages harvesting with the links of separating-cleaning of flax threshing material.In this paper,mainly in the following aspects of a more in-depth study:(1)Grasping the physical and mechanical characteristics of flax and its threshing material.It is important to analyze and recognize the character of each component of thresh material in the mature stage of flax.The tensile,bending and shear mechanical properties of different parts(root,middle part and neck part)of the flax stem in mature stage were tested.In order to explore the change law of mechanical parameters of flax capsule under different types of external forces.In this paper,the shape and characteristics of flax threshing material under different operation modes are discussed,and two types of flax threshing material(underthreshing and fully threshing)are defined.The triaxial size of flax seed,the parameters of stacking angle and sliding angle to characterize the flow friction characteristics of flax threshing material were determined.Discrete element models of different components of flax threshing material were established by means of spherical particle polymerization.And the accuracy and reliability of the discrete element models for different components of flax threshing material were verified by numerical simulation of the stacking angle and sliding angle parameters.(2)The suspension velocity of each component of flax threshing material in different suspension stages was determined.The suspension state,distribution interval and corresponding suspension velocity of the four components in flax threshing material were measured and calculated by the suspension velocity measuring device of agricultural materials.Among them,the total suspension velocity of flax seeds was between 4.55 m/s and 8.64 m/s,the overall suspension velocity of the capsule was between 6.46 m/s and 10.90 m/s,the total suspension velocity of the flax short stem was between 3.23 m/s and 7.75 m/s,the overall suspension velocity of the flax capsule shell was between 1.06 m/s and 4.21 m/s.It's going to clear up the migration rule and motion state of the components of flax threshing material under the action of vertical air flow field of the material suspension velocity measuring device.And also development key equipment and provide the basic characteristic parameters for the research on the theory of air-flow and air-screen separating-cleaning of flax threshing material.(3)The mechanism of air-flow separating-cleaning of flax threshing material was expounded,and the air-flow separating-cleaning machine of flax threshing material was developed.Through designing and selecting of key operating components of the prototype,the important working parameters of the vibration feeding system,grain separation device,suction fan and cyclone separator were determined.The operation process of automatic discharging discarded device was analyzed and the necessary work conditions to ensure automatic discharge were obtained,and completed the prototype performance test.the single factor parameters obtained with the analysis were determined by numerical simulation test,the parameters such as feeding amplitude,adjust thickness of material layer and rotate speed of impurity suction fan were selected as independent variables,which would influence the seed impurity rate and the cleaning loss rate.The mathematical model was established,and then the effects of various parameters and their interactions were analyzed as well.The optimal working parameters were feeding amplitude of 16.5 mm,adjust thickness of material layer of 7.0 mm and rotate speed of impurity suction fan of 1775 r/min(that is,the speed conversion frequency of the impurity suction fan is 59.2 Hz).(4)In order to study the motion law and characteristics of components of flax threshing material,the numerical simulation was carried out on the separating cleaning process of flax threshing material based on CFD-DEM method.And the components of flax threshing material were separated and cleaned under the influence of airflow field.Meanwhile,variation curves of quantity and mean velocity of flax seeds in the separating cleaning system were obtained.By referring to streamline distribution of gas-solid coupling,the quantity variation law of components of flax threshing material with time was explored and their motion curves and variation tendency of average velocity were studied.(5)The mechanism of double duct air-screen separating-cleaning of flax threshing material was expounded,and double duct system of air-screen separating-cleaning machine for flax threshing material was developed.Through design selection and parameter calculation of key operating components of the prototype,the important working parameters of the three stage vibrating screen,double duct impurities aggregate device and dust suction device were determined.The working conditions for the parameters of the crank connecting rod of the vibrating screen are obtained with the different components of flax material are in different motion state on three-stage screen surface,and completed the prototype performance test.The single factor parameters obtained with the analysis were determined by practical test method,the parameters such as vibration frequency of screen box,air volume adjustment gear in front duct and air volume adjustment gear in rear duct were selected as independent variables,which would influence the seed impurity rate and the cleaning loss rate.The mathematical model was established,and then the effects of various parameters and their interactions were analyzed as well.The optimal working parameters were vibration frequency of screen box is 2 Hz,the front duct air volume control handle in 2 gear and the rear duct air volume control handle in 4.5 gear.(6)In order to study the motion law and characteristics of components of flax threshing material in winnowing-screening process,by using CFD and EDEM unidirectional coupling principle,the gas-solid coupling simulation model of double duct system of air-screen separating-cleaning machine was established and the numerical simulation was carried out on the separating-cleaning process of flax threshing material.The distribution pattern and migration trajectory of flax threshing material were obtained in the wind screen system,three stage vibrating screen,double duct impurities aggregate device and dust suction device,the quantity variation law of components of flax threshing material with time was explored and their motion curves and variation tendency of average velocity were studied.To sum up,the air-flow separating-cleaning mode and the duct system of air-screen separating-cleaning mode were put forward for flax threshing material with different threshing degrees,and the supporting key operating equipment was developed through in-depth research and disclosure of the operation mechanism of these two modes.The test results showed that the air-flow separating-cleaning mode and its equipment were more suitable for the separating-cleaning of flax threshing material with no complete capsule and longer stalk after full threshing.The double air duct air-screen separating-cleaning mode and its equipment have good adaptability to the flax threshing materials,which can be applied to separating and cleaning the flax threshing materials under different threshing effects. |
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