Simulation Analysis And Experimental Study Of The Screening Porcess Of A Swing-bar Sieve Based On The Coupling Of DEM With MBK

Vibrating screen is an important device for the separation of granular materials according to their sizes.In agricultural production,the widely used vibrating screens are swing-bar screens.The motion of the screen deck in the swing-bar screen is a complex planar motion.In the past,the screening process was studied by means of performing experiments.Conducting experiments is time-and labor-consuming,and it is hard to get the kinetic information of particles on the screen.Due to the lack of a better understanding of the particle flow and its passing behaviour on the swing-bar screen,the design and optimisation of this type of screen is extremely limited.In this paper,a coupled model of DEM(Discrete Element Method)with MBK(Multi-Body Kinematics)is proposed.The screening process of soybean particles on an LA-LK laboratory scale swing-bar screen is simulated using this coupled model.The parameters used in the simulations are determined mainly based on the material properties of soybean kernel and the interaction properties between soybean kernel and other materials.The coupled model is verified by the good agreement between the simulated results and the experimental results in terms of the distribution of the percentage passing along the screen for particles in different size-ranges.The operating and structural parameters of the swing-bar screen are optimized using the quadratic orthogonal rotation combination design.The effects of the crank length,incline angle,aperture size,crank speed,and feeding rate on the screening performance are studied.The simulated results are analysed in terms of the particle bed volume,averaged particle velocity parallel to the screen deck,and the particle-particle collision.These analysis data contribute to the better understanding of the motions of particles and their passing behaviour on the swing-bar screen,and this work provides a reference for the design and optimisation of the vibration screens with a complex motion.The following conclusions can be drawn from this study.(1)The material properties of soyben kernel(i.e.,particle shape,particle size,moisture content,particle density,stiffness,modulus of elasticity)and the interaction properties between soybean kernel and other materials(i.e.,coefficient of restitution between soybeans and between soybean and organic glass,static and rolling frictional coefficients between soybean and organic glass)for five varieties of soybean kernels are determined and analyzed.(2)A modified algorithm is proposed to calculate the radius of curvature of a soybean particle.The calculated results from this modified algorithm are compared to those reported in the literature.It can be found that the modified algorithm gives a more realistic radius of curvature of a soybean particle at the contact point than other methods.(3)A modelling method of soybean particles is proposed in this work.1 An algorithm is developed for the filling of spheres that are internally tangential to an ellipsoid.The particle model of soybean kernels is built based on the Multi-Sphere(MS)method.The index named filling error is defined.The particle model of soybean kernels is then optimized by a developed algorithm to obtain a new particle model with less filling error.2 The models of a single soybean particle and the models of a soybean particle assembly for five varieties of soybean kernels are built based on the abovementioned method.The DEM simulations of the pilling process for these five varieties of soybean kernels in a “sandpile” formation box are performed.The modelling methods of soybean particles are verified by the comparison of the simulated results with the experimental results,and the appreciate numbers of spheres in a particle model for these five varieties of soybean kernels are obtained.(4)The screening process of soybean kernels on an LA-LK laboratory scale swing-bar screen is studied by conducting experiments.The effects of the aperture shape,aperture configuration,the radio of the screen length to its width,and the crank speed on the screening performance are analysed.The results are as follows:1 The phenomenon of particle accumulation is more likely to occur on the circular aperture screen than on the rectangular aperture screen with the same aperture size;2 The composition of the soybean kernels retained on the screen after screening is analysed.It shows that the crank speed has little effect on the composition of the soybean kernels retained on the screen.The particles in the size-ranges of 6.85 mm ~ 7.00 mm and 7.00 mm ~ 7.30 mm are the ones pegging into the apertures;3 The partition curve is used to analyze the overall screening performance.It demonstrates that the crank speed has little effect on the partition curve.The ratio of the screen length to its width has a significant effect on the partition curve;4 The distributions of the percentage passing along the screen deck for particles in different size-ranges are analysed.For the circular aperture screen,with the decrease of the size of soybean particles in underflow on the first two parts of the deck,the percentage passing increases.With the increase of the crank speed,there is a decreasing tendency of the percentage passing on the first part of the deck,while there is an increasing tendency of the percentage passing on the third part of the deck.For the rectangular aperture screen,the distribution of the percentage passing is the same to the circular aperture screen.The difference between them is the composition of the underflow.The rectangle aperture screen has a strong passing performance for the particles in the size-ranges of 7.30 mm ~ 7.60 mm and 7.00 mm ~ 7.30 mm.In addition,the effect of the ratio of the screen length to its width on the distribution of the percentage passing is also analysed.It shows that the ratio of the screen length to its width only affect the percentage passing after the positions where the apertures are perforated,and it has little effect on the percentage passing before the positions where the apertures are perforated.(5)A coupled model of DEM with MBK is proposed.A CAD-model-based planar multi-body kinematics modelling method is proposed.Combined with the CAD-model-based boundary modelling method,it can be used for the boundary modelling of machines with a complex motion.Based on the idea of coordinate transformation,the formulae used for the calculation of local coordinates of particles in a domain moving with a body is derived,and the criteria judging the positon relationship between a spherical particle and a domain are also given.A modified exact algorithm is proposed to calculate the porosity of spherical particles in a rectangular box.Eight indices are developed for the quantitative analysis of the simulated results of screening process.They are the particle residence time,weight percentage passing,partition number,particle-wall collision,particle-particle collision,particle flow volume,averaged particle velocity along a given direction and porosity,respectively.On the basis of the above work,the software AgriDEM-MBK,which is used for the simulation and analysis of the particle flow on a vibrating screen with complex motions,is developed.(6)The soybean particle flow and its passing behaviour on an LA-LK laboratory scale swing-bar screen is simulated using the AgriDEM-MBK,and the conclusions are as follows:1 The boundary model of the LA-LK laboratory scale swing-bar screen is built using the combined method of the CAD-model-based planar multi-body kinematics modelling method with the CAD-model-based boundary modelling method.2 The planar multi-body kinematics model is verified by the good agreement between the simulated results and the experimental results in terms of the trajectory of a specific point on the sieve-boat.3 The coupled model of DEM with MBK is verified by the good agreement between the simulated results and the experimental results in terms of the particle flow on the screen deck as well as the distributions of the percentage passing for particles in different size-ranges along the screen deck.3 The effects of the crank length,incline angle,aperture size,crank speed,and feeding rate on the screening performance are studied by means of the simulations,and the conclusions are as follow:a)With the increase of feeding rate or the decrease of incline angle or aperture size,the particles are more likely to accumulate on the first part of the screen deck.Moreover,an increase in feed rate,or a decrease in incline angle contributes to an increase in the percentage passing on the first part of the screen deck;b)A decrease in crank length or an increase in crank speed leads to an increase in the particle flow volume on the second part of the screen deck;c)The partition curve becomes flat with an increase in feed rate or a decrease in crank length or incline angle.This is helpful for the penetration of the particles smaller than the apertures,but it inhibits the particles larger than the apertures from flowing out.The increase of aperture size can improve the separation performance.The crank speed has little effect on the separation performance.5 The operating and structural parameters of the LA-LK laboratory scale swing-bar screen are optimized using the quadratic orthogonal rotation combination design,and the optimal parameter combination is obtained.