Research On Briquetting Mechanism Of Extruding Corn Stalk Powder Based On Discrete Element Method

Every year, a large number of corn stalk and some other biomass resources are produced in the fields. Owing to the disadvantages of biomass resources, such as their low density, unconventional transportation and high transportation cost, most of the stalk resources are being wasted or burned on the spot mainly because of these existing difficulties. The compression molding of straw resources is badly imminent.Currently, it is difficult to generalize straw briquetting machine or granulator in a large scale, due to the briquetting mechanism of corn stalk powder is not definitely understood. In response to this situation, the mechanical behavior and the briquetting mechanism of corn stalk powder were studied, in order to further deepen the understanding of the molding process and provide a theoretical basis for the design of new kinds of structures and molds.For studying the microcosmic mechanical behavior and the briquetting mechanism of corn stalk powder in single-hole die under the uni-axial compression process, the discrete element method, which is suitable for the numerical simulation of the discrete particle materials, is choose to simulate the forming process after analyzing this process. This paper is mainly including the following aspects:(1) Theoretical study and model building of the discrete element method. This method’s basic solution thoughts, contact theory between particles, and the simplified soft-sphere contact model were studied. The three-dimensional particle simulated model of corn stalk powder, which is based on the soft-sphere contact model of discrete element method, was established. And the contact walls of the discrete element method model were completely consistent with the extrusion cavity boundary conditions in geometric shape and dimension of tests, and the loading speed in the simulated model was also set to the same value.(2) Particular mechanical parameters by artificial neural networks. The initial numerical ranges of the mechanical parameters, such as the normal stiffness (kn), shear stiffness (ks) and friction coefficient (μ) between the two contact simulated particles in the discrete element method model, were obtained by debugging. The Compressive force could fluctuate widely, when the mechanical parameters were set to the arbitrary values in these numerical ranges. Therefore, the artificial neural networks, which would be used to particular mechanical parameters, were established by 60 group data of the simulated DEM model and trained.(3) Verified tests and application of the discrete element method. The compressive and relaxed force data between actual tests and DEM simulation and the validity of the simulation was verified by the hypothesis test. The consistency of the two kinds of data was fairly good on the whole, and there’s little statistical significance at 5% level in the significant difference analysis. The optimal numerical ranges of the mechanical parameters of simulated particles were obtained, the normal stiffness is (1.2～1.8)×104N/m, the shear stiffness is (0.8～1.3)×104 N/m and the friction coefficient is 0.10～0.12.(4) Application of the discrete element method. The compressive and relaxed force curves at different compressive velocities, compressive displacements, diameters of cavity and cone angles were studied. The influence of compressive velocity can be ignored, and in order to increase the stabilization of compressed straw stalk, the cavity’s diameter of 8 mm and the the cone angle of 45° is suggested to be the best parameter values, and the compressive displacement should be increased appropriately.(5) Motion and force analyses of random particles and contact walls. It was conducted that, when the loading surface stopped, the stalk powder particles in the system didn’t stop immediately, stress relaxation behavior occurred until it reached equilibrium stress. The end of compression bar and cone angle sustained larger force than other faces in the forming process, and its material properties should be improved.The study of the compression process of corn stalk powder based on artificial neural networks and discrete element method showed that the discrete element method is an efficient and effective method to study the briquetting mechanism and analyze mechanical parameters.