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The dense molding process of biomass straw looks very simple, but the extrusion deformation of the material in the mold is affected by many factors, which makes the dense molding of the material complicated. In the dense forming process of straw, the material is sent to the compression chamber through the feeding mechanism. Chapter 4 Finite Element Analysis of Dense Forming of Biomass Straw
4.1 Model establishment
4.1.1 Establish an analytical model
The dense molding process of biomass straw looks very simple, but the extrusion deformation of the material in the mold is affected by many factors, which makes the dense molding of the material complicated. During the dense molding process of the straw, the material is sent to the compression chamber through the feeding mechanism, and under the action of the pressure roller, the particles are mutually squeezed and deformed and finally bonded together to form. Since the establishment of the actual geometric model depends on many factors, in order to facilitate the research problem in the dense molding process, the model is appropriately simplified, and the analysis model is established, as shown in Figure 4-1.
This paper makes the following assumptions based on actual conditions:
1) Simplifying the dense forming process of biomass into a non-smooth contact calculation model of elastoplastic and rigid bodies;
2) Does not consider the physical properties, stirring, chemical reactions, etc. inside the material;
3) treating the biomass straw raw material as a continuous and uniform substance;
4) The temperature structure coupling problem is not considered.
The large deformation of the straw material after being squeezed in the mold is a nonlinear problem. Nonlinear problems can be divided into three categories: geometric nonlinearity, state change, and material nonlinearity. The straw deformation studied in this paper belongs to the state of large deformation contact problem. The analysis model is mainly composed of mold and straw material, and the mold inlet has a certain taper angle to facilitate the material to enter the extrusion. This paper mainly studies the influence of forming holes on dense forming, considering the symmetry of structure and load, simplifying the actual device, adopting the axisymmetric compression model, and using ANSYS software to establish the two-dimensional geometric model as shown in Figure 4-2.
4.1.2 Setting finite element types and material properties
Before meshing, first define the cell attributes that will be used in the finite element model. The unit properties generally include: cell type, real constant, and material properties.
The discretized finite element model consists of two parts: one is to select the appropriate unit type, mainly based on the size and shape of the structure, so that the structural changes and geometric features are well represented in these units; the second is to determine the discretization The number of units used.
The basic idea of ​​finite element is to discretize the solution domain. For different solution domains, select different cell properties, which can reflect the characteristics of the solution and obtain the optimal analysis effect. Therefore, it is important to choose a finite element type. If the choice is appropriate, the analysis and calculation will be very fast and accurate; if the selection is not appropriate, the analysis results may have large errors, and may even be completely wrong.
To accommodate different analytical needs, the ANSYS cell library offers more than 190 different cell types, from common line cells, face cells, solid cells (including 2D and 3D cells) to special contact cells, gap cells and surfaces. Effect unit, etc. The unique unique representation of each unit type, the unit type determines the unit's degree of freedom and the space in which it resides.
In this paper, the structural nonlinear analysis of biomass straw is mainly involved in the nonlinear analysis of the structure. Considering the large deformation of the material during the extrusion process, the selected element must have plasticity and large deformation capability. In this paper, the 2-dimensional 4-node quadrilateral surface element PIAN182 is used. Each node has a translational freedom in the X and Y directions and has a high structural precision. The unit can be used as an axisymmetric unit or as a planar unit with large deformation and large strain capability. The extrusion model was simulated using the TARGET 169 unit. Extrusion molds and biomass straws are also different in material properties. The given material parameters are shown in Table 4-1.
4.13 Model meshing
After the compression model is established, the model is meshed. Meshing is one of the most important steps in the entire analysis process. When meshing, set the corresponding material properties for different material components. The quality of the meshing has a great influence on the accuracy of the calculation results and the calculation progress. If the meshing is unreasonable, the calculation result may be inaccurate, and if it is serious, it will not converge.
There are two ways to general meshing: free meshing and mapped meshing. There is no specific rule for free meshing. There is no requirement for the shape of the element. It is basically applicable to all models, and the mesh is not irregular. The mapping meshing comparison rule, the general mapping volume mesh only contains hexahedral elements, and the mapping surface mesh contains both triangles and quadrilateral elements. Map meshing applies to faces or bodies of shape rules. Choosing a good mesh type is important for meshing the built model, which has a great impact on the calculation time and results. According to the actual situation, it is determined whether to use free or mapped grid for analysis. In this paper, due to the comparison of the shape of the model, a map mesh that is easy to converge is used. The divided grid is shown in Figure 4-3.
4.1.4 Definition of contact unit
There is friction and extrusion between the material and the mold during the dense molding process of the straw material, which involves contact problems, so it is necessary to establish a contact unit for contact analysis.
Contact problems are generally divided into two types: rigid body and soft body contact, semi-flexible body and soft body contact. When the rigid body and the soft body are in contact, one or more of the contact faces are treated as rigid bodies. When the soft material is in contact with the hard material, it can be regarded as a contact problem of a soft body and a rigid body. Many non-metal and metal forming problems are contact problems of a soft body and a rigid body. The other type is the contact between soft materials and soft materials, that is, the soft body-soft body contact, which is a common type of contact. Usually, in this case, the two contact bodies have approximate stiffness, they are both It belongs to the deformed body.
In the above-mentioned biomass straw, there is a phenomenon of extrusion and sliding friction between the extrusion die and the corn stover during the molding extrusion process, and a contact pair must be established in the finite element analysis.
ANSYS supports three contact methods: one point of contact, one point of contact, and 'face-to-surface contact.
When analyzing the problem, you must confirm which parts of the model may touch each other, and then model the contact problem. If there is a point in contact with each other, the corresponding component of the model is a node. If one of the contacts is a face, then the corresponding component of the model is the cell. The finite element model contact matching pair is identified by the specified contact unit, which is a layer unit, which is mainly attached to the [55l,
1) contact unit on one side
Units for dealing with rigid body and soft body-soft body surface contact problems are provided in ANSYS, and the physical contact relationships are simulated by the "target faces" and "contact pairs" of these elements.
The rigid surface was treated as a "target surface", which was simulated with a Targe 169 and a Targe 170, respectively, and the surface of the flexible body was regarded as a "contact surface", which was molded with Conta 171, Conta 172, Conta 173, Conta 174. A "contact pair" is intended to include a target unit and a contact unit. The shared real constant implements the program of the contact pair, usually specifying the same real constant for the established "contact pair".
One side contact has the following advantages compared to point-to-surface contact:
1 Support large deformation, coordinate stiffness matrix calculation, calculation of asymmetric element stiffness matrix, etc.
2 supports low-order units and high-order units.
3 Provides more suitable contact results for engineering purposes such as normal pressure and frictional stress.
4 There is no limitation on the shape of the surface of the rigid body, allowing surface discontinuities caused by natural or mesh dispersion.
5 Compared with a point contact unit, more contact units are not required, resulting in less disk space and shorter CPU calculation time.
6 allows for a variety of modeling controls, such as the automatic movement of the target surface to the initial contact position, binding contact, translational contact surface and support unit life and death.
2) point contact unit
The contact behavior on one side is mainly modeled by point contact units. Contact175, Contact26, Contact48, and Contact49 are point-to-point contact units that support relative sliding and large deformation, and there is no need to maintain a consistent mesh between the contact surfaces. The contact problem on one side can be simulated by defining a set of node-defined contact surfaces on one side of the contact unit.
3) One point contact unit
The point contact behavior is mainly simulated by a little touch unit. In order to use a point contact unit, the contact position must be known, and the point contact problem only applies to a small relative slip between the contact surfaces.
In this paper, the stiffness of the mold is much larger than the stiffness of the straw material. The contact between the mold and the straw material is the contact between the soft material and the hard material, and belongs to a pair of rigid body-soft body contact. The friction behavior is handled by Coulomb's law and the coefficient of friction is 0.2. Using a two-dimensional face contact unit (TARGET169 and CONTACT172), the mold is a rigid body, the target surface is TARGET169 unit; the corn stover is a flexible body, and the contact surface is made with CONTACT 172 unit. The contact pair model is shown in Figure 4-4.
Granulator straw briquetting machine
4.2 Applying a load and solving
The load in ANSYS software includes all boundary conditions (constraints, support or boundary field parameters), external or internal effects. The loads we use are generally divided into six different types:
1) DOF constraint: used to define the degree of freedom of the model. A known quantity represents a given degree of freedom.
2) Force: A concentrated load applied to a model node.
3) Surface load: The distributed load applied to a unit, including line distribution and surface distribution.
4) Body load: is the volume or field load acting in the volume or field.
5) Inertia loads: mainly used in structural analysis, is the load caused by inertia.
6) Coupled-fidld loads: A special case of an upper load, one result acting as a load for another analysis.
This paper mainly simulates the stress-strain distribution during the dense forming process of straw and the rheological law of the material in the mold, and analyzes the stress and deformation of the material. Apply a fixed constraint to the outer surface and bottom of the mold to apply a displacement load in the Y direction to the top surface of the material, as shown in Figure 4-5.
Granulator straw briquetting machine
Completion of the constraint and the application of the load, enter the ANSYS solver (solution), because the dense formation of biomass is a nonlinear problem, it is prone to non-convergence, in order to avoid the convergence of the calculation results, the options for nonlinear analysis in this analysis Linear search and deformation prediction are turned on, Largedisplacement static is set and the automatic time step is turned on in the solution option. The Full N-Runsyrnm solver is selected for the calculation, as shown in Figure 4-6.
Static structure finite element analysis can be performed after the solution setup is completed.
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