玉米秸秆揉丝破碎过程力学特性仿真与试验

Simulation and experiment on mechanical characteristics of kneading and crushing process of corn straw

由于玉米秸秆揉丝破碎过程缺乏有效数值模拟方法,在一定程度上降低了加工设备结构改进效率。该文基于离散元法(discrete element method,DEM)建立了玉米秸秆离散元模型,并通过物理试验和虚拟试验相结合对玉米秸秆粘结接触模型(bonded particle model,BPM)进行了参数校核。最后针对玉米秸秆离散元模型进行了破碎仿真以及试验验证。结果表明:以 5 mm/min 为加载速度进行秸秆压缩和剪切试验时,最大临界载荷分别为 2 260 和 110 N;对 BPM 粘结模型进行参数校核后,得到法向刚度系数、切向刚度系数、临界法向应力、临界切向应力及粘结半径分别为 9.60×10^6 N/m、6.80×10^6 N/m、8.72 MPa、7.5 MPa、2 mm,此时离散元模型力学特性与含水率为 87.8%的收获期玉米秸秆相接近;仿真结束后,物料可分为短型、标准型、长型及未完全破碎型 4 种,与试验结果相一致,不同类型物料质量与试验结果数据偏差保持在 10%以内。研究结果表明离散元法应用于玉米秸秆揉丝破碎过程仿真是可行的。

As a large agricultural country, China has abundant straw resources, and the annual output of corn stalks is 3 billion tons. Corn straw could be used as silage after mechanized processing, it could effectively alleviate the current shortage problem of animal feed. At present, the research on kneading and crushing of corn straw mainly focused on physical experiment, but less on numerical simulation. Therefore, the lack of effective numerical simulation method in the crushing process of corn straw and the improvement efficiency for the structure of processing equipment is reduced to some extent. Based on the above background, in this paper, a discrete element model for bimodal distribution of corn straw was established based on discrete element method, which was originally applied to the field of geotechnical mechanics. This process included setting particle size, filling geometric model, obtaining particle group coordinate information and so on. All of the numerical simulations were carried out in EDEM 2018 version, and users were allowed to customize the settings. Then the parameters calibration for the bonded particle contact model of corn straw was carried out that combined with physical experiment and virtual experiment. The mechanical parameters were obtained through compressing and shearing experiments of corn stalk samples. This parameter could provide basis for theoretical calculation of bonded particle model parameters of corn stalks. The mechanical properties of bonded particle model were determined by 5 parameters: normal stiffness coefficient, tangential stiffness coefficient, normal critical stress, shear critical stress and bonded radius which could be obtained through theoretical formula calculation. In the virtual experiment, the corn stalk model was subjected to axial compression experiment and radial shear experiment, respectively. The final BPM (bonded particle model) parameters were determined by the phenomenological simulation method which was compared physical experiment and virtual experiment. Finally, the discrete element model of corn straw was simulated and verified by physical experiment. This process involved the kneading and crushing simulation of corn straw model, classifying the types of materials and weighing the mass of materials, and using API plug-in to realize particle replacement and adding bonded particle model in the simulation process. After the simulation was completed, the change of bonded number and the shape classification of broken materials were analyzed. The whole research results showed that the maximum critical loads were 2,260 N and 110 N, respectively when the straw was compressed and sheared at a loading rate of 5 mm/min;After calibrating the parameters of bonded particle model, it was found that the normal stiffness coefficient, tangential stiffness coefficient, critical normal stress, critical tangential stress and bonded radius were 9.60×10^6 N/m, 6.80×10^6 N/m, 8.72 MPa, 7.5 MPa and 2 mm, respectively. At this time, the mechanical properties of the discrete element model were close to the harvested corn straw that had a moisture content of 87.8%. After the simulation, material could be divided into four types: short type, standard type, long type and incomplete crushing type. At the same time, the simulation mass of 4 materials were 5.2, 5.8, 8.2 and 8 g, respectively. After the physical experiment, the mass of different types of materials were respectively 4.8, 5.3, 7.6 and 9.1 g. The deviation of the mass for different materials were 8.3%, 9.4%, 7.8% and 7.7%, respectively. The mass deviation for different types of materials between the physical experiment and virtual experiment was kept within 10%. This research could not only prove that it was feasible to establish corn straw model by discrete element method and simulate the kneading and crushing process, but also provide guidance for kneading other straws.