Improved design of the classification structure of vertical mechanical impact superfine grinder

Mechanical impact superfine grinder is characterized by the wide working condition, high efficiency, and simple operation. The vertical mechanical impact superfine grinder has been commonly used to crush agricultural by-products in recent years. However, the processed material often contains coarse particles with a wide distribution range, due to the current classification chamber structure. The classification performance can also depend mainly on some changes in the vertical position relationship between the current-guide cover and the classification wheel. In this study, two optimization schemes were proposed for the classification chamber using the initial structure, in order to examine the influence of different position relationships on classification performance. Optimized structure 1 involved the current guide that attached to the upper cover, thereby blocking the flow path between them, while maintaining the height of the classification wheel blades. Optimized structure 2 retained the original position of the current-guide cover in consistence with the initial structure, but increased the height of classification wheel blades. Additionally, 3mm was adjusted for the distance between the top edge of the classification wheel blades and the bottom of the discharge port. Firstly, the performance tests were conducted on three classification chamber structures. Secondly, the CFD software and numerical simulation of gas-solid two-phase flow were employed to analyze the flow of fluid within the classification chamber. Finally, the simulation was used to explain the test findings. The following conclusions were also drawn. Optimized structure 1 yielded a more uniform clipping velocity outside of the classification wheel. However, there was a small variation gradient of axial velocity and flow field velocity in the classification chamber. Consequently, the product with the smaller median particle size was obtained in the optimized structure 1, compared with the others. Classification efficiency and accuracy were only changed slightly. The optimized structure 1 resulted in the absence of large particles in the products, due to the blocked flow path in the attachment of the current-guide cover to the upper cover. Meanwhile, the radial velocity decreased gradually from the outer to the inner of the grading wheel in the optimized structure 2. The reverse vortices were generated between the grading wheel blades, leading to a more uniform axial velocity distribution within the grading wheel and a significant variation gradient of axial velocity and flow field velocity within the classification chamber. Consequently, the product with a larger median particle size was yielded, compared with the others, whereas, the classification efficiency and accuracy were enhanced significantly. Moreover, the increasing height of the grading wheel blades impeded the entry of large particles into the finished material. The reason was that the higher tangential velocity outside the grading wheel promoted the material dispersion and the ejection of large particles. In summary, both optimized structures 1 and 2 resulted in the products devoid of large particles. A comparative analysis was performed on the three classification performance indexes, namely product particle size, classification efficiency, and classification accuracy index. Therefore, the optimized structure 2 demonstrated better classification accuracy and efficiency than before, although was yielded a coarser product particle size. Conversely, the optimized structure 1 enabled the production of ultrafine powder with a smaller particle size, although at the cost of lower classification efficiency. The findings can provide a strong reference and theoretical foundation for the flow field investigation and structural optimization of the classification chamber in the CWFJ vertical mechanical superfine grinder.