Numerical analysis of the rice transport uniformity in vertical friction rice mill based on discrete element method

Rice milling is a crucial step to determine the final rice quality in rice processing. The uniformity of rice transportation in milling chamber is the key factor affecting the milling uniformity. Unfortunately, the cause of the uneven transport of rice in milling chamber is still poorly understood, which constrains the development of rice milling industry. In this study, the movement process of rice in vertical friction rice mill was numerically simulated by discrete element method. The macroscopic motion law of rice was obtained first and then quantified by residence time distribution (RTD). Then, the transport mechanism of rice in the milling chamber was analyzed from the kinematic perspective. Finally, a mathematical model involving the relationship between the mill parameters and the rice transport uniformity was established. The results showed that the axial dispersion of rice occurs during the transport process in milling chamber. The RTD curve indicates that the residence time of most particles in the milling chamber is short while that of a few particles is long. Besides, with the increase of rotational speed, the axial motion consistency of the particles becomes worse. The particle bulk density distribution shows that particle flow is affected by both axial backflow and radial centrifugation. There are complex convection and local vortices flow in particle vector field. In addition, the trough value and peak value of the axial velocity of the particle flow layer decrease and increase respectively with rotational speed. The swept volume method containing the structural and operating parameters of the rice mill is established. There is a strong corresponding relationship between swept volume and the uniformity of flow layer. Specifically, the larger the swept volume, the stronger the axial convective mixing, the more uneven the rice transportation. Finally, it is verified by experiments that the milling uniformity of rice mainly depends on the swept volume. The research result is of guiding significance for the design of relevant parameters of similar equipment.