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Paddy usually requires a series of processing techniques to become edible polished rice. Among them, rice milling is a necessary processing connection closely related to the nutrition content and economic value of rice. How to accurately control rice milling on demand has always been a frontier topic. In this work, the milling process of rice grains in friction milling cavity with different parameter configurations was simulated numerically. The feasibility of the established simulation model was verified through experiments. Kinematic and dynamic similarity of rice grains in milling cavity were analyzed. The main contribution mechanism affecting the energy loss of rice grains was explored. The scale-up rule that can predict the milling degree of rice was proposed for the first time. The results showed that when the structural or operating parameters change, the flow structure of the particle bed will change significantly, which directly determines rice force characteristics. Besides, the energy dissipation between rice depends more on the configurational temperature dominated by the compactness of particle bed than on the granular temperature. The fluctuation of displacement field of particles will be more intense with the decrease of local space porosity and the increase of external input energy. Finally, a parameter scale-up rule – Relative compaction volume (RCV) suitable for friction rice mill is established. The average dissipated energy of rice grains can be characterized as a linear function of RCV. The characteristic parameter RCV is determined by the geometric characteristics and operating conditions of the rice mill. The research results can provide analytical ideas and theoretical direction for the design and parameter selection of rice mill and similar equipment. [ABSTRACT FROM AUTHOR]

• DEM can accurately simulate the movement of particles in vertical rice mill. • The rice in milling chamber is transported dispersedly rather than layer by layer. • The particle transport mechanism is revealed from the perspective of kinematics. • A method that can establish link between mill parameters and uniformity is proposed. • This study can provide guidance of the parameter design for similar equipment. 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. [ABSTRACT FROM AUTHOR]