Discrete element method modelling of impact breakage of ellipsoidal agglomerate
Breakage of cereal grains approximated ellipsoidal shape such as rice and wheat kernel is an extremely common phenomenon for harvest and post-harvest processing in food industries. Unfortunately, there are few investigations about the breakage of cereal grains neither by means of experiment nor simulation methods. Moreover, it is of importance to understand the breakage mechanisms of cereal grains for controlling the final product quality. In this regard, this paper is to apply the discrete element method (DEM) combining bonded particle model (BPM) for simulating impact breakage of rice kernel and revealing its breakage characteristics. The rice kernel was represented as agglomerate composed of a set of mono-size primary spherical particles connected by ideal elastic solid bonds. Parameters characterizing strength of agglomerate were calibrated by comparing force-displacement curves from the uniaxial compression test. The critical impact velocities corresponding to the critical breakage state during the milling were compared through the theoretic, experimental and simulated methods. The effects of impact velocity represented the strength of impact and moisture content characterized the strength of agglomerate on its breakage behaviors, including damage ratio and size ratio of the largest fragment were analyzed. The results indicate that according to the change in breakage extent with impact velocity, breakage pattern can be divided into four modes. For the mode of local disintegration, the damage ratio and size ratio of the largest fragment change linearly with the impact velocity and moisture content, whereas for the mode of fragmentation the damage ratio and size ratio of the largest fragment vary exponentially with the impact velocity and moisture content. Furthermore, the relationship between difference in kinetic energy and damage ratio fits the linear function well for the mode of local disintegration, whereas it fits the Exp3p2 exponential function well for the mode of fragmentation. The findings are useful for providing theoretic guidance to the study on the impact breakage of other cereal grains and the revelation of breakage mechanism in a rice mill.
Keywords: Discrete element method, Bonded particle model, Ellipsoidal agglomerate, Breakage characteristic,