DEM study of rice grain flow in an alternating mixed-flow dryer: Effects of moisture content and discharge roller speed

Drying is essential for the safe storage and subsequent processing quality of rice grains. Particle flow characteristics in mixed-flow dryers are a key factor affecting grain drying uniformity. To investigate the effects of moisture content and discharge roller speed on the flow behavior of rice grains, multi-sphere (MS) discrete element method (DEM) models of rice grains with different moisture contents were developed. A geometric model of an alternating mixed-flow drying unit was also established. The particle trajectories, velocity field, spatial orientation, and porosity distribution during discharge were systematically analyzed at the particle-assembly scale. The results showed that the rice grain assembly exhibited a typical mass-flow regime in the drying unit, with differential motion characterized by spatial S-shaped trajectories. Particles with higher moisture content showed more pronounced overall nonuniformity in descent, whereas the effect of discharge roller speed was relatively weak. Both parameters had limited effects on the spatial orientation distribution characteristics of the particles. Meanwhile, the rice grains were predominantly oriented in inclined postures, with no tendency toward a preferential vertical orientation. The porosity exhibited spatial heterogeneity. Changes in moisture content and discharge roller speed did not significantly alter the spatial distribution of the rice grain bed porosity. When the normal distance from the wall exceeded 1.5 d p , the wall effect on local porosity was no longer significant. The findings provide insight into the flow characteristics of rice grains during discharge and serve as a theoretical basis for the structural optimization of grain dryers.