Dynamic regimes in granular mixing: Effect of sliding friction and stirrer rotational frequency

Abstract

The dynamics of granular mixing in a vertical cylindrical vessel with a flat-blade stirrer has been investigated by systematic discrete element method simulations. Three distinct dynamic regimes have been identified by examining stirrer rotational frequencies ranging from gentle stirring to conditions inducing substantial surface deformation and particle accumulation near vessel walls, and sliding friction coefficients ranging from effectively frictionless to very rough particles. In the low-friction regime, the bed behaves as a quasi-rigid body corotating with the agitator, resulting in minimal homogenization. Poor homogenization is also observed at high rotational frequencies and moderate to high friction, where strong centrifugal forces deform the granular bed into partially isolated regions. Optimal mixing is achieved at intermediate conditions, where balanced centrifugal and gravitational forces allow effective particle redistribution by sustained frictional contact rather than particle collision. Analysis of surface deformation, volume fraction, and particle velocities reveals how these regimes arise from competition between mechanical energy input and the internal resistance of the granular bed. These results provide quantitative guidance for the optimization of mixing processes.