The effect of rotational speed on granular flow in a vertical bladed mixer

摘要

Granular mixing is an important unit operation used to ensure the uniformity of mixture properties. Unfortunately, the mechanisms of particles' motion and the role of operation parameters remain poorly understood. In this work, granular mixing is studied using computer simulations via OEM (discrete element method). The examined process is the mixing of approximately 42 thousand glass spherical particles with a 2 mm diameter in a vertical cylindrical mixer with two opposed flat blades with a 45 degrees rake angle. The effect of different blade rotational speeds (varying from 0.1 rpm to 960 rpm) on the formation and evolution of flow patterns is investigated. Examining individual particle trajectories shows that the particles exhibit two basic types of periodic movements. The first one, characterized by a higher frequency, is related to the motion of the stirrer. The second one (lower frequency) is associated with recirculations in the vertical plane. A methodology for their detection is proposed. The observed recirculation zones are the secondary flow commonly occurring in liquid mixing cases and also the mechanism of their emergence is similar. Unlike liquids, the mixing of granular systems exhibits a greater diversity of recirculation zones. The tangential motion does not create complex structures in the angular direction, because the height of the granular layer is several times greater than the height of the blade. For this reason, we have focused only on the tangential velocity distribution in the radial and axial directions. Three global particle transport characteristics describing the mixing process in the tangential, axial and radial directions were proposed. Significant changes in the behavior of these variables were used to distinguish six dynamical regimes of granular mixing depending on the blade rotational speed. These regimes are characterized by different flow patterns. (C) 2015 Elsevier B.V. All rights reserved.