Analysis of the flow of granular materials through a screw conveyor

Abstract

Screw conveyors are widely employed in industry for the bulk transport of particulate materials. Several studies have attempted to correlate the discharge rate with the rotation speed of the screw via experiments and particle dynamics simulations. However, a detailed mechanical model that would assist in the optimal design of screw conveyors has not been attempted. In this study, we first construct a simple model that assumes the entire granular medium to move as a rigid body sliding along the surfaces of the screw and barrel. By enforcing the balances of linear and angular momentum to a suitably chosen continuum element, we show that under certain limiting conditions, the discharge rate for a given angular velocity and screw geometry can be obtained. Further, we show that the discharge can be maximized by setting the ratio of the pitch to barrel diameter to a particular value. We then study the detailed flow within the conveyor using the Discrete Element Method, which reveals that a significant fraction of the material exhibits solid body motion, in agreement with the simple model. We assess the effect of relaxing the limiting conditions employed in the model, thereby determining the connection between the friction at the walls and the kinematics of flow. Finally, the trend in variation of average axial velocity with the pitch to barrel diameter ratio in the presence of gravity is compared with that from our model and simulations in the absence of gravity. We observe that both the trends are qualitatively the same indicating that the dependence of average axial velocity on the geometry of the conveyor is not altered by the introduction of gravity.