Bulk handling of powders and granular solids is common in many industries and often gives rise to handling difficulties especially when the material exhibits complex cohesive behavior. For example, high storage stresses in a silo can lead to high cohesive strength of the stored solid, which may in turn cause blockages such as ratholing or arching near the outlet during discharge. The wall friction and the internal friction of the stored solid are expected to influence the flow pattern which develops within a silo. The varying flow pattern and the possibility of cohesive arching in the solid can lead to significant variation in the wall pressures observed in a silo during discharge.
This paper deploys the Discrete Element Method to study the discharge of a granular solid from a flat-bottomed silo with varying levels of cohesion and varying levels of internal friction which arises from a combination of particle interlocking and contact friction (sliding and rolling). The DEM simulations were conducted using the commercial EDEM code with a recently developed DEM contact model for cohesive solids implemented through an API. The contact model is based on an elasto-plastic contact with adhesion and uses hysteretic non-linear loading and unloading paths to model the elastic-plastic contact deformation. The adhesion parameter is a function of the plastic contact overlap. The model has previously been shown to be able to predict the stress history dependent behavior depicted by a flow function of the material.
The effects of cohesion and friction on the discharge rate and flow pattern in the silo are investigated. The predicted discharge rates are compared for the varying levels of cohesion and the effect of adhesion is evaluated. The effect of varying wall friction on the flow patterns observed and the resulting wall pressure profiles is evaluated. Small levels of asymmetry in the flow channel can lead to large variations in silo wall pressures and the effect of varying friction on the flow channel is studied. The ability of the contact model to qualitatively predict the phenomena that are present in the discharge of a silo has also been shown with the salient features of mixed flow from a flat bottomed hopper identified in the simulation.