During the first term of the project, our investigation on screw feeder performance had three components. The first was to formulate two mechanics-based models for the kinematics and mechanics of non-cohesive powders: a simple model that relies on several simplifying approximations, and a more detailed continuum model that predicts the variations of velocity, packing fraction and stress within the feeder. Both models make the interesting prediction that the feed rate is maximum for a specific value of p/(2R!), the ratio of the screw pitch to barrel diameter of the feeder. The second component of our work was to conduct DEM simulations to validate the models and guide experimental efforts. The third component was to conduct experiments on a custom-built feeder assembly to test the model predictions and to extend/refine the model. Overall, we find good agreement between the experimental data, DEM simulations, and model predictions for non-cohesive powders (such as glass beads). In particular, the simulations and experiments verify the model predictions of the feed rate being maximum at a certain ratio of pitch to diameter of the screw. The experiments also identify the free surface at the feeder exit as being responsible for feed rate fluctuations for dry powders, and show that the fluctuations may be mitigated by appropriate end-cap design. The combination of theoretical analysis, DEM simulations, and experiments yielded substantial insight.
In the current (renewed) term of the project, we have focused on understanding the flow of cohesive powders in feeders. To quantify the effect of cohesion, we first created powders of controlled cohesion by combining a small quantity of glycerol to dry glass beads. Interestingly, the experiments show that the feed rate as a function of p/(2R!) shows the same qualitative trend as for dry powders. However, the feed rate fluctuations for cohesive powders are quite different from those seen in dry powders. A lacuna in the current understanding of cohesive powders is that there is no reliable constitutive model. To address this, we have initiated a study to determine the flow rule, a relationship between the strain rate and stress, for cohesive powders. Our initial studies on horizontal rotating drums point to the formation of clusters or agglomerates that crucially affect the flow of cohesive powders. On the modelling front, we have shown that the non-local model correctly predicts a complex dilatancy-driven secondary flow seen in experiments and DEM simulations, thereby increasing confidence on the model.
In ongoing work, we are conducting DEM simulations and experiments to quantify the formation and persistence of clusters in cohesive powders and determine how they influence their flowability.