In the first year of this project, we had derived a mechanics-based model for the feed rate in a single-screw feeder, making several simplifying assumptions. The model makes the prediction that the feed rate is maximum at a particular value of the ratio of pitch to diameter p/2R of the screw. This prediction matches exactly with the results obtained by DEM simulations under the same conditions. When the simplifying assumptions are relaxed in the DEM simulations, the dependence of the feed rate on p/2R was found to be qualitatively similar, suggesting that the simple model captures the essential physics of the problem.
In the second year, our work was extended in several directions: (i) experimental measurement of the feed rate for different p/2R, the stress at the barrel surface using sensitive force sensors, and the velocity profile adjacent to the (transparent) barrel surface by flow imaging; (ii) application of a newly developed non-local constitutive model to the screw feeder problem and solving the governing equations to obtain the velocity and stress fields; (iii) DEM simulations to study the effect of particle cohesion on the feed rate and obtain the detailed spatial variation of the stress and velocity in the particulate medium. Overall, we found good agreement between results of the DEM simulations, model predictions, and experimental data. The important conclusion was that combination of the three components of our investigation, namely theoretical analysis, DEM simulations and experiments, led to substantial insight.
In the third year (for which this report is written) we have further extended our experimental studies in some directions: we first completed the determination of the feed rate for a larger range of p/2R for glass beads and confirmed the existence of a maxima in the feed rate at an optimum value of p/2R. We then measured the feed rate for two cohesive powders – though measurements for sufficient large p/2R are yet to be made, the data strongly suggest the presence of the maximum in the feed rate. The experiments also throw light on the feed rate fluctuations, which are quite different for non-cohesive and cohesive powders. Our earlier DEM simulations restricted the feeder length to one pitch and assumed periodic inlet and outlet conditions. We have now conducted simulations for the full feeder, from the inlet hopper to the feeder exit. The results show a gradual fall in the fill level with axial distance from the inlet, as observed in the experiments.
Our ongoing work is to obtain solutions of the non-local model for the more general cases of finite screw friction in the presence of gravity. We are measuring the feed rate of cohesive powders for a large range of p/2R to confirm the presence of the maxima. We will soon conduct DEM simulations for cohesive powders for the full length of the feeder.