In the current (renewed) term of the project, we have focused on understanding the flow of cohesive powders in feeders. To study the system experimentally, we synthesized model cohesive powders by combining small quantities of glycerol to dry glass beads. Our experiments showed that the feed rate has the same qwualitative dependence on the ratio of pitch to diameter of the screw as for dry powders. A lacuna in the current understanding of cohesive powders is that there is no reliable constitutive model. During the last year we have addressed this issue by conducting experiments and DEM simulations of flow in a cylindrical Couette cell to measure the velocity and stress fields in the non-inertial slow flow regime.
Our results show a systematic dependence of the velocity and stress fields on the Bond number Bo (a dimensionless measure of the magnitude of the cohesive force). Importantly, the form of the velocity profile is quite similar to that of a non-cohesive powder. The main influence of cohesion is to alter the wall slip and the sharpness with which the velocity decays with radial distance. Another key observation we have made in the past year is that cohesion fundamentally alters the rheology of the powder: for relatively loose powders, cohesion causes a transition from the rapid (inertial) to slow flow regimes, and for relatively dense powders, cohesion extends the range of shear rate of the slow flow regime.
In the previous years of this project, we had presented a non-local constitutive model for non-cohesive powders and demonstrated its efficacy in accurately predicting the velocity field in simple flows and a complex dilation-driven secondary flow. Over the past year, we have concentrated on extending the model to cohesive powders. We have used the results of our experiments and simulations to determine the influence of cohesion on the parameters in the nonlocal model. We now have data for the dependence of the decay length of the velocity on Bo. Our ongoing work is directed at determining the dependence of all the parameters of the model on Bo. By the end of the project, we hope to be able to give recommendations on operating conditions and strategies that will enhance the precision and reliability of screw feeding of powders.