(55e) Use of a Couette Shearing Cell As a “Powder Rheometer” to Predict Stress and Flow Fields of Dense Granular Matter

The basic motivation for this work was to develop a reliable continuum model that relates stresses inside the system to shear rates and is valid for both quasi-static and intermediate regimes of powder flow. To achieve the goal, a traditional Couette shearing cell was modified by providing continuous, low flow-rate axial powder feed and discharge, which allows for the bed dilation and provides space for powder particles to collide. A visco-plastic constitutive equation was developed from the Couette data, and this was then generalized by considering the effects of size, shape, surface texture, compressibility, deformability, rigidity and elasticity of particles on the rheological index. As a second step, this constitutive equation was validated in a more complicated geometry that of the spheronizer: a device widely used in the pharmaceutical industry. We used a computational fluid dynamics (CFD) approach to model the free surface centripetal flow of granular matter in the spheronizer. We constructed a lab-scale spheronizer and measured all relevant parameters. Experimental results, such as surface height and shape, solid volume fraction and wall normal stress as a function of rotational speed were compared with those obtained from computations and showed a good agreement.

The picture that emerges from all the experimental, theoretical and computational work is the fact that one can use a continuous flow Couette cell as a “Powder Rheometer” and find powder flow characteristics to enable the prediction of flow and stress fields in complex systems such as spheronizer. This is a significant first step since flows in high shear granulators and mixers are somewhat similar and, in general, pose similar problems.