We have examined the effects of powder cohesion and particle size distribution on mixing and segregation processes. As agreed in our Year 1 work plan, we have completed the development of experimental and computational procedures for this study and we have conducted an extensive characterization of non-segregating systems in order to provide a baseline for segregating mixtures (to be examined in years 2 and 3 of this project). We have met all of the stipulated milestones, summarized as follows.
1. We have built a computer-controlled lab scale double-cone blender.
2. We have developed and debugged discrete element simulation code for the double-cone blender.
3. We have simulated mixing in the double-cone over the course of up to 12 tumbler revolutions, using 15,000 monodisperse and 13,000 bidisperse particle blends.
4. We have measured the mixing rate as a function of fill level and vessel speed.
5. We have evaluated effects of filling level and vessel speed on mixing and segregation, both experimentally and computationally.
6. We have examined the effects on mixing and segregation of three-way interactions between particle size, fill level and vessel speed.
At this stage of the project, we have found the following.
1. Mixing in the double-cone occurs by a combination of radial-azimuthal convection and axial diffusion. The chief bottleneck to mixing in the double-cone is diffusion acrass the symmetry plane; we have demonstrated that judicious baffle placement can significantly improve mixing.
2. As particle sizes are reduced below about 200mu, steady and regular flow in tumbling blenders gives way to intermittent and chaotic mixing. This results in a dramatic improvement in mixing rates, overwhelmingly exceeding what would be possible by traditional mixing mechanisms. Moreover, this work demonstrates that traditional analysis cannot be applied, even qualitatively, to the study of flow and mixing of fine grains.
3. Several new segregation modes have been identified. We have charted the phase space of these modes, and we have begun an analysis of a preliminary model of segregational mechanisms which seems to show promise for developing a predictive understanding of flow and transport of polydisperse granular mixtures.