Particle Diffusion and Mixing during Silo Drainage

Clark University, Worcester MA

Investigators

Abstract

Abstract CTS-0334587 A. Kudrolli, Clark University Mixing and segregation in particulate flows will be investigated using experiments with high-frame rate digital imaging and direct particle tracking. Besides measuring diffusion of the particles near the side walls of a silo, the PI proposes to also investigate the motion of the particles deep inside the silo using index-matching fluids. The investigation will provide rigorous answers to a problem that plagues the design of modular pebble-bed reactors, which is of current interest to nuclear engineers. A central question is the amount of diffusion among billiard ball sized fuel pebbles containing radioactive material that are slowly cycled through a reactor. The study is important to many other industrial systems where competition between mixing and segregation in particulates is important. In preliminary experiments conducted with MIT graduate student Jaehyuk Choi, the positions of particles were tracked using high frame rate digital imaging over a long period inside the silo as it drains. Although kinematic models formulated in terms of packing geometry and void movement have served as one-parameter descriptions of the mean flow of the grains inside a silo, they do not correctly predict the mixing rate observed in the experiments. Other modeling approaches derived from plasticity theory do not address diffusion and mixing rates. The PI proposes to experimentally investigate diffusion rates not only as a function of flow rates but also shear gradients that can be varied by changing the nature of the orifice. Then the effect of the shape of the silo and the half angles of the hopper will be studied. Discontinuity in the boundary condition and other obstacles induces shock fronts which can enhance mixing. The effect of segregation mechanisms on mixing rates will be investigated when two species that differ in size or density are present. The experiments will be part of a collaborative effort to understand slow granular flow. In addition to the experiments, modeling and simulations with Professors Martin Bazant and Ruben Rosales of the Applied Mathematics Department at MIT will be performed. A new spot model incorporating the observed correlation is being developed which complements this experimental study. Applicability of new concepts such as \effective" temperature defined using the fluctuation dissipation theorem to slow particulate flow will be tested. The proposed activity will also involve significant educational and science outreach components. A post-doctoral associate will be trained who is interested in pursuing a career in an environment where undergraduate research participation is encouraged. The proposed work will also impact the research of two female graduate students. The grant will enhance summer research experience of undergraduate students and help accomplish their honor thesis projects in the laboratory. The PI also plans to continue speaking at neighboring high schools to introduce the challenges posed by particulate technology from nano to macro scale, and to excite interests in the sciences.