Granular Transport Group

Why study granular mixing? The simple answer lies in its undeniable industrial importance. A more subtle reason is that mixing flows generate unique qualitative patterns that are easily visualized and verified; because of this, mixing studies allow relatively modest computational experiments to effectively probe complex flows and to test the robustness of model theories. In particular, particle dynamics simulations allow access to information which would be very difficult or impossible to measure in a physical experiment such as, instantaneous force distributions, three dimensional concentration and velocity profiles, etc. This project ostensibly focuses on cohesive mixing, yet insight into granulation and aggregate break-up processes -- with implications far beyond granular mixing -- will be gained.

Computer simulation of heap formation both with and without cohesion. As expected, cohesive effects become important near Bo~1 (i.e., when the cohesive force between the particles is roughly equal to the particle weight). The static angle increases markedly with Bo < 1 (i.e., cohesive force > particle weight), while the dynamic angle -- obtained in a rotated tumbler -- remains relatively flat (the angle increases for Bo > 1 and decreases for Bo < 1).

Many of the industries which deal with particulate materials are in some way affected by cohesion; most notably the pharmaceutical, metallurgical, and pigment industries. Cohesion between particles arises from a variety of sources: van der Waals forces, electrostatic forces, and liquid bridging (capillary forces), to name a few. These interparticle forces become increasingly important as particle size decreases and, as cohesion becomes important, substantial departure from the behavior of free-flowing particulate systems becomes evident. For the case of free-flowing powders, it is necessary to attempt to restrict particle motion in order to minimize segregation or de-mixing. In contrast, in cohesive systems, particles tend to segregate into aggregates that must be broken apart to attain a quality mixture. Despite this fundamental difference, some of the tools that have proven effective for the study of cohesionless materials can still be employed for cohesive systems -- specifically, particle dynamics simulations. To that end, a cohesive-solid particle dynamics code is being developed to study the change in stable heap angle as a function of global Bond number (the ratio of particle weight to attractive force) as well as the evolution of mixing in a rotated drum. Here, we are concerned with moderately sized particles, ~1mm, and so we devote our attention to the forces attributable to interstitial liquid -- the dominant interaction effect at these length scales.

A comparison of mixing at different Bo numbers. At left is a qualitative comparison, after 2 revolutions, of the mixing within a drum at different Bo numbers. A quantitative measure, based on the interfacial area As, is plotted as a function of revolutions at right. Note that the mixing rate for Bo = 0.025 is faster.

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