Bridging the Scales of Wetted Particulate Flows: Experiment, Theory, and Simulation

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

CBET-0754825, Davis Intellectual Merit. Processes involving the flow of solid particles are ubiquitous in both nature (landslides, avalanches, planetary rings, etc.) and industry (pharmaceuticals, food products, chemical process industries), though a predictive understanding of their behavior remains an elusive goal. Of particular interest in the proposed effort are systems involving particles coated with a thin layer of viscous fluid ? i.e., wet solids. Such systems are found in a wide range of applications (fluidized-bed granulation, mixing of pharmaceuticals, pollen transport, filtration, etc.), and are known to display characteristics atypical of their dry counterparts, namely the presence of particle agglomerates. A predictive knowledge of agglomeration formation, rearrangement, growth, and break-up is a key element in the rational design of wet-solid processes, though such a predictive tool is not currently available. The current effort aims to address the aforementioned need, with a particular focus on viscous (dynamic) effects occurring between wetted particles, which are relatively unexplored as compared to the capillary (static) effects of wetted systems and viscous effects of fully-immersed systems. Preliminary experiments display two surprising behaviors: (i) an oblique collision between 2 wetted particles initially forms a rotating agglomerate, which may separate at a later time, and (ii) a normal collision between 3 wetted particles in a Newton?s cradle setup, when run at a series of impact velocities, displays all possible geometric outcomes except that displayed by a traditional (dry) Newton?s cradle. It is hypothesized that (i) is due to the role of centrifugal forces and (ii) is due to the simultaneous, fluid-mediated interaction between all 3 particles (whereas the dry Newton?s cradle is modeled as a series of 2-body collisions). A combination of experiments and fundamental theory based on lubrication, capillary forces, and solid mechanics will be used to develop stick/separate and breakup criteria for small agglomerates. The theory will require only measurable solid and liquid properties (no adjustable parameters). A stepwise approach will be followed, where 2-particle experiments and theory are the initial focus, followed by an extension of both to 3-particle systems. To bridge this micro-level physics with macro-level behavior, the microphysical theory will be incorporated into discrete-particle simulations. The effect of the liquid layer on continuum quantities like stress will be assessed via an examination of simple shear flow of wet grains. Furthermore, discrete-particle simulations will also be carried out for two specific unit operations, namely rotating drums and fluidized-bed granulation (or enlargement), in an effort to describe non-intuitive behaviors observed in granular and gas-solid systems, respectively. This work will be a collaboration between the PI (Prof. Robert Davis) and the co-PI (Prof. Christine Hrenya), who have extensive background in the theoretical, experimental, and simulation aspects associated with fluid and particulate flows. Broader Impacts. The broader impacts of the work include the following: (i) a more fundamental understanding of wetted particulate systems, (ii) incorporation of the new theory into the MFIX framework, a no-cost, open-source code for modeling multiphase systems, available to researchers worldwide, (iii) training of students in the area of particle technology, which has been identified as a national need [1-3], (iv) sharing of learned information with the technical community (via presentions, peer-reviewed publications, and web sites), the student community (via incorporation into Particle Technology course developed by the co-PI and outreach), and the non-scientific community, and (v) active encouragement of underrepresented minorities. A unique aspect of this work is the use of a ?Stokes? cradle? (similar to the conventional Newton?s cradle, but with liquid added to the colliding balls) as an experimental apparatus; the widespread familiarity with this desktop toy will be capitalized upon in demonstrations to K-12 students and teachers, as well as the non-scientific community.