Polydispersity

The general focus of our research is to further the fundamental understanding of particulate flows. Flows involving solid particles are used extensively in industry including areas such as pollution control, pharmaceuticals, energy production, and materials synthesis, and are also found in natural environments like landslides, avalanches, and planetary rings. However, since the fundamental flow behavior of such systems is not well understood, the prediction, design and operation of related systems are often based on experience rather than on scientific principles. As a result, processes employing particulate flows often operate below design capacity and exhibit undesired flow behavior. These challenges motivate our goal to better resolve the elusive particulate phenomena. 

Notably, flows found in both industry (mixers, fluidized beds) and natural settings (avalanches,motion in planetary rings) are typically not monodisperse. The presence of a non-uniform size distribution gives rise to de-mixing, or segregation. Although such a phenomenon may be beneficial to operations involving separation, it may prove detrimental if a well-mixed system is desired, as is common in the pharmaceutical industry. For example, a tablet is made from two powder substances – the medication and the binder which holds the medication together. If these two substances are not-well mixed prior to tablet formation, a patient may be over- or under-medicated. A better understanding of polydisperse flows will allow for an improvement of such existing operations and an efficient design of new operations.

Polydispersity is well-known to have a strong impact on the performance of fluidized, gas-solid systems. Empirical correlations for polydisperse, fluidized beds are highly unreliable, with typical errors between predictions and experiments on the order of 100%. As a result, computational fluid dynamics (CFD) tools have been identified as crucial for the improved performance of coal-based technologies. To date, continuum models for polydisperse systems have been largely based on ad-hoc modifications of monodisperse theory. Not surprisingly, recent comparisons between model predictions and experimental data indicate that such ad-hoc approaches are inadequate.

To address the aforementioned shortcomings, the overall aim of our work is to develop a fully-specified, continuum, gas-solid model targeted specifically at materials with differences in size and/or density. In particular, balance equations and closures for both solid-solid interactions (stress, etc) and gas-solid interaction (drag force) will be rigorously derived for polydisperse systems.

               

Representation of Granular Mixtures for Continuous Particle Distributions 

The overall aim of my project is to develop a mathematical model for flows composed of a distribution of particle sizes. Motivation for this project is twofold: (i) continuum theories (i.e., Navier-Stokes-like equations) for the particle phase are developed for a discrete number of species, and (ii) use the fewest number of discrete particles in the approximation for an accurate representation in order to reduce the computational load that will be needed to solve the model equations. Recently, I have been focusing on the constitutive relations, or more specifically the transport coefficients, for moderately dense binary mixtures. Significant contributions have been made in this field with regards to dilute binary mixtures; however, my focus is to analyze the importance of a dense-phase extension to its dilute counterpart.

Impact of Particle Size Distribution on Bubbling Bed and Riser Characteristics 

Investigate experimentally the impact of polydispersity on bubbling fluidized beds and circulating fluidized beds. With regards to bubbling fluidized beds, an understanding of impact of widths of particle size distributions on segregation and bubbling characteristics will be sought. With regards to circulating fludized beds, how riser characteristics (including mass flux, particle velocity, clustering, etc profiles) vary with respect to particle material and shape of distributions will be investigated on two separate scales.

Collaborators: PSRI, Millenium

              

                  

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Past Students: J. Aaron Murray, Brent Rice, Jia Chew, Drew Parker, Jeff Wolz, Kat Potter

Relevant Papers:

Axial segregation in bubbling gas-fluidized beds with Gaussian and lognormal distributions of Geldart group B particles, J. W. Chew, J. Wolz, and C. M. Hrenya, AIChE Journal, in press, 2010.