Advanced Research:  Development, Verification and Validation of Multiphase Models for Polydisperse Flows

 

DE-FC26-07NT430098 Project Description


Polydispersity is well-known to have a strong impact on the performance of fluidized, gas-solid systems such as coal combustors.  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 (Workshop on Multiphase Flow Research, DOE NETL, Morgantown, WV, June 6-7, 2006).  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.

 

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To address the aforementioned shortcomings, the overall aim of this 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 obtained for polydisperse systems.  Furthermore, the impact of polydispersity on both solid-phase instabilities (clustering) and gas-phase instabilities (turbulence) will be probed.


These goals will be achieved using state-of-the-art methodologies tailored to polydisperse systems.  For solid-solid closures, two complementary approaches will be pursued for use in isolation and in hybrid form.  Namely, a kinetic theory of granular flows (KTGF, Hrenya) and a direct quadrature method of moments (DQMOM, Fox) will be developed in which the reliance on over-limiting assumptions will be eliminated.  To describe gas-solid interactions, a drag law will be developed via a combination of the lattice-Boltzmann method (LBM, Sundaresan) and the discrete-time immersed boundary method (DTIBM, Subramaniam).  Novel aspects of this approach are the incorporation of the effects of random particle motion as well as a nonzero mean velocity between species.  Finally, for the first time, the effect of polydispersity on instabilities in the gas-phase and solid-phase will be explored using direct numerical simulation (Subramaniam) and coarse-graining (Sundaresan) techniques.


Collectively, this effort will result in a fully-specified, continuum model for polydisperse systems, which will be implemented into the MFIX framework.  Model validation will proceed in a stepwise manner ranging from simplified systems designed to isolate the constitutive quantity of interest to complex problems of industrial importance.  Both simulation (Hrenya, Subramaniam) and experimental (Cocco, Hrenya) data will be obtained for binary mixtures and continuous distributions.

A Department of Energy National Energy Technology Laboratory Project

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