(22c) Meso-Scale Structures in Binary Gas-Solid Suspension Flows

Gas-particle flows are ubiquitous in chemical process industries. These flows often manifest inhomogeneous structures such as clusters and streamers in dilute particle laden flows (such as those encountered in riser flows) and bubble-like voids in dense fluidized beds. In most practical devices a distribution of particle sizes is quite common, and in some cases the particle size distribution (PSD) is tailored to mitigate the extent of inhomogeneities generated in the flows.

Much of the early studies on analysis of gas-particle flows were based on continuum models where the fluid and particle phases are treated as inter-penetrating continua [1], and these studies further assume that the particles are all of uniform size. Constitutive models for gas-particle flows where the particles primarily interact through binary collisions have been developed in the literature by adapting the kinetic theory of gases to granular materials [2,3]. It is now well known that these model equations capture inhomogeneous flow structures such as clusters and bubbles [2,4] and that the details of the structures depend markedly on grid resolution [4,5]. Coarse-grained equations that can be simulated using coarse numerical grids have also been developed in the literature by filtering these kinetic theory based models for uniformly sized particles [5].

The present study is concerned with the influence of PSD on the details of inhomogeneous flow structures. As an initial step in this direction we have carried out simulations of bidisperse gas-particle systems in periodic domains and compared the results with equivalent monodisperse cases. In these simulations we have employed the kinetic theory model framework developed by Garzó, Hrenya, and Dufty which treats polydisperse systems using one momentum balance and one fluctuating energy balance equation for the particle mixture and determines individual particle species velocities and granular temperatures through algebraic constraints on particle mass flux and cooling rate [6,7]. These equations have been implemented in MFIX [8], along with drag laws for bidisperse systems developed by Holloway et al. [9].

The simulations reveal that bidisperse systems readily form meso-scale structures such as clusters and bubbles just like the corresponding monodisperse cases. Decreasing the coefficient of restitution increases the size of the clusters; in addition, the scale of these structures decreases with increasing grid resolution both of which are similar to what is observed for monodisperse systems. This suggests that coarse-grained models for the polydisperse systems are likely to be necessary for large scale process analysis, just as in the monodisperse case.

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