(633d) Validation Studies On Filtered Model Equations for Gas-Particle Flows in Risers

Gas-particle flows in fluidized beds and riser reactors are well known to exhibit large fluctuations in velocities and local suspension density. In riser flows, these fluctuations are associated with the random motion of the individual particles (typically characterized through granular temperature) and with the chaotic motion of particle clusters, which occur over a wide range of time and length scales. The clusters form as a result of inherent instabilities of the fluidized state and/or inelastic particle-particle collisions; the two-fluid model equations are able to capture their existence in a robust manner; however, to resolve the clusters at all length scales, extremely fine spatial grids are necessary [1]. Yet, due to computing limitations, gas-particle flows in large fluidized beds are invariably simulated by solving discretized versions of the two-fluid model equations over a coarse spatial grid. Such a coarse-grid simulation will clearly not resolve the structures which exist on sub-grid length scales. The need to account for the consequences of these unresolved structures through suitable sub-grid models is now well established [1-3].

In the present study, we have constructed a filtered model appropriate for 3D coarse-grid simulations by filtering results obtained through kinetic theory model simulations of uniformly sized particles [1-5]. We have simulated the PSRI challenge problem in order to validate the approach. The experimental data for the challenge problem were generated in a 14.2 m tall circulating fluidized bed riser with a 0.2 m i.d. located at the PSRI experimental research facility in Chicago [6]. Several research groups have studied the system and the detailed description of the experimental unit can be found elsewhere [6-8]. We have focused on 3D simulations employing a simplified 3D geometry [9]. The 3D coarse-grid simulations (of filtered model equations) manifested nearly grid-size independent time-averaged solutions. In addition, the simulations corresponding to different filter sizes yield nearly similar results. The details of these results and a comparison of the simulation results with the experimental data will be described in the presentation.

References:

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