Modeling of the turbulent gas-particle flow structure in a two-dimensional circulating fluidized bed riser
Laboratoire Matériaux, Procédés et Energie Solaire, CNRS-PROMES, 7 rue du Four Solaire, 66120 Font-Romeu Odeillo, FranceChemical Engineering Science (Impact Factor: 2.34). 01/2007; 62(1-2):269-280. DOI: 10.1016/j.ces.2006.08.066
The gas–particle turbulent flow in a circulating fluidized bed (CFB) riser is investigated numerically by large eddy simulation (LES) coupled with Lagrangian approach. The gas phase model is based on locally averaged two-dimensional Navier–Stokes equations (N–S Eqs.) for two-phase flow with fluid turbulence calculated by LES, in which the effect of particles on subgrid-scale (SGS) gas flow is taken into account. The particles’ motion is treated by a Lagrangian approach, in which the particles are assumed to interact through binary, instantaneous, and non-elastic collisions. The model predicts the heterogeneous particle flow structure and the gas and particle mean velocities and turbulent intensities. The gas turbulence induces particle lateral dispersion. The instantaneous gas turbulent intensity is attenuated by the formation of clusters, whereas the intensity is enhanced by the increase of gas–particle interactions. Globally, the presence of clusters enhances the gas turbulent intensity. Finally, the turbulent model is noticeably affected by an empirically assigned gas constant Ck and it consistently predicts values of the gas and particle turbulent intensities higher than those predicted by Smagorinsky's method.
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ABSTRACT: The axial and radial solids holdup profiles were studied by measuring with an optical fibre probe in a 7.6 m rectangular circulating fluidized bed with a narrow 19x114 mm cross-section riser under a wide range operating conditions. Comparing the rectangular riser with the other cylindrical columns, it is found that the general shapes of the axial and radial profiles of solids holdup in rectangular riser are quite similar to that in cylindrical risers, but more uniform. It is concluded that the geometry of the riser is a factor affects the solids distribution due to the perimeter per unit cross-sectional area and the distance from the wall to the centre. In some extent, the two-dimensional and three-dimensional risers are more comparable under fast fluidization conditions. Generally, the solids distribution along the axial and the radial direction in rectangular riser are close to that in cylindrical risers, and the main differences have been known by the study of this paper.
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ABSTRACT: In large-eddy simulations (LES) of gas-particle flows most investigators use single-phase subgrid scale (SGS) stress models. The Interaction between the two-phase SGS stresses is not fully taken into account. In this paper, a unified second-order moment (USM) two-phase SGS stress model for the LES of gas-particle flows is proposed, in which the interaction between the two-phase SGS stresses and anisotropy of the two-phase SGS stresses is fully taken into account. The proposed model is used in the LES of swirling gas-particle flows, together with RANS modeling using the USM two-phase turbulence model. The instantaneous results exhibit the multiple recirculating gas flow structure similar to that of the single-phase swirling flows, but the particle flow structure shows less vortices. The two-phase time-averaged velocities and RMS fluctuation velocities predicted by both LES-USM and RANS-USM models are almost the same and are in good agreement with the experimental results. However, for two-phase RMS fluctuation velocities, the LES-USM results are somewhat better than the RANS-USM results.
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