Modelling and simulation of a gas–solids dispersion flow in a high-flux circulating fluidized bed (HFCFB) riser (original) (raw)
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CFD-simulation of a circulating fluidized bed riser
Particuology, 2009
In the current work, a model of the fluid mechanics in the riser of a circulating fluidized bed (CFB) has been implemented using computational fluid dynamics (CFD). The model developed shall be used in future as the basis of 3D-reactor model for the simulation of large scale CFB combustors. The two-fluid model (TFM) approach is used to represent the fluid mechanics involved in the flow. The computational implementation is accomplished by the commercial software FLUENT. Different closure formulations are tested on a simplified geometry. Two different turbulence formulations, namely the swirl modified RNG k-ε model and the Realizable k-ε model, are tested in combination with two different approaches to solid phase turbulence, namely the dispersion and per phase approach. One focus of the current work is put on the study of different drag correlations. Besides the drag correlations by Syamlal et al. [Syamlal, M., Rogers, W., & O'Brien, T. J. (1993). MFIX documentation theory guide. Technical Report DOE/METC-94/1004, U.S. Department of Energy (DOE). Morgantown Energy Technology Center: Morgantown, WV] and Gidaspow [Gidaspow, D. (1994). Multiphase flow and fluidization. New York: Academic Press] the EMMS model has been used to determine the momentum exchange between the two phases. The resulting formulation is then used to simulate a 1-m × 0.3-m cold CFB setup and is validated by experimental results [Schlichthärle, P. (2000). Fluid dynamics and mixing of solids and gas in the bottom zone of circulating fluidized beds. Unpublished doctoral dissertation, Technische Universitaet Hamburg-Harburg, Shaker Verlag: Aachen].
Powder Technology, 2008
CFD modeling of air and fluid catalytic cracking (FCC) particles in the riser of a high density circulating fluidized bed (HDCFB) has been performed. The implementation of correct inlet conditions was found to be critical for the successful simulation of the hydrodynamics. The simulated profiles of gas and solid velocity and volume fraction were overall in good agreement with experimental data reported in the literature. However, due to the difficulties in accurate modeling of the solid segregation toward the wall, the solid volume fraction was under predicted near the walls. The effect of modeling parameters including different drag models, wall restitution coefficient values, and solid slip conditions have been evaluated. While the wall restitution coefficient did not exhibit a significant effect on the riser hydrodynamics, the appropriate slip condition aided in predicting the solid segregation toward the wall.
CPFD simulation of circulating fluidized bed risers
Powder Technology, 2013
This study investigated the applicability of computational particle fluid dynamics (CPFD) numerical schemes for simulating flows in circulating fluidized bed (CFB) risers. Gas-solid flows were simulated in CFB risers containing Geldart A particles with both low and high solid fluxes as well as in a CFB riser containing Geldart B particles for three flow conditions using CPFD. The results are compared to experimental data and previous two-fluid model (TFM) simulations. The time-averaged axial and radial distributions of the solid concentration show that the bottom-dense, upper-dilute and core-annulus heterogeneous structures were successfully captured by the CPFD calculations, but only qualitatively. The results differ from experimental data for Geldart A particles and high solid fluxes, although they were more accurate than two-fluid simulations with conventional drag models. Two-fluid modeling with the EMMS (energy minimization multi-scale) drag model gave more accurate results than the CPFD simulations. The results indicate that the drag force in the CPFD scheme is still overestimated, although the cumulative method used to compute drag force is more accurate than the proportional method in the two-fluid model. An EMMS drag model which takes into account the intrinsic heterogeneity in the CFB risers is needed for the CPFD scheme. The effect of the realistic particle size distribution was seen in the by CPFD results.
Chemical Engineering Communications, 2005
A comprehensive investigation was carried out to study hydrodynamics aspects of secondary air injection in circulating fluidized beds. This article presents modeling and results of computational fluid dynamics simulations of gas-solid flow in the riser section of a laboratory-scale (ID ¼ 0.23 m, height ¼ 7.6 m) circulating fluidized bed with a radial secondary air injector. The gas-solid flow model is based on the two-fluid (Eulerian-Eulerian) approach, where both gas and solids phases are treated as interpenetrating continua. A granular kinetic theory model is used to describe the solids phase stresses. The simulation results are compared with measured pressure drop and axial particle velocity profiles; reasonable agreement is obtained. Qualitatively, excellent agreement is obtained in predicting the increase in solids volume fraction below secondary air ports, the accumulation of solids around the center of the riser due to momentum of secondary air jets, and the absence of the solids down-flow near the wall above the secondary air injection ports, which are the prominent features of secondary air injection observed in the experiments.