CFD simulations of circulating fluidized bed risers, part II, evaluation of differences between 2D and 3D simulations

Two-dimensional (2D) numerical simulations have been widely reported in the literature for qualitative, even quantitative, study of the complex gas–solids flow behavior in circulating fluidized bed (CFB) risers. It is generally acknowledged that there exist quantitative differences between 2D and three-dimensional (3D) numerical simulations. However, no detailed study evaluating such differences can be found for simulations of CFB risers. This paper presents 2D and 3D numerical simulations of three different CFB risers. Axial pressure gradients from both 2D and 3D simulations are compared with the experimental data. It has been clearly demonstrated that the 2D simulation cannot satisfactorily reproduce the 3D simulation results. A further comparison of radial profiles of void fraction and solids velocity for an axi-symmetric riser configuration is reported and the quantitative differences between 2D and 3D simulations are analyzed. In conclusion, 2D simulation is only recommended for qualitative evaluation and 3D modeling is recommended for predictive simulations.