CFD study of exit effect of high-density CFB risers with EMMS-based two-fluid model

It has been widely recognized that the exit geometry of circulating fluidized bed (CFB) risers may have significant effects on the hydrodynamics of gas-solid flow. However, a systematical study of exit effect is still lack after extensive experimental studies, possibly due to the fact that systematical modification of exit geometry in experimental study is an expensive and time-consuming task. In this study, after further validation of the recently developed EMMS-based two-fluid model (Wang et al., 2012. Chem. Eng. Sci. 75, 349-358; Zhou et al., 2014. Chem. Eng. Sci. 107, 206-217), the model is used to systematically investigate the effect of exit geometry of high-density CFB risers, fully taking the advantage that bed geometry can be easily modified in computational fluid dynamics study. It is shown that (i) the type of exit has a significant effect on hydrodynamics, the use of abrupt exit results in an increased solids concentration, not only in the immediate vicinity of the exit but also for a considerable distance down to the riser; (ii) the cavity height of abrupt exit, the curvature diameter of smooth exit and the length of the horizontal tube connecting the exit and the primary cyclone only have a minor effect or have no influence on the bed hydrodynamics of the studied risers; (iii) more importantly, the decrease of the diameter of abrupt exit tube results in a remarkable increase of solids holdup, the symmetry of radial solids concentration near the exit can also be enhanced significantly. However, in case of smooth exit, the diameter of the exit tube has no effect on the bed hydrodynamics at all. All of those results are in agreement with conclusions obtained from previous experimental studies, thus offering further validation of the EMMS-based two-fluid model for modelling heterogeneous gas-solid flow.