Steady-state multiscale CFD simulation of a circulating fluidized bed riser

Abstract

Compared to transient simulation, steady-state simulation of circulating fluidized bed risers is more efficient, but is also harder to perform due to the complex scale-dependency of dense gas-solid flows. In this work, steady-state computational fluid dynamics (CFD) simulation of a riser is performed using the steady energy-minimization multi-scale (EMMS) drag. It is found that the steady state corresponds to an extremely large scale of length and time, thus the grid size required in steady-state simulation is larger than that in transient one. The time-averaged two-fluid model (TFM) coupled with the steady-state EMMS/1M drag model enables a good prediction of the S-shaped, axial solids distribution and the choking transition, whereas the two-phase turbulence and solids stress models are important in predicting the radially core-annular distribution of solids. So far as we know, this is the first time that one can predict the choking transition in a steady-state CFD simulation. Further improvement may need an EMMS modeling of the time-averaged solid stresses.