Simulation of gas-solid flow behavior in downers using a new drag model based on the spatial superposition assumption

Abstract

A drag model based on the “spatial superposition” assumption is built to better simulate the local heterogeneous structure of the downer in a triple-bed circulating fluidized bed system. Because of this special assumption, fewer structure parameters will be required to describe the local structure, and the number of corresponding governing equations derived to converge the model calculation decreases, significantly saving calculation time when compared with the classic multi-scale model. To improve the efficiency and ensure the stability of the coupling calculation, we calculate the heterogeneous index (Hd) by introducing a matrix and then further couple it with the momentum equation in the two-fluid model. The Hd values exhibit a valley-shaped surface trend, which agrees positively with other reported works. The coupling simulation results indicate that this model can more accurately predict the axial solids holdup distribution than the Gidaspow drag model. Additionally, based on the calculated axial pressure and pressure gradient distribution under different superficial gas velocities or solid mass fluxes, the widely accepted flow pattern in a downer can be clearly captured. Through comparison with results from the classic multi-scale drag model, we confirmed that this developed model is useful and can be further applied to other gas-solid systems with heterogeneous structures.