Numerical simulation of gas-solid two-phase flow behaviors in a coal mill separator

Abstract

In order to understand the behavior of gas-solid two-phase flow motion and the process of pulverized coal separation in coal mill separator, the numerical simulations on the gas-solid two-phase flow in the separator of a laboratory-scale coal mill were carried out. In the framework of the Eulerian-Lagrangian method, the realizable k-ɛ turbulence model was used to describe the gas-phase field and the discrete phase model with the random walk model was used to describe the pulverized coal particle motion in the simulations. On the basis of cell independence verification, the results of one-way coupling and two-way coupling simulations were compared with the experiments. It is found that the particle size distribution of pulverized coal obtained by the two-way coupling method agrees better with the experimental data than that obtained by the one-way coupling method. The gas phase flow behavior, particle motion characteristics and separator performance in the laboratory-scale coal mill separator were further investigated using a two-way coupling method. The results show that the particle overflow occurs later and the overflow lasts longer when using the two-way coupling method. Because of the effect of particles, the airflow velocity difference between the inner wall of the separator and the outer wall of the inner cone of the separator becomes large, and the uniformity of airflow velocity distribution in the inner cone becomes poor, and there are obvious backflow regions in the primary and secondary separation zones. With the increase of the inlet airflow velocity, the particle overflow rate increases, and the particle discharge rate decreases linearly. The inlet airflow velocity has a small influence on the total particle return rate but has a profound influence on the grade return rate. The grade return rate exhibits a unimodal distribution at the lower airflow velocity, and it displays a bimodal distribution at the higher airflow velocity. When the inlet airflow velocity remains constant, the grade separation efficiency increases as the particle diameter increases. The greater the inlet airflow velocity, the slower the grade separation efficiency increases with the particle diameter, and the larger the cut-off diameter of the separator.

Keywords

• gas-solid two-phase flow
• coal mill
• separator
• two-way coupling