Numerical Simulation On The Airflow And Gas-Solid Flow Of Turbo Air Classifiers

Turbo air classifier, which uses air as the classification medium, is one kind of important equipments for the mechanical preparation of ultra fine powder. Movements of particles in the classifier depend on the airflow motion characteristics, which thus affect the classification performance. Investigation of air and particles' motion features in classifiers has been a key avenue for the improvement of classification precision. In this paper, simulations of flow field in the turbo air classifiers were carried out using FLUENT, a dedicated CFD software for fluid-flow problems in complex geometric regions, to determine the optimum condition parameters for establishing smooth and steady flow field. Furthermore, material trajectories were calculated based on FLUENT discrete phase model, providing a comprehensive and intuitionistic method to comprehend the classification process. These investigation results may provide reference for the optimization of inner flow field and the determination of turbo air classifiers' cut-size.Results of flow field simulation showed that (a) the increase of rotor cage speed could help facilitate the tangential velocity of inner air flow, but not affect the distribution of tangential velocity between the vanes, which kept increasing along the radial direction; (b) uniform radial velocity distribution appeared only at the critical speed, higher or lower cage speed would cause fluctuations of radial velocity. The steadiest distribution of radial velocity appeared in the condition of 60 vanes in the rotor cage and ratio (rotor cage speed/inlet air speed) of 100.Adding different numbers of horizontal plates into the volute affected axial velocity, radial velocity and turbulent dissipation rate(ε) of flow field. A critical value (3 horizontal plates) existed making a lowest axial velocity with a most uniform distribution. The tangential velocity of air flow could be enhanced by the addition of horizontal plates, which may weaken the impact of inertial counter rotating vortex on the front radial velocity, and led a steady flow field between the blades. However, excessive plates would be counterproductive-the turbulent dissipation rate(ε) was greatly enhanced with the increase of horizontal plates.The particles'trajectories with variable injection positions were simulated based on FLUENT discrete phase model. Particle's initial injection position had a great effect on its trace in the classifier. For particles of the same size, different injection positions caused different trajectories. The closer injection point approached the inner edge, the higher possibility large particles went into the fine. The values of cut-size calculated for different injection points of annular region were:inner edge> middle edge> outer edge. The experimental investigation showed that the cut-size obtained by injecting particles from the inner of annular region was closest to the actual one.