Flow Field Analyses And Structure Optimization For Turbo Air Classifiers

In the field of powder preparation, classification is a key mechanical method to produce fine powder with narrow particle size. In recent years, turbo air classifiers have been the mainstream dynamic classifiers because of their simple structure, high reliability and controllable product granularity. Given the surge in the demand for fine powder, there is a growing trend for classification to obtain smaller cut size and higher classification accuracy. And that puts forward a higher requirement of turbo air classifiers' classification performance greatly. In this paper, the flow field characteristics in a turbo air classifier are analyzed using both simulated and theoretical methods. The core components of turbo air classifier, including rotor cage and volute, are optimized to improve the flow field in the classifier and increase the classification performance. Meantime, the design theories on turbo air classifier are enriched. The main contents in this paper are summarized as follows.The applications of FLUENT software on flow field and particle tracks simulation in turbo air classifier were investigated deeply. Combining the characters of turbo air classifier and the properties of calculation models in FLUENT, a reasonable numerical simulation was scheduled, which would improve the reliability of simulation. In this project, the turbo classification is divided into such three regions as region I,region II and region III. The type of Hex/Wedge mesh and Cooper meshing method are used to mesh. The model of ENGk-? is chosen as the turbulence model after comparing the common turbulence models. Meanwhile, the near wall is treated as standard wall function. The uncoupled mode in DPM is selected to simulate the particles motion, according to the calculation results of gas-solid ratio.Analyses were focused on the velocity distributions of flow field in the channel between rotor blades and at the inlet of rotor cage. The relative velocity distribution formula was derived theoretically, indicating the relationship between the rotor blade profile and relative velocity distribution. The relative velocity angle at the inlet of rotor cage was calculated based on the simulation results and velocity triangle theory. According to these analyses, a novel rotor cage with radial arc blade was designed with concave pressure side, whose installed angle was equal to inlet relative velocity angle. Numerical simulations by ANSYS-FLUENT 14.5, as well as material classification experiments, were implemented to verify this novel design. Simulation results indicate that the relative velocity gradient in the channel between rotor blades and incidence angle at the inlet of the rotor cage decrease significantly, and no air anti-vortex is present in the channels of the rotor cage. The simulative cut size decreases. The material classification experiment results demonstrate that, in design case (12-1200), the cut size decreases 11.5%, with no reduction of classification accuracy than that of straight blade rotor cage.The installed angles at the inlet and outlet of rotor cage were calculated totally by theory methods. Combining the installed angles and bending direction of the blades, a theory design method of rotor cage with non-radial arc blades was obtained. Numerical simulation and classification experiment were carried out to verify the improvements of a non-radial arc blade rotor cage which was designed using this method. Simulation results indicate that there is a significant improvement of flow field in the rotor cage with non-radial arc blades. The incidence angle at the inlet of the rotor cage decreases significantly. Airflow streamlines match the profile of the non-radial arc rotor blade perfectly, and no air anti-vortex is present in the channels of the rotor cage. The material classification experiment results demonstrate that the classification accuracy increases by 10.6%-40.8%, and the fine powder yield increases by 12.5%-40.1%, with an almost changeless cut size. The experimental results agree with the simulation results, thus verifying the feasibility of the modified rotor blades in practice.The path line formula of airflow in volute of the turbo air classifier was derived by making dramatic analysis on airflow. From this, a novel volute profile was designed and it was log spiral curve. This profile coincides with airflow path line. The simulation results indicate that the new designed volute has a favorable guidance function. Airflow rate is uniform at different position of guide vanes'inlet boundary. The velocity at different position of one circle is uniform. The discrete phase simulations indicate that the particles with the same size, which are released from different position of annual region, have the similar classification result. That will result in an improvement in classification accuracy.