A Study On Numerical Simulation Of Classifying Flow Field In Turbo Classifier

Turbo air classifier is a important style of accurate classifying facilities. The flow field of inner cavity in turbo air classifier is a gas-solid two phase flow of air and ultrafine particles, and the state of its movement is quite complicated. However, whether the flow field is steady and is in favor of the effectual disperse between the coarse and fine powder and so on, are some problems which can determine whether the design for the ultrafine classifying equipments is successful or not; and besides, in order to design reasonable classifying chamber structure and adjust optimal operation parameters, a good grip on the flowing characteristics of classifying flow field and motion rules of the particles is very essential and significant.First, in this paper, as for a new type of turbo air classifier, FW- 4> 150 horizontal turbine superfine classifier, the theories of hydrodynamics and numerical calculation is applied to numerically simulate the gas-solid two phase flow field of the inner cavity between two blades in classifier by virtue of a mature commercial CFD code PHOENICS .In the course of the simulation , the theoretical model of two fluid and the stochastic trajectory model of particles in two-phase flows are used respectively. The calculation results shows that , (1) when the rotor rotates at the working speed, a local regurgitant whirlpool that is rightabout to the rotor's rotating direction appears in the classifying region, which makes the separated granules come back to the outside of the classification district and influences the classification efficiency of the turbine classifier;(2) in the condition of a low rotating speed of turbo rotor, ultrafine particles can follow the gas phase by and large, but when the rotating speed increases, a phenomenon of velocity slippage between two phases appears evidently;(3) A operating parameter, air flow rate is so important that can directly influence the stability of flow field in classifying chamber, the bigger the air flowrate is , the steadier the flow field is;(4) Compared with straight blades, the new shape of blades with backward crooked elbow are helpful to improve the stability of flow field and form whole stream easily, also beneficial to advance the classification accuracy and classification efficiency;(5) The motion trace of particles are very complicated when the turbo rotor rotates at a high speed, the constant bounce of the particles appear between the blades and the coarse particles are even bounce back from the inlet, only the finer can pass through the classification area from the oulet;(6) Along with the increase of the rotor's rotating speed, radiah axial and tangential velocity all begain to fluctuate repetitively like sine-wave, the smaller the particles are, the more violently they fluctuates.Second, a theoretic analysis of the flow movement in the flow chunnel between two blades is made, and the result reveals that the axial circumfluence phenomenon often exsists in revolving machines is the main reason that result in the local circumfluence votex in the classifying flow field, however, when the shape of blade is designed in reason, especially the backward crooked blades are used, which can improve the stability of flow field to some extend.Finally, some simulation results are compared with the theoretic calculation and the experimental data, and they shows a good consistency. These simulation results described the actual flowing state of the air and the ultrafine particles in classifying chamber from different sides, which can provide some theoretic evidence for optimizing the structure of the turbo rotor and adjusting correlative operation parameters appropriately in the future.