Study On Design Theory Of Centrifugal Air Classifier

Centrifugal air classifier is an important device in powder processing.The flow field distribution is a key factor affacting their performance.Up to now,various types of centrifugal air classifiers have been developed.They usually have more than one air inlet,leading to the complex flow filed distribution.Numerous studies were focused on the specific air classifier like the third-generation turbo air classifier.The flow field in turbo air classifier has been described clearly,and much attention was paid to the optimization study of flow field in the key zone.However,what kind of overall flow field generated by the air inlets to be more suitable for particle classification has not been well solved.That is,there a lack of comprehensive investigation on creating the flow field from overall point of view.In this study,the Computational Fluid Dynamics technology and experimental technique were applied to find out the reasonable vertical and horizontal classifying vortex.Then effects and mechanism of the elutriation air on the primary flow field were studied.Moreover,design theory for improving the classification accuracy were put forward after summarizing the effects of flow field distribution and particle motion on the classification performance.Based on this theory,the new dynamic horizontal vortex classifier was developed.Main contents and results are as follows:(1)A comparison flow field characteristic of the new vertical vortex classifier and traditional cyclonic classifier was obtained by numerical simulation.A pair of vortex was formed in the new classifier,named the upper vortex and lower vortex.The downward air volume of new classifier decreases near the wall.The upward swirling flow composes the upper vortex,which elutriates the near-wall fine particles and classifies them again in the axial direction.The experimental results show that compared with traditional cyclonic classifier,the new classifier' classification size ratio increases by 27% and pressure drop reduces by 42% on average,respectively.It is suggested that the new classifier has the features of high efficiency and low energy consumption.The elutriation air has less influence on the primary flow field characterized by double-vortex but mainly affects its stability and central axial velocity distribution.The elutriation gas and primary airflow competes for classification space and forms their respective flow field.The proper elutriation air velocity improves the stability of the primary flow field and restrains the vortex end swing,which is favorable for the secondary classification and discharge of the fine particles.(2)The choice of feeding position is coupled with flow field distribution,and effects of the three representative feeding positions on particle motion and classification performance were investigated.The results indicate that feeding near-wall causes much fine particle reporting to the coarse product and impairs the classifying accuracy.This is not appropriate when the key problem is particle classification.It increases coarse product loss when the raw material was fed at the centre.Feeding at the region with high force improves the particle dispersion and shortens the particle residence time,which helps to achieving a rapid separation of fine and coarse particles.Based on the particle force analysis,it was found that the centrifugal force and axial drag force are the dominant driving factor to make coarse and fine particles separated.However,the radial drag force has less influence on the particle motion.It is considered that the optimal feeding position locates at the peak of the tangential velocity.A calculation formula of the optimal feeding position is deduced and checked.(3)The effect of operational parameters and elutriation air type on flow field and performance of the typical horizontal air classifier was explored.The rotor cage speed and air volume has limited influence on the general flow pattern and tangential velocity near the rotor cage.Moreover,the swirling elutriation air causes the non-uniform airstream of the primary horizontal vortex.In extreme cases,a double-layer vortex with opposite directions appears in the classifying chamber,which results in the appearance of small local vortex and weakens the centrifugal force.In addition,the secondary counter-flow generated by the shutter-shaped air inlet has less distraction on the primary flow and improves the primary flow field distribution of horizontal vortex.The yield of coarse product could increase by more than 3% and Newton efficiency increases by about 6%.(4)Design theory for high efficiency centrifugal air classifier was put forward based on the above investigation and our preliminary exploratory work.It can be explained as follows.The gas forms a primary flow field and another flow field for elutriating the entrained fine particles in the coarse fraction.They work together to classify the particles.The swirling gas dominates the primary flow field with moderate intensity.Both of the flow filed evenly distributed with less velocity fluctuation.Besides,there is no small-scale vortex structure.The flow field for elutriation the fines should match the primary classification field,and the pathline of elutriation air changes as little as possible when joining with the primary air flow.The primary and elutriation flow field are relatively independent on the spatial distribution and have less interaction,which can be generated by the separated airflow.The feeding position should be far away from the wall to avoid fine particles directly captured by the wall and far away from the gas outlet to decrease the coarse particle loss.The feeding position locates at the strong force field,offering high initial acceleration for the separation of fine and coarse particles.This theory offers macro guidance for optimizing the geometry of centrifugal air classifier.(5)A novel horizontal air classifier was designed and simulated.The flow field distribution is more reasonable.The primary air forms the jet effect to disperse the particles at the inlet,and then creates uniformly distributed flow field in the classifying chamber.The secondary air forms the elutriation flow field characterized by counter plug flow.Hardly any of them experience interactions.The gradient of axial velocity is small,indicating a two dimensional flow field distribution.According to the experimental results,maximum Newton efficiency of the new horizontal air classifier reaches 87% and the classification accuracy index can be as low as 1.53.Considering the theoretical force analysis on particle and effects of the elutriation air and particle concentration,the correction formula of cut size was proposed,and the predicted values match well with the experimental dates.It thus provided guidance for the design and application of the new classifier.