Diagnosis And Control Of Flow Field In Turbo Air Classifier Based On Entropy Generation Theory

Fine powder is widely used in chemical industry,food,materials,medicine and etc.With the rapid development of technology,the demand for fine powder with narrow particle size distribution is increasing.Turbo air classifier is an important equipment in particle preparation,and the flow field distribution affects its performance significantly.The flow field characteristic,the entropy production and the structure optimization of the turbo air classifier are investigated in our study.Firstly,CFD method is used for simulating the flow field.The error of the tangential air velocity between the simulated and the experimental data is within 10% and the small vortex appears between the rotor cage blades,which verifies the reliability of the CFD method.Secondly,the entropy generation analysis is established based on the CFD results and entropy generation theory.The results show that the energy loss value/total energy varies slightly and the classification accuracy is good when the inlet air velocity is 8.6m/s and the rotating cage speed is 800r/min ～ 1000r/min.This verifies the feasibility of entropy generation theory for the classifier.Specifically,the turbulent entropy production mainly appears around the rotating cage.The strong wall shear at the air inlet and outlet leads to the wall entropy production.The ratio of turbulent entropy production and wall entropy production to the total entropy production is 56.41% and 43.11%,respectively.Finally,the structure of the rotating cage blade is optimized to improve the flow field distribution based on the entropy generation analysis and DPM model.The wedge-shaped,inverted wedge-shaped and shuttle shaped rotating cage blades are designed.The results show that thinning the front section of the blade is beneficial to the stability and uniformity of the radial air velocity among the blades,while thinning the rear section aggravates the flow field fluctuation.Compared with the rectangular cage blade,the effective area of the wedge-shaped blade increases by 7.5%.The production of the total entropy,turbulent entropy and wall entropy decreases by 17.3%,25.86% and 3.34%.Correspondingly,less coarse particles mix into the fine powder and the classification performance has been improved.The above study not only offers strategy for the flow field analysis but also lays theoretical foundation for the structure optimization of the turbo air classifier.