A Study On The Characteristics And Mechanism Of Short-circuit Flow In A Gas Cyclone

Gas cyclone,which is widely used in industrial filed,especially for gas(liquid)-solid separation,is one of the most important equipments.Although its working principle is simple,its internal flow is extremely complicated.In addition to the main vortex,there are secondary flows in the cyclone separator,among which the shortcircuit flow has the greatest influence on the separation performance.The short-circuit flow exists near the mouth of the vortex finder of the cyclone separator,and its high centripetal velocity can cause a part of the airflow to directly enter the vortex finder,and escape from the cyclone.Theoretically,short-circuit flow can cause at least 10% of the inlet air flow to escape from the vortex finder of the cyclone,which not only reduces the separation efficiency of the cyclone,but also causes unnecessary energy dissipation,resulting in industrial costs production.In order to further improve the separation performance of the gas cyclone for fine particles,it is necessary to thoroughly study the mechanism and characteristics of the short-circuit flow.Regarding the generation of short-circuit flow,most scholars believe that it is caused by the asymmetry of the inlet of the ordinary cyclone separator.The configuration of single-inlet makes the flow field in the annular space of a gas cyclone non-axisymmetrical,and the radial velocity at the lower mouth of the vortex finder is slightly greater than the rest of the same cross section,resulting in the air flow passing here easily escaping from the vortex finder,which is regarded as asymmetric hypothesis.However,some scholars address that the airflow at the inlet of the cyclone can collide with the swirling flow inside the separator before it enters the mainstream.Such collisions inevitably produce local eddy flow near the wall of the cylinder,which eventually leads to short-circuit flow,which can be called as collision hypothesis.At present,the mechanism of short-circuit flow is just hypothesis,and there is still no theoretical support and verification.In this study,an ordinary Lapple cyclone separator is selected to investigate the mechanism and characteristics of short-circuit flow through numerical simulation,and the calculation of mass flow ratio is performed to clarify the efficiency and energy loss caused by the short-circuit flow.Meanwhile,this study can provide theoretical support for optimization and improvement of the cyclone separator.In the numerical simulation study,the Reynolds stress model(RSM)is first selected to simulate the characteristics of gas-phase in the cyclone,such as the distribution of static pressure,tangential velocity,axial velocity,radial velocity,turbulence intensity,and energy dissipation.Then,the distribution of radial velocity is used to analyze the formation area and characteristics of short-circuit flow,and calculate the proportion of its mass flow.After that,for two hypotheses of the mechanism of short-circuit,design the corresponding cyclone separator structure respectively.For asymmetric hypothesis,which believes that the single inlet structure of the cyclone separator causes the flow field asymmetry to form a short-circuit flow,the characteristics of short-circuit flow in the double-inlet cyclone separator are simulated and analyzed.For collision hypothesis,which believes that the air flow collision causes a short-circuit flow,the characteristics of the shortcircuit flow in the pent-roof cyclone separator are simulated and analyzed.Subsequently,the characteristics of short-circuit flow of the two cyclone separators are compared with the cyclone to reveal the mechanism of short-circuit flow.Finally,based on the mechanism of short-circuit flow,the optimized configuration cyclones,ring-type,volute cyclones and second-flow cyclones,are analyzed as well.The main research conclusions of this study are listed below: 1.The flow characteristics of the short-circuit flow in the cyclone separator show that the radial centripetal velocity has a greater impact on the formation of the short-circuit flow.The short-circuit flow exists in the annular zone formed by the large radial centripetal velocity at the lower mouth of the vortex finder,the height range is-0.14 m~-0.17 m in this study.2.The analysis of the flow rate of the short-circuit flow shows that the dividing line between the radial centrifugal velocity and the centripetal velocity can be used to determine the velocity distribution.In this paper,the radial velocity distribution of the short-circuit flow is-2.3 m/s~-5.6 m/s.Calculations based on the size and velocity of the short-circuit flow area show that the short-circuit flow rate accounts for 21.72%.3.The results of the mechanism of short-circuit flow show that the airflow entering the cyclone separator obtains a downward axial velocity after the airflow collides,causing it to form a downward flow along the outer wall of the vortex finder,and short-circuit occurs at the lower mouth of the vortex finder.That is,airflow collision is the cause of short-circuit flow.At the same time,the symmetrical structure of the double-inlet cyclone does not reduce or destroy the short-circuit flow.On the contrary,the mass flow ratio of the short-circuit flow(25.33%)is greater than that of the Lapple cyclone(21.72%),while there is no short-circuit flow area composed of radial centripetal velocity at the lower mouth of the vortex finder in the pent-roof cyclone,which further verifies that air collision is the main reason for the formation of short-circuit flow.4.The analysis of the effect of the cyclone optimization on short-circuit flow shows that the ring-type cyclone does not destroy short-circuit flow,otherwise this rate is increasing,the separation efficiency is decreased;The volute cyclone makes it easier for the airflow to merge into the existing rotating airflow in the cyclone,the collision of the airflow is significantly reduced,and the separation efficiency is increased;The second-flow cyclone introduces high-speed new air to impact the airflow,thereby destroying the formation of short-circuit flow.There is no complete ring area composed of radial centripetal velocity at the lower mouth of the vortex finder,and the separation efficiency is increased.Generally,the formation of short-circuit flow in a cyclone separator is mainly caused by the collision of airflow.The circular area composed of radial centripetal velocity can be used to analyze the morphology and calculation of the short-circuit flow.As the geometric configuration changes,the morphology of the short-circuit region can change to a certain extent.For example,the addition of a reverse inlet to the doubleinlet cyclone can increase the collision of airflow in the other direction,the circular area becomes symmetrical,and the proportion of short-circuit flow increases.At the same time,the short-circuit flow analysis of the ring-type,volute and second-flow cyclone proves that reducing or destroying the airflow collision can reduce the short-circuit phenomenon.This paper clarifies the flow characteristics and mechanism of the shortcircuit flow in the cyclone separator,and improves the configuration of the cyclone separator to reduce or destroy the short-circuit flow to improve its separation performance and reduce the emission of fine particles.The flow field analysis of this study can provide a theoretical basis for further optimization of cyclone separators.