Large Eddy Simulation Of Turbulent Flow Characteristics In Dual-swirling Inflow

In actual engineering applications,in order to promote complete combustion,swirling inflow is often used,and the central recirculation zone caused by the swirling flow to generate vortex crushing can achieve sufficient mixing while stabilizing the flame.The swirling combustion technology is stable,efficient,and clean.The characteristics have become a means of great development potential.At the same time,in order to reduce the emission of NOx and particulate matter,the swirling combustion technology has been widely used in aerospace and ground gas turbine combustors and various industrial devices.The swirling flow is generated by the cyclone,where the spiral vanes guide the airflow to form a spiral state,and at the same time,the flow velocity is also slowed down,effectively reducing the pressure fluctuation in the horizontal straight pipe.This paper uses UG and ICEM-CFD software to model the double cyclone designed by our group and to divide the high-quality block structure,including the double-swirl device of the inner and outer pipes.Large eddy simulation was carried out to study the mixing speed of air with methane and air with nitrogen in the device by using OpenFOAM calculation program.Three-dimensional cold flow field calculations were performed on the device using OpenFOAM software,including co-rotating and counter-rotating cyclone with swirl angles of 60,90 and 120 degrees,with swirl numbers of 0.25,0.36 and 0.51,respectively.On the premise that the momentum flux of the inner and outer pipes is the same,methane in the inner pipes and air in the outer pipes have different velocity inflow.The results show that under the circumstance of the counter-rotating cyclone,the velocity in the inner and outer pipes produces strong shear force,which accelerates the mixing of flow field,so it develops more completely downstream.In the middle and lower reaches of the hydrocyclone,stronger vortex breakdown are produced.Compared with the flow field at the same position,the mixing of the counter-rotating cyclone is more complete and the development of the flow field is faster than that of the corotating cyclone.In addition,the flow field expansion angle of the co-rotating cyclone is larger than that of the counter-rotating cyclone and the streamwise vorticity of the counter-rotating cyclone are stronger than that of the co-rotating cyclone.The existence of precession vortex core can be observed in a counter-rotating cyclone,but not in a corotating cyclone.Kelvin-Helmholtz instability in the swirl shear layer is the cause of this phenomenon.By comparing three different swirl numbers of methane flow field,it was found that the increasing swirl number accelerated the mixing of flow field and the diffusion of components to a certain extent.At the same time,for the cyclone with a swirl angle of 60 degrees,the mixing process of air with methane and nitrogen was studied.It is found that there are some differences in the mixing process of gas and air of different densities.Due to the difference of diffusion coefficients,the distribution of components at the outlet of the cyclone is different,and the flow vortex and plane vortex in the flow field also have certain differences.Based on the results of this study,it can be concluded that the swirl intensity has a great influence on the development of the flow field,and the enhancement of the swirl intensity promotes the mixing of the flow field.The difference of shear force between the co-rotating cyclone and the counter-rotating cyclone also leads to the difference of shear force.Obviously,the flow field of the counter-rotating cyclone develops faster.The difference of fluid density also affects the development of flow field to a certain extent.