Experimental And Large Eddy Simulation Study Of The Isothermal State Turbulent Characteristics Of A Two-stage Axial Swirl Combustor

In engineering practice,rotating turbulent flow technology is usually used to stabilize the flame in the combustor by using the central recirculation zone generated by the combustion device.At the same time,it can also promote the rapid mixing of fuel and oxidant to achieve the purpose of improving combustion efficiency and reducing pollutant emissions.However,the coherent structures such as vortex breakdown and precession vortex core existing in the swirl flow field will have a very complicated effect on the flow field,which requires in-depth study through a combination of experimental measurements and numerical simulation calculations.This paper systematically studies the cold flow field characteristics in a two-stage axial swirl combustion chamber independently designed by our research group.The swirler includes 8 blades,the inner and outer swirler blades have the same rotation direction,and the number of swirler is 0.53.OpenFOAM software was used to study the large eddy simulation?LES?calculation of the flow field,and particle image velocimetry?PIV?was used to measure the velocity field.The LES results and PIV results were compared and found to be in good agreement.With the same Reynolds number of the inner and outer pipes,two working conditions were studied:1)CO₂ passes into the inner tube,and the outer tube carries air;2)the inner tube is connected with air and the outer tube with CO₂.Studies have shown that a large central recirculation zone appears in case 1,while a small scattered recirculation zone appears only in case 2 away from the outlet of the swirler.The reason is that due to the difference in gas density and viscosity,the ratio of the outflow speed of the inner pipe and the outer pipe under the working condition 2 is3.5 times that of the working condition 1,and the circumferential velocity shows a small distribution outside and a large distribution central region.The overall rotational momentum is reduced,which is not conducive to the formation of low pressure areas.In the case of the same volume-to-flow of the inner and outer pipes,two inflow conditions were also studied:3)the air flows into the inner tube,and the CO₂ passes into the outer tube;4)the inner and outer tubes are connected with CO₂ and air,respectively.The research shows that a large central recirculation zone appears in working condition 3,but there is no central recirculation zone in working condition 4,only local recirculation zones appear on both sides of the combustion chamber.The reason is that under working condition 4,the ratio of the outflow velocity of the inner tube of the outer tube is very large.The circumferential velocity cannot be coupled with the axial velocity.A low-pressure region cannot be formed in the combustion chamber,making it difficult to form a central recirculation zone.Through the analysis of the pulsation velocity and Reynolds stress,it is found that there is a strong shear layer at the junction of the inner and outer tubes in the upstream regions of the flow field.Under the condition of the central recirculation zone,there appears between the central recirculation zone and the mainstream in another shear layer.Vortex breakdown mainly occurs in the upstream regions.The vortex structure develops in the axial and radial directions,but under operating conditions 2 and 4,due to the relatively larger inner tube speed,the vortex structure extends far downstream and even reaches at the exit position of the combustion chamber.No obvious precession vortex core phenomenon is seen in the four working conditions.Through the research results of this paper,it is found that the two-stage axial swirl burner designed by this our research group has good flow field characteristics,which provides a good experimental platform for studying the turbulent combustion phenomenon in near future.This study provides the deep understanding of the cold flow field characteristics in this swirl combustor,which lays the foundation for the next study of turbulent combustion characteristics.