Large Eddy Simulation Of Turbulent Combustion In A Double-Swirl Combustion Chamber

In the design of gas turbine,a rotating inflow is usually adopted,through swirler which a central recirculation zone can be generated in the upstream of the combustion chamber so as to fully mix the fuel and improve the stability of the combustion flame.However,there are coherent structures such as precession vortex core and vortex breakage in the turbulent field of rotating inflow,which are accompanied by the generation and transformation of different flame structures under different working conditions.Therefore,at the beginning of the design of the combustion chamber,it is necessary to analyze and improve the combustion state through numerical simulation and experimental research.In this paper,the research designed two double-stage axial swirl burners have carried on the experiment and simulation.Study are given priority to with LES large eddy simulation,verified the accuracy of grid,turbulent model and combustion model by using Particle Image Velocimetry(PIV)experimental data.Finally,3.5 million grid was selected for the subsequent simulation study.Two kinds of structures are studied in this paper:1.A combustion chamber with a width of 100mm and a height of 150mm plus a swirl with a center blunt body of 4mm at a rotation Angle of 100 degrees;2.Combustion chamber 120mm wide and 150mm high plus 2mm cyclone with center bluff body at 120 degree rotation Angle.Large eddy simulation of turbulent combustion in a twin-swirl combustion cham ber was carried out to analyze the flow field variation,flame structure and vortex during combustion.In the first structure,four different working conditions were compared by changing equivalence ratio and inflow velocity.Studies show that in a certain range,with the increase of inflow velocity and equivalence ratio,the spatial position of IRZ will move to the downstream of the flow field.The position of the ORZ depends on the extrusion effect caused by heat release and the expansion acceleration effect of the working medium.The size of IRZ in the combustion chamber is related to the structure of the cyclone and is affected by the inflow velocity.It also increases with the increase of the inflow velocity,but there is a limiting size.The OH mass fraction and flame surface increased with the increase of inflow velocity and equivalent ratio,and the flame shape was M-shaped.The main reason was that the tensile coefficient K increased due to the flame hitting the wall,and the outer flame branch appeared in the ORZ region when the flame ignited the fuel.The spiral vortex structure was obvious in all the four working conditions,and the existence of PVC structure in the flow field was found in working conditions A and C.The vortex intensity is directly related to the inflow velocity,and the peak value of the vorticity is in the annular region.The formation of the two large-scale vortex rings is caused by the action of the shear layer on both sides of the inner and outer swirls.The size of the vortex rings increases along the radial direction with the development to the downstream,and the intensity decreases until the final crushing.In the second structure,select the conditions of flame structure transformation in simulation and use Q criterion,POD,FFT and other to analysis.It is concluded that the existence of internal and external shear layer directly affects the flame structure.The range of internal and external shear layer in V-shaped flame structure is larger than that in M-shaped flame structure,so the flame is more likely to be stable in it and less affected by the shear of both sides.The position of internal and external shear layer directly affects the flame structure.The flame structure is under the dual action of internal and external vortexes,and the different shedding frequencies of internal and external vortexes in the shear layer affect the constant change of the number of vortexes,which eventually leads to the transformation of flame structure.Therefore,the transformation of flame structure has an important relationship with the vortex shedding mode.The conversion process is accompanied by an increase in the frequency at the crest.In addition,the pressure pulsation amplitude in the time domain diagram of V-shaped flame structure is smaller when it tends to be stable.Therefore,V-shaped flame structure is considered to be a more stable flame structure.