Experimental Investigation On Thermoacoustic Oscillation Of Lean Premixed Flames In A Two-stage Axial Swirl Combustor

Gas turbines mostly use lean premixed combustion to achieve low emission targets.However,it brings thermoacoustic instability.These oscillations can produce high intensity noise,which will affect the normal operation of the gas turbine.Experimental system of thermoacoustic instability is built,the main fuel is premixed methane and air and a number of experimental investigation are carried out for the dual swirl burner.Based on the pressure waveform and power spectral density diagram,combined with the analysis of flame images taken by high-speed camera.New processing methods are used,including Phase space reconstruction,Recurrence plots,Flame image processing method based on Lab VIEW and Proper Orthosonal Decomposition(POD)method for combustion flows measured by PIV.The above methods are used to analyze the thermoacoustic oscillation phenomenon.Three processes are tested.Process 1: The equivalence ratio is changed by decreasing and increasing step by step.The change of equivalence ratio is an important cause of thermoacoustic instability.When the initial equivalence ratio is 0.90,the intensity of combustion chamber oscillation is the strongest.In the two sub-processes,the same conditions show different results.There may be a certain time memory effect in the transition of unstable combustion state,there is a hysteresis phenomenon.Process 2: when the Reynolds number ratio of the inner and outer tubes is 1,the oscillation intensity of the combustor is the strongest.When the Reynolds number ratio of the inner and outer tubes increases from 0.62 to 1,the oscillation increases;when the Reynolds number ratio of the inner and outer tubes increases from 1 to 1.8,the oscillation decreases.When the ratio of Reynolds number of inner and outer tubes is1.8,the oscillation disappears and the system returns to stability.The dominant frequency of thermoacoustic instability increases with the increase of Reynolds number ratio of inner and outer tubes,but it does not increase linearly.There are two kinds of thermoacoustic instability modes dominated by intermediate frequency in the dual swirl combustor,which are about 258.0 Hz and 400.0 Hz.Process 3: a stratified combustion is designed to invest low frequency oscillation.The low frequency oscillation frequency disappears completely when the delamination ratio is 1 and 1.03,and it still exists in other condition.The state at different times and the development of limit cycle oscillation can be clearly reproduced through the phase space,there is a clear correlation between PSD analyses.From RP diagram,the oscillation of the two intermediate frequency oscillations in the combustor are obvious.The main reason for the intermediate frequency oscillation of the thermal and acoustic coupling is the flow field structure fluctuation in the inner and outer circulation region of the dual swirling flame.The main reason for the low frequency oscillation is the heat release fluctuation in the central recirculation zone.The POD method is used to reconstruct the combustion flows measured by PIV.It is found that the development of the vortex structure inside the system expands and develops until it breaks down near the shear layer.Then,oscillations are generated by influencing the local equivalence ratio fluctuation,which affect the heat release fluctuation and pressure fluctuation of the combustor.