Large Eddy Simulation Combined With System Identification For Partial Premixed Flame Dynamics Investigation

In order to deal with increasingly stringent emission regulations,it's necessary to reduce the gas turbines' emissions of using lean premixed combustion with its wide application.However the defect is that gas turbines may exhibit an increased risk in generating thermo-acoustically induced combustion instabilities.Combustion instability is a resonance phenomenon that arises due to the coupling between the system acoustics and the unsteady heat release.These instabilities which might cause damage of combustor parts and limit the stable operating window.Therefore it is important to investigate the combustion instability,and the key of which is the analysis of flame dynamics.This research focuses on the partially premixed flame response to the perturbations,and studies the partially premixed flame dynamics in a simplified combustor without bluff body,swirler and some certain type of combustion chamber with swirler and bluff body.The main research contents and conclusions are as follows:The methodology of this study is using Large Eddy Simulations(LES)of incompressible reacting flow in combination with System Identification(SI)method.And three different broadband excitation signals are used to investigate the influence of the signal type on the results of the identification at first,including a superposition of sine waves(SINE),broadband white noise signal(BBMWN)and discrete random binary signal(DRBS),with the theoretical flame response model proposed by Cuquel as basis for verification.And DRBS is chosen as excitation signal for the subsequent calculation.Then a relatively more accurate excitation amplitude is selected by comparing with the theoretical model in a simplified combustor model,and the excitation level of 5%average incoming velocity is in good agreement with the theoretical model results.And then the effects of basic boundary conditions such as different inlet velocities and inlet temperatures on the flame response characteristics are calculated and analyzed.It is found that with the increase of inlet velocity or temperature,the response amplitude of conical flame increases first and then decreases in the low frequency domain,and the phase saturation occurs earlier at relatively lower and higher inlet velocities or temperatures.Afterwards,in some certain type of swirling combustion chamber with a swirler and a bluff body,the effects of different inlet velocities,inlet temperatures,thermal boundary conditions,inlet pressures,fuel compositions,combustor geometries on the flame response characteristics are calculated and analyzed.With the increase of the inlet velocity,it is obvious that the recirculation zone is strengthened,and the flame length and flame speed increase.In the range of 80-350 Hz,the response amplitude of the flame is greater than 1.When the frequency is greater than 400 Hz,the response of the flame decreases,and the amplitude decreases until it reaches 0.It is proved that the flame is similar to a low-pass filter.With the increase of the inlet velocity,the flame amplitude increases first and then decreases,and the frequency response range also increases first and then decreases.The flame is more stable in the outer shear layer,and the flame length decreases gradually.The time for the perturbation to transfer to the flame front decreases,so the phase difference of FTF decreases gradually with the increase of the flame velocity.By changing the combustion chamber wall to adiabatic wall,the external shear layer will react strongly and the flame will become M-shaped topology.Compared with the non-adiabatic V-shaped flame,the adiabatic M-shaped flame has a wider frequency response(amplitude greater than 1).At the same time,the phase difference in the non-adiabatic state is larger than that in the adiabatic state.This is due to the instability of the external recirculation zone and the extension of the flame wall,which lengthens the flame and prolongs the time when the disturbance passes to the front of the flame.With the increase of inlet pressure,the central recirculation zone increases gradually,and a slight decrease in flame length can be observed.The amplitude of transfer function reaches a local peak at similar frequencies.The amplitude in low frequency region decreases with the increase of pressure,while the amplitude in high frequency region increases with the increase of pressure.Under all the research pressures,the trend of phase is almost the same before 330 Hz,and then there is a linear deviation and phase increase under higher pressures.With the increase of inlet temperature,the flame velocity increases,and V-shaped flame changes into M-shaped flame.With the increase of the inlet temperature,the amplitude increases first and then decreases in the region larger than 1,and the phase difference decreases gradually.This is due to the unstable V-shaped flame in the external recirculation zone at low temperature,which leads to the flame prolongation.The two flame transfer functions of methane and propane show similar trend,but at the highest point of amplitude,the propane flame is larger than the methane flame.At the same time,the difference of phase response can be observed when the frequency is greater than 600 Hz.Because of the higher flame velocity of propane,the propane flame disturbance can be transmitted to the leading edge faster under the same boundary conditions,so there is a slightly larger phase difference.When hydrogen is added,the maximum amplitude of flame decreases slightly,but in terms of frequency response width,the range of instability is obviously enlarged.Similarly,because of the higher volume calorific value of hydrogen and the faster laminar flame speed,the flame morphology changes after hydrogen is added,and the time of disturbance transfer to the flame front is relatively shorter,thus the flame phase difference is smaller than that of pure methane.The influence of combustor restriction on flame transfer function is studied by numerical simulation of two combustors with different cross-section.At about 200-350 Hz,the amplitude under high restriction is greater than 1,while that under low restriction is less than 1.Therefore,in this frequency band,it is easier for flame to capture the influence of different restriction ratios on response.At the same time,due to the elongation of the flame along the wall,the phase difference of the flame with high restriction is larger.Finally,the stability of the system under different working conditions is analyzed by using state space modeling method combined with the flame transfer function obtained previously.It is proved that the difference of the identified flame transfer function will have a significant impact on the stability prediction and cycle increment.The stability behavior of the system can be modified by modifying the width of the combustor and the heat flux on the wall of the combustor.