Study On Self-induced Instabilities Of Methane Premixed Flames In An Unconfined Reaction Zone

In recent years,with the enhancement of energy consumption and the environmental protection,in order to effectively reduce the level of NOx emissions,the lean premixing technology has higher efficiency and lower pollution is favored,but it can cause a serious problem during combustion that is to induce combustion self-induced instabilities.The direct harm caused by self-induced instabilities is the increase of thermal NOx content in the flue gas and the vibration of the equipment structure.Sometimes it may lead to devastating consequences,that seriously threaten the safe operation of the system.Therefore,Thermoacoustic oscillations in the development of the lean premixing technology is the most urgent issue.An experimental system with the premixed combustor was established in an unconfined reaction zone.On the basis of experimental research,the self-excited oscillation characteristics and driving mechanism of combustion had been studied in detail.The main research results are as follows:(1)The heat release rate cannot be obtained by direct measurement,a high-speed camera was employed to continuously capture the flame images.By setting appropriate intensity thresholds,the impact noise can be eliminated from the image and the flame front boundary can be determined.The binary diagram of the image was morphologically processed to obtain the normalized area fluctuations of the flame front in a temporally resolved manner.After comparing with the measurement results of the photomultiplier,which fully confirms the feasibility of this method,a calculation method is provided to realize the rapid measurement of the heat release rate fluctuations.(2)The thermoacoustic coupling self-oscillation analysis of a combustor with similar behaviors as a Helmholtz Resonator.For the first time in the derivation process,the actual shape of the vessel,the quality of the internal gas and the influence of the external gas on the characteristic frequency were considered,and the calculation method of the characteristic frequency of the HRB burner was improved,comparing with the classical equation of eigenfrequencies,it is found that there are differences between them.The phase relationship related to the self-oscillation parameters was verified through theoretical analysis and experiments.For the stationary flame self-excited oscillation phenomenon formed on the HRB burner,the Rayleigh criterion and the energy balance relationship were used to establish an unstable combustion theoretical model,combined with the burner acoustics and the flame transfer function,the theoretical prediction results are consistent with the experiment.The correctness of the theoretical model and the feasibility of the joint analysis method were verified.(3)Analyze different combustion conditions of burners with behavior characteristics similar to the organ pipes.The results show that the hole depth and aperture of the burner head significantly affect the combustion state.The larger hole depth has a wider cavity length that can achieve stable combustion;the larger aperture has the larger the stable combustion interregional and the lower noise.The flame transfer function was studied by changing the parameters of the fluid and the burner head.The higher mean velocity with the same driving frequency and the burner head has the higher amplitude of the flame transfer function.Combustion oscillations are more likely to occur when the combustion operation is closer to lean limits.The larger aperture has the better damping effect of the flame transfer function amplitude and the better anti-interference ability of the combustion system.The greated pitch of hole has the weaker mutual effect between adjacent flames and the faster amplitude of its decay rate with frequency,and the stronger the low-pass filtering behavior of the flame.(4)The theoretical model of unstable combustion was established by using the analogy circuit method,the phase of the flame transfer function was combinated that gave the frequency range of the self-excited oscillation with the different burner head;the energy balance relationship during the limit cycle was analyzed,and a calculation example was given based on the experimental conditions.The equation is basically closed,revealing that the energy balance is the reason that the combustion oscillation reaches the limit cycle.