Investigations On Low Frequency Combustion Oscillations In Supersonic Flow

The low- frequency oscillation characteristics of combustion inside different co mbustion engines with different injection schemes upstream of the flameholder under the simulated flight conditions of Ma 4 and Ma 5.5 are investigated experimentally to analyze the mechanism of the combustion oscillation in the paper, combined with quasi-one-dimensional and two-dimensional numerical method.The experimental results with the short injection distance between the injector and the leading edge of the cavity suggest that in the combustion chamber there exists an apparent low frequency oscillation of combustion which results from thermoacoustic instability with a wide range of the frequency distribution and the weak amplitude. Analysis combined with high-speed photography of flame and schlieren of flow field infers that under the condition in the combustion chamber there is a wide range of subsonic region in which disturbance of unsteady heat release in the downstream of the combustion zone can effectively influence the upstream pre-ignition or fuel injection shock wave mixing process, and lead to a closed- loop feedback. While the pulsations of fuel and air flow play little effect on the combustion of low frequency oscillations in the combustion chamber.According to the experimental results and related theoretical analysis, a analysis model is established by connecting disturbance of heat release in the combustion cha mber with location of flame oscillations with a correction factor. Quasi-one-dimensional numerical calculation by using the model can be able to assess the thermoacoustic instabilities inside the combustion chamber preliminary. The calculation result shows that the acoustic-convection feedback loop between the pre-combustion shock and flame zone is an important source of pressure oscillations. Two-dimensional numerical calculation results of the combustion and flow field of parallel cavities with short injection distance suggest a little strong fluctuations of heat release exists in combustion chamber and leads to the changes of temperature field,shock train and fuel jet, which is likely to result in combustion oscillation.The experimental results with the long injection distance between the injector and the leading edge of the cavity suggest that a apparent low-frequency combustion oscillation appears in the combustion chamber which is characterized by the flame flash-back and blow-off processes, and is accompanied by significant periodical pressure oscillation with strong amplitude and a clear fixed dominant frequency Pressure oscillation is lead by the flame flash-back process. The mechanism of the flame flash-back is analyzed by analyzing the auto- ignition and separation area, and evaluating the flame propagation speed during flame flash-back which is compared with the CJ detonation speed and CJ deflagration speed. The results show that long injection dis-tance is in favor of the fuel injected mixing well with inflow and forms a premixed zone which lead to the turbulent flame flash-back with high speed. It is essentially the process of flame acceleration which is the stage of deflagration-to-detonation transition process. The flame flash-back process coupled with flame blo w-off leads to an oscillation with a specific frequency which can be given by a reasonable explanation. Two-dimensional numerical calculation results of the combustion and flow field of parallel cavities with long injection distance suggest that the local detonations appear once in the combustion chamber, which is likely to be formed by the oblique shock wave.