Study On Self-sustained Mechanism Of High Frequency Combustion Instability In Air Heater

Air heater is the key facility of ground-level testing of hypersonic vehicle and its propulsion system,which is used to produce a test gas that resembles air flow as the test medium at total temperature,Mach number,and components that properly simulate the stagnation conditions associated with real flight conditions.However,up to now,there exists a lack of understanding of transient spray combustion and combustion instability in air heater chamber.Motivated by the high frequency combustion instability in fullscale air heater,a subscale single-injector air heater was designed in present thesis.The mass flow rate is small and the chamber pressure is low.The lateral side walls comprise observation windows.In order to identify the self-sustained mechanism of high frequency combustion instability,droplet evaporation,combustor acoustic oscillations,self-excited oscillation of transient spray combustion and its response to combustor acoustic modulations were investigated by experiment and numerical simulation.The main research contents and results are outlined as follows.?1?The ignition processes were experimentally investigated.The developments of flame and LO₂/C₂H₅OH sprays from the moment of pre-ignition to steady combustion were investigated by analyzing the high speed visualizations and the time traces of chamber pressure.The ignition-quenching–reignition behaviors and flash evaporation phenomena during one entire firing duration were captured.The effects of chamber length and preparation time prior to combustion on the ignition processes were analyzed.It was found that ignition performance was improved by increasing chamber length and preparation time prior to combustion and the ignition-quenching–reignition behaviors were no more visualized.The reverse traveling of spray and flame to the annual gap of air injector were observed due to high pressure peak during the ignition transient.This process disturbs the flow field in chamber and is one of the key factors causing the injector ablation.Due to the high dissipation of pressure oscillations in subscale air heater chamber,it is difficult to sustain high amplitude pressure fluctuations.Thus,in order to investigate the self-sustained mechanism of high frequency combustion instability,it is necessary to adopt effective technique to sustain pressure perturbation.The ignition process in air heater may be split into three main phases:?a?The flammable recirculation zone formed between the faceplate and chamber wall is ignited by the hot jet gases exhausted from the torch igniter.?b?The combustion flame located in the flammable recirculation zone propagates quickly within the chamber.?c?The air/LO₂/C₂H₅OH jets relay the flame.The recirculation zone formed between the faceplate and chamber wall plays an important role in the ignition process in air heater.?2?A new thought for studying the combustion instability in air heater was proposed and the acoustics in fullscale and subscale air heater were investigated.The influence of flame spatial distribution on predictions of resonance frequency and mode was analyzed.It was found that resonance frequency and mode closely rely on the length of heat release zone.Comparison of different heat release zones indicates that increment of heat release length exhibits an increased tendency toward lower order longitudinal mode,when heat release zone locates near the faceplate where it is the pressure antinode of the longitudinal mode.Combining the results of modal dynamics with CFD?Computational Fluid Dynamics?,it was thought that non-uniform distribution of heat release is the key bifurcation parameter that is responsible for the standing or spinning mode nature.The heat release is rather annular uniform for the shorter heat release,which would likely to excite lower order spinning mode.However,the heat release is rather annular non-uniform for the longer heat release,which is likely to excite higher order standing mode.The CFD results show that the second order transverse acoustic mode may be easily formed when the injector is located in the center of the faceplate for the subscale air heater.While the first order transverse acoustic mode is easily formed when the injector is located off-center in the faceplate.?3?An unsteady droplet heating and vaporization model was developed.Oscillatory vaporization of a single droplet exposed to pressure oscillations atmosphere was investigated over a wide range of amplitudes and frequencies.The results show that frequency of the externally imposed pressure oscillation is a key parameter in determining the phase lag between evaporation and pressure fluctuation and droplet vaporization is only capable of driving combustion instability in some certain frequency domains.However,this phase lag appears to be independent to the amplitude of the pressure fluctuation.When the frequency of pressure oscillation is fixed,the phase lag of vaporization rate with respect to pressure oscillation is mainly determined by the droplet thermal inertia.?4?Transient spray combustion in air heater was investigated,the impacts of key injection parameters on combustion processes were analyzed,such as droplet injection diameter and the pressure drop across the air-injection annular gap.It was found that steady combustion flame can be obtained for the smaller droplet diameter.When the atomization situation is worse,that is to say,the droplet diameter becomes rather bigger,transition to unstable spray combustion occurs,which creates air injection coupled low frequency combustion instability.Increasing pressure drop across the air-injection annular gap can eliminate this type of combustion instability.?5?Self-excited oscillation of transient spray combustion and its response to combustor acoustic forcing were investigated.The physical route feeding energy to the combustor acoustic modes from spray combustion was elucidated.The results showed that combustion flame,species,heat release locations and spray density distributions were observed to move in unison with transverse velocity under acoustic forcing.This process drives high frequency combustion instability in air heater.The coupling between air injection velocity and chamber pressure oscillation can not only drive low frequency combustion instability but also sustain high frequency combustion instability.