Spontaneous Longitudinal Combustion Instability in a Continuously-Variable Resonance Combustor

An experimental study of spontaneous longitudinal high-frequency combustion instabilities in a high-pressure model rocket combustor is described. The experiment is designed to efficiently obtain stability data under variable resonance conditions in a single test. A traversing choked axial oxidizer inlet is used to vary the coupled resonances between the variable-length oxidizer tube and the fixed-length combustion chamber, thereby changing the temporal and spatial relationship between the fluid mechanical resonances in the oxidizer tube and the local pressure oscillations at the head end of the combustor. Marked transitions between pressure-fluctuation amplitudes and spectral contents are observed as the injector tube length changes. Stable-to-unstable and unstable-to-stable transitions are observed. Depending on the injector tube length, the first dominant injector-coupled resonance ranged from 1200 to 1500 Hz. The high-frequency pressure oscillation amplitudes varied from less than 10% of mean pressure to greater than 30% of chamber pressure. Spectrograms are used to assess the global frequency contributions, and power spectral densities are used as a quantitative measure of the stability characteristics and to identify the most unstable geometry. The combustor was also operated in fixed-tube-length mode to measure linear growth rates and compare limit cycle amplitudes. The experimental results provide a basis for comparison with stability models, and their use in conjunction with reliable computational fluid dynamics models can potentially provide detailed insight into the physics of self-excitation and limit cycle behavior in a practical system.