Injector-Driven Combustion Instabilities in a Hydrogen/Oxygen Rocket Combustor

Self-excited combustion instabilities of the first tangential mode have been found in a research combustor operated with the cryogenic propellant combination of hydrogen/oxygen. In a series of consecutive test campaigns, the influence of operating conditions on these self-excited combustion instabilities was examined. This included a variation of the combustion chamber pressure, the mixture ratio, and the propellant temperatures. It has been shown how these operating parameters influence the resonance frequencies of the combustion chamber. The analysis of the influence of operating conditions on the oscillation amplitude of the first tangential mode indicated that the instability occurred when the frequency of the first tangential mode of the combustion chamber was shifted into the frequency of the second longitudinal mode of the liquid oxygen injector. With a variation of the injector length, and therefore its longitudinal resonance frequencies, this hypothesis has been tested. Based on the experimental observations it was concluded that the observed instabilities were a result of the interaction of combustion chamber resonance frequencies with injector resonance frequencies. The heat release rate oscillation was shown to be a result of the injector acoustics and not of the combustion chamber pressure oscillations.