Abstract
A shift in transverse eigenmode frequency was observed in an experimental combustion chamber when exposed to large amplitude acoustic oscillations during oxygen–hydrogen combustion tests. A shift in eigenmode frequency under acoustic conditions representative of combustion conditions is of critical importance when tuning acoustic absorbers or investigating injection coupled combustion instabilities. The experimentally observed frequency shift was observed both in the frequency domain and as an asymmetric amplitude response to a linear frequency ramp of an external excitation system in the time domain. The frequency shift was found to be dependent on amplitude and operating condition. A hypothesis is presented for the frequency shift based on change in speed-of-sound distributions due to flame contraction when exposed to high amplitude pressure oscillations. A one-dimensional (1D) model was created to test the hypothesis. Model parameters were based on relationships observed in experimental data. The model was found to accurately recreate the frequency shifting asymmetric response observed in test data as well as its amplitude dependence. Further development is required to investigate the influence of operating conditions and chamber design on the quantitative modeling of the frequency shift.
Highlights
High-frequency (HF) combustion instabilities are an ongoing challenge to rocket development programs
The frequency of the pressure oscillations is de¦ned by the speed-of-sound and the combustion chamber geometry and is of critical importance when analyzing coupling as both injector resonance coupling and combustion chamber processes have been shown to be sensitive to oscillation frequency [3]
The frequency shift was observed as an asymmetric amplitude response to a linear excitation ramp
Summary
High-frequency (HF) combustion instabilities are an ongoing challenge to rocket development programs. Signi¦cant progress in understanding has been made since combustion instabilities were ¦rst identi¦ed in the 1940s. Due to the complicated processes present during rocket combustion, further research of the physical process that leads to combustion instabilities as well as development of tools to predict and limit the risk of combustion instabilities are still required [1]. © The Authors, published by EDP Sciences. High-frequency combustion instability occurs when combustion processes couple with the local acoustic ¦eld and is characterized by high-amplitude pressure oscillations that can lead to the rapid degradation of the rocket combustion chamber [2]. The frequency of the pressure oscillations is de¦ned by the speed-of-sound and the combustion chamber geometry and is of critical importance when analyzing coupling as both injector resonance coupling and combustion chamber processes have been shown to be sensitive to oscillation frequency [3]
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