High-frequency combustion instability mechanism of a liquid oxygen/methane swirl coaxial injector in an optically accessible chamber

Injector elements are an elemental sub-component of any liquid rocket engine, as they are critical in influencing engine efficiency and combustion stability. Therefore, characterizing such elements is of great importance. In regeneratively cooled rocket engines, significant changes in fuel injection temperature can occur between load points or during the development phase. The impact of such changes on combustion dynamics, however, is not fully understood, especially for swirl injectors. Therefore, this study presents results from a single swirl coaxial injector element hot-fire test campaign where fuel injection temperature is varied between 175 K and 240 K. The injector element injects liquid oxygen and natural gas into an optically accessible combustion chamber at pressures between 35 and 80 bar. It is observed that under high-pressure, low-fuel-temperature conditions, high-frequency chamber modes with frequencies above 15 kHz are increasingly excited. Additionally, high-amplitude oscillations occur for these modes, with peak-to-peak oscillations approaching 20% of chamber pressure. Analysis concludes that, for low fuel injection momentum scenarios, the liquid oxygen injectors couple with the first radial combustion chamber mode, while an additional coupling effect with the fuel injector leads to these high-amplitude instabilities. Combined with the outward flame pulsation observed in the optical data, a mechanism explaining the flame-acoustic interaction for a swirl coaxial injector is proposed. The article concludes by defining stability boundaries for common injection parameters, including velocity and momentum flux ratio.