High-Frequency Combustion Instability Mechanism of a LOX/Methane Swirl Coaxial Injector in an Optically Accessible Chamber

Kurzfassung

Injector elements are an elemental sub-component of any liquid rocket engine, as they are critical in influencing an engine’s efficiency and combustion stability. Therefore, characterizing such elements is of great importance. In regeneratively cooled rocket engines, significant changes in the fuel injection temperature can occur between load points or during the development phase of such an engine. The impact of such a change in fuel injection temperature on t he combustion dynamics however is not fully understood, especially for swirl injectors. Therefore this study presents the results from a single swirl coaxial injector element hot-fire test campaign where fuel injection temperature is varied between 175 and 240 K. The injector element injects liquid oxygen and natural gas in 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 upwards of 15 kHz are increasingly excited. Additionally, high amplitude oscillations for these modes occur - with peak-to-peak oscillations close to 20 percent of the chamber pressure. The analysis of these phenomena concludes that for low fuel injection momentum scenarios the LOX injectors couple with the first radial combustion chamber mode with an additional coupling effect with the fuel injector leading to these high amplitude instabilities. In combination with the outward flame pulsation observed in the optical data, an mechanism explaining the flame-acoustic interaction is proposed. The paper concludes with the definition of stability boundaries for common injection parameters such as velocity and momentum flux ratio.