Influence of Injector and Chamber Geometry on the Thermoacoustic Behaviour in Proximity to the Stability Boundary of LOX/CH4 Rocket Combustors

Abstract

A shear-coaxial-element is a prominent choice in terms of injection technology for bipropellants with a high density ratio. While this type of injector performed well for liquid-oxygen and hydrogen combustion in an optically accessible single-element combustor and a multi-injector thrust chamber, the same combustion devices featured instabilities using liquid-oxygen/natural-gas. Both experiments showed short-lived events of oscillatory combustion and high-amplitude and high-frequency limit-cycle combustion instabilities within the analyzed tests. The stochastic distributed occurrence of such intermittent events of heightened excitation are typically related to operation of the combustion system close to its stability boundary. Analyzing the high-speed imaging of the optical accessible experiment during these phases showed injector generated hydrodynamic phenomena preceding the short-lived combustion instabilities. Finally, depending on the chamber configuration and operating conditions – primarily the momentum flux ratio – these events triggered high-frequency combustion instabilities. Consolidation of the data obtained from different hardware configurations highlighted the significant role of recessed injector elements in developing combustion instabilities.