Influence of Injector and Chamber Design on LOX/CH4 Combustion Instabilities

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 is typically related to the operation of the combustion system close to its stability boundary. Analyzing the high-speed imaging of the optically 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.