Effect of Recess in High-Pressure Liquid Oxygen/Methane Coaxial Injection and Combustion

The effect of a recessed liquid oxygen tube in shear coaxial injection has been investigated experimentally using an optically accessible subscale rocket combustor operated at pressures between 40 and 60 bar. Different single-shear coaxial injectors have been used to inject liquid oxygen and methane at relevant operating conditions covering sub-, near-, and supercritical pressures with respect to the critical point of oxygen. Liquid oxygen was injected at 120 K and the injection temperature of gaseous methane was about 275 K. Detection of spontaneous OH and CH chemiluminescence has been performed to characterize the flame-anchoring zone near the liquid oxygen post tip. In addition, the influence of the injector geometry on the combustion roughness and stability has been investigated during steady-state operating points. An increased flame expansion was observed with a recessed injector element. At a low momentum flux ratio, the pressure drop across the injector increases with a recessed liquid oxygen tube compared with a flush tube. Furthermore, a recessed liquid oxygen tube led to a smoother combustion in general; however, this configuration also led to additional resonant frequencies in the chamber acoustics.