Injection and Mixing Processes in High-Pressure Liquid Oxygen/Gaseous Hydrogen Rocket Combustors

The injection, mixing, and combustion processes in a LOX/gaseous hydrogen (GH2) rocket engine combustor at high chamber pressures (10 MPa) are studied and modeled. An experimental LOX/GH2 rocket motor consisting of a single coaxial shear injector element and a cylindrical chamber with optical access was used for flow visualizations and measurements. Cold-flow injection tests utilizing liquid nitrogen and gaseous helium at elevated pressures were done for flowfield characterization by different diagnostic methods, such as flashlight photography and high-speed cinematography, using a shadowgraph setup. The injection visualizations and studies under cold-flow and combusting conditions revealed a remarkable difference between subcritical spray formation and evaporation and the supercritical injection and mixing process. The study shows that approaching supercritical chamber pressure injection can no longer be regarded as a spray formation but rather as a fluid/fluid mixing process. As the flow visualizations indicate, the effect of the coaxial atomizer gas is less effective than previously expected. The flame is attached to the LOX post and develops in the LOX post wake. The observed flame holding mechanism is discussed. An evaluation of the radiation spectrum of the flame inside the combustion chamber revealed that radiation in the visible range is mainly due to water vapor. (Author)