Flame dynamics of an injection element operated with LOX/H2 and LOX/CNG in a high-pressure rocket combustor with large optical access
Creators: name:  J. MARTIN 
name:   J. HARDI 
name:   D. SUSLOV 
name:   M. OSCHWALD 
name:   W. ARMBRUSTER 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
FUTURE FUELS

description: The requirements of future space missions demand the development of new rocket engines. Due to their high performance the propellant combinations of liquid-oxygen/hydrogen (LOX/H2) and liquid-oxygen/methane (LOX/CH4) are of special interest. While the LOX/H2 propellant combination offers the highest specific impulse under all flown chemical propellants [1, 2], LOX/CH4 is predicted to be cheaper and easier to handle. A crucial part in the development process of a new engine is the injection element. The injector directly affects the mixing of the propellants and thus also the combustion dynamics and overall engine performance. A common injector type in liquid propellant rocket engines (LPREs) is the shear-coaxial injection element. The high-density propellant (LOX) is injected through a central tube (LOX post) and the propellant with a lower density (H2/CH4) is injected via an annular gap surrounding the LOX post. While the resulting shear forces from the propellants injected with different velocities have a major impact on the mixing, the reaction kinetics resulting from the used propellant combination affects the combustion of the mixed species. Therefore, both the geometry and the propellant combination determine the oscillating heat release of the flame. If the heat release fluctuates at frequencies close to at least one acoustic eigenmode of the chamber and the phase of both processes are in phase the necessary condition for growing amplitudes of hazardous high-frequency (HF) combustion stabilities is given. The mathematical description of correlation is provided by the Rayleigh-Criterion Due to the high-power density in LPREs the transition of only a small amount of the chambers heat release is sufficient to develop damagingly high amplitudes of HF combustion instabilities and thus harm the engines safe operation by the increased thermal loads and mechanical stresses [2]. The highly modular and optically accessible sub-scale combustor model N (BKN) serves as a valuable tool to characterize the performance of injection elements as well as provide data necessary for the validation of injector design tools. BKN features the Full-Length Optical Window (FLOW), with a length of 255 mm, and densely distributed high and low frequency pressure and temperature sensors along the chamber axis. BKN, equipped with the FLOW, has already been tested with the cryogenic propellant combination of LOX/H2 and LOX/CNG, various ratios of oxidizer to fuel (ROFs) and chamber pressures up to 70 bar. The same injector geometry was used for both propellant combinations. Within this test campaigns, high-speed imaging of the ultraviolet (OH*) and blue (CH* with CNG as fuel) radiation has been conducted. This paper will first provide a brief description of the experimental hardware. Then the injection characteristics and the resulting flame dynamics of the campaigns with LOX/H2 and LOX/CNG will be presented, contrasted and discussed. References [1] Huzel, D. K., and Huang, D. H., Modern Engineering for Design of Liquid Propellant Rocket Engines American Institute of Aeronautics and Astronautics, Washington, DC, 1995. [2] Sutton, G. P., and Biblarz, O., Rocket Propulsion Elements, 8th ed., John Wiley Sons, New York, 2010.