Methane-oxygen rotating detonation exhaust thermodynamics with variable mixing, equivalence ratio, and mass flux

Time-resolved in situ measurements of thermodynamic properties (pressure, temperature and species density) were performed in the annular exhaust of a methane-oxygen rotating detonation rocket engine (RDRE) using high-speed laser absorption spectroscopy. Bias-tee circuitry was coupled with a distributed feedback quantum cascade near 5 μm to spectrally-resolve a cluster of rovibrational transitions in the fundamental vibrational band of CO at MHz measurement rates, from which temperature and density were inferred from the line areas and pressure from line-width. The laser source was fiber-coupled for remote light delivery to the exhaust plane of the RDRE combustor. A single-ended optical interface with retro-reflection off of the annulus center body was utilized for stand-off in situ detection. The test article consisted of a 76.2 mm diameter annulus with a annular width of 0.5 cm and doublet impingement injection. Time-resolved CO absorption data was analyzed to examine the evolution of gas properties during engine test firings of 0.5–1.0 s in duration. Start-up transients were examined and intra-cycle profiles of gas properties were compared over a range of equivalence ratios and mass fluxes comprising approximately 25 test conditions. The effects of variable mixing were also examined through a staggered doublet injector configuration. The novel thermodynamic dataset was compared with thrust measurements, wave speed visualizations and first-order detonation models, revealing non-ideal behavior such as parasitic deflagration and modulating oxidizer-to-fuel ratio related to post-detonation injector recovery.