Flame dynamics of a subscale rocket combustor operating with gaseous methane and gaseous, subcritical or transcritical oxygen

This article describes an experimental investigation into the flame dynamics of a subscale rocket combustor operating with five shear-coaxial injectors fed with methane and oxygen. We describe three cases labeled after the thermodynamic state at which oxygen is injected into the combustor: gaseous, subcritical or transcritical, whereas methane is continuously injected at a gaseous state. We employ the backlit and OH center dot chemiluminescence imaging techniques to describe the oxygen atomization and flame dynamics. We provide, with high time-resolution, visualizations of the transitional dynamics of the oxygen injection across the saturation line and through the critical pressure. The Gaseous Case presents a Low-Frequency instability which manifests by an apparent flapping of the flame. The instability is analyzed by means of the Spectral Proper Orthogonal Decomposition technique to reveal the space/time coherence of the instability. It appears that the instability could lock itself inside the combustor as a result of a coupling mechanism between the 1L acoustic resonance of the oxygen feed line and a confinement-induced hydro-dynamic instability of the combustion chamber. We observe another flapping instability when the oxygen is in the liquid state (Subcritical and Transcritical cases). This apparent self-induced instability seems to occur at a constant Strouhal number of St = 0 . 051 . Finally, we provide quantitative descriptions of the atomization process in terms of (i) the dynamic evolution of the liquid core length and (ii) a probabilistic characterization of the oxygen spray. (C) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.