Experimental Investigation of Shear-Coaxial Injector Flame Stability on Flow Parameters

Abstract

*† ‡ Flame-acoustic interaction is a critical mechanism in many types of combustion instabilities, because it amplifies small acoustic disturbances during the onset of combustion instability leading to self-sustaining combustion oscillations. In this paper, a parametric study investigating the sensitivity of flame-acoustic interaction on various physical parameters that govern the shear-coaxial injector operation is reported. Three key parameters were chosen for this investigation; density ratio, velocity ratio, and a measure of chemistry. Results showed that flame-acoustic interaction was significantly affected by changes in fuel-oxidizer density ratio while remaining relatively insensitive to changes in velocity ratio or fuel mixture fraction, at least within the ranges tested. The results were consistent with the intermittent baroclinic vorticity mechanism that was proposed in our earlier study as a new physical mechanism that could play significant role during the onset of acoustic instabilities in liquid-fueled rockets. This work essentially highlights the role of density gradient between fuel and oxidizer in combustion instability and suggests the tailoring of propellant density as a possible method for instability suppression. High speed imaging and spectral measurements of flame oscillations under transverse forcing further show multiple parameters influencing flame instability simultaneously and highlights the efficacy of the partial dependency evaluation method used in this work as a practical approach in identifying strong and weak players among a host of flow parameters that influence flame acoustic interaction.