Flame response of a sub- and supercritical LOX/H2 and LOX/LNG rocket combustor with large optical access

Abstract

Hot fire tests including flame radiation and acoustic measurements were performed in a single-element rocket combustor with large optical access (255x38 mm) for sub- and supercritical injection conditions. Three test campaigns were conducted with the propellant combination of liquid oxygen and hydrogen (LOX/H2), liquid oxygen and compressed natural-gas (LOX/CNG), as well as liquid oxygen and liquefied natural gas (LOX/LNG) at conditions relevant for main and upper-stage rocket engines. High-speed imaging of the flame in blue radiation wavelengths (capturing CH* when natural gas was the fuel) was conducted. For the analysis of the flame-acoustic interaction, these measurements are related with the interpolated acoustic pressure distribution in the chamber. While three load points (LPs) feature high-amplitude, self-sustaining oscillations of the longitudinal acoustic resonance modes, one LP is intermittently unstable and two LPs are stable. Three different regions of interest (ROIs) were used to calculate flame transfer functions (FTFs) from the 2D imaging data, each increasing successively in their extent. While the phase difference between the radiation intensity and acoustic pressure is consistent for all three ROIs, the gain values differ. The largest ROI including the complete extent of the optically accessible region provided gain values qualitatively most consistent with expected distributions.