Combustion Instability Characteristics of a Methane–Oxygen Shear Tri-Coaxial Rocket Injector

Shear coaxial rocket injectors, including a novel tri-coaxial design, were experimentally tested in a single-element high-pressure combustor to characterize combustion instabilities. Dynamic pressure within the combustor and the oxidizer feed system was measured using high-speed piezoelectric sensors. High-speed OH* chemiluminescence videography was simultaneously recorded to capture fluctuations in the spontaneous emission from each coaxial flame. Characteristic frequencies of combustion instability modes and their associated amplitudes of pressure fluctuations were determined. Thermoacoustic coupling between the heat release from combustion and pressure fluctuations was also quantified. Measurements associated with the tri-coaxial injector design were compared to two traditional shear bi-coaxial geometries with and without a recessed central oxidizer post over a range of chamber pressures from 35 to 69 bar. Methane and oxygen served as the primary propellant combination. The recessed and flush bi-coaxial injectors exhibited large pressure fluctuations at a low-frequency chugging mode and strong injection-coupled instability characteristics. The tri-coaxial design showed stronger excitation of longitudinal acoustic modes, although usually at lower amplitudes than the chugging mode of the bi-coaxial injectors. The compiled findings of this study indicate that the enhanced-mixing tri-coaxial injector design has a relative resistance to injection-coupled instabilities over a range of flow conditions.