Combustion Instability with a Single-Element Swirl Injector

A single injector element liquid rocket combustion experiment was designed and conducted to investigate the combustion dynamics of a gas-centered, liquid-swirled coaxial injector element. The oxidizer was a mixture of superheated water and oxygen, and kerosene was used as the fuel. The mean chamber pressure ranged from 2.14 to 2.38 MPa. The combustion chamber length was discretely varied between 25.4 and 88.9 cm to determine the dependence of combustion stability characteristics on resonant frequency and mode shape. Strong spontaneous instabilities were measured with peak-to-peak amplitudes of 0.69 to 1.38 MPa, and wave slopes on the order of 1000 MPa/s. The frequencies of the strongest instabilities ranged from 1184 to 1721 Hz. The most amplified modes ranged from the first longitudinal for the 38.1-cm chamber to the third longitudinal for the 88.9-cm chamber. One test, with a 25.4-cm chamber, was classically stable with pressure oscillation amplitudes less than 5% of the mean pressure. Resonant frequencies calculated with a model of the chamber acoustics compared well with measured values. For this injector, the data suggest that the observed stability behavior is a result of the combined effects of chamber mode shape and a driving combustion mechanism that limits the frequency range over which instability occurs.