An experimental investigation on high-frequency combustion oscillations

Abstract

An experimental study was conducted with a research rocket motor, burning premixed gases as the propellant, for investigating the effects of several variables upon the high-frequency combustion oscillations in detail and making clear the driving mechanism of these oscillations. The effects on the longitudinal-mode pressure oscillations of the combustion-chamber length, the equivalence ratio of the propellant, the combustion-gas temperature, the mean chamber pressure, and the injector configuration, were examined. Three modes of the longitudinal-pressure oscillations—namely, the fundamental-, the second harmonic-, and the third harmonic-mode oscillations—were observed in the combustion chamber, and the measured periods of each mode oscillation were found to satisfy the relation of the acoustic-mode oscillations. Therefore, the high-frequency combustionpressure oscillations may be treated essentially as the standing waves of the longitudinal mode, though the propagating shock-type pressure waves were observed. The fundamental- and the higher harmonic-mode pressure oscillations occurred only for certain conditions of the chamber length, the propellant-equivalence ratio, and the mean chamber pressure. With increasing of the chamber length and the chamber pressure, the instability regions of each mode oscillation became larger. It was confirmed that there existed the lower critical oscillation period which was of the same value for each mode oscillation, and that the critical period was the essential parameter for the initiation of the combustion oscillations.