Mean Burning Rate Variation in Composite Propellant Combustion Due to Longitudinal Acoustic Oscillations

Abstract

This paper focuses on investigating “combustion instability,” a phenomenon that mainly involves the interaction of the propellant flames with the acoustic oscillations prevalent in full-scale rocket motors. The effect of excited acoustic pressure oscillations on the mean burning rate of solid propellants was estimated over the pressure ranges from 1 to 7 MPa using a window bomb facility coupled with a rotary valve. Both non-aluminized and aluminized composite propellants were used. A rotary valve was used to drive the acoustic oscillations at frequencies of 140, 180 and 240 Hz with the pressure amplitude perturbations ranging from 0.1 to 1.4% of mean chamber pressure. Frequency shift due to propellant combustion was also investigated for both the types of propellants. The acoustic oscillations enhance the heat transfer between the flame and propellant burning surface, altering the mean burning rate. The acoustic pressure oscillations influence the dynamics of aluminum particles and its agglomeration processes, which modifies the mean burning rate. The variations in excited frequencies show a significant impact on the mean burning rate. The burning rate augmentation factor shows that acoustic excitation is more predominant at low pressures and high frequencies, whereas it is relatively marginal at high pressures. The evaluated maximum augmentation factors are 1.45 and 1.51 for nonaluminized and aluminized propellants when compared with those without oscillations.