Flame stabilization of liquid oxygen/kerosene bi-swirl injector at elevated pressure

Abstract

Throttleable liquid-propellant rocket engines have wide range and economical applications. However, low frequency combustion instabilities were often excited in the oxygen-rich preburners of liquid oxygen (LOx) and kerosene rocket engines as the engines throttling. To understand the mechanisms and eliminate these instabilities, the detail features of the instabilities need to be described. Bi-swirl injectors with deep recess are used in the oxygen-rich preburners to spray propellants. This paper presents the non-premixed flame dynamic of a LOx/kerosene bi-swirl injector under high-pressure conditions. High-speed cameras captured OH* and CH* chemiluminescence images of the oxygen-rich flame for the first time. Results indicate that a stable flame was anchored inside the recess region and featured by a V-shaped, swirl flame. The flame-spreading angle under supercritical conditions was slightly larger than the liquid spray angle. Intense low-frequency combustion instabilities at 16–37 Hz were developed when the LOx/kerosene mixing ratio (ROF) was greater than threshold value. The flame oscillation process was coupled to the oscillations of the mass flow rate and the ROF. As the oxygen-rich level decreased, the flame oscillation pattern gradually shifted from flame blow-out/auto-ignition mode to flame length oscillation mode. Both modes were characterized by violent flashback and blow-off movement of the flame and large pressure variations. The instability was one type of “lean-limit” instability. A discussion of the oscillation periods qualitatively explains why the oscillation periods were found to decrease as the ROF decreases. The axial velocity of the kerosene is a controlling parameter of the oscillation period.