Observation of LOX/Hydrogen Combustion Flame in a Rocket Chamber during Chugging Instability

To obtain concrete information on the mechanism of unstable combustion of liquid oxygen-hydrogen rocket engines, a rectangular rocket chamber with four glass windows was developed. The designed maximum chamber pressure was 3.5 MPa and the thrust was 12.6 kN. The chamber included 12 actual size coaxial injection elements. The height of the chamber was designed to simulate first tangential mode instability of a 100-kN-sized cylindrical rocket chamber with the first transverse mode instability of the rectangular chamber. Combustion tests were conducted at a chamber pressure of 1.7 MPa. Two kinds of low frequency instability were observed in the tests. One was found to be related to the two-phase flow of liquid oxygen, which was observed during the start-up transient, and the other was related to low propellant injection pressure drops, which were observed at steady state. Combustion flames and oxygen jets were visualized for both conditions with a high-speed video at a rate of 4000 frame/sec. Oxygen jet images were obtained with a backlight. The results indicated interesting phenomena. For example, the combustion chamber pressure, the intensity of combustion flame and the breakup length oscillated in phase and the flame always attached behind the oxygen post during the chugging instability. Stability analysis based on the double time lag model by Szuch was conducted to clarify the cause of the start-up transient instability.