Behavior of a Rocket-Like Coaxial Injector in an Acoustic Field
AIR FORCE RESEARCH LAB EDWARDS AFB CA PROPULSION DIRECTORATE
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A non-reacting-flow experimental investigation was undertaken to gain a better understanding of some of the underlying physics associated with the interaction of acoustic waves and a coaxial-jet injector similar to those used in cryogenic liquid rockets. Liquid nitrogen the round inner jet and gaseous nitrogen the annular outer jet were used under subcritical, near critical, and supercritical chamber pressures, with and without acoustic excitation. High-speed digital imaging provided information on the dynamic behavior of the jet under a variety of conditions. It is found that when the jet is at the pressure node, an externally-imposed acoustic field excites the dark-core of the jet to a wavy-shaped structure consistent with the fields instabilities characteristics. Mean and root mean square RMS values of the dark-core length fluctuations were measured from images. It is seen that as the outer-to-inner-jet velocity ratio increases, the RMS of the dark-core length fluctuations decreases both with and without the existence of the acoustic field. It is thought that a connection to the rocket instability may be established from these data through examination of the RMS values. It is possible that decreases in the fluctuation levels, shown to occur at higher velocity ratios, could weaken a key feedback mechanism for the self-excitation process that could be driving combustion instability in rocket engines. This could offer a possible explanation of the combustion stability improvements experienced in engines when a transition to higher values of the outer-to-inner-jet velocity ratio is made. Finally, after a careful review of relevant data taken here and those by others, there appears to be a good correlation between the dark-core length and the momentum flux ratio.
- Combustion and Ignition
- Rocket Engines