A Rocket-Like Coaxial Injector in an Acoustic Field Under Sub- and Supercritical Conditions
AIR FORCE RESEARCH LAB EDWARDS AFB CA PROPULSION DIRECTORATE
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An experimental investigation was undertaken under non-reacting condition to gain a physical understanding of 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 presence of an external acoustic field. High-speed 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 characteristics. Mean and root mean square RMS values of the dark-core length fluctuations were measured from images. It is found that when 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. 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, observed 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 can 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 elsewhere, there appears to be a good correlation between the dark-core length and the momentum flux ratio.
- Combustion and Ignition
- Jet and Gas Turbine Engines
- Fluid Mechanics