Measurements in an Acoustically-Driven Coaxial Jet Under Sub-, Near-, and Supercritical Conditions (PREPRINT)
AIR FORCE RESEARCH LAB EDWARDS AFB CA PROPULSION DIRECTORATE
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An experimental investigation was conducted on a coaxial jet, similar to those used in cryogenic liquid rockets, under sub-, near-, and supercritical pressures, with the ultimate intent of gaining a better understanding of an aspect of combustion instability that pertains to interactions of an externally-imposed acoustic field with the jet. Past research works on this subject have shown both the relevance and importance of geometrical changes in an injectors exit-area and its nearby physical and fluid mechanical processes. On this basis, special attention is paid in collecting spatially-resolved time averaged temperatures and documenting the aforementioned interactions at the exit of this injector. Short-duration and high-speed framing digital images provided information on the behavior of this jet under a variety of conditions. Mean and rms values of the dark-core length fluctuations were measured from the acquired images via a computer-automated method and the ensuing results are discussed. There appears to be a good correlation between this length and the outer-to-inner jet momentum ratio, but the form of this dependence was found to be different at subcritical pressures than the rest of the conditions. The rms values of the dark-core length fluctuations suggested a possible explanation for the observed improvement in instability limit at increasingly higher outer-to-inner jet velocity ratios.
- Fluid Mechanics
- Combustion and Ignition