Measurements in an Acoustically Driven Coaxial Jet under Sub-, Near-, and Supercritical Conditions

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 intent of gaining a better understanding of an aspect of combustion instability pertaining to interactions of an externally imposed acoustic field with the jet. Past research on this subject has shown both the relevance and importance of geometrical changes in an injector's exit-area and its nearby physical and fluid mechanical processes. 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 various conditions. Mean and root mean square values of the length fluctuations were measured from the acquired images via a computer-automated method, and 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 root mean square values of the dark-core length fluctuations suggested possible explanation for the observed improvement in instability limit at increasingly higher outer-to-inner-jet velocity ratios.