On the behavior of a shear-coaxial jet, spanning sub- to supercritical pressures, with and without an externally imposed transverse acoustic field

In the past, liquid rocket engines (LRE) have experienced high-frequency combustion instability, which impose an acoustic field in the combustion chamber. The acoustic field interacts with the fluid jets issuing from the injectors, thus altering the behavior of the jet compared to that of stable operation of the LRE. It is possible that this interaction could be a substantial feed back mechanism driving the combustion instability. In order to understand the problem of combustion instability, it is necessary to understand the interaction of the jet with the acoustic waves. From past combustion instability studies of the liquid oxygen and hydrogen propellant combination in a shear-coaxial injector configuration, a design guideline of outer-to-inner jet velocity ratio greater than about ten was proposed in order to avoid high-frequency acoustic combustion instability problems. However, no satisfactory physical explanation was provided. To promote this understanding, a cold-flow experimental investigation of a shear-coaxial jet interacting with a high-amplitude non-linear acoustic field was undertaken under chamber pressures extending into the supercritical regime. Liquid nitrogen (LN2) flowed from the inner tube of a coaxial injector while gaseous nitrogen (GN2) issued from its annular region. The injector fluids were directed into a chamber pressurized with gaseous nitrogen. The acoustic excitation was provided by an external driver capable of delivering acoustic field amplitudes up to 165 dB. The resonant modes of the chamber governed the two frequencies studied here, with the first two modes being about 3 and 5.2 kHz. High-speed images of the jet were taken with a Phantom CMOS camera. The so-called "dark core" of the jet is among the most salient features in the acquired images, and therefore, was defined and measured. The core length was found to decrease with increasing velocity and momentum flux ratio. Because of the ability of the camera to capture thousands of images and an automated routine to measure the dark core of the jet, meaningful statistics and time histories of the core length were determined. The root mean square (RMS) fluctuation of the dark-core length decreases and approaches a low constant value as the velocity ratio of the jet increases. The RMS of the dark core length, in some fashion is related to variations in mixture ratio within the combustion chamber. By decreasing this variation, at high velocity ratios under cold-flow conditions, this may lead to a physical explanation of the observed stable behavior of the LRE at high velocity ratios. The decreased RMS fluctuations at high velocity ratios reduce mixture ratio variations, which in turn leads to a more uniform heat release zone in the chamber, thus possibly weakening a key feed back mechanism to drive the combustion instability. Comparisons of the dark core length measured here to those reported by others for both single-phase coaxial jets (i.e. gas-gas or liquid-liquid) and two-phase coaxial jets (i.e. water-air or LN2-GN2) establish two regimes on the dependence of this length to the outer-to-inner jet momentum flux ratio (M). The core length of single-phase jets quantitatively agrees with the dark-core length and its M-dependence measured in our studies under supercritical conditions. However, under subcritical conditions the dark core tends to be much longer and depends more weakly on M than when under supercritical conditions. At M < 1 the two regimes meet and approach the values reported for single round turbulent jets (at the limit of M = 0). However, at higher values of M, divergence between the two regimes is observed.