Mode assessment of a single-element shear coaxial injector

This experimental research characterized the modes of combustion instability produced by a shear coaxial injector burning in an open, cylindrical chamber. Matching scaling parameters from full-scale testing, the device is being explored as a laboratory analogue for unsteady mixing processes in supercritical rocket injectors and compared to full-scale test results. The Injector burned gaseous oxygen and gaseous methane at equivalence ratios from 1.0 to 3.8 and total mass flow rates from 0.56 to 2.19 grams per second. The radial position of the injector was also varied from 0% to 88% of the chamber radius. Six high-frequency pressure transducers, located around the circumference and end of the chamber, simultaneously recorded pressure fluctuations at each operating point. A detailed phase analysis among the transducers allowed determination of the modes of instability. Fast Fourier transforms determined the amplitude and frequency of the primary instability at each operating point. Tests were repeated at the injector location showing the highest pressure amplitudes and displayed significant variance between tests. The results show that this single element injector exhibits spontaneous first radial, first radial-first tangential, or second tangential modes at fluctuation amplitudes up to 4.5% of chamber pressure. Changing the radial location of the injector changed the operating points at which maximum instability occurred. The tangential instability modes occurred at injector radial locations greater than 50% and radial instability modes occurred at injector radial locations less than 50%. Operational conditions from each injector location with the highest pressure amplitude were also repeated multiple times to evaluate experiment data uncertainty. The uncertainty of the mass flow rate values for oxidizer and fuel were relatively low, whereas the uncertainty of the values recorded by the high frequency pressure transducers displayed a relatively high level of uncertainty in comparison.