Experimental injector element stability characterization and combustion imaging
This research evaluated the stability characteristics of a swirl-coaxial injector in an atmospheric combustion chamber using gaseous oxygen and gaseous methane as the propellants. The experiment studies the mixing processes in a laboratory-scale analogue to full-scale engine operating conditions. The injector was tested over a range of mass flow conditions that matched specified scaling parameters for a notional space engine. The scaling parameters used were mixture ratio, velocity ratio, momentum ratio, and momentum flux ratio. A maximum amplitude of 3.52% of chamber pressure was measured when the injector was placed 19 mm from the chamber wall. The associated instability mode—determined by phase and amplitude analysis from high-frequency pressure measurements—corresponded to a second tangential mode. The scaling parameters that most closely correlated to combustion instability were the design exit velocities and mixture ratio. CH* chemiluminescence emissions were shown to occur in phase with pressure oscillations, and during unstable combustion, the combustion zone appeared to lift off the injector. Fuel annulus resonant frequencies possibly caused repeated flame blowout at lower fuel flow rates.