Onset of combustion instabilities during transition to supercritical fuel injection in high pressure combustor

This paper describes a study of the onset of severe combustion instabilities (i.e., with peak to peak amplitudes of up to 0.97MPa) in a high pressure (P-c > 500psia or 3.45MPa) air breathing combustor as the inlet temperature of the injected liquid fuel (n-heptane-C7H16) was varied over the 20-335 degrees C range, achieving supercritical conditions when the temperature exceeded 300 degrees C. The attainment of supercritical operation was determined by a specially developed probe that illuminated the liquid spray with a laser beam and collected the light scattered off the spray. As the temperature of the fuel was increased, the spray disappeared when the fuel attained a supercritical state and the scattered signal could be no longer detected. Two different unstable modes (i.e., similar to 100 and similar to 400Hz) were excited in the combustor as the fuel temperature was increased from low to supercritical conditions and then cooled again. The lower frequency instability (similar to 100Hz) was excited and then disappeared when the fuel temperature was well below supercritical values (i.e., T-FUEL < 200 degrees C) whereas the higher frequency mode (similar to 400Hz) was typically excited at sub- and super-critical conditions (i.e., T-FUEL > 250 degrees C). It's shown that the dynamics of the excitation and disappearance of these modes as well as their limit cycle amplitudes strongly depend upon the direction in which fuel temperature varies and also upon the temperature of the combustion air. Significantly, the results of this study strongly suggest that combustion instabilities may be excited in future high performance aircraft combustor that will operate at very high-pressures and with supercritical fuel injection.