One impediment to substantially further the reductions in NOx emissions for aviation gas turbine engines is thermal-acoustic instabilities, also referred to as combustion dynamics. Dynamics arise due to the coupling of heat and pressure fluctuations in such systems. Numerous passive and semi-active control schemes, including performance de-rating and fuel staging, have been developed for land-based gas turbine engines. However, many of these schemes are not well suited to aviation engines, as a result of their weight and bulk. Observations of several combustors operating on either gaseous or liquid fuels show that the dominant dynamic frequencies have a special relation to specific non-coherent lower frequencies. Experiments show that combinations of two of these non-coherent frequencies form the dominant tones of the combustor. As part of NASA’s intelligent engines program, active combustion control is used to mitigate dynamics, as the combustor’s bulk fuel-air ratio (FAR) is made leaner in an effort to reduce NOx emissions by about 85% below the Committee on Aviation Environmental Protection (CAEP) 6 limit. In the feedback control scheme suggested in this paper, a small percentage of the overall fuel flow is pulsed at a given non-coherent frequency and with varying amplitude. The effectiveness of the dynamics reduction approach has been demonstrated via preliminary open loop control tests on a liquid-fuelled partially premixed high-pressure combustion test rig at GE Aviation in Evendale, Ohio.
Active Combustion Control for Aircraft Gas-Turbine Engines - Experimental Results for an Advanced, Low-Emissions Combustor Prototype