Combustion instability control block diagram. GV, GF, and GA are transfer functions of the associated combustion processes reflected in the figure; NL is a damping nonlinearity that restricts the amplitude of the opened-loop self-excited instability.
Long description.
This year, an adaptive feedback control method was demonstrated that suppresses thermoacoustic instabilities in a liquid-fueled combustor of a type used in aircraft engines. Extensive research has been done to develop lean-burning (low fuel-to-air ratio) combustors that can reduce emissions throughout the mission cycle to reduce the environmental impact of aerospace propulsion systems. However, these lean-burning combustors are susceptible to thermoacoustic instabilities (high-frequency pressure waves), which can fatigue combustor components and even the downstream turbine blades. This can significantly decrease the safe operating lives of the combustor and turbine. Thus, suppressing the thermoacoustic combustor instabilities is an enabling technology for lean, low-emissions combustors under NASA's Propulsion and Power Program. This control methodology has been developed and tested in a partnership of the NASA Glenn Research Center, Pratt & Whitney, United Technologies Research Center, and the Georgia Institute of Technology. Initial combustor rig testing of the controls algorithm was completed during 2002. Subsequently, the test results were analyzed and improvements to the method were incorporated in 2003, which culminated in the final status of this controls algorithm.
This control methodology is based on adaptive phase shifting. The combustor pressure oscillations are sensed and phase shifted, and a high-frequency fuel valve is actuated to put pressure oscillations into the combustor to cancel pressure oscillations produced by the instability.
Amplitude spectral density of uncontrolled versus controlled instability.
Long description.
Combustor instability suppression poses a challenging feedback controls problem due to unmodeled dynamics, large dead-time phase shift in excess of 1000°, large-amplitude wide-band noise in comparison to the amplitude of the instability (~ 6/1 ratio for the combustor rig), severe amplitude modulations, frequency and phase-shift randomness, and a system that continuously transitions through inherently unstable operation at increased suppression levels. To overcome these difficulties, NASA researchers developed a sophisticated controls method that does not depend on detailed modeling of system dynamics. The controls method is named "adaptive sliding phasor averaged control." The controller phase continuously slides back and forth inside the boundaries of an effective stability region that lies within a restricted control region in a stationary frame of reference. In this control algorithm, the combustor pressure oscillations are sensed through a band-pass filter to isolate the instability from noise. Then, the filtered pressure oscillations are continuously phase shifted at a rate of 40 Hz in the direction that suppresses the instability and are output to the fuel actuator at a rate of 10 kHz in order to suppress the instability. Also, discontinuous exponential gain modulation and control parameter adaptation is employed.
This active combustion instability control method was shown to reduce thermoacoustic-driven combustor pressure oscillations and was demonstrated for a high-frequency (530 Hz) instability on a single-nozzle combustor rig at United Technologies Research Center. This is the first known successful demonstration of high-frequency combustion instability suppression in a realistic aircraft engine environment. This rig, which emulates an actual engine instability experience, has many of the complexities of a real engine combustor (i.e., an actual fuel nozzle and swirler, dilution cooling, etc.).
Kopasakis, George; and DeLaat, John C.: Adaptive Instability Suppression Controls in a Liquid-Fueled Combustor. NASA/TM-2002-211805 (AIAA Paper 2002-4075), 2002. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2002/TM-2002-211805.html
Kopasakis, George: High Frequency Adaptive Instability Suppression Controls in a Liquid-Fueled Combustor. NASA/TM-2003-212535 (AIAA Paper 2003-4491), 2003. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2003/TM-2003-212535.html
Kopasakis, G.: Systems Characterization of Combustor Instabilities With Controls Design Emphasis. AIAA-2004-0638, To be presented at the 42nd AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 2004.
Find out more about this research:
Active Combustion Control:
http://www.grc.nasa.gov/WWW/cdtb/projects/combustor/
Glenn's Combustion Branch: http://www.grc.nasa.gov/WWW/combustion/
Glenn contact: George Kopasakis, 216-433-5327, George.Kopasakis-1@nasa.gov
Author: George Kopasakis
Headquarters program office: OAT
Programs/Projects: Propulsion and Power, SEC, UEET
Last updated: January 21, 2005
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