As researchers work toward drastically reducing particulate and NOx emissions, future aeroengine combustors will likely have lean-burning front ends. However, as the ground-based gas-turbine field has experienced, lean-burning combustors are more susceptible to combustion instabilities. Thermoacoustic resonances can produce large pressure oscillations inside the combustor, leading to premature mechanical failures. Consequently, active control of combustion instabilities was identified as an enabling technology for superior engine performance. Among the many control mechanisms investigated, properly phased fuel modulation to regulate the combustion heat release has been recognized as a very promising, and possibly the most practical way, to suppress combustion instabilities in aero-engines. Toward this end, the multiscale extended Kalman (MSEK) method--a novel, model-based control approach developed in-house at the NASA Glenn Research Center--was successfully used in the design of fuel-modulation logic for the demonstration of an active combustion control technique. The digital control logic formulated in this approach demonstrated steady suppression of a high-frequency (>500-Hz) combustion instability emulated in a liquid-fuel combustion test rig operating with realistic engine conditions.
MSEK logic for combustion instability suppression by fuel modulation.
Long description.
Effective fuel modulation for combustion instabilities suppression has been known to be a very challenging control problem because of the large time delay in the fuel flow mechanism and heat-release dynamics and because of the noisy environment of the combustor. The MSEK controller (see preceding figure) combines a waveletlike multiscale analysis of combustion pressure perturbations and an extended Kalman observer (a state predictor) to predict the thermoacoustic states to make up for the severe delay in the effects of fuel modulation on heat release. The commanded fuel modulation is composed of a broadband acoustics damping action based on the predicted states, and a tone-suppression action based on an estimation of thermal excitations and other transient disturbances. The multiscale analysis logic also calculates a type of time-scale-averaged pressure variance of the combustion zone that the controller minimizes by performing automatic adjustments to the control gains and phases.
In demonstrations of active combustion control on the rig in June and September 2002 at the United Technologies Research Center, this controller was shown to consistently perform automatic self-tuning and to suppress the oscillation amplitude peak by 30 percent (see the following graph). This accomplishment completed an important milestone in the Smart Efficient Components program being conducted by Glenn in collaboration with Pratt & Whitney and United Technologies. High-fidelity simulations done in 2003 showed that this controller could possibly suppress the instability peak down to the "noise-floor" if the background disturbances were at a (lower) level comparable to that seen in an actual, low-emission engine under study (ref. 1). It has potential applications at Glenn to lean-direct-injection combustors and can be generalized for effective model-based control of many challenging types of acoustic dynamics, structural dynamics, or even rotordynamics.
Amplitude spectra showing the effects of active combustion control by the MSEK method over
the combustion instability peak pressure oscillations (Sept. 11, 2002, test).
Find out more about this research.
Glenn contacts: Dr. Dzu K. Le, 216-433-5640, Dzu.K.Le@nasa.gov; and John C. DeLaat, 216-433-3744, John.C.DeLaat@nasa.gov
Author: Dr. Dzu K. Le
Headquarters program office: OAT
Programs/Projects: SEC, SRF (fiscal years 2001 to 2003)
Last updated: January 20, 2005
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