Online identification approach for adaptive control of combustion instabilities

This paper describes theoretical and experimental studies that focus on an identification procedure for determining the control parameters needed for damping oscillations in unstable combustors. It consists of an online system identification algorithm that determines the optimum phase for controlling the instabilities. The performance of this algorithm is investigated theoretically using a model of a Van der Pol oscillator, and experimentally in a positive feedback acoustic setup and an unstable combustor. Predicted and measured results demonstrate that the developed algorithm effectively and rapidly identifies the optimal control phase, which is applied to control the instability immediately after completion of the identification procedure. Introduction The onset of detrimental combustion instabilities has hindered the development of propulsion systems, power generating gas turbines and industrial combustion systems for many years. Such instabilities generally occur when energy supplied by the combustion process excites one or more natural acoustic modes of the combustor. In the past, these instabilities were generally eliminated with the use of passive approaches consisting of one or a combination of the following: 1. Modification of the combustion process in a way that reduced its driving, e.g., increasing the pressure drop across fuel injectors to reduce their coupling with the oscillations, 2. Increasing the acoustic damping within the system by, e.g., the addition of acoustic liners to the combustor, and 3. Modification of the combustor geometry in a way that prevented excitation of unstable modes by, e.g., the welding of baffles to the injector plate. Unfortunately, due to limited understanding of the processes that control combustion instabilities, development of passive control generally required considerable time and resources and produced solutions # Patent application pending for described identification algorithm l Graduate Research Assistant and AIAA/ASME Member + Senior Research Engineer and Adjunct Professor * David S. Lewis Jr. Chair, Regents Professor, and AIAA Fellow Copyright Q 1999 by C.E. Johnson, Y. Neumeier, and B.T. Zinn. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. 1 of limited applicability; i.e., they were generally effective for a specific system and only for a limited range of operating conditions. Furthermore, a passive control approach that was effective in one system would on occasion exacerbate the instability problem when applied in another system. These limitations of passive control approaches have stimulated interest in the development of active control approaches that could be effectively applied to various combustion systems over wide ranges of operating conditions. This paper describes progress made toward the development of an active control system (ACS) for controlling combustion instabilities. This ACS rapidly identifies critical parameters of the unstable combustor and then applies active control that adapts to slow variations in the system while continuously controlling unwanted pressure oscillations. The investigated identification process will be implemented in a previously developed ACS whose effectiveness had been demonstrated on a number of systems including a simulated gas rocket and an industrial gas turbine’“. The developed ACS uses real-time mode observation4 and periodic modulation of a fraction of the injected fuel to generate combustion process heat release oscillations that are out of phase with respect to the unstable pressure oscillations, thus damping the instability in agreement with Rayleigh’s criterion’. A schematic of this ACS is shown in Figure 1. It consists of a sensor, an observer, a controller and a fuel injector actuator (FIA). During operation, the sensor continuously measures the combustor pressure and sends the measured signal to the observer. The latter analyzes the measured pressure and determines the amplitudes, frequencies and phases of the most unstable combustor modes in real time4. The controller then uses these data to generate a control signal for the FJA. Specifically, the controller uses data describing the open loop response of the control system, which had been previously determined in open loop control test8, to determine the gain and phase of the control signal. Studies conducted to date have demonstrated that this American Institute of Aeronautics and Astronautics (c)l999 American Institute of Aeronautics & Astronautics