Simulation and experimental results from an annular Rijke tube are presented. This system is a thermoacoustic surrogate system of an annular gas turbine combustor which, despite its simplicity, possesses the basic mechanisms to feature unstable azimuthal modes. A thermoacoustic network model is set up and used to derive low-order models for modal control of the system. The derived controllers are successfully applied in simulation and experiment. With the modal controllers, all unstable acoustic modes can be eliminated individually. A simultaneous use of all controllers results in a complete stabilization of the system.

In extremum seeking control, the gradient estimation is the key enabler for a successful online optimization. For this purpose, the classical algorithm uses a combination of high- and low-pass filters. In this investigation extended Kalman filters (EKF) are used instead. The approach is explained in detail and advantages of Kalman filtering will become apparent. A novel approach for the gradient estimation for dual-input single-output systems is presented. The proposed EKF incorporates the coupling of the output to both inputs, thus, enabling a superior gradient estimate. A simulation study shows that faster convergence of the extremum-seeking controller can be achieved using this estimator. The feasibility of the proposed algorithm in an experimental setup is demonstrated by control of thermoacoustic instabilities in an atmospheric combustor test rig. (C) 2012 Elsevier Ltd. All rights reserved.

This paper presents an overview of the methodology developed to predict, control and optimize the NOx emissions and stability of lean premixed combustors. Investigations are performed firstly in cold flow and are validated with reacting flow measurements. A new cold flow mixing model describes the relevant characteristics of the fuel/air mixing, i.e. the mixing quality and convective time delays, for different operating points of the system. Measurements in the combustor are performed to correct the flame position effect or calibrate the cold flow results. The model is for the first time implemented in an extremum seeking controller to optimize the emissions and pressure pulsations of the combustor by adjusting the fuel mixing profile. A further increase of the fuel/air mixing, necessary for further NOx reductions, with pulsating fuel injection, is demonstrated. At the end, the developed adaptive control strategies demonstrate opportunities for future efficiency increases in industrial combustors.

A mixing model of a swirl inducing premixed burner is derived from non-reacting investigation and used to control the fuel staging of the burner to ensure stability and low NO(x) emissions. The convective time delays, critical for the combustor stability, are obtained after identification of a step response of the outlet concentration with a one dimensional mixing model. The steady mixing is used to evaluate quantitatively the mixing quality which correlates with NO(x) emissions. Time delays as well as scalar unmixedness criteria derived from those measurements are used to predict the combustor stability and NO(x) emissions maps for different injection configurations at one operating point. The resulting model is used to extend an Extremum Seeking Controller, which adjusts the fuel repartition to reduce the pressure pulsations and NO(x) emissions.

Extremum seeking control was applied to modify the fuel distribution of a premixed atmospheric swirl stabilized gas combustor to suppress combustion instabilities without increasing NOx emissions. The overall fuel flow was split into one premixed and two secondary injections. Dierent injection repartitions were tested and the best configuration regarding instability suppression was selected for closed loop control. Two dierent types of extremum seeking controllers were implemented and tested on symmetric and asymmetric secondary fuel injections at dierent operating points of the combustion chamber. To increase the controller speed, a fast NOx sensor based on chemiluminescence was implemented. The controller was able to suppress the instability of two operating points of the combustion system within a few minutes, mainly limited by the response time of the fuel flow sensors. The control scheme was also able to maintain the combustion stable during transients.

Extremum seeking control based on phase-shifted pressure feedback was applied to suppress combustion instabilities in an atmospheric swirl-stabilized combustor. Acoustic actuation up- and downstream of the flame was used to attenuate pressure oscillations. Gain and delay of the phase-shift controller were tuned by the adaptive scheme. The controller was able to successfully drive the control parameters to the values corresponding to the highest suppression of the instability. During transients, the scheme could maintain control by tracking the optimal control parameters. The controller was further used to study the linear growth of the instability at dierent preheat temperatures. The growth rates were compared to the corresponding limit cycle amplitudes. A monotonic relationship was found.