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Self-excited systems are open loop unstable plants having a nonlinearity that prevents an exponentially increasing time response. The resulting limit cycle is induced by any slight disturbance that causes the response of the system to grow to the saturation level of the nonlinearity. Because there is no external disturbance, control of these self-excited systems requires that the open loop system dynamics are altered so that any unstable open loop poles are stabilized in the closed loop. This work examines a variety of adaptive control approaches for controlling a thermoacoustic instability, a physical self-excited system. Initially, a static feedback controller loopshaping design and associated system identification method is presented. This design approach is shown to effectively stabilize an unstable Rijke tube combustor while preventing the creation of additional controller induced instabilities. The loopshaping design method is then used in conjunction with a trained artificial neural network to demonstrate stabilizing control in the presence of changing plant dynamics over a wide variety of operating conditions. However, because the ANN is designed specifically for a single combustor/actuator arrangement, its limited portability is a distinct disadvantage. Filtered-X least mean squares (LMS) adaptive feedback control approaches are examined when applied to both stable and unstable plants. An identification method for approximating the relevant plant dynamics to be modeled is proposed and shown to effectively stabilize the self-excited system in simulations and experiments. The adaptive feedback controller is further analyzed for robust performance when applied to the stable, disturbance rejection control problem. It is shown that robust stability cannot be guaranteed because arbitrarily small errors in the plant model can generate gradient divergence and unstable feedback loops. Finally, a time-averaged-gradient (TAG) algorithm is investigated for use in controlling self-excited systems such as the thermoacoustic instability. The TAG algorithm is shown to be very effective in stabilizing the unstable dynamics using a variety of controller parameterizations, without the need for plant estimation information from the system to be controlled. / Ph. D.

This last decade experienced a sudden increase of interest in the control of thermo-acoustic instabilities, in particular through the use of fuel modulation techniques. The primary goal of this research was to design, construct and characterize a high bandwidth proportional fuel injection system, which could be used to study the effect of specific levels of fuel modulation on the combustion process and the reduction of thermo-acoustic instabilities. A fuel injection system, incorporating the use of a closed loop piston and check valve, was designed to modulate the primary fuel supply of an atmospheric liquid-fueled swirl stabilized combustor operating at a mean volumetric fuel flow rate of 0.4 GPH. The ability of the fuel injection system to modulate the fuel was examined by measuring the fuel line pressure and the flow rate produced during operation. The authority of this modulation over the combustion process was investigated by examining the effect of fuel modulation on the combustor pressure and the heat release of the flame. Sinusoidal operation of the fuel injection system demonstrated: a bandwidth greater that 800 Hz, significant open loop authority (averaging 12 dB) with regards to the combustor pressure, significant open loop authority (averaging 33 dB) with regards to the unsteady heat release rate and an approximate 8 dB reduction of the combustor pressure oscillation present at 100 Hz, using a phase shift controller. It is possible to scale the closed loop piston and check valve configuration used to create the fuel injection system discussed in this work to realistic combustor operating conditions for further active combustion control studies. / Master of Science

High-bandwidth fuel modulation is currently one of the most promising methods for active combustion control. To attenuate the large pressure oscillations in the combustion chamber, the fuel is pulsed so that the heat release rate fluctuations damp the pressure oscillations in the combustor. This thesis focuses on the development and implementation of a high-bandwidth, proportional modulation system for liquid-fuel active combustion control. The throttle valve modulation system, discussed in this thesis, uses a 500-um piezoelectric stack coupled with an off-the-shelf valve. After comparing three other types of actuators, the piezoelectric stack was selected because of its compact size, bandwidth capabilities, and relatively low cost. Using the acoustic resonance of the fuel line, the system is able to achieve 128% pressure modulation, relative to the mean pressure, and is capable of producing more than 75% flow modulation at 115 Hz. Additionally, at 760 Hz the system produces 40% pressure modulation and 21% flow modulation with flow rates between 0.4 and 10 gph. Control authority was demonstrated on a single-nozzle kerosene combustor which exhibits a well-pronounced instability at ~115 Hz. Using the modulation system, the fundamental peak of the combustion instability was reduced by 30 dB, and the broadband sound pressure levels inside the combustor were reduced by 12 dB. However, the most important conclusion from the combustion control experiments was not the system?s accomplishments, but rather its inability to control the combustor at high global equivalence ratios. Our work indicates that having the ability to modulate a large percentage of the primary fuel is not always sufficient for active combustion control. / Master of Science

The first part of this work focuses on control experiments performed on an unstable kerosene-fueled turbulent combustor. Using a phase shift controller and primary fuel modulation stability is successfully gained for a wide band of global equivalence ratios allowing the limitations of the control scheme to be characterized. It is shown that control signal saturation can significantly impact the ability of the control scheme to stabilize the system. Three different regions of controllability are defined based on the degree of saturation. A hysteresis behavior is also found to exist for the controller settings depending on whether stability is being maintained or realized for an unstable system. The second part of this work focuses on the impact that primary fuel modulation has on the fuel spray. Measurements for a simplex nozzle and an air-assist nozzle are taken under both static and dynamic operating conditions with a Phase Doppler Anemometry system. The dynamic modulation is found to significantly impact the spray properties of both nozzles. / Master of Science

Thermoacoustic instabilities are a problem in modern pre-mixed combustors causing reduced performance and leading in the extreme to combustor failure from excessive pressure cycles. Much work has been done using linear controllers to eliminate these instabilities. Many experimenters in the field have used pulsed and subharmonic fuel controllers to eliminate these instabilities, but very little investigative work has been done on these controllers. The goal of this work is to explain the mechanism of control behind pulsed controllers. It is shown that the combustion system can be treated as a linear system, thus meaning that frequency components of the control signal at the desired instability frequency are the dominant means of control, with nonlinear effects only serving to slightly reduce the gain necessary for control. Fourier analysis is thus performed on pulsed signals and the components analyzed, showing that there will indeed be a component of a pulsed signal at the frequency of the instability, aside from a few select cases. It is then proven that this frequency component is largely responsible for control of the thermoacoustic system using proportional height pulse train signals, which will change pulse height based on the amplitude of the instability. This analysis is then used to predict the height of instabilities resulting from the use of fixed height pulse control signals. Finally, numerical simulations and experimental observations support the analytical constructs. Acoustic control is mainly used for these experiments, although some preliminary work with liquid fuel controllers is also presented. / Master of Science

Lean premixed fuel-air conditions in large gas turbines are used to improve efficiency and reduce emissions. These conditions give rise to large undamped pressure oscillations at the combustor's natural frequencies which reduce the turbine's longevity and reliability. Active control of the pressure oscillations, called thermoacoustic instabilities, has been sought as passive abatement of these instabilities does not provide adequate damping and is often impractical on a large scale. Phase shift control of the instabilities is perhaps the simplest and most popular technique employed but often does not provide good performance in that controller induced secondary instabilities are generated with increasing loop gain. This thesis investigates the general underlying cause of the secondary instabilities and shows that high average group delay through the frequency region of the instability is the root of the problem. This average group delay is then shown to be due not only the controller itself but can also be associated with other components and inherent characteristics of the control loop such as actuators and time delay, respectively. An "optimum" phase shift controller, consisting of an appropriate shift in phase and a low order, wide bandwidth bandpass filter, is developed for a Rijke tube combustor and shown to closely match the response of an LQG controller designed only for system stabilization. Both the optimal phase shifter and the LQG controller are developed based on a modified model of the thermoacoustic loop which takes into account the change in density of the combustion reactants at the flame location. Additionally, the system model is coupled with a model of the control loop and then validated by comparison of simulated results to experimental results using nearly identical controllers. / Master of Science

Este trabalho descreve o desenvolvimento de um protótipo para o controle em malha fechada da temperatura e estequiometria de um secador vertical cerâmico alimentado a gás natural, instalado na linha de produção de revestimentos cerâmicos da empresa Eliane Revestimentos Cerâmicos - Unidade II, em Cocal do Sul - SC. É apresentada a fundamentação teórica da combustão e os dados experimentais do comportamento do protótipo aplicado no secador, com o objetivo de reduzir a emissão de poluentes e aumentar a eficiência de queima do gás natural, tendo como conseqüência a diminuição do consumo do mesmo. Foi aproveitada a estrutura do secador referente à instrumentação e aos mecanismos de combustão. Sendo concluído o sensoriamento, com a adição do sensor de oxigênio em um tubo fixado ao queimador. A interface de potência do protótipo foi realizada com a inclusão do inversor de freqüência para o controle da rotação do ventilador de combustão. Com a conclusão da implementação da instrumentação, e interfaces de potência do protótipo, foi inserido o sistema de supervisão desenvolvido com a ferramenta Delphi no computador industrial do secador, o qual contém as telas referentes às monitorações dos sensores e parametrização do controlador Com o estudo do comportamento do processo em questão, foi obtido o modelo matemático através da análise dos dados experimentais do secador. A partir deste, desenvolveu-se o projeto e a estratégia de controle, bem como, o algoritmo de controle. O modelo teórico do sistema de controle foi implementado em um microcontrolador. Realizaram-se simulações do modelo teórico de controle para validar o mesmo referente à estabilidade, tanto na temperatura, quando na estequiometria. Após a validação do controle pelas simulações realizadas, foi implementado no secador o protótipo. Ao longo de um período pré-determinado foi analisado o comportamento do secador com o protótipo, visando comparar com o sistema original os ganhos obtidos vinculados a etapa de secagem como: aumento da qualidade final das peças cerâmicas e redução do consumo de gás natural. Conclui-se, que o trabalho atingiu as metas propostas, pois a validação do controle desenvolvido foi comprovada através da verificação da estabilidade da temperatura e estequiometria, aumento da qualidade final das peças cerâmicas e redução dos níveis de emissão de poluentes comparados ao sistema original do secador. / This work describes the development of a prototype for the closed loop control of temperature and composition of a vertical ceramic dryer fueled with natural gas, installed on the production line of Coating Ceramics Tiles Eliane Company - Unit II in Cocal do Sul - SC. It presented the theoretical foundation of combustion and experimental data of the behavior of the prototype used in the dryer, with the objective of reducing the emission of pollutants and increase the efficiency of burning natural gas, and consequently the decrease in consumption of it. It used the structure of the hair on the instrumentation and the mechanisms of combustion. Since the sensing completed with the addition of the oxygen sensor in a tube attached to the burner. The interface power prototype was made with the inclusion of the frequency inverter to control the rotation of the combustion fan. With the completion of the implementation of instrumentation, interfaces and power of the prototype, inserted the supervisory system tool developed with Delphi in the dryer industrial computer, which contains the paintings concerning monitoring of sensors and controller parameterisation. With the study of the behavior of the process in question, was carried out a mathematical model by analyzing the experimental data of the dryer. From this, it was developed the design and control strategy, and the algorithm of control. The theoretical model of the control system was implemented in a microcontroller. There were simulations of the theoretical model of control to validate the same on the stability, both in temperature, when in stoichiometry. After the validation of control by the simulations carried out, has been implemented in the prototype dryer. Over a pre-determined period has been analyzed the behavior of the dryer with the prototype, to compare with the original gains tied to the stage of drying as: increasing the quality of the final ceramic goods and reduce consumption of natural gas. It was concluded that the work has reached the goals, because the validation of the developed control was confirmed by checking the stability of temperature and stoichiometry, increase the quality of the final pieces of ceramics and reducing emission levels of pollutants compared to the original system dryer.

The work reported in this thesis explores innovative control structures and controller design for a heavy duty Caterpillar C6.6 diesel engine. The aim of the work is not only to demonstrate the optimisation of engine performance in terms of fuel consumption, NOx and soot emissions, but also to explore ways to reduce lengthy calibration time and its associated high costs. The test engine is equipped with high pressure exhaust gas recirculation (EGR) and a variable geometry turbocharger (VGT). Consequently, there are two principal inputs in the air-path: EGR valve position and VGT vane position. The fuel injection system is common rail, with injectors electrically actuated and includes a multi-pulse injection mode. With two-pulse injection mode, there are as many as five control variables in the fuel-path needing to be adjusted for different engine operating conditions.

Este trabalho descreve o desenvolvimento de um protótipo para o controle em malha fechada da temperatura e estequiometria de um secador vertical cerâmico alimentado a gás natural, instalado na linha de produção de revestimentos cerâmicos da empresa Eliane Revestimentos Cerâmicos - Unidade II, em Cocal do Sul - SC. É apresentada a fundamentação teórica da combustão e os dados experimentais do comportamento do protótipo aplicado no secador, com o objetivo de reduzir a emissão de poluentes e aumentar a eficiência de queima do gás natural, tendo como conseqüência a diminuição do consumo do mesmo. Foi aproveitada a estrutura do secador referente à instrumentação e aos mecanismos de combustão. Sendo concluído o sensoriamento, com a adição do sensor de oxigênio em um tubo fixado ao queimador. A interface de potência do protótipo foi realizada com a inclusão do inversor de freqüência para o controle da rotação do ventilador de combustão. Com a conclusão da implementação da instrumentação, e interfaces de potência do protótipo, foi inserido o sistema de supervisão desenvolvido com a ferramenta Delphi no computador industrial do secador, o qual contém as telas referentes às monitorações dos sensores e parametrização do controlador Com o estudo do comportamento do processo em questão, foi obtido o modelo matemático através da análise dos dados experimentais do secador. A partir deste, desenvolveu-se o projeto e a estratégia de controle, bem como, o algoritmo de controle. O modelo teórico do sistema de controle foi implementado em um microcontrolador. Realizaram-se simulações do modelo teórico de controle para validar o mesmo referente à estabilidade, tanto na temperatura, quando na estequiometria. Após a validação do controle pelas simulações realizadas, foi implementado no secador o protótipo. Ao longo de um período pré-determinado foi analisado o comportamento do secador com o protótipo, visando comparar com o sistema original os ganhos obtidos vinculados a etapa de secagem como: aumento da qualidade final das peças cerâmicas e redução do consumo de gás natural. Conclui-se, que o trabalho atingiu as metas propostas, pois a validação do controle desenvolvido foi comprovada através da verificação da estabilidade da temperatura e estequiometria, aumento da qualidade final das peças cerâmicas e redução dos níveis de emissão de poluentes comparados ao sistema original do secador. / This work describes the development of a prototype for the closed loop control of temperature and composition of a vertical ceramic dryer fueled with natural gas, installed on the production line of Coating Ceramics Tiles Eliane Company - Unit II in Cocal do Sul - SC. It presented the theoretical foundation of combustion and experimental data of the behavior of the prototype used in the dryer, with the objective of reducing the emission of pollutants and increase the efficiency of burning natural gas, and consequently the decrease in consumption of it. It used the structure of the hair on the instrumentation and the mechanisms of combustion. Since the sensing completed with the addition of the oxygen sensor in a tube attached to the burner. The interface power prototype was made with the inclusion of the frequency inverter to control the rotation of the combustion fan. With the completion of the implementation of instrumentation, interfaces and power of the prototype, inserted the supervisory system tool developed with Delphi in the dryer industrial computer, which contains the paintings concerning monitoring of sensors and controller parameterisation. With the study of the behavior of the process in question, was carried out a mathematical model by analyzing the experimental data of the dryer. From this, it was developed the design and control strategy, and the algorithm of control. The theoretical model of the control system was implemented in a microcontroller. There were simulations of the theoretical model of control to validate the same on the stability, both in temperature, when in stoichiometry. After the validation of control by the simulations carried out, has been implemented in the prototype dryer. Over a pre-determined period has been analyzed the behavior of the dryer with the prototype, to compare with the original gains tied to the stage of drying as: increasing the quality of the final ceramic goods and reduce consumption of natural gas. It was concluded that the work has reached the goals, because the validation of the developed control was confirmed by checking the stability of temperature and stoichiometry, increase the quality of the final pieces of ceramics and reducing emission levels of pollutants compared to the original system dryer.