SUMMARY
Manufacturers of commercial, power-generating, gas turbine engines continue to develop combustors that produce lower emissions of nitrogen oxides (NOx) in order to meet the environmental standards of governments around the world. Lean, premixed combustion technology is one technique used to reduce NOx emissions in many current power and energy generating systems. However, lean, premixed combustors are susceptible to thermo-acoustic oscillations, which are pressure and heat-release fluctuations that occur because of a coupling between the combustion process and the natural acoustic modes of the system. These pressure oscillations lead to premature failure of system components, resulting in very costly maintenance and downtime. Therefore, a great deal of work has gone into developing methods to prevent or eliminate these combustion instabilities. This dissertation describes the results of an investigation of a novel Fuel System Tuner (FST) that can be used to change the acoustic response of a fuel supply system in order to damp detrimental combustion oscillations in gas turbine combustors. The FST accomplishes this by altering the amplitude and phase of the fuel flowrate as it is injected into the air stream. Thus, the device provides for control of the amplitude and phase of the fuel-air ratio oscillations that eventually reach the combustion zone. When properly tuned, these fuel-air ratio oscillations alter the amplitude and phase of the combustion oscillations in such a way that stable combustor operation is achieved. A feasibility study was conducted to prove the validity of the basic idea and to develop some basic guidelines for designing the FST. Nonlinear, acoustic models for the subcomponents of the FST were developed, and these models were experimentally verified using a two-microphone impedance tube. Models useful for designing, analyzing, and predicting the performance of the FST were developed and used to demonstrate the effectiveness of the FST in a model combustor. The FST was tested and shown to reduce the acoustic pressure amplitude for a wide range of operating conditions and combustor configurations. Combustor acoustic pressure amplitude measurements were used in conjunction with model predicted fuel system impedances to determine the optimal magnitude and phase of the fuel system impedance for stable combustor operation. The FST concept and design methodology presented in this dissertation can be used on existing gas turbine combustors to reduce, or eliminate altogether, thermo-acoustic oscillations. Furthermore, the concepts presented here can be used in the initial combustor design process to develop tunable fuel systems including the necessary controls to prevent, or eliminate, thermo-acoustic oscillations.