Characterisation of acoustic oscillations in annular systems
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Gas turbine engines are widely used to provide propulsion in aircraft and for power generation on land and sea. Currently, engine manufacturers are working to reduce the emission of environmentally harmful combustion products by operating these engines under lean conditions (with more air than is needed for complete combustion). However, such operating conditions can make the combustion process unstable, resulting in large, potentially damaging pressure and heat release oscillations, which need to be understood. This project is aimed at improving our understanding of this unstable process by making experimental measurements in a model annular combustor (modern gas turbines almost exclusively use annular architecture). The thesis work involves controlling a number of loudspeakers and the analysis of pressure measurements (using LabVIEW and MATLAB) to determine the modes and amplitudes of oscillation in an annular combustion chamber. The thesis therefore is a mix of experimental, analytical and computational work. Recent research has shown that combustion instabilities in annular combustion chambers can lead to a mixture of standing and spinning azimuthal modes. The nature of these modes is not yet fully understood. The goal of this work is to demonstrate the excitation of standing, spinning and mixed modes in a cold, flow-less combustion chamber and to gain a better understanding of their nature. Moderate control over the sound field in the annular system is demonstrated in the present work. While the excitation of standing waves is relatively simple, spinning modes could only partially be generated. It should be pointed out, however, that the methods applied seem to be promising if explored further and refined.