Combustion noise has become a major research interest within the aerospace community. Stricter pollutant emission regulations have forced the introduction of lean premixed pre-vaporised combustors, which produce less NOx but burn more unsteadily, generating more noise and creating the potential for combustion instabilities. Pressure fluctuations in combustion chambers are traditionally classified as direct and indirect noise. The former arises directly from the heat release rate perturbations

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... the flame. The latter is generated indirectly from the acceleration of regions of non-uniform temperature, density and composition (entropy and composition spots) through narrow passages such as nozzles or turbine guide vanes. Entropy-generated sound waves are both transmitted downstream of the acceleration point, contributing to the overall noise emission, and reflected upstream, where they may couple with the acoustics of the system, potentially triggering instabilities. Presently, the processes causing the generation of indirect noise and the triggering mechanisms for secondary instabilities are still not completely understood and need further investigation. The aim of this thesis is to shed light on the behaviour of accelerating entropy spots, providing analytical and experimental tools to identify and isolate travelling entropy spots and the sound generated from their acceleration. The physical mechanisms of entropy-to-sound conversion are investigated using a model non-reacting flow system (the Entropy Generator Rig, EGR), designed to mimic the behaviour of a combustor in a controlled environment. The contribution of indirect noise can be easily identified and isolated in the acquired pressure traces, allowing the first direct comparison between experimental data and a newly developed analytical model for the entropy-to-sound conversion in non-isentropic systems. An important challenge that has limited the understanding of indirect noise is the lack of experimental techniques capable to detect and characterise the p [...]