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 in 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 production and decay of entropic perturbations in combustion chambers. Laser-Induced Grating Spectroscopy (LIGS) is a promising optical technique for the diagnostics in the gas phase, which measures the local speed of sound, temperature and composition. LIGS has been previously applied as a low frequency (10 Hz) diagnostic technique in flows containing a seeded or natural absorbing molecule. In the present work, two advances are demonstrated. Firstly, LIGS is applied in a pressurised reacting flow environment using the fundamental Nd:YAG laser wavelength, taking advantage of the absorption line of the water in the flame products. Secondly, high repetition rate lasers (1-100 kHz) are used to obtain signals at high frequency in non-reacting and reacting flows, enabling time resolved measurements. This work opens up a new avenue to capture the evolution of unsteady scalars involved in the convection of entropy spots and turbulence.
Thermoacoustics, Laser-Induced Grating Spectroscopy (LIGS), Indirect noise, Combustion
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