Sound produced by entropic and compositional inhomogeneities
Combustion noise is central to several efforts to curb aircraft emissions. Indeed, acoustic waves originating in the combustor are a major contributor to aircraft noise. Moreover, they can act as a trigger for thermoacoustic instabilities, the consequences of which may range from decreased efficiency to outright failure. Modern engines designed to lower NOx emissions are particularly susceptible to this phenomenon. Unsteady combustion generates acoustic waves — direct noise — as well as convected flow disturbances, such as entropic, vortical or compositional inhomogeneities. These disturbances generate additional acoustic waves — indirect noise — if they are accelerated. The main objectives of this thesis are to examine the validity of current theoretical models for indirect noise, and to propose new ones where needed. First, a one-dimensional theoretical framework for the direct and indirect noise produced in a reflective environment is presented. The direct noise produced by the addition of mass, momentum and energy to a flow is determined analytically. A model for the entropic and compositional noise generated at a compact nozzle is then derived, accounting for nozzles with non-uniform entropy. Finally, the effect of reverberation (i.e. repeated acoustic reflections) is determined analytically. This enables direct and indirect acoustic sources to be identified and separated within experimental data, while eliminating the effect of acoustic reflections. The framework is applied to a model experiment — the Cambridge Wave Generator — in which direct, entropic and compositional noise are generated. Direct and indirect noise models are validated using experimental measurements of the sound field resulting from air injection and extraction, heat addition and helium injection. For the first time, direct, entropic and compositional noise are clearly identified in the experimental data, and shown to be in line with theoretical predictions. The results provide the first experimental demonstration of the compositional noise mechanism, and show that isentropic nozzle models are inadequate in predicting the indirect noise generated at nozzles with substantial losses.
Combustion noise, Indirect noise, Thermoacoustics, Entropic noise, Compositional noise, Reverberation, Acoustics, Fluid Mechanics
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