Combustion instabilities (thermo-acoustic pressure oscillations) have been recognised for some time as a problem limiting the development of low emissions (e.g., lean burn) gas turbine combustion systems, particularly for aviation propulsion applications. Recently, significant research efforts have been focused on acoustic damping for suppression of combustion instability. Most of this work has either been experimental or based on linear acoustic theory. The last 3-5 years has seen application of density based CFD methods to this problem, but no attempts to use pressure-based CFD methods which are much more commonly used in combustion predictions. The goal of the present work is therefore to develop a pressure-based CFD algorithm in order to predict accurately acoustic propagation and acoustic damping processes, as relevant to gas turbine combustors. The developed computational algorithm described in this thesis is based on the classical pressure-correction approach, which was modified to allow fluid density variation as a function of pressure in order to simulate acoustic phenomena, which are fundamentally compressible in nature. The fact that the overall flow Mach number of relevance was likely to be low ( mildly compressible flow) also influenced the chosen methodology. For accurate capture of acoustic wave propagation at minimum grid resolution and avoiding excessive numerical smearing/dispersion, a fifth order accurate Weighted Essentially Non-Oscillatory scheme (WENO) was introduced. Characteristic-based boundary conditions were incorporated to enable accurate representation of acoustic excitation (e.g. via a loudspeaker or siren) as well as enable precise evaluation of acoustic reflection and transmission coefficients. The new methodology was first validated against simple (1D and 2D) but well proven test cases for wave propagation and demonstrated low numerical diffusion/dispersion. The proper incorporation of Characteristic-based boundary conditions was validated by comparison against classical linear acoustic analysis of acoustic and entropy waves in quasi-1D variable area duct flows. The developed method was then applied to the prediction of experimental measurements of the acoustic absorption coefficient for a single round orifice flow. Excellent agreement with experimental data was obtained in both linear and non-linear regimes. Analysis of predicted flow fields both with and without bias flow showed that non-linear acoustic behavior occurred when flow reversal begins inside the orifice. Finally, the method was applied to study acoustic excitation of combustor external aerodynamics using a pre-diffuser/dump diffuser geometry previously studied experimentally at Loughborough University and showed the significance of boundary conditions and shear layer instability to produce a sustained pressure fluctuation in the external aerodynamics.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this study, using the Method of Characteristics and numeric resources, presents a solution to the equation Spectral Density Evolution of Turbulent Kinetic Energy for a Convective Boundary Layer (CBL) in the morning. It presents three models for the evolution of spectral energy density. The first model, based on the assumption of a system of isotropic turbulence, considering only terms of energy transfer inertial and viscous dissipation. The second model adds the term energy production due to the onset of action of the parameter of heat flux on the surface, but consider it a term of energy transfer inertial anisotropy. The third model employs a mixed configuration of the two previous models, assuming thus distinct regions of operation to inertial transfer terms. The results shaped the evolution of the CLC. In this case, the growth of the energy spectrum is modeled by inserting energy in the region of low wave numbers, a region in which the term of anisotropic energy transfer can not transfer the energy introduced by the energy production term. It is observed that in a region of wave number higher there is a stabilization of the parameter variation temporal on the plane characteristics curves (PCC), indicating that the variation of wave number govern the evolution of the energy spectrum. This fact establishes a kind of criterion for stationarity of turbulent flow regimes. / No presente trabalho, utilizando o Método das Característica e recursos numéricos, apresenta-se uma solução para a Equação de Evolução Espectral de Densidade de Energia Cinética Turbulenta para uma Camada Limite Convectiva (CLC) no período da manhã. Apresenta-se três modelos para a evolução espectral da densidade de energia. O primeiro modelo, baseado na suposição de um regime de turbulência isotrópica, considera apenas termos de transferência de energia inercial e de dissipação viscosa. O segundo modelo adiciona o termo de produção de energia devido o início da ação do parâmetro de fluxo de calor na superfície, porém considerá-se um termo de transferência de energia inercial anisotrópico. O terceiro modelo emprega uma configuração mista dos dois modelos anteriores, admitindo, desta forma, regiões distintas de atuação para os termos de transferência inercial. Os resultados obtidos modelaram a evolução da CLC. Neste caso, o crescimento do espectro de energia modelado se deu pela inserção de energia na região de baixos números de onda, região em que o termo de transferência de energia anisotrópico não consegue transferir a energia inserida pelo termo de produção de energia. Observa-se que em uma região de número de onda mais alto existe uma estabilização da variação do parâmetro temporal sobre as curvas características planas (CCP), indicando que a variação do número de onda governará a evolução do espectro de energia. Fato que estabelece um tipo de critério de estacionariedade para de regimes de escoamento turbulento.