No abstract

A numerical model is presented for the precise prediction of carbon monoxide (CO) emissions in gas turbine combustors. All models are based on Computational Fluid Dynamics (CFD). This work starts with an introduction of fundamental mechanisms, which are responsible for CO emissions. As we will show, there is a need of CO-models as standard combustion models fail to predict CO-emissions precisely. For the purpose of validation, experiments are conducted. High ratios of secondary air is bypassing the burner in order to induce interaction of the flame front with secondary air, as the flame brush gets diluted and decreases in reactivity. Note, this is an important mechanism for elevated CO emissions in multi-burner systems with high staging ratio. Five operating points with each having a different adiabatic flame temperature were measured. They include equilibrium (complete burnout) and superequilibrium CO (incomplete burnout). In summary, it is shown that the prediction of CO with the presented models lead to a significant improvement as it captures the transition from equilibrium to superequilibrium CO. Furthermore, the strong underestimation of CO predicted by standard combustion models is shown.

A novel methodology for linear stability analysis of high-frequency thermoacoustic oscillations in gas turbine combustors is presented. The methodology is based on the linearized Euler equations, which yield a high-fidelity description of acoustic wave propagation and damping in complex, non-uniform, reactive mean flow environments, such as encountered in gas turbine combustion chambers. Specifically, this work introduces three novelties to the community: (1) Linear stability analysis on the basis of linearized Euler equations. (2) Explicit consideration of three-dimensional, acoustic oscillations at screech level frequencies, particularly the first transversal mode. (3) Handling of non-compact flame coupling with LEE, that is, the spatially varying coupling dynamics between perturbation and unsteady flame response due to small acoustic wavelengths. Two different configurations of an experimental model combustor in terms of thermal power and mass flow rates are subject of the analysis. Linear flame driving is modeled by prescribing the unsteady heat release source term of the linearized Euler equations by local flame transfer functions, which are retrieved from first principles. The required steady state flow field is numerically obtained via CFD, which is based on an extended Flamelet-Generated Manifold combustion model, taking into account heat transfer to the environment. The model is therefore highly suitable for such types of combustors. The configurations are simulated, and thermoacoustically characterized in terms of eigenfrequencies and growth rates associated with the first transversal mode. The findings are validated against experimentally observed thermoacoustic stability characteristics. On the basis of the results, new insights into the acoustic field are discussed.

The work presented in this paper comprises the application of an extension for the Flamelet Generated Manifold model which allows to consider elevated flame stretch rates and heat loss. This approach does not require further table dimensions. Hence, the numerical overhead is negligible, preserving the industrial applicability. A validation is performed in which stretch and heat loss dependent distributions are obtained from the combustion model to compare them to experimental data from an atmospheric single burner test rig operating at lean conditions. The reaction mechanism is extended by OH*-kinetics which allows the comparison of numerical OH*-concentrations with experimentally obtained OH*-chemiluminescence. Improvement compared to the Flamelet Generated Manifold model without extension regarding the shape and position of the turbulent flame brush can be shown and are substantiated by the validation of species distributions which better fit the experimental in situ measurements when the extension is used. These improvements are mandatory to enable subsequent modeling of emissions or thermoacoustics where high accuracy is required. In addition to the validation, a qualitative comparison of further combustion models is performed in which the experimental data serve as a benchmark to evaluate the accuracy. Most combustion models typically simplify the combustion process as flame stretch or non-adiabatic effects are not captured. It turns out that the tested combustion models show improvement when stretch or heat loss is considered by model corrections. However, satisfactory results could only be achieved by considering both effects employing the extension for the Flamelet Generated Manifold model.

The hybrid Computational Fluid Dynamics/Computational AeroAcoustics (CFD/CAA) approach represents an effective method to assess the stability of non-compact thermoacoustic systems. This paper summarizes the state-of-the-art of this method, which is currently applied for the stability prediction of a lab-scale configuration of a perfectly-premixed, swirl-stabilized gas turbine combustion chamber at the Thermodynamics institute of the Technical University of Munich. Specifically, 80 operational points, for which experimentally observed stability information is readily available, are numerically investigated concerning their susceptibility to develop thermoacoustically unstable oscillations at the first transversal eigenmode of the combustor. Three contributions are considered in this work: (1) flame driving due the deformation and displacement of the flame, (2) visco-thermal losses in the acoustic boundary layer and (3) damping due to acoustically induced vortex shedding. The analysis is based on eigenfrequency computations of the Linearized Euler Equations with the stabilized Finite Element Method (sFEM). One main advancement presented in this study is the elimination of the non-physical impact of artificial diffusion schemes, which is necessary to produce numerically stable solutions, but falsifies the computed stability results.