Browsing by Author "Kostrzewa, Krzysztof"

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Open Access
Advanced computational methods in identification of thermo-acoustic systems
(2011) Kostrzewa, Krzysztof; Aigner, Manfred (Prof. Dr.-Ing.)
The use of fossil fuels in power generation has a significant impact on the environment. Even the burning of natural gas, considered as the cleanest of all commonly available fossils fuels, is generally associated with emissions of many dangerous air pollutants. In the last decades, additional environmental constrains have been introduced to reduce the emissions from gas turbines. The primary focus has been especially placed on nitrogen oxide (NOx) to minimize its emissions to acceptable values. This reduction has been partially accomplished by utilizing a lean-premixed flame rather than the standard diffusion type flame. The biggest disadvantage of these combustion systems is that they are very prone to thermo-acoustically induced oscillations as a result of a complex feedback mechanism between pressure and heat release fluctuations, which may lead to uncontrolled high-pressure amplitude oscillations inside the combustor at certain operating conditions In order to predict the linear stability of partially premixed combustion systems in industrial-scale gas turbines, a detailed acoustic characteristic for each of the major components is required. Sudden changes in the combustion process of gas turbines in order to reduce emissions may result in large amplitude pressure oscillations associated with a coupling between the natural acoustic modes of the combustor and the unsteady heat release from the flame. Detailed one-dimensional acoustic network models have to be built up to represent entire combustion systems. These models consist of ducts, diffusers, junctions and a flame element. Time delay elements have to be applied to describe properly the interactions between acoustics and heat release fluctuations and to investigate the stability of the system using linear stability theory. The main goal of this work is to develop and to validate technically relevant tools to mitigate the consequence of an occurrence of combustion instabilities. From the acoustic modeling point of view, the combustion system elements can be characterized either by their acoustic transfer matrices or by flame transfer functions. A description of the burner and flame are of main importance. In this study, acoustic characteristics of a sudden change of area, a truncated teardrop specimen, an atmospheric generic swirl burner, and a prototype industrial burner at elevated pressure have been made by means of unsteady flow simulations and system identification. A comprehensive process for the computation of such CFD/SI based characteristics has been shown. Whenever possible the results have been validated and compared with the experimental and theoretical data. In the case of the prototype industrial burner at elevated pressure it has been demonstrated that using the CFD based flame transfer function it is possible to improve the overall gas turbine combustion system stability prediction. In addition to these calculations, LES-like computations of an atmospheric swirl burner, and a prototype industrial burner at elevated pressure have been performed. It allows for the determination of flame and flow dynamics, which might drive the stability of the entire combustion system. In the case of an atmospheric swirl burner, the LES-based flow field has also been forced at discrete frequencies to visualize the development of coherent structures in the combustion chamber. Similarly, to the experiments, the evaluation of coherent structures is captured. The numerical visualizations are then compared with the experimental findings. Finally, an exemplary calculation of self-excited oscillations has been made in order to demonstrate that commercial CFD codes are able to capture these kinds of instabilities. The unstable frequencies found during the computation have been benchmarked employing one-dimensional acoustic network code.