CFD Modeling of Turbulent Flows in Industrial Applications with Emphasis on Premixed Combustion in Gas Turbines
Abstract: Increasing restrictions on hazardous emissions combined with demands for good profitability from the operators in the energy market have contributed strongly to the development of innovations in which the emphasis is on low emissions and high efficiency. For gas turbine-based power plants, emission of NOx in particular is governed by stringent legislation. To meet these restrictions, most of the modern gas turbines for power production are equipped with a lean premixed combustion system. Increased awareness of global warning has helped to focus development on concepts that reduce the emission of CO2 into the atmosphere, since this gas is believed to be a major contributor to the problem. The maintenance of an existing power plant plays an important role in keeping emissions at a constant low level over time, since the performance of a power plant starts to decline directly after it has been put into operation. A major contributor to this decline is fouling of the compressor. The Evaporative gas turbine cycle is one example of new concepts that focus on high efficiency and low emissions. Other examples are cycles focusing on CO2 emissions, such as bio-fuel based cycles and cycles involving CO2 removal. The combustion process is an essential part of these new concepts. In contrast to the traditional cycles, the majority of these new high-efficiency and low-emission power cycles operate with combustion atmospheres that differ from the traditional hydrocarbon-air mixtures. Detailed investigations of such combustion systems are necessary for further development and/or modification of existing systems. Investigations of this type are often based on both experiment and numerical modeling, the latter studies being based on computational fluid dynamics codes and/or chemical kinetics codes. This thesis is based on computational fluid dynamics modeling of turbulent flows mainly related to premixed combustion. The methodology in this thesis was first to establish a method by which it would be possible to investigate premixed turbulent combustion in combustors of industrial character. Here, computations with different turbulence and combustion models have been applied and these computations have been validated against experimental data. The emphasis has been on accurate prediction of emissions, especially NOx and CO, in gas turbine combustors using the level-set flame library approach. Later, the method that was established was applied to combustion processes of relevance concepts dealing with low emission, such as the EvGT cycle. The influence of dilution on the emissions has been investigated for combustors under different operating conditions. Two different flame configurations have been investigated, namely a bluff body stabilized-flame and a swirl-stabilized flame. The two-phase flow in the compressor inlet of an industrial gas turbine during off-line washing conditions has also been investigated in this thesis, since a fouled compressor results in an efficiency drop which may lead to an increase in emissions.
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