ThermoacousticInstabilities in a Gas Turbine Combustor

Sammanfattning: Stationary gas turbines are widely used today for power generation and mechanical drive applications. The introduction of new regulations on emissions in the last decades have led to extensive development and new technologies used within modern gas turbines. The majority of the gas turbines sold today have a so called DLE (Dry Low Emission) combustion system that mainly operates in the leanpremixed combustion regime. The lean-premixed regime is characterized by low emission capabilities but are more likely to exhibit stability issues compared to traditional non-premixed combustion systems. Thermoacoustic instabilities are a highly unwanted phenomena characterized by an interaction between an acoustic eld and a combustion process. This interaction may lead to self-sustained large amplitude oscillations which can cause severe structural damage to the gas turbine if it couples with a structural mode. However, since a coupled phenomena, prediction of thermoacoustic stability is a complex topic still under research. In this work, the mechanisms responsible for thermoacoustic instabilities are described and a 1- dimensional stability modelling approach is applied to the Siemens SGT-750 combustion system. The complete combustor is modelled by so called acoustic two-port elements in which a 1-dimensional ame model is incorporated. The simulations is done using a generalized network code developed by Siemens. The SGT-750 shows today excellent stability and combustion performance but a deeper knowledge in the thermoacoustic behaviour is highly valued for future development. In addition, measurement data from an engine test is evaluated, post-processed and compared with the results from the 1-dimensional network model. The results are found to be in good agreement and the thermoacoustic response of the SGT-750 is found to be dominated by both global modes including all cans as well as local modes within the individual cans.