Identification and modeling of coherent structures in swirl stabilized combustors at dry and steam diluted conditions

Gas turbines are the key technology for the backup of fluctuating renewable electrical energy sources. Future requirements are low pollutant emissions, high cycle efficiencies at fast startup and turn-down times, and increased fuel flexibility. Advanced cycles, such as the ultra-wet cycle, are developed to fulfill these requirements, but at the same time impose new challenges to the gas turbine combustor design. In the ultra-wet cycle, steam is produced from the hot exhaust gases and is injected into the combustion process. Thereby, the cycle efficiency is increased and the pollutant emissions are significantly reduced. However, the addition of steam to the combustion further increases the range of reactivities of the fuel–air–steam mixture. This leads to a multitude of different flame shapes in the combustor with different flow fields and flow field dynamics. In the present thesis the flow fields and flow field dynamics of swirl-stabilized combustors are experimentally investigated and analytically modeled. The focus is placed on the occurrence of large-scale coherent flow structures and their impact on the combustion process. In the first chapters of this thesis, a well-known helical, self-excited coherent flow structure, denoted as the precessing vortex core (PVC), is assessed. Its occurrence is shown to be linked to different flame shapes, which are demonstrated to strongly depend on the reactivity of the fuel–air– steam mixture. The importance of the PVC for flame fluctuations, mixing processes, and the flame stabilization is experimentally demonstrated. In an analytical study employing linear hydrodynamic stability analysis, the PVC is modeled and the key parameters for its excitation and suppression are identified. Furthermore, the modeling allows for the identification of control strategies for the suppression of the PVC by small flow field modifications. The second type of coherent flow structures investigated in this thesis are axisymmetric, ring-shaped vortices that are excited by the coupling of the flame with the acoustics of the combustion system. This coupling can lead to dangerous high amplitude acoustic pressure oscillations and heat release fluctuations, called thermoacoustic instabilities. One key driver for the flame oscillations is the interaction of the flame with these axisymmetric vortical coherent flow structures. In an experimental and analytical study, the growth of these vortical structures in the linear and non-linear regime is investigated and their important role for the flame oscillation is pointed out. Furthermore, the analytical study reveals an important and new saturation mechanism for the vortical structures and, thus, for the thermoacoustic instabilities. Finally, the interaction of both types of coherent structures is analyzed. The experiments reveal a strong influence of the axisymmetric structures on the PVC. Employing the same analytical tools as in the previous parts, the mechanisms for the influence are identified and a simple model analogy is presented, which features the most important dynamics and allows for a better insight into the interaction mechanisms.