Investigation of the Global Mode in Swirling Combustor Flows: Experimental Observations and Local and Global Stability Analysis

Self-excited helical flow oscillations are frequently observed in gas turbine combustors. In the present work a new approach is presented tackling this phenomenon with stability concepts. Three reacting swirling flows are investigated. All of them undergo vortex breakdown, but only two of them show self-excited global flow oscillations at well-defined frequencies. The oscillations feature a precession of the vortex core and synchronized Kelvin-Helmholtz instabilities in the shear layers. Based on the mean flow fields, local and global linear hydrodynamic stability analyses are carried out. The dampening effect of the Reynolds stresses is accounted for by an eddy viscosity estimated from the experimental results. Both the local and the global analysis successfully identify linear global modes as being responsible for the large-scale flow oscillations and successfully predict their frequency. However, only the global analysis accurately predicts a globally stable flow field for the case without the oscillation, while the local analysis overpredicts the global growth rate. The predicted spatial distribution of the amplitude functions agree very well to the experimentally identified global mode. This successful application of global and local stability concepts to a complex and practically relevant flow configuration paves the way for the application of theoretically-founded passive and active control strategies

Identifikation und Modellierung von kohärenten Strukturen in drallstabilisierten Brennkammern bei trockenen und dampfverdünnten Betriebsbedingungen

Gasturbinen sind die Schlüsseltechnologie für eine Sicherstellung der zuverlässigen Stromversorgung, wenn große Teile des Stroms aus fluktuierenden erneuerbaren Energieformen stammen. Um zukünftige Anforderungen, wie Effizienzsteigerungen, Schadstoffreduktionen, schnelle Regelzeiten und Treibstoffflexibilität zu erreichen, werden neue Verbrennungskonzepte wie die "Ultra-Nasse" Verbrennung benötigt. Bei diesem Konzept wird die Wärme des Abgasstrahls genutzt, um Wasserdampf zu erzeugen, der in die Brennkammer eingedüst wird, um die Effizienz zu erhöhen und die Emissionen zu verringern. Dampfzugabe und Brennstoffflexibilität führen zu einer großen Variation der Reaktivität des zu verbrennenden Gemisches und in der Konsequenz zu verschiedenen Flammenformen und Strömungsfeldern in der Brennkammer. In der vorliegenden Arbeit wird das Strömungsfeld in drallstabilisierten Gasturbinenbrennkammern experimentell und analytisch untersucht. Der Fokus der Arbeit ist auf das Auftreten und die Modellierung von großskaligen kohärenten Strömungstrukturen und deren Auswirkungen auf die Verbrennung gerichtet. In den ersten Kapiteln dieser Arbeit wird eine bekannte selbsterregte helikale kohärente Strömungsstruktur, der sogenannte präzessierende Wirbelkern (precessing vortex core, PVC) untersucht. Es wird gezeigt, dass das Auftreten des PVC eng mit der Flammenform verbunden ist. Die hohe praktische Relevanz des PVC wird durch seinen Einfluss auf Mischungsvorgänge, Flammenoszillationen und die Flammenstabilisierung verdeutlicht. Die Analyse der hydrodynamischen Stabilität der gemittelten Strömungsfelder ermöglicht die Modellierung des PVC und die Identifikation der Schlüsselparameter, die die Anfachung oder Unterdrückung des PVC bestimmen. Auf Basis der Modellierung können weiterhin Kontrollmechanismen entwickelt werden, um den PVC gezielt zu beeinflussen. Bei der zweiten Form von kohärenten Strukturen, die im Rahmen dieser Arbeit untersucht werden, handelt es sich um achsensymmetrische Ringwirbel. Die Wirbel werden durch eine Kopplung der Flamme mit der Systemakustik angeregt, interagieren mit der Flamme und können thermoakustische Instabilitäten verbunden mit starken Druckschwankungen hervorrufen. In einer experimentellen und analytischen Untersuchung wird das lineare und nichtlineare Anfachen der Wirbel in den Scherschichten des Strömungsfeldes bestimmt und modelliert. Die Ergebnisse zeigen die Relevanz eines neuartigen Sättigungsmechanismus für die Ringwirbel und damit auch für thermoakustische Instabilitäten. Basierend auf den vorherigen Ergebnissen wird im letzten Teil der Arbeit die Interaktion der selbsterregten helikalen kohärenten Struktur (PVC) und der Ringwirbel untersucht. Die Experimente zeigen einen starken Einfluss der Ringwirbel auf den PVC. Die Interaktionsmechanismen werden unter Berücksichtigung der hydrodynamischen Stabilität der Strömungsfelder identifiziert und für eine Modellbildung genutzt. Die Modellbildung erlaubt ein tiefergehendes Verständnis der experimentell beobachteten Phänomene.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 start-up 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

Large Eddy Simulations of Hydrogen Oxidation at Ultra-Wet Conditions in a Model Gas Turbine Combustor Applying Detailed Chemistry

Humidified gas turbines (HGT) offer the attractive possibility of increasing plant efficiency without the cost of an additional steam turbine as is the case for a combined gas-steam cycle. In addition to efficiency gains, adding steam into the combustion process reduces NOx emissions. It increases the specific heat capacity (hence, lowering possible temperature peaks) and reduces the oxygen concentration. Despite the thermophysical effects, steam alters the kinetics and, thus, reduces NOx formation significantly. In addition, it allows operation using a variety of fuels, including hydrogen and hydrogen-rich fuels. Therefore, ultra-wet gas turbine operation is an attractive solution for industrial applications. The major modification compared to current gas turbines lies in the design of the combustion chamber, which should accommodate a large amount of steam without losing in stability. In the current study, the premixed combustion of pure hydrogen diluted with different steam levels is investigated. The effect of steam on the combustion process is addressed using detailed chemistry. In order to identify an adequate oxidation mechanism, several candidates are identified and compared. The respective performances are assessed based on laminar premixed flame calculations under dry and wet conditions, for which experimentally determined flame speeds are available. Further insight is gained by observing the effect of steam on the flame structure, in particular HO2 and OH* profiles. Moreover, the mechanism is used for the simulation of a turbulent flame in a generic swirl burner fed with hydrogen and humidified air. Large eddy simulations (LES) are employed. It is shown that by adding steam, the heat release peak spreads. At high steam content, the flame front is thicker and the flame extends further downstream. The dynamics of the oxidation layer under dry and wet conditions is captured; thus, an accurate prediction of the velocity field, flame shape, and position is achieved. The latter is compared with experimental data (PIV and OH* chemiluminescence). The reacting simulations were conducted under atmospheric conditions. The steam-air ratio was varied from 0% to 50%. [DOI: 10.1115/1.4007718