Analysis of flow dynamics and flame stabilization in gas turbine related combustors

Sammanfattning: The regulations on the emissions from combustion devices are getting more and more strict for environmental and human health reasons. Modern combustion research faces two major challenges; reduction of the pollutants, such as CO, NOx and unburnt hydro- carbons and increase of the combustion efficiency. Current trend for stationary power generation is to use lean premixed and/or pre-vaporized combustors, close to blow-off limit. Besides the beneficial effects of lower emissions and improved performance, the use of lean, near blow-off combustion gives rise to combustion instabilities that can lead to flashback, blow-off as well as acoustical and mechanical vibrations. A clear under- standing of the dynamics of the flame and the flow under lean conditions has not been reached yet. This is where the contributions of this thesis lie. In this work, advanced techniques, namely, Reynolds Averaged Navier Stokes (RANS) based models, Large Eddy Simulation (LES) models, Proper Orthogonal Decomposition (POD), Dynamic Mode Decomposition (DMD) and wavelet-analysis have been applied to analyze both stable and unstable lean flames.Maintaining stable flame is difficult when the flame speed of any practical fuel is several orders of magnitude smaller than the flow velocity and a flame holding mechanism is required. Stabilization may be achieved primarily by swirl or jet induced recirculating flow or by a bluff-body flame holder. Bluff-bodies as flame-holders are found in a wide range of high-speed reacting environments, e.g. ram-jets, scram-jets and turbojet after- burners. Primary goals of this study were to examine in detail the possibility of modeling a flame close to blow-off and during the blow-off, and furthermore to extract the dynamics of a bluff body flame under stable flame and close to blow-off conditions. In the end, based on the LES results, some of the modern theories of bluff-body flames approaching blow-off were critically assessed. The findings were summarized and a hypothesis on the full sequence of events leading to blow-off was proposed.Real gas turbine combustors are typically characterized by much more complex geometries than bluff-bodies. Apart from stability issues, operation flexibility, efficiency and fuel flexibility need also to be addressed, especially when dealing with alternative fuel gas mixtures. One of the purposes of this work was also to design a burner that can, experimentally and numerically address some of these issues. The burner should be similar to a modern gas turbine burners, downscaled and experimentally feasible, yet possessing all of the complex interactions between fundamental combustion phenomenon and fluid mechanics of an industrial gas turbine combustor. This was done by using advanced computational tools in conjunction with CAD tools for identifying the operating limits of such a burner. Further, several experimental set-ups were re-designed for operation under various conditions.Throughout the thesis, LES-based models showed capabilities of predicting flames under stable and unstable conditions. RANS-based models were proved to be useful for design purposes. It was shown how POD, DMD and wavelets can be used for not just identifying important flame-flow features but also exploring the flow-flame interaction.