Experimental studies of turbulent flames at gas turbine relevant burners and operating conditions
Sammanfattning: With the increasing demand of using alternative and renewable fuels, it becomes of vital importance to consider the fuel flexibility when designing a new burner for gas turbines. Hydrogen-enriched fuel and ammonia are two of these potential fuels, and they can significantly change the operability range of the gas turbines. Thus, it is necessary to enhance both the fundamental understanding on turbulent combustion of these fuels and their combustion performance in practical combustors. Due to its advantages of in-situ measurement, non-intrusiveness and high spatial and temporal resolution, laser-based diagnostics technology has been regarded as one of the best measurement methods for researching combustion processes and phenomena. In this thesis work, experimental studies have been conducted to investigate the turbulent flames of different fuels at various gas turbine related burners, employing laser diagnostics measurement. The measurement methods include planar laser-induced fluorescence (PLIF) for various species, particle image velocimetry (PIV), laser doppler anemometry (LDA), etc.A newly designed gas turbine model combustor had been developed at the Swedish National Centre of Combustion and Technology, so it was named the CECOST burner. One of the main objectives of this thesis is to improve the premixing effect of the CECOST burner by changing part of its internal configuration and investigate its fuel flexibility by using natural gas and hydrogen-enriched methane mixtures as fuels. The experiment was conducted at an atmospheric rig, and high-speed OH' chemiluminescence imaging, simultaneous OH-/CH2O PLIF, and PIV were employed. The operability range and flame structures were investigated for different fuels at various Reynolds numbers (Re). The operability range was found to be highly sensitive to Re, as well as the fuel. For natural gas/air flames, the lean blowout (LBO) limit was approximately independent of Re, while flashback showed obvious dependence on Re and no flashback was observed for higher Re. For hydrogen-enriched methane/air flames, a comparison of combustion characteristics between pure methane and hydrogen-enriched methane with two mixing ratios, 25% and 50% in volume, was investigated. It was found that the flame stabilized in an M shape for all pure methane/air flames, whereas the flame shape transits to a П shape at a specific equivalence ratio ("ϕ" ) for hydrogen-enriched methane flames. Besides, the flashback events with two different mechanisms, combustion-induced vortex breakdown (CIVB) and boundary-layer flashback, were observed. By statistical analysis, we can get that the CIVB flashback took place only for pure methane flames with M shape, while the boundary-layer flashback happened for all hydrogen-enriched flames with П shape.Aiming to achieve stable combustion in lean conditions, a plasma-assisted flame control system is a potential way to help stabilize the flame. An industrial gas turbine combustor, known as Siemens dry low emission (DLE) burner, was modified to place a high-voltage electrode in the rich-pilot-lean (RPL) section and was used for investigation of a rotating gliding arc (RGA) discharge effect on swirl flames stabilized in the gas turbine combustor. In the unmodified DLE burner configuration, fuel and air are injected into the RPL to hold a premixed flame which can help stabilizing the main flame, but in the modified configuration, only air/O2 was injected into the RPL. The flame emissions were measured by a gas sampling probe and emission analyzer. The CO emission results were used to identify the improvement of the LBO limit with plasma assistance. NOx emissions were slightly increased by the RGA plasma, but still, less than the same main flame with RPL flame assisted. Flame emission spectra were also measured. Ammonia combustion is recently one of the hot research topics due to its promising future of carbon-free emission. To deepen our knowledge on turbulent ammonia flames, a jet burner with a large scale was constructed and used to investigate the flame structure of premixed ammonia/air flames, by employing simultaneous OH-/NH-PLIF and LDA measurements. Most of the studied flames are located in the regime of the distributed reaction zone (DRZ), determined by their Karlovitz numbers that are larger than 100. Results of simultaneous OH-/NH-PLIF show that the NH and OH layer can coexist in a thin boundary and the NH signal appears evolving to the reactants side. In addition, the practical gas turbine combustors are all operated at elevated pressure conditions, but it is not easy to perform an experiment at elevated pressure in the lab. A co-axial jet burner was installed and studied in the high-pressure combustion rig (HPCR) at Lund University to investigate the characteristics of methane/air inverse diffusion flames (IDF) at elevated pressure (up to 5 bar). The flame structure and its lift-off height influenced by pressure increasing were discussed. More wrinkles and larger curvature of the flame front were found in the inner flame structure at higher pressure.
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