Transient Phenomena in Annular Combustors
This thesis presents a detailed assessment of the transient phenomena related to the operation of annular gas turbine combustors, namely ignition and blow-off. The experiments were conducted on complex multi-burner geometries, close to realistic combustors, to provide a wide data set including stability limits, flame regimes and physical mechanisms of ignition and extinction. The first part of the investigation focused on the lean blow-off (LBO). Experiments were carried out on a premixed annular combustor, comprising a variable number of injectors, obtaining flame regime diagrams and stability curves for the system. Highspeed imaging (5 kHz) of OH* chemiluminescence revealed the peculiar flame shapes and behaviours that can be observed for different mixture velocities and equivalence ratios. Close to extinction, a pattern of blow-off and detachment of the flame was observed, due to flame-to-flame interactions, which was never reported for single burner systems. Further, a linear 5-burner combustor was built to investigate whether the same flame characteristics could be observed in a simpler and cheaper configuration. Laser diagnostics was employed, in the form of low-speed (10 Hz) CH2O planar laser induced fluorescence (PLIF). An analysis of the CH2O layer showed that the change in flame shape with the equivalence ratio was associated with the build up of formaldehyde in the side recirculation zone. The pattern of blow-off and detachment close to LBO could be explained with CH2O accumulating in the inner recirculation zone of a single burner causing the flame to lift, while restabilisation was triggered by flame pockets being convected from an adjacent flame to reignite the partially burnt products. In the linear configuration a non-premixed mode was also tested, resulting in a shorter flame with different characteristics, that showed a pattern of blow-off/reignition, instead of flame lift-off. A comparison between these multi-burner configurations and single burner data from the literature using a simple correlation for the LBO conditions revealed that simple configurations are not capable of reproducing the behaviour of more realistic gas turbine combustors. The second part of the dissertation reports the experimental and numerical investigation of the ignition transient in a premixed annular combustor. The lean ignition vii limits of the annular combustor were obtained and compared to the lean blow-off limits previously reported. The ignition probability in the combustor was calculated from 20 individual tests for multiple mixture velocities, equivalence ratios and spark locations. It was found that increasing mixture velocity or reducing the equivalence ratio is detrimental for achieving successful ignition, as expected, while moving the spark downstream from the bluff body or placing it in the middle between two burners increased the ignition probability. The latter is in contrast with the observations reported on single burner systems and depended on the possibility of a flame kernel to propagate from the inner recirculation zone of the first burner to the adjacent one, as captured with high-speed imaging (10 kHz) of OH* chemiluminescence. Following, the focus shifted towards the peculiar part of the ignition transient in multi-burner configurations, which is the burner-to-burner propagation process, known as “light-round”. For stable conditions, a wide range of mixture velocities and equivalence ratios were tested, along with two spark locations (close to a single burner and in the middle of the combustion chamber). Two fuels, methane and ethylene, were employed, to match the laminar flame speed of mixtures characterised by different thermal power and density ratios. High-speed imaging (10 kHz) of OH* chemiluminescence from two simultaneous views allowed to track the light-round process and analyse the flame propagation mechanism. It was found that the flame front travelled around the combustion chamber forming two flame branches and its trajectory was influenced by the swirl direction and the spark location. The prevailing operating conditions affecting the light-round time were assessed. It was discovered that light-round time decreased when: (i) mixture velocity increased, (ii) the spark was placed close to the bluff body and (iii) the laminar flame speed of the mixture increased. In particular, when fixing the mixture velocity and thus the convection component of the light-round, it was found that the laminar flame speed had a strong correlation with the light-round speed, compared to the thermal power and the density ratio. This suggests that turbulent flame propagation has a first order influence on the light-round process, stronger than dilatation. Lastly, low-order simulations were performed using the stochastic code SPINTHIR to model the light-round process in the annular combustor. The aim was to reproduce the behaviour of a complex combustor using a small fraction of the computational cost of a complete CFD simulation. The results with the original code were satisfactory in terms of light-round mechanism, as the simulations showed the key elements of the flame propagation. However, the trends of the light-round time with the laminar flame speed were not reproduced. Thus some modifications were proposed to implement flame speed and dilatation inside the particle propagation equation and to provide an viii alternative formulation for the extinction criterion in the code. The results showed that the new versions of the code increased the fidelity of the simulations, being able to fairly reproduce the key elements of the light-round process, both qualitatively (mechanism) and quantitatively (light-round time). Understanding ignition and lean blow-off is fundamental for the design and the safe operation of the new generation of lean burning gas turbine combustors. Moreover, the results of this doctoral project can form a database of experimental data on complex multi-burner combustors, to help validate combustion models and CFD simulations.
Combustion, Annular combustor, Ignition, Blow-off
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