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Abstract: Lean premixed (LP) combustion systems are currently used for most modern power generation gas turbines. Though this method reduces emissions, specifically nitrogen oxides, and is more efficient than non-premixed systems, LP systems are susceptible to blowoff. The goal of this study is to find out how centerbody geometry plays a role in the lean blowoff process for swirl-stabilized flames. We find that cylindrical centerbodies have higher lean blowoff equivalence ratios than tapered centerbodies. We also find that the dominant flame shape for both centerbodies is M-shape when not anchored and tulip shaped when anchored, though the tapered centerbodies induce V-shape flames as well. The blowoff equivalence ratio and blowoff process are strongly coupled ith the flame shape. 

This study considers the effect of exhaust gas recirculation diluents on the static and dynamic stability of a swirl-stabilized flame in a model gas turbine combustor. The test matrix is designed such that the unstretched laminar flame speed of the mixture is maintained constant as the diluent level and composition is varied. Results show that the constant flame speed test matrices exhibit similar flame stability and shape. This can be attributed to the similar stretched flame properties that these conditions exhibit when the unstretched laminar flame speed is matched. 

This work focused on understanding how swirl influences the blowoff limit and process in lean-premixed, swirl-stabilized flames. Two initial equivalence ratios (Ф0 = 0.8 and 1.0) were used to study the effect of swirl number (S = 0.80, 0.95, 1.15, and 1.43) on the lean blowoff limits. It was seen that at higher Ф0, the blowoff equivalence ratio of flames with lower swirl levels was typically more sensitive to bulk flow velocities than flames stabilized at lower Ф0. The blowoff Ф of flames stabilized at higher swirl levels did not vary much with an increase in bulk flow velocity. Global CH* chemiluminescence was done to study the lean blowoff process further. At lower Ф0 and swirl levels, the occurrence of the extinction/reignition events in the shear layers seemed more prominent. 

Swirl-stabilized flames are used in many gas turbine combustor configurations due to their enhanced static stability. The effects of combustor geometry, fuel composition, and bulk velocity on flame stability in swirling flows are well studied, but the effects of centerbody temperature have not been rigorously considered. The purpose of this study is to understand the impact of centerbody temperature on flame shape and dynamics. A newly instrumented variable-angle swirl-stabilized combustor was used to perform a repeatability study, and blowoff equivalence ratio was measured at centerbody temperatures ranging from 150 to 350°C and bulk velocities ranging from 16 to 55 m/s. Blowoff equivalence ratio generally decreases with centerbody temperature. Two structures were observed during blowoff: a cone shape and flame chugging. Blowoff equivalence ratio was consistently lower when the cone structure occurred, though the mechanism that excites these behaviors is still under investigation. 

The presence of large-scale coherent structures can significantly impact the dynamics of a turbulent flow field and the behaviour of a flame stabilized in that flow. The goal of this study is to analyse how increasing free-stream turbulence can change the response of the flow to longitudinal acoustic excitation of varying amplitudes. We study the flow in the wake of a cylindrical bluff body at both non-reacting and reacting conditions, as the presence of a flame can significantly alter the global stability of the flow. The frequency of longitudinal acoustic excitation is set to match the natural frequency of anti-symmetric vortex shedding for this configuration and we vary the free-stream turbulence using perforated plates upstream of the bluff body. The results show that varying the level of free-stream turbulence can influence not only the amplitude of the coherent flow response, but also the symmetry of vortex shedding in the presence of longitudinal acoustic excitation. Increasing the turbulence intensity can fundamentally change the structure of the time-averaged flow and can directly impact the coherent flow response in two ways. First, increasing turbulence intensity can enhance the amplitude of the natural anti-symmetric vortex shedding mode in the wake. Second, increasing turbulence intensity weakens the symmetric response of the flow to the longitudinal acoustic excitation. In the non-reacting and reacting conditions, both symmetric and anti-symmetric modes are present and are characterized using a spectral proper orthogonal decomposition. We see evidence of interaction between the symmetric and anti-symmetric modes, which leads to an interference pattern in the coherent vorticity response in the shear layers. We conclude by presenting a conceptual model for the influence that turbulence has on these flows. 

Global instabilities in swirling flows can significantly alter the flame and flow dynamics of swirl-stabilized flames, such as those in modern gas turbine engines. In this study, we characterize the interaction between the precessing vortex core (PVC), which is the consequence of a global hydrodynamic instability, and thermoacoustic instabilities, which are the result of a coupling between combustor acoustics and the unsteady heat release rate. This study is performed using experimental data obtained from a model gas turbine combustor employing two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at five bar pressure. The flow split between the two streams is systematically varied to observe the impact of flow structure variation on the system dynamics at both non-reacting and reacting conditions. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence and acetone planar laser-induced fluorescence are used to obtain information about the velocity fields, flame and fuel flow behaviour, respectively. Spectral proper orthogonal decomposition and a complex network analysis are used to identify and characterize the dominant oscillation mechanisms driving the system. In the non-reacting data, a PVC is present in most cases and the amplitude of the oscillation increases with increasing flow through the centre nozzle. In the reacting data, three dominant modes are seen: two thermoacoustic modes and the PVC. Our results show that in the cases where the frequency of the PVC overlaps with either of the thermoacoustic modes, the thermoacoustic modes are suppressed. The complex network analysis coupled with a weakly nonlinear theoretical analysis suggests the mechanisms by which this coupling and suppression of the thermoacoustic mode occur. 

Abstract Combustion instability, which is the result of a coupling between combustor acoustic modes and unsteady flame heat release rate, is a severely limiting factor in the operability and performance of modern gas turbine engines. This coupling can occur through different pathways, such as flow-field fluctuations or equivalence ratio fluctuations. In realistic combustor systems, there are complex hydrodynamic and thermo-chemical processes involved, which can lead to multiple coupling pathways. In order to understand and predict the mechanisms that govern the onset of combustion instability in real gas turbine engines, we consider the influences that each of these coupling pathways can have on the stability and dynamics of a partially premixed, swirl-stabilized flame. In this study, we use a model gas turbine combustor with two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at an elevated pressure of 5 bar. The flow split between the two streams is systematically varied to observe the impact on the flow and flame dynamics. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence, and acetone planar laser-induced fluorescence are used to obtain information about the velocity field, flame, and fuel-flow behavior, respectively. Depending on the flow conditions, a thermoacoustic oscillation mode or a hydrodynamic mode, identified as the precessing vortex core, is present. The focus of this study is to characterize the mixture coupling processes in this partially premixed flame as well as the impact that the velocity oscillations have on mixture coupling. Our results show that, for this combustor system, changing the flow split between the two concentric nozzles can alter the dominant harmonic oscillation modes in the system, which can significantly impact the dispersion of fuel into air, thereby modulating the local equivalence ratio of the flame. This insight can be used to design instability control mechanisms in real gas turbine engines.