| dc.description.abstract | The aviation industry, like many others, has to meet stringent pollution and emission norms due to climate change and global warming. Aircraft engines burning fossil fuels emit pollutants such as NOx, CO, and soot, impacting the atmosphere's air quality. These increasing demands for air transport, coupled with strict emission norms, necessitate the development of next-generation aero engines that can burn fossil fuels more efficiently while meeting emission standards. Combustor plays a crucial role in determining the overall performance of the gas turbine engine. Moreover, recent time requires the combustors to operate on multiple types of fuels. Development of next-generation combustors with the desired properties requires a thorough understanding of the processes occurring within the combustor. The work is divided into two parts, where the first part discusses the experimental results on the effect of flare angle on the resulting flame and flow structure of the high shear injector while the second part explores the flow dynamics within the gas turbine sector combustor.
Swirl flow is utilized to anchor the flame in the compact zone within the combustor. High shear injectors which comprise multiple coflowing swirlers are utilized to enhance fuel-air mixing. The design of such high-shear injectors is complex where multiple parameters need to be optimized to get the desired operability of the combustor. This study experimentally investigates the effect of one such design parameter called flare angle (β), which is the angle of the diverging part attached at the end of the high-shear injector. Three flare angles (0˚, 30˚ and 50˚) are examined, keeping the other design parameters of the high-shear injector constant. In the high-shear injector, the primary and secondary swirl air are in opposite directions to each other (hence referred to as counter-rotating flows). The fuel in the high-shear injector is introduced into the primary swirl air. The experiments, conducted under non-reacting and reacting conditions using methane as fuel, utilize Stereo-PIV for flow field diagnostics. The non-reacting flow field results reveal that increasing the flare angle enhances the mixing between primary and secondary swirling air. The central recirculation zone (CRZ) size and recirculation strength increases with an increase in β. Additionally, the mixing length, defined as the axial distance downstream of the dump plane where the azimuthal momentum of the primary swirler completely decays, decreases with increasing β, indicating improved mixing of primary and secondary swirling air. Furthermore, the study finds that the inlet Reynolds number has a negligible effect on the CRZ size across all flare angles. The local equivalence ratio reduces with an increase in mixing between the primary (fuel-rich) air and secondary air with increasing β. Thus, with increasing β, the rich blow-off limit improves and the lean blow-off limit deteriorates. Under reacting conditions, the flow field changes significantly compared to non-reacting conditions. For β = 0° and β = 30°, the azimuthal momentum of primary swirling air increases significantly in reacting conditions compared to on-reacting conditions. For β = 50°, the secondary swirling air mixes more effectively with the primary air, lowering the local equivalence ratio at higher fuel inlets. This results in sustained combustion at higher global equivalence ratios (φ) for β = 50° compared to β = 30° and β = 0°. The time-averaged flame images show two distinct types of flame structure: i) Attached flame structure and ii) lifted flame structure which is only observed for β = 50° above a certain equivalence ratio. The attached flames exhibit self-excited longitudinal thermoacoustic instability which becomes more pronounced with increasing β. The lifted flames do not show such instabilities and are quieter in operation. Further, the work provides insights into the acoustics and pressure fluctuations, showcasing the effect of β on their spectral characteristics.
The second part of this dissertation focuses on the design and flow field diagnostics of a sector of an annular gas turbine combustor. The experimental rig is a full-scale sector of an actual gas turbine combustor, featuring core components such as the liner, dome, cowl, and atomizer. Experiments are conducted at high inlet mass flow rates of up to 0.5 kg/s and pressures reaching 5 bar. The aim of the study is to apply advanced diagnostics tools within the combustor resembling an actual gas turbine combustor at high inlet mass flow rated and high pressures. The results provide detailed insights into the major flow field structure and the dynamics occurring within a real gas turbine combustor. Simultaneous high-speed Particle Image Velocimetry (PIV) is performed in both the primary and exit zones of the combustor, elucidating the dynamic interaction between these two zones. The work on the sector combustor is divided into three chapters described below. In the first part of this work, the characterization of the experimental rig is performed where it is observed that the total pressure drop across the combustor is a strong quadratic function of the inlet Mach number. The major flow field structures in the primary zone, such as the injector's swirling flow field and the inner and outer dilution jets, are captured. The study finds that crucial flow structures within the combustor like the Central Recirculation Zone (CRZ) size and various flow field parameters like the recirculation strength, the flow distribution in the swirler and dilution jets, and the momentum ratio of the swirler and dilution jet, remain invariant across different combustor operating conditions. Additionally, the flow field within the combustor scales linearly with the inlet Mach number. The second part of the sector combustor work focuses on the flow field dynamics within the sector combustor using proper orthogonal decomposition (POD) on time-resolved flow field data. The major coherent structures in the POD eigenmodes are found to be similar across all cases. Detailed discussions are provided on the dynamical flow features in the swirler, dilution jet, and exit zones. The dynamics of the dilution jets are highlighted, illustrating the shifting stagnation point and the flapping behaviour of opposed dilution jets. The resulting dynamics manifest as convective patterns at the combustor's exit. The spectral signatures of the flow field and dynamic pressure data exhibit sharp peaks at characteristic frequencies originating from the swirl flow dynamics. These spectral signatures vary with different operating conditions of the combustor, which is discussed in detail in the third part of the sector combustor work. The spectral dynamics within the combustor are governed by the inlet Mach number rather than the inlet Reynolds number or inlet pressure. The spectral-POD is applied to the flow field data to decompose the dominant dynamics occurring at the characteristic frequency within the combustor. The dynamics at M = 0.1 show a frequency peak at 467 Hz, while the M = 0.18 case shows a peak at 1350 Hz. The spatial scales of these dynamics differ, with larger scales observed at M = 0.1 and smaller scales at M = 0.18. Two major types of dynamical behaviours are identified resembling to: i) helical instability and ii) shear layer instability. The overall dynamics at peak frequencies reflect a combination of these behaviours, where the shear layer instabilities dominate at lower Mach numbers and helical instabilities are predominant at higher Mach numbers.
In conclusion, this thesis delves into the intricate dynamics and performance characteristics of a high-shear injector and a full-scale sector of a gas turbine combustor. Through comprehensive experimental investigations and advanced diagnostics, significant insights have been gained into the effect of design parameters, such as flare angle, on both non-reacting flow structures and reacting flame dynamics. The study highlights the critical role of flare angle in injector design for enhancing combustion stability and efficiency, crucial for advancing next-generation gas turbine technologies. Furthermore, the detailed analyses of flow field structures in a full-scale gas turbine combustor and the complex underlying flow field dynamics are elucidated. Such insights into the gas turbine combustor will help in optimizing future combustor designs which could meet stringent emissions regulations while improving operational performance. | en_US |
