Multi-Scale Flow and Flame Dynamics at Engine-Relevant Conditions
thesisposted on 20.04.2022, 18:37 authored by John PhiloJohn Philo
The continued advancement of gas turbine combustion technology for power generation and propulsion applications requires novel techniques to increase the overall engine cycle efficiency and improved methods for mitigating combustion instabilities. To help address these problems, high-speed optical diagnostics were applied to two different experiments that replicate relevant physics in gas turbine combustors. The focus of the measurements was to elucidate the effect of various operating parameters on combustion dynamics occurring over a wide range of spatio-temporal flow and chemical scales. The first experiment, VIPER-M, enabled the investigation of coupling mechanisms for transverse instabilities in a multi-element, premixed combustor that maintains key similarities with gas turbine combustors for land based power generation. The second experiment, COMRAD, facilitated the study of the effect of fuel heating on the combustion performance and dynamics in a liquid-fueled, piloted swirl flame typical of aviation engine combustors.
Two different injector lengths were tested in the VIPER-M experiment, and high-speed CH* chemiluminescence imaging and an array of high-frequency pressure transducers were used to characterize the overall combustor dynamics. For all conditions tested, the longer injector length configuration exhibited high-amplitude instabilities, with pressure fluctuations greater than 100% of the mean chamber pressure. This was due to the excitation of the fundamental transverse mode, with a frequency around 1800 Hz, as well as multiple harmonics. Shortening the injector length significantly lowered the instability amplitudes at all conditions and excited an additional mode near 1550 Hz for lower equivalence ratio cases. The delineating feature controlling the growth of the instabilities in the two injector configurations was shown to be the coupling between the transverse modes in the chamber and axial pressure fluctuations in the injectors.
Heated fuels were introduced into the COMRAD experiment, and simultaneous 10 kHz stereoscopic particle image velocimetry and OH* chemiluminescence imaging were performed over a range of equivalence ratios and combustor pressures to study the influence of fuel temperature on the flow and flame structure. The main flame was found to move upstream as the fuel was heated, while no changes in the pilot flame location were observed in the field of view at the exit of the injector. The upstream shift of the main flame corresponded to a local increase in the axial velocity, which caused the shear layer between the pilot/main flames and the central recirculation zone to move downstream. Direct comparison of the mean velocity fields relative to the mean flame location showed that heating the fuel caused the velocity normal to the flame front to increase, which is indicative of an increase in flame speed. The changes to the fuel injection and chemical kinetics help explain the local changes to the flow and flame structure, which contribute to an overall increase in combustion efficiency as well as NOx emissions.
Lastly, the effect of fuel injection temperature on the presence of an 800 Hz combustion instability in the COMRAD experiment was investigated. High-frequency pressure and high-speed chemiluminescence measurements revealed a decrease in the instability amplitude as the fuel was heated. The coupling between the fuel flow and the unsteady heat release was studied using independent 10 kHz stereoscopic particle image velocimetry and 10 kHz Mie scattering measurements. The variations in the fuel flow entering the combustor over the acoustic cycle decreased as the instability amplitude weakened. 100 kHz burst-mode, two-component particle image velocimetry was then applied to the unstable condition with ambient temperature fuel. This measurement was capable of resolving both the large-scale changes to the structure of the inner recirculation zone occurring at 800 Hz as well as the time-evolution of small-scale vortex structures. The vortices were shown to correspond to a characteristic frequency in the range of 4-5.5 kHz, and the strength of the vortex structures fluctuated with the global 800 Hz combustion dynamics. These results highlight the importance of performing measurements capable of resolving the wide range of scales present in the flow-fields typical of gas turbine combustors to improve current understanding of flame-flow coupling mechanisms.