Flow characterization of lifted flames in swirling, reacting flows
Chterev, Ianko Pavlov
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Swirl stabilized combustors are commonly used in gaseous fueled land-based gas turbines and liquid fueled aerospace combustors to achieve simultaneously high efficiency, low emissions, wide operability limits, and low thermal and mechanical hardware loadings. Flame shape and location are critical to successful design, and are, therefore, the general focus of this work. In premixed swirl combustion aerodynamically stabilized flames are sometimes observed and desirable as they potentially reduce hardware heat loadings. However, their understanding is largely phenomenological and geometry specific. First, aerodynamically stabilized flames are subject to flow perturbations such as a precessing vortex core (PVC), and therefore, this thesis studies how a precessing flow field affects time-averaged quantities such as flame location. Second, in swirling flowfields with no interior time-averaged stagnation point, flames are sometimes aerodynamically stabilized by instantaneous stagnation points created by large scale structures such as the PVC. Since this places the flame in a time-averaged reverse flow, natural questions are what the flame and flow characteristics are at the flame stabilization location, such as flame stretch, and why the flame does not flash back. Experiments in high pressure, multi-phase, hydrocarbon fueled, reacting flows are highly complex, and quantities such as liquid and gas phase fuel distribution, heat release and flowfield are difficult to obtain. Thus, another focus of this work is experimental development to study the internal physics. First, this thesis finds that precession in radial-axial planar measurements can result in the time-averaged stagnation point to be located in a highly negative region of the flow. Since the time-averaged flow field is often used to determine the flame location, these findings indicate that time-averaged treatments may lead to erroneous results. Precession can also alter the general flow field topology by inducing asymmetries and can cause time-averages to converge slower. Second, the local flow field of a flame aerodynamically stabilized by instantaneous stagnation points is characterized using planar velocity and flame location measurements, conditioned using a line-of-sight technique to capture the flame global leading in the imaging plane. The flame stretch is measured, indicating that the stretch the flame experiences has a high dependence on nozzle velocity. However, the scaling is not understood, and further study is proposed. The time-averaged flame stretch is much higher than opposed diffusion flame extinction stretch rate calculations, which also requires further study. Furthermore, the stretch is not correlated strongly with location and flow velocity. Last, a simultaneous stereo-PIV and fuel/OH-PLIF technique is developed using a single PLIF laser (and a PIV laser) to characterize the spray distribution, flame shape and location, and dual phase flow field for two different jet fuels, at pressures from 2 to 5 bar. Two different flame shapes are observed, with a stability behavior different than with gaseous fuel. Furthermore, the flames extends into the annular jet core, a phenomenon not observed in premixed systems, and mentioned as needing verification in the liquid fueled combustion literature.