Swirl Effects on Compactness of a Peripherally Piloted Reheat Combustor
Miller, Alex Benjamin
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In aircraft turbine engines, it is desirable to minimize size and maximize efficiency. One of the largest components found in high-performance aircraft turbine engines is the thrust augmentor, which traditionally uses bluff body flameholders. One advanced concept instead uses a peripheral pilot to stabilize a flame around the outside of a swirling flow. Removal of the bluff body reduces pressure losses and initial experiments suggest that the swirling flow creates a shorter flame, allowing for a more compact device. Little detailed work has been done to elucidate the fundamental mechanisms controlling flame stabilization in this architecture. This thesis shows how swirl affects flame length and tests current hypotheses that describe the fundamental mechanisms behind this effect. A test facility is developed followed by an experimental investigation in a swirl combustor with outer-diameter flame stabilization and inlet conditions relevant to practical devices. Premixed flame configuration and dynamics are captured by advanced instrumentation, high-speed OH-PLIF, and CH* chemiluminescence imaging. High-speed stereoscopic PIV is used to quantify the unique flow field near the trailing edge of a tapered center body in a confined swirling flow. Parametric studies identified the core equivalence ratio and swirl level as the parameters that control flame length. The flame length was significantly reduced by swirl, but exhibits a non-monotonic relationship to core equivalence ratio when swirl is present. This behavior was found to depend on two important features: the outer flame, stabilized by the pilot, and the inner flame, anchored on the conical trailing edge of the center body. Helical vortices allow the flame to propagate upstream and flow separation on the center body promotes flame stabilization. Large-scale distortions in the outer flame allow the flame to propagate significantly faster under some conditions. Several hypotheses are examined that could explain the outer flame behavior, including a Rayleigh-Taylor mechanism, a Kelvin-Helmholtz mechanism, a baroclinic push mechanism, and vitiation effects. The experimental data set obtained in this study is extremely valuable to the scientific community for further analysis of the physics important to novel compact combustion systems and validation of advanced CFD models.