Development of high-speed laser diagnostics for the study of advanced propulsion systems

Combustion in next-generation high-speed propulsion systems involves highly turbulent reactive flow conditions, often beyond the limits of our physicochemical understanding. It is a challenge to produce reliable combustion as localized extinction and instabilities are highly transient in nature. It is in this effort that the progressing capabilities of laser diagnostics open new possibilities of exploring the chemistry and fluid dynamics of complex reacting flows. The primary objective of this study is to develop and apply kHz-rate planar imaging to further our understanding of ignition and flame dynamics in turbulent flames relevant to next-generation propulsion systems. High-repetition-rate planar laser-induced fluorescence (PLIF) is used to produce temporally resolved two-dimensional concentration fields of the molecular species targeted. Novel advancements in this study include kHz-rate hydroxyl radical (OH) PLIF of combustion phenomena within a supersonic wind tunnel cavity flameholder, the development and demonstration of a 50-kHz OH PLIF system using a small-scale turbulent flame, the development and demonstration of 10-kHz PLIF of the nitric oxide (NO) molecule, and the development and demonstration of a novel strategy for PLIF of the methylidyne (CH) radical at 10 kHz. The methods and diagnostics presented here are on the forefront of the field, extending the limits of the current capabilities to greater imaging rates and expanding the pool of molecular species that can be imaged at high rates. The diagnostics methods are demonstrated in a range of laboratory combustors and propulsion systems. These include (1) an extensive study of a direct microwave plasma coupled flames developed for use in supersonic combustion systems, (2) high enthalpy supersonic combustors, and (3) high shear turbulent combustors. Laser diagnostics are used to examine direct-coupled, plasma ignited and sustained flames, for multiple flame types and nozzle geometries. OH radical number densities are quantified using PLIF and temperature measured by Rayleigh scattering thermometry. High-repetition-rate laser diagnostic methods are implemented to simultaneously record OH PLIF and chemiluminescence within the plasma-enhanced flame, allowing for temporally resolved observation of OH radicals in the plane of the thin laser sheet as well as volume-integrated excited state emission. High-speed OH PLIF is also applied to a Mach 2 combustor to characterize the behavior of the flameholding cavity. The effects of cavity fueling rate are explored and discussed. High-shear turbulent combustors produce convoluted reaction zones, and approach the boundaries between theoretical combustion regimes. Visualization of CH radicals is an excellent way to probe the nature of such flows, and validate theory and computational models. Heretofore, CH PLIF framing rates have been constrained because the strategies have required very high laser pulse energies. In this work a new approach is presented, using the C-X (0,0) transition, which produces excellent signal even with the low laser pulse energies typical of kHz-rate Nd:YAG and dye laser systems. The novel capabilities of the diagnostics in this study will provide new insights that can help to optimize new combustor geometries and flame enhancement technologies in our future