Multiparameter optical diagnostics for high-speed flow applications
Keywords: Rayleigh scattering
polarised-depolarised Rayleigh scattering
particle image velocimetry
planar laser-induced fluorescence
optical diagnostics
laser diagnostics
transpiration cooling
ultra high temperature ceramic
infrared thermography

This thesis details the development of two multiparameter spatially-resolved optical diagnostic techniques for research in high-speed flow applications. This is motivated by the need to extract as much information as possible in physics-coupled, multidimensional flows, and flows generated in short duration test facilities.
Fluorescent Particle Image Velocimetry (FPIV) was developed with the aim of being widely accessible as a simultaneous velocimetry and thermometry tool for coupled fluid momentum and heat transport investigations. Alternative strategies to provide such simultaneous measurements exist, but these are typically more complex, expensive and invasive to implement, making them less conventional. To achieve this, droplet-based Planar Laser-Induced Fluorescence (PLIF) was added to PIV, enabling the use of a single tracer for measuring both velocity and temperature. This work focused on the novel discovery of temperature-dependent fluorescence properties of di-ethylhexyl sebacate (DEHS), an established PIV seeding substance. Liquid DEHS was found to be excitable by the fourth harmonic of an Nd:YAG laser, leading to a weak, but measurable ultraviolet (UV) fluorescence signal. Two Colour PLIF applied to liquid DEHS demonstrated a temperature-dependent red-shift, yielding a sensitivity of 0.019 K−1. A change in fluorescence properties was evident in droplet form, but this was still temperature-dependent albeit with a reduced sensitivity of 0.011 K−1. This discovery has the potential to make joint velocity and temperature measurements available to a wide audience of existing PIV users.
Polarised-depolarised Rayleigh Scattering was developed for simultaneous composition and temperature measurements in transpiration cooling, to enhance understanding of flow mixing interactions and cooling mechanisms. Previous experimental investigations of this cooling technology have focused mainly on surface temperature measurements, so this development would provide additional useful information about the effect of coolant coverage and effusion pathways. A bench-top demonstration of the technique achieved good quality results, with a mean signal-to-noise ratio of 7.5. The technique was then applied in a simultaneous experimental framework with infrared thermography to obtain combined gas composition, gas temperature, and surface temperature measurements in order to infer the internal cooling mechanisms of transpiring porous media. This framework was applied in a 1-D manner to extract the volumetric heat transfer coefficient for candidate transpiration cooling materials. Results demonstrated the enhanced ability of carbon-fiber-reinforced carbon composites over zirconium diboride Ultra High Temperature Ceramics for internal cooling, both of which are strong contenders for this technology. This discovery improves the accuracy of complete transpiration cooling systems-level designs, bringing the technology one step closer to implementation.