Master of Engineering / Turbulent propagating premixed flames are encountered in spark ignition engines, gas turbines, industrial burners, as well as in vented gas explosions. In all these applications, the flame fronts interact with complex solid boundaries which not only distort the flame structure but directly affect the propagation rate in ways that are not yet fully understood. This thesis aims to provide both a quantitative and qualitative understanding of the link between overpressure, flame front wrinkling and turbulence levels generated in the propagating medium. This is an issue of importance for the provision of improved sub-models for the burning rates of premixed flames. An experimental chamber was constructed where controlled premixed flames were ignited from rest to propagate past solid obstacles and/or baffle plates strategically positioned in the chamber. Laser Doppler Anemometry was used to measure the velocity field and turbulence fields while pressure transducers were used to obtain pressure-time traces. In addition to this Laser-Induced Fluorescence of the Hydroxyl radical is was to image the flame front as it consumes the unburnt fuel captured in the re-circulation zone behind the main obstruction. The thesis reports on the effects of various parameters such as the inclusion of grids and obstructions, blockage ratio, and repeated obstacles to explore possible correlations between the pressure and the flow-fields. Pressure, velocity and LIF images were correlated and analysed to prove the significance of grid location and number on overall turbulence intensity. Corresponding flow field parameters such as flame front wrinkling, peak overpressure and RMS all combine to conclusively demonstrate their interaction and influence to turbulence intensity. By progressively positioning more grids further downstream, consequent rises in the flow field parameters and the establishment of positive trends indicates the overall significance of kernel development and flow disturbances in relation to turbulence generation.

Master of Engineering / Turbulent propagating premixed flames are encountered in spark ignition engines, gas turbines, industrial burners, as well as in vented gas explosions. In all these applications, the flame fronts interact with complex solid boundaries which not only distort the flame structure but directly affect the propagation rate in ways that are not yet fully understood. This thesis aims to provide both a quantitative and qualitative understanding of the link between overpressure, flame front wrinkling and turbulence levels generated in the propagating medium. This is an issue of importance for the provision of improved sub-models for the burning rates of premixed flames. An experimental chamber was constructed where controlled premixed flames were ignited from rest to propagate past solid obstacles and/or baffle plates strategically positioned in the chamber. Laser Doppler Anemometry was used to measure the velocity field and turbulence fields while pressure transducers were used to obtain pressure-time traces. In addition to this Laser-Induced Fluorescence of the Hydroxyl radical is was to image the flame front as it consumes the unburnt fuel captured in the re-circulation zone behind the main obstruction. The thesis reports on the effects of various parameters such as the inclusion of grids and obstructions, blockage ratio, and repeated obstacles to explore possible correlations between the pressure and the flow-fields. Pressure, velocity and LIF images were correlated and analysed to prove the significance of grid location and number on overall turbulence intensity. Corresponding flow field parameters such as flame front wrinkling, peak overpressure and RMS all combine to conclusively demonstrate their interaction and influence to turbulence intensity. By progressively positioning more grids further downstream, consequent rises in the flow field parameters and the establishment of positive trends indicates the overall significance of kernel development and flow disturbances in relation to turbulence generation.

Turbulent flows and turbulent flames are inherently multi-dimensional in space and transient in time. Therefore, multidimensional diagnostics that are capable of resolving such spatial and temporal dynamics have long been desired; and the purpose of this dissertation is to investigate three such diagnostics both for the fundamental study of flow and combustion processes and also for the applied research of practical devices. These multidimensional optical diagnostics are a 2D (two dimensional) two-photon laser-induced fluorescence (TPLIF) technique, a 3D hyperspectral tomography (HT) technique, and a 4D tomographic chemiluminescence (TC) technique. The first TPLIF technique is targeted at measuring temporally-resolved 2D distribution of fluorescent radicals, the second HT technique is targeted at measuring temperature and chemical species concentration at high speed, and the third TC technique is targeted at measuring turbulent flame properties. This dissertation describes the numerical and experimental evaluation of these techniques to demonstrate their capabilities and understand their limitations. The specific aspects investigated include spatial resolution, temporal resolution, and tomographic inversion algorithms. It is expected that the results obtained in this dissertation to lay the groundwork for their further development and expanded application in the study of turbulent flow and combustion processes. / Ph. D.

<div>Chirped-probe pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs CARS) was used to study high pressure gas-phase thermometry. The experimental measurements were</div><div>mostly performed in a static gas cell and in a canonical flat flame burner. The purpose of this study is to provide insights for the future rocket relevant combustion measurements. </div><div><br></div><div><div>Chirped-probe pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs CARS) was used to study high pressure gas-phase thermometry. The experimental measurements were mostly performed in a static gas cell and in a canonical flat flame burner. The purpose of this study is to provide insights for the future rocket relevant combustion measurements. </div></div><div><br></div><div><div>The optical effects associated with ultrashort pulse propagation in the high-pressure system were investigated. For example, the femtosecond pulse can receive large amount of frequency chirp when transmitting through thick glass windows of the optical section in the high-pressure system. The effects of pump and Stokes frequency chirp were investigated both experimentally, by inserting disks of SF11 glass into the pump and Stokes beam paths to study the flame thermometry, and theoretically by incorporating pulse chirp into the TDDM simulations to calculate the Raman excitation efficiency. Meanwhile, the ultrashort pulses can experience self-phase modulation in the high-pressure gas medium. The effects of self-phase modulation (SPM) on the power spectra of femtosecond pulses will have significant impact on the fs CARS profile. On the other hand, the extend and the behavior of SPM reply on the laser intensity and are also species-specific. The optimal laser intensities in high-pressure gas mediums like N2, O2, CO2 and CH4 were investigated.</div></div><div><br></div><div><div>To prepare for future rocket relevant combustion studies, CPP fs CARS thermometry was developed for CO2, O2 and H2. Especially for CO2 and O2, they have close vibrational frequencies but very different coherence dephasing rates. Relative concentration between CO2 and O2 can then be extracted by using a short probe delay, and the temperature information can be determined by using long probe delays and the O2 transitions will not interfere with CO2 and nonresonant contribution of the CARS signal can be suppressed. CO2 CPP fs CARS measurements inside the high-pressure high-temperature gas cell were presented and discussed. Collisional narrowing effects for CO2 especially for high gas number density situation were discussed. </div></div>

No description available.

The study of high-speed flows represents a challenging problem in the fluid dynamics field due to the presence of chemical reactions and non-equilibrium effects. Hypersonic flights, where speeds reach Mach 5 and above, are particularly influenced by these effects, resulting in a direct impact on the flow and consequently on the aerodynamic performance of a vehicle traveling at these speeds. The study of hypersonic flow conditions requires the experimental capability of determining local temperatures, pressures and velocities using non-intrusive techniques. Furthermore, the simultaneous measurement of two or more variables in a complex flow boosts the amount of information that is obtained since valuable correlations can be established. This research includes the design, construction and characterization of a hypersonic flow apparatus explicitly intended as a tool for advanced laser diagnostics development. This apparatus is characterized by its pulsed operation mode that translates into a significant reduction in mass flow rates and can be operated for long periods at Mach numbers ranging from 2.8 to 6.2. The flow conditions during the uniform flow time interval of each pulse vary by less than 1%, generating a flow of sufficient quality for quantitative measurements. The development of a laser diagnostic technique, the VENOM technique, which is a non-intrusive method to provide simultaneous 2-D measurements of the mean and instantaneous fluctuations in two-component velocity and temperature is also presented. This technique represents the first single diagnostic capable of instantaneous two-component velocimetry and thermometry in a gaseous flow field by combining two Nitric Oxide Planar Laser Induced Fluorescence methods: two-component Molecular Tagging Velocimetry and two-line thermometry, employing the nascent NO(v"=1) arising from the NO2 photodissociation as a molecular tracer. The VENOM technique is expected to be not only applicable to cold high-speed flows, which is the focus of the present work, but also to combustion and other reactive or high-enthalpy flow fields.

No description available.

Understanding reacting flows in conditions relevant to practical combustion devices is a challenging but critically important task. In such devices, combustion nearly always occurs in a turbulent flow field and at high pressure. The formation of soot is highly sensitive to these parameters. However, little research has been conducted in conditions that replicate the complex physics of such devices in simplified configurations. This body of work focuses on the development of a rig suitable for investigating turbulence-chemistry interactions in simple jet flames at high pressure and high Reynolds numbers and discusses results from the initial experiments in that rig. First, the flame structure of syngas flames at pressures up to 12 bar and at Reynolds numbers up to 83,500 is investigated using OH-PLIF. A corrugation factor is used to characterize the wrinkling of the flame fronts and PDFs of this factor show that the corrugation of the flame front is a very strong function of the Reynolds number, but in most cases, the pressure has no effect. Separations in the OH layers become less probable as the pressure increases if the Reynolds number remains constant. Next, the flame structure of nitrogen-diluted ethylene flames at pressures up to 5 bar and Reynolds numbers up to 50,000 are examined using OH-PLIF. Again, the corrugation factor is used to show that the flame fronts become more wrinkled as the Reynolds number increases. Further analysis shows that the extent of wrinkling is limited and further increases in turbulence result in more frequent breaks in the OH layer. Lastly, two soot studies on the ethylene flames are presented. The soot particle size distribution is characterized in two flames at atmospheric pressure. The time-averaged, mean particle diameter on the centerline increases as the distance from the nozzle increases. Soot volume fraction measurements are made with LII in three flames at different pressures and Reynolds numbers. Soot production is found to be much more sensitive to changes in pressure than changes in Reynolds number. Increases in the mean soot volume fraction as the pressure increases are due to higher instantaneous soot concentrations and lower soot intermittency.

The work described focuses on characterization of fluorescence dye-doped polystyrene latex particles (PSLs) dispersed in air as a candidate for advanced flow diagnostic techniques for applications in wind tunnels. PSLs with mean diameter on the order of 1 μm were provided by collaborators at NASA Langley Research Center and measurements of simultaneous Mie scattering and laser induced fluorescence (LIF) signals were obtained with the goal of providing a capability for velocity and scalar flow measurements. Three organic fluorescent dyes, Rhodamine B (RhB), Dichlorofluorescein (DCF), and Kiton Red (KR), were doped into PSLs and studied in benchtop experiments. A major interest in the present research is the application of safe dyes, thus DCF and KR are of particular interest, while RhB is used as a benchmark. The study is broken up into two major elements: response of the particles to continuous wave (CW) excitation, and response to pulsed excitation. The former was examined for mean LIF imaging and single-point single-shot measurements, the latter for planar single-shot measurements. The LIF signal has several potential uses, though the primary interest herein is the reduction of flare from wind tunnel walls which occurs at the laser excitation wavelength and the use of the fluorescence signal for sensing temperature. It is demonstrated that the LIF signal under CW excitation may be used for velocimetry with the laser-Doppler velocimetry technique, thus effective in filtering out the laser flare in the vicinity of wind tunnel models or walls. A two-band LIF technique was also demonstrated for one of the DCF batches, indicating that the technique effectively removes interfering inputs such as particle diameter and dye concentration variation. Temperature measurement uncertainties are estimated based upon the variance measured for the two-band LIF intensity ratio and the achievable dye temperature sensitivity, indicating that particles developed to date may provide about 12.5 degrees C precision. Pulsed excitation with an Nd:YAG laser at about 200mJ/pulse at 532 nm yield no fluorescence above the detection threshold of the cameras used from the DCF batch examined. However, a batch doped with Kiton Red 620 was shown to provide sufficient intensity of LIF for single-shot planar measurements such as PIV via planar LIF. The KR batch also exhibited highly non-uniform yield of fluorescence signal on a particle-to-particle basis - this despite a highly uniform Mie scattering signal. Given the uniform Mie signal and a relatively narrow Gaussian distribution of the particles size, a possible mechanism is mentioned for the LIF non-uniformity as lasing within individual particles due to whispering gallery mode resonance. / Master of Science

This work describes the development of a Raman spectroscopy system for measuring aerothermochemistry in high-speed jets and flames. A transmissive grating spectrometer was newly developed for capturing pure rotational Raman and rotationalvibrational Raman with a single CCD camera. Previous state-of-the-art experiments applied line imagining in known flowfields of constant pressure. The system described herein is designed to provide local measurement of pressure, with full thermochemistry, along a line. In every point, temperature will be measured by examining the Boltzmann decay of the rotational spectrum, while molar fraction will be measured from the vibrational Raman spectrum. The temperature and concentrations will then be combined to obtain partial pressure measurements via the equation of state. This work examines the phenomenology of rotational and vibrational Raman scattering and proposes algorithms that can be used for data extraction.