Master of Science / Department of Mechanical and Nuclear Engineering / Steven Eckels / In this thesis, a two-color ratiometric planar laser-induced fluorescence (PLIF) technique for the measurement of temperature fields in liquids is described. The method uses the temperature sensitive rhodamine B and temperature insensitive rhodamine 110 fluorescent dyes. The ratio of the fluorescent emission intensity of these two dyes is inversely proportional to temperature and is independent of laser intensity variation in the flow field. Because the emission spectra of these two dyes overlap, a correction was developed to disentangle the two signals. In addition, the absorption spectra of rhodamine B and rhodamine 110 and emission spectrum of rhodamine 110 overlap, leading to the self-attenuation of the rhodamine 110 signal by the dye solution. A correction with respect to path length was developed for self-attenuation. This thesis presents the calibration process for a PLIF thermometry system and visualization of temperature gradients in a glass water tank with motion induced by large temperature gradients. A step-by-step procedure of the final calibration process is also presented.

The development of Laser-Induced Thermal Grating Spectroscopy (LITGS) for diagnostics of combusting and non-combusting flows is described. The first use of LITGS to provide in situ calibration of 2-Dimensional temperature distributions generated using Two-Colour Planar Laser-Induced Fluorescence (TC-PLIF) is reported. Time-resolved measurements of temperature distributions in a firing GDI optical engine obtained by TC-PLIF were made during the compression stroke and calibrated to the absolute temperature scale by simultaneous LITGS measurements. The accuracy and precision of the temperatures derived from LITGS data are evaluated using alternative methods of data analysis - Fast Fourier Transform and Fitting to theoretical models of the experimental data. The relative merits of the two methods are examined for analysis of weak LITGS signals obtained under engine conditions of low pressure and high temperature. The combined TC-PLIF and LITGS system was demonstrated by performing repeated single-shot measurements for 1 in every 10 four-stroke cycles showing excellent correlation of the temperatures derived from both techniques. Direct measurement of the effect of 'charge cooling', of order 5 K, for operation with direct injection is reported. Inhomogeneous temperature distributions were observed during the compression stroke for fired operation with Port Fuel Injection (PFI) and also with Gasoline Direct Injection (GDI). The effects of varying the relative concentrations of toluene and iso-octane in the two-component fuel were investigated. Extension of the LITGS technique to multi-point measurements along a 1-D line is described. By recording signals from 4 points on separate detectors using a fibre-coupled photodiode array the limitations of Streak Cameras used previously for 1-D LITGS measurements were overcome. Demonstration of principle experiments are reported in which simultaneous 4-point measurements were made with 1 mm spatial resolution and a precision of 0.7 % in temperature gradients in gas flows and in boundary layers at surfaces.

A new technique is developed that uses planar laser-induced fluorescence (PLIF) imaging of sublimated naphthalene to image the transport of ablation products in a hypersonic boundary layer. The primary motivation for this work is to understand scalar transport in hypersonic boundary layers and to develop a database for validation of computational models. The naphthalene is molded into a rectangular insert that is mounted flush with the floor of a Mach 5 wind tunnel. The distribution of naphthalene in the boundary layer is imaged by using PLIF, where the laser excitation is at 266 nm and the fluorescence is collected in the range of 320 to 380 nm. To investigate the use of naphthalene PLIF as a quantitative diagnostic technique, a series of experiments is conducted to determine the linearity of the fluorescence signal with laser fluence, as well as the temperature and pressure dependencies of the signal. The naphthalene fluorescence at 297 K is determined to be linear for laser fluence that is less than about 200 J/m². The temperature dependence of the naphthalene fluorescence signal is found at atmospheric pressure over the temperature range of 297K to 525K. A monotonic increase in the fluorescence is observed with increasing temperature. Naphthalene fluorescence lifetime measurements were also made in pure-air and nitrogen environments at 300 K over the range 1 kPa to 40 kPa. The results in air show the expected Stern-Volmer behavior with decreasing lifetimes at increasing pressure, whereas nitrogen exhibits the opposite trend. Preliminary PLIF images of the sublimated naphthalene are acquired in a Mach 5 turbulent boundary layer. Relatively low signal-to-noise-ratio images were obtained at a stagnation temperature of 345 K, but much higher quality images were obtained at a stagnation temperature of 380 K. The initial results indicate that PLIF of sublimating naphthalene may be an effective tool for studying scalar transport in hypersonic flows. / text

This thesis describes experiments to investigate high-altitude relight of a lean direct injection (LDI) combustor. The features that make LDI technology less polluting in terms of NOx compared to conventional combustors are expected to impede relight performance. Therefore an improved understanding of ignition behaviour is required to ensure that stringent relight requirements can be satisfied. Realistic operating conditions are simulated in a ground-based test facility. The application of laser diagnostics presents particular difficulties due to the large quantities ofliquid fuel that impinge on the combustor walls during relight. Advances are made in the application of planar laser-induced fluorescence (PLIF) to monitor fuel placement in a combustor under these conditions. A novel apparatus is developed to deliver a laser sheet to the combustion chamber while protecting all optical surfaces from contamination. The PLIF images are compared with the cold flow field obtained from CFD modelling. These results indicate that fuel becomes trapped inside the central recirculation zone in highconcentrations. High-speed flame imaging performed simultaneously with the PLIF measurements provides important insights into the motion and breakup of flame during relight. An algorithm developed to track the flame activity reveals that the initial spark kernel is convected downstream, before breaking apart and moving upstream towards a recovery origin close to the fuel injector. Analysis of many ignition events has revealed several distinct modes of ignition failure.

Fuel-air mixing enhancement in axisymmetric jets using an array of synthetic jet actuators around the perimeter of the flows (primarily parallel to the flow axis) was investigated using planar laser-induced fluorescence of acetone. The synthetic jets are a promising new mixing control and enhancement technology with a wide range of capabilities. An image correction scheme that improved on current ones was applied to the images acquired to generate quantitative mixing measurements. Both a single jet and coaxial jets were tested, including different velocity ratios for the coaxial jets. The actuators run at a high frequency (~1.2 kHz), and were tested with all of them on and in other geometric patterns. In addition, amplitude modulation was imposed at a lower frequency (10-100 Hz). The actuators generated small-scale structures in the outer (and inner, for the coaxial jets) mixing layers. These structures significantly enhanced the mixing in the near field (x/D less than 1) of the jets, which would be useful for correcting an off-design condition in a combustor. The amplitude modulation generated large-scale structures that became apparent farther downstream (x/D greater than 1). The impulse at the start of the duty cycle was responsible for creating the structures. The large structures contained broad regions of uniformly mixed fluid, and also entrained fluid significantly. In addition, highly asymmetric forcing geometries displayed the power of the actuators to control the spatial distribution of jet fluid. This spatial control is important for the correction of hot spots in the pattern factor. In order to extend quantitative acetone PLIF to two-phase flows, the remaining unknown photophysical properties of acetone were identified. Tests showed that the technique could simultaneously capture acetone vapor and acetone droplets. A model of droplet fluorescence was developed, and applied to images acquired in a dilute spray. The sensitivity of the model to the value of the unknowns was evaluated, including a best and worst case. The results revealed that several liquid acetone photophysical properties must be measured for the further development of the technique, especially the phosphorescence yield. Quantitative two-phase acetone PLIF will provide a powerful new tool for studying spray flows.

Low-emissions combustor design is crucially important to gas turbine engine manufacturers. Unfortunately, many designs are susceptible to unsteady oscillations that can result in structural fatigue and increased noise. Computational approaches that resolve flow unsteadiness, for example Large Eddy Simulation (LES), are being explored as one avenue to help understand such phenomena. However, in order to quantifY the accuracy of LES predictions, benchmark validation data in suitably chosen test cases are required. Comprehensive experimental data covering both time-averaged and timeresolved features are currently scarce. It was the aim of this thesis, therefore, to provide such data .in a configuration representing the near-field of a typical gas turbine fuel injector. It was decided to focus on the fuel injector since many unsteady events are believed to originate because of the transient interactions between the fuel injector flow and the main combustor flow. A radial fed two-stream fuel injector, based on a preexisting industrial gas-turbine Turbomeca design was used, since this geometry was known to be susceptible to unsteadiness. The fuel injector was investigated under isothermal conditions to place emphasis on the fluid mechanical behaviour of the fuel injector, including detailed capture of any unsteady phenomena present. Light Sheet Imaging (LSI) systems were used as the primary experimental technique to provide high quality spatially and temporally resolved instantaneous velocity and scalar field information in 2D planes (using ParticieImage Velocimetry (PIV) and Planar LaserInduced Fluorescence (PUF) techniques). Several methods were employed to extract information quantifYing the flow unsteadiness and improve visualisation of timedependent large-scale turbulent structures. Proper Orthogonal Decomposition (POD) analysis enabled clear identification of the dominant modes of energy containing structures. The results indicated that periodic high-energy containing vortex structures occurred in the swirl stream shear layer, emerging from the fuel injector. These formed a two-strong two-weak rotating vortex pattern which propagated down the main duct flow path. The formation of these vortices was found to be a function of the swirl number and originated due to an interaction between the forward moving swirl flow and the furthest upstream penetration point ofthe recirculation zone present in the main duct flow. Dependent on the magnitude of the swirl number (influencing the swirl stream cone angle) and the geometry of the fuel injector, the vortex formation point was sometimes found inside the fuel injector itself. If the vortices originated inside the fuel injector they appeared much more coherent in space and time and of higher energy. A second unsteady high energy containing phenomenon was also identified, namely a Precessing Vortex Core (PVC), which was damped out if the fuel injector contained a central jet. The dynamics of the PVC interacted with the dynamics of the swirl stream shear layer vortices to reduce there strength. Transient scalar measurements indicated that there was a clear connection between the unsteady vortex pattern and the rate of mixing, resulting in bursts of high heat release and is therefore identified as one source of combustor oscillations. Future fuel injector designs need to pay close attention to these unsteady features in selecting swirl number and internal geometry parameters.

The mixing performance of multiple transverse jets has been evaluated experimentally. Measurement techniques included laser Doppler velocimetry and planar laser induced fluorescence. Basic findings are consistent with results presented in literature for single jet mixing behavior. Mixing performance has been compared to literature for the single jet case and the Holdeman parameter has been re-evaluated for effectiveness at low jet numbers. A single jet in a confined crossflow was found to have a local minimum at B(d⁄D) = 0.721. Results for two jets indicate monotonically decreasing unmixedness for the range of conditions tested, with no local optimum apparent. Data for three jets indicate a local optimum at B(d⁄D) = 0.87and relatively flat range of mixing performance in the range of 0.75 < B(d⁄D) < 1.5. Six jets indicate a minimum unmixedness near B(d⁄D) = 0.5, but exhibited poorer mixing performance than all other configurations at the highest values of B(d⁄D)tested. The most optimum configuration tested was six jets at B(d⁄D) = 0.5, resulting in an unmixedness of 0.0192. This value was 76% lower than the next lowest configuration (three jets) at the same B(d⁄D).Total momentum was found to collapse the data well, as configurations more closely matched a historical correlation for second moment of a single confined jet more closely.

<p dir="ltr">Examination of gaseous detonation flow-fields represents a unique experimental challenge. High-speed shock interaction within a reactive mixture manifests combustion modes across a range of spatial-temporal scales. While the kinetics along the leading front are often characterized by adiabatic compression, simultaneously strong shear induces turbulent mixing in downstream portions of the flow. This all occurs within a wave structure typically traveling near 2000 m/s. To advance fundamental understanding, high-resolution diagnostics are required to make quantitative, time-resolved measurements of the unsteady detonation propagation.</p><p dir="ltr">In this work, detonations are experimentally studied in a single-shot, narrow channel using non-intrusive optical diagnostics. The change in wave structure between mixtures fueled by methane and natural gas was characterized using 175 kHz schlieren and CH* chemiluminescence imaging. The effect of the higher order alkanes in natural gas is speed up the reaction kinetics and produce a wave structure with smaller spatial scales and in which reaction occurs closer to the leading shock front.</p><p dir="ltr">A schlieren system operating at a rate of 5 MHz is then implemented to resolve the spatial-temporal oscillation of the leading shock front. These images are used to compute the lead shock normal speed, which enables a statistical analysis of the oscillating shock velocity. The moments of distribution are compared with computed instability levels of sixteen mixtures and a positive correlation is found. Simultaneous chemiluminescence is used to create joint distribution of shock speed and chemical length scale, which are then compared with the quasi-steady reaction zone solution.</p><p dir="ltr">Experiments are performed with highly nitrogen diluted mixtures of methane and oxygen to examine specific flow features. Different regimes of transverse wave reactivity are observed, from nonreactive to detonative. The transverse detonation wave structure is modeled using oblique shock relations and good agreement is found. The chemical length scales within the configuration are compared to the relevant expansion scales to explain the observed near-steady propagation. Distinct reactive processes following transverse wave collision are also captured. In one instance an explosion immediately occurs, while in the other a reactive gas jet grows from the point of collision. An unsteady reaction zone model is applied to understand the reaction mode within the jet.</p><p dir="ltr">Lastly, 300 kHz OH PLIF is performed to study small scale and weak reaction structures within the flow. The evolution of deflagrative burning mechanisms becomes resolvable using this technique, which highlights the benefit of its use.</p>

[pt] Entender o processo de produção de fuligem é crucial para o projeto de novos queimadores, como os de fornos industriais. Estes queimadores, que utilizam processos de combustão turbulenta, dependem de transferência de calor via radiação das partículas de fuligem para as paredes do forno para seu bom funcionamento. A fuligem formada na região de radiação deve ser oxidada para evitar problemas de saúde e meio ambiente. Mesmo tendo havido significativo progresso no decorrer das duas últimas décadas em relação às chamas laminares, a interação entre a turbulência e a produção de fuligem ainda é um problema em aberto. Este trabalho apresenta resultados experimentais recentes da distribuição instantânea e média da distribuição de fuligem em chamas turbulentas de etileno/ar não prémisturadas estabilizadas em um queimador do tipo bluff-body. A intensidade de turbulência na região de esteira deste queimador é muito alta, levando a uma presença de fuligem intermitente e a estruturas de fuligem altamente distorcidas. A distribuição de fuligem é medida usando incandescência induzida por laser (LII), com uma excitação em 266 nm a 10 Hz e fluência de 0,8 J/cm2 e medição em 400 nm por uma câmera intensificada. Os resultados da técnica LII são comparados à técnica clássica de extinção da luz. Resultados da distribuição de hidrocarbonetos aromáticos policíclicos também são apresentados. Os resultados obtidos permitem caracterizar a distribuição da função de densidade de probabilidade de fuligem. Nas situações de escoamento onde a turbulência da esteira é controlada pelo escoamento de ar, demonstra-se que a PDF da fração volumétrica de fuligem corresponde a uma distribuição lognormal. / [en] Understanding the soot production process is crucial to the design of new burners, such as those in industrial furnaces. Indeed, these burners, which use turbulent combustion processes, rely on radiative heat transfer from the soot particles to the furnace walls to operate properly. The soot formed within the radiation region must the be oxidized in order to avoid health and environment issues. Although there has been significant progress over the past two decades in relation to laminar flames, the interaction between turbulence and soot production is still an open problem. This works presents recent experimental results of the instantaneous and mean soot distribution on non-premixed turbulent ethylene/air flames stabilized at a bluff-body burner. The turbulence intensity in the wake region of this burner is very high, leading to a soot intermittent presence and to highly distorted soot structures. The soot distribution is measured using laserinduced incandescence (LII), with 266 nm excitation at 10 Hz, 0.8 J/cm2 fluence and collected at 400 nm by an intensified camera. The results of the LII technique are compared to those of a classical of light extinction technique. Polyciclic aromatic hydrocarbon distribution results are also presented. The results obtained allow to characterize the soot probability density function distribution. In flow situations where the wake turbulence is controlled by the air flow, the soot volume fraction PDF is shown to correspond to a lognormal distribution.

The Multidisciplinary University Research Initiative (MURI) research at Texas A and M University is concerned with the experimental characterization of non-thermal and non-chemical equilibrium systems in hypersonic (Mach greater than 5) flowfields using experimental diagnostics, and is an interdisciplinary collaboration between the Chemistry and Aerospace Engineering departments. Hypersonic flight conditions often lead to non-thermochemical equilibrium (NTE) state of air, where the timescale of reaching a single (equilibrium) Boltzmann temperature is much longer than the timescale of the flow, meaning that certain molecular modes such as vibrational modes, may be much more excited than the translational or rotational modes of the molecule leading to thermal-nonequilibrium. A nontrivial amount of energy is therefore contained within the vibrational mode, and this energy cascades into the flow as thermal energy, affecting flow properties through the process of various vibrational-vibrational (V-V) and vibrational-translational (V-T) energy exchanges between the flow species. The research is a fundamental experimental study of these NTE systems and involves the application of advanced laser and optical diagnostics towards hypersonic flowfields. The research is broken down into two main categories: the application and adaptation of existing laser and optical techniques towards characterization of NTE, and the development of new molecular tagging velocimetry techniques which have been demonstrated in an NTE flowfield, but may be extended towards a variety of flowfields.