L’optimisation des systèmes de production d’énergie par combustion requiert une connaissance précise de la cinétique de combustion. Cependant, la majorité des systèmes de production d’énergie par combustion fonctionnent à haute pression et il est reconnu que la pression a une influence sur la cinétique de combustion. En laboratoire, l’analyse de la structure de flamme laminaire se présente comme un outil puissant pour étudier la chimie de la combustion. A ce jour, la plupart des travaux menés ont été réalisés à des pressions inférieures ou égales à la pression atmosphérique. Au cours de cette thèse, un dispositif expérimental pour l’analyse de structure de flammes laminaires, à contre-courants et à haute pression a été mis en place. Il permet de stabiliser des flammes de CH4/air et CH4/CO2/air jusqu’à 0,7 MPa et l’étude de leur structure par diagnostics laser. Les profils de concentration de OH dans les flammes CH4/air et CH4/CO2/air à différentes richesses (=0,7-1,2) et différentes pressions (P=0,1-0,7 MPa) ont été mesurés par Fluorescence Induite par Laser et calibrés en concentration par absorption laser. Pour cela, la longueur du milieu absorbant a été déterminée par Fluorescence Induite par Plan Laser (PLIF). Une attention particulière a été portée aux corrections du signal de fluorescence prenant en compte l’élargissement de raie et le taux de collisions, qui augmentent avec la pression. Les profils expérimentaux obtenus ont été comparés à la modélisation à l’aide du code de calcul OPPDIF et des mécanismes cinétiques GRI-Mech3.0 et GDFKin®3.0. En parallèle, une analyse spectroscopique des flammes de CH4/air à haute pression a été entreprise. / The optimisation of practical combustion devices requires a detailed knowledge of the combustion kinetic. However, most practical combustion systems operate at high pressure and it is known that pressure has an influence on combustion kinetics. In laboratory, the analysis of laminar flame structure is a powerful tool for studying combustion chemistry. However, most of studies have been realised at pressures under or equal to atmospheric pressure. During this thesis, an experimental device has been implemented for the study of the structure of high pressure counterflow flames. It allows the stabilisation and the study of CH4/air and CH4/CO2/air flame structure through laser diagnostics until 0.7 MPa. CH4/air and CH4/CO2/air flames have been studied for a various range of stoichiometry (equivalence ratios from 0.7 to 1.2) and pressures (0.1 MPa to 0.7 MPa). Experimental OH concentration profiles have been measured by Laser Induced Fluorescence and calibrated by laser absorption. To do this, absorption path length has been determined by Planar Laser Induced Fluorescence (PLIF). Great care has been attached to the determination of the fluorescence signal by taking into account the line broadening and de-excitation by quenching which both arise at high pressure. Experimental data were compared with modeling results obtained through the OPPDIF calculation code with GRI-Mech3.0 and GDFKin®3.0 kinetic mechanisms. In parallel, a spectroscopic analysis of the CH4/air flames has been undertaken.

Les systèmes de combustion à prémélange pauvre (PP) sont l’une des technologies les mieux adaptées pour la réduction des émissions de polluants, mais ils sont très sensibles aux phénomènes d’extinction, aux retours de flamme (flashback) dans l’injecteur et aux instabilités de combustion. La plupart des chambres de combustion des turbines à gaz utilisent de swirleurs pour stabiliser des flammes compactes et permettre une combustion efficace et propre avec des densités de puissance élevée. Une meilleure connaissance des mécanismes de la dynamique de la combustion d’écoulements swirlés PP présente un intérêt aussi bien pratique que fondamental. Ce travail est une contribution pour atteindre ce but. Le brûleur Noisedyn, avec une geometrie modifiable, a été spécialement conçu pour répondre à cet objectif. Une analyse expérimentale a etait conduite pour examiner les paramètres qui reduisent la sensibilité des systèmes PP aux phénomènes dynamiques. Mesures de fonction de transfert de flamme (FTF), diagnostiques laser (LDV et PIV) et imagerie des flammes sont les principaux techniques utilisé dans ce travail. Large eddy simulation sont aussi utilisé pour expliquer les mécanismes derrière les observations experimentaux. / Lean premixed (LPM) combustion systems achieve low pollutant emission levels, with compact flames and high power densities, but are highly sensitive to dynamic phenomena, e.g, flashback, blowout and thermoacoustic instabilities, that hinder their practical application. Most LPM gas turbine combustors use swirling flows to stabilize compact flames for efficient and clean combustion. A better knowledge of the mechanisms of steady and unsteady combustion of lean premixed swirled mixtures is then of practical, as well as fundamental interest. This thesis is a contribute towards the achievement of this goal. A burner, made of several components with variable geometry, was specifically designed for this scope. An experimental analysis was conducted to investigate the main parameters leading to a reduction of the sensitivity of LPM systems to dynamic phenomena. The diagnostics applied include flame transfer function (FTF) measurements, laser diagnostics (LDV and PIV) and flame imaging. Large eddy simulations were also exploited to elucidate the mechanisms behind the experimental observations.

The quenching of undesired flames by cold surfaces has been investigated for more than a century. The current quenching theory can predict simple configurations, this is not the case for real environments such as fuel management systems. Flames are sensitive to numerous parameters, such as fuel, mixture fraction, pressure, temperature, flow properties, acoustics, radiation, and surface interactions. The effects of some of these parameters are very well documented but there is a lack of information regarding the effects of acoustics and flow. This dissertation work will focus on improving the understanding of flow effect on the quenching of premixed gaseous flames. First, the effect of apparent velocity on flame quenching was investigated for different fuels and equivalence ratios. An experimental facility is designed such that the apparent flame velocity at which the flame enters and propagates through the channel can be varied without changing the initial mixture condition. High-speed (15,000 frames per second (FPS)) Schlieren and dynamic pressure measurement were used to measure the apparent flame velocity and to assess the flame quenching, respectively. This study showed that the high-speed laminar flames are harder to quench compared to self-propagating and turbulent flames. A similar trend was obtained for all the conditions investigated, lean and stoichiometric methane-air, lean propane-air, and lean ethylene-air mixtures. Further investigation was carried out to understand the quenching of high-speed laminar flames. The flame propagation through the channel was investigated using Hydroxyl (OH) planar laser induced fluorescence (PLIF). This study showed that the OH intensity fell below the detection threshold in the later part of the channel when quenching is observed. Then, the influence of heat transfer was investigated using spatial and temporal evolution of the temperature in the quenching channel. A high-speed (10 kHz) filtered Rayleigh scattering (FRS) technique was used to measure the one-dimensional time-resolved temperature profile. Three different channel heights (H = 1.3, 1.5, 2.0 mm) were investigated. Based on the evolution of the temperature profile in the quenching channel, a new parameter was identified and the importance of its evolution on the flame quenching was discussed.

Sustainable and low emission combustion requires new combustion paradigms and solutions to increase efficiency, comply with more stringent regulations on pollutants, and cope with the varying qualities of renewable fuels. Plasma Assisted Combustion (PAC) could be one of the tools to achieve these goals in practical combustion systems. Previous studies showed that PAC can be used in a variety of applications: to improve ignition in difficult environments, to extend the operating range of burners to leaner conditions, to contrast thermoacoustic instability, to allow flame-holding in extreme conditions, and more. While applications keep being proposed, there are efforts to model and understand the coupling between flames and plasma discharges. This work contributes to the unraveling of the action of plasma discharges on flames by performing a number of investigations on a simple PAC burner. Trends and temporal evolution of key chemical species and electric fields are measured during plasma actuation of the flame. Experimental datasets resulting from this work are meant to be used in cross-validating numerical simulations. The considered PAC burner generates a lean methane-air stagnation flame, across which discharges are applied, developing partially in the fresh and partially in the burned gases. Time-resolved 2D imaging of atomic hydrogen and oxygen is obtained by using two-photon absorption planar laser induced fluorescence (TALIF) while OH and CH radicals are measured by using planar laser induced fluorescence (PLIF). To measure the electric field, the Electric Field Induced Second Harmonic generation (EFISH) technique is used. A novel deconvolution-like post-processing procedure is proposed and used to calibrate the measurements and improve the spatial resolution, overcoming limitations and distortions typical of EFISH measurements. Presented results quantify the effect of the plasma actuation on the flame and lend themselves to the validation of numerical models.

No description available.

This thesis investigates the oxidation chemistry and ignition properties of formic acid (FA). The study reports experimental measurements of ignition delay time (IDT) and CO/CO2 time histories during FA oxidation in a shock tube. The initial concentration of FA was measured with a laser to minimize uncertainties arising from its low vapor pressure and tendency to form dimers. Shock tube experiments were carried out at two pressures, around 1.7 and 3.5 bar, and temperatures ranging from 1194 to 1658 K, with two equivalence ratios, 0.72 and 1.47. The results show a noticeable dependence of IDTs on temperature and pressure, while there was insignificant dependence on equivalence ratio. Six kinetic models for FA oxidation available in the literature were tested against the obtained data to evaluate their accuracy and suggest potential improvements. We found that 4 models performed well in predicting IDTs and CO/CO2 profiles with some overprediction at certain conditions. Sensitivity analysis revealed that the IDTs of FA are governed by unimolecular decomposition, H abstraction, and radical consumption (HOCO) reactions. The concentration of HO2 is higher at low temperatures, which is favorable for the system’s reactivity as it makes IDTs more sensitive to the reaction HOCHO + HO2 = H2O2 + HOCO. CO formation is controlled by two reactions: CO + OH = HOCO and HOCHO (+M) = CO + H2O, while the second reaction is more pronounced at high temperatures. Moreover, the dissociation of HOCO is faster at higher pressures, leading to higher initial CO concentrations. The formation of CO2 is determined by CO + OH = CO2 + H, while at higher temperatures, HOCHO (+M) = CO2 + H2 (+M) becomes more important, resulting in higher initial CO2 concentrations.

<p> Understanding boundary layer transition is fundamental to hypersonic vehicle design as the significant heating induced by the transition process informs the development of vehicle thermal protection systems. Carbon-based thermal protection systems have been shown to decrease thermal loads and delay transition by absorbing thermal energy during ablative mass transfer into the boundary layer. To better understand this process, a high-repetition rate measurement technique is needed to temporally resolve carbon species concentrations as they propagate through the boundary layer at frequencies where boundary layer instabilities occur. Carbon monoxide is a dominant product from the chemical reactions that take place during the ablation process and is the species of interest considered in this work. A proposed approach is applying carbon monoxide two-photon laser-induced fluorescence (CO TP-LIF) at 100 kHz+ during a simulated ablation experiment where CO is injected into the boundary layer of an axisymmetric slender-body cone model in the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. To develop this capability, a custom-built optical parametric oscillator (OPO) was designed and used in conjunction with a burst-mode laser system to achieve narrowband excitation of CO at 100 kHz. The OPO was designed, built, and characterized through measurements of cavity energy efficiency, spectral bandwidth, and beam quality. Initial demonstrations to show the OPO could successfully achieve CO LIF were conducted in a vacuum cell at 10 Hz. The feasibility of performing CO LIF at 100 kHz in the BAM6QT was then assessed on a bench-scale using a burst-mode laser, a high speed camera, and an imaging intensifier. CO number densities in a vacuum cell were related to those that would be expected within the boundary layer of a 3 degree half-angle cone in the BAM6QT, and a series of measurements were made at these representative conditions. Appreciable signal levels were attained for single dimensional focused line measurements demonstrating high potential for using this technique in the BAM6QT at 100 kHz. The potential for a two-dimensional planar measurement was also assessed with decent promise for success for planar laser sheets of small dimensions (2 mm tall or less). Additionally, an initial BAM6QT test entry was carried out to gain experience with experimental setup; lessons learned from this experience are examined and discussed. To date, CO TP-LIF has only been applied up to 1 kHz repetition rates. This work represents a 100 fold increase over the current CO LIF state of the art and the first reported measurements, bench scale or otherwise, of 100 kHz-rate CO LIF. This lays the foundation for future CO LIF experiments in the BAM6QT at kHz-MHz repetition rates. </p>

A novel method has been implemented for measuring the concentration of gas species, water vapor and carbon dioxide, within a narrow channel flow field non-invasively using tunable diode laser absorption spectroscopy (TDLAS) in conjunction with a laser modulated at a high frequency [Wavelength Modulation Spectroscopy (WMS)] tuned to the ro-vibrational transition of the species. This technique measures the absorption profile which is a strong function of the species concentration across short path lengths and small time spans, as in PEM fuel cells during high load cycles. This method has been verified in a transparent circular flow 12 cm path length and a 12 mm rectangular flow channel. Distinct absorption peaks for water vapor and carbon dioxide have been identified, and concentrations of water vapor and carbon dioxide within the test cells have been measured in situ with high temporal resolutions. A comparison of the full width at half maximum (FWHM) of the absorption lineshapes to the partial pressure of water vapor and carbon dioxide showed a predominantly linear relationship, except in the lower partial pressure regions. Test section temperature was observed to have very minimal impact on these curves at low partial pressure values. A porous media like a membrane electrode assembly (MEA) similar to those used in PEM fuel cells sandwiched between two rectangular flow channels was also tested. Some of the scattered radiation off the MEA was observed using a photodiode at high gain, allowing for more localized species detection. The technique was used to monitor the humidity on either side of the MEA during both temperature controlled and super-saturated conditions. The measurements were observed to be repeatable to within 10 %.

No description available.

<p>Multiphase flow dynamics are integral to many propulsion, sprays, energetics, and industrial processes. Practical systems, especially in combustion, typically involve multidimensional spatial structures and complex and coupled physics interactions. At some operating conditions, flow mixing, combustion chemical reactions, and flow residence time scales are relatively similar and therefore coupled (i.e., each affects the other). For example, the combustion and atomization of liquid fuel govern the performance of combustors. In addition to spray-air interactions, injection strategies may rely on spray-wall interactions to achieve improved mixing and performance. Understanding and predicting these flows requires advanced experimental diagnostics that provide information on local state variables with high spatiotemporal resolution. However, multiphase flow dynamics integral to these combustion systems may not be fully resolved with conventional one or two-dimensional diagnostics. Tomographic reconstructions yield 3D spatial information and may provide high-fidelity data to fill the technology gap. Performing these 3D diagnostics with adequate time-resolution is necessary to capture the full dynamics of high-speed flows. This work focuses on developing, applying, and evaluating non-intrusive 4D (x,y,z,t) volumetric imaging in challenging combustion environments. Each optical diagnostic approach probes a different phase of combustion experiments in a non-instructive manner. For example, Schlieren imaging visualizes the index of refraction gradients corresponding to density changes in the gas phase. This work uses various optical approaches (e.g., scattering, Schlieren, or fluorescence) with 4D imaging to provide quantitative measurements of different combustion phenomena. Parallel ray-tracing simulations are utilized to guide diagnostic development and quantify measurement capabilities. This work presents significant high-speed diagnostic improvements for combustion applications relevant to defense, energy generation, and propulsion.</p>