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.

An extension of the current acetone Planar Laser-Induced Fluorescence (PLIF) was formulated for mixing studies of fluids at subcritical and supercritical conditions. The new technique, called Planar Laser-Induced Fluorescence and Phosphorescence (PLIFP), employs the difference in the mass diffusivity of the denser (liquid) to the less dense (vapor/supercritical fluid) to delineate the interface where a phase change occurs. The vapor/supercritical acetone fluorescence signal is utilized to measure of the acetone vapor density, the mixture fractions and liquid acetone phosphorescence signal to determine the location of the phase interface. The application of the technique requires the photophysical properties of vapor and liquid acetone to be known. Therefore, a series of controlled experiments were done to determine their photophysics at elevated temperatures and pressures up to T/TC = 1.2 and p/pC =1.25. The demonstration of the techniques shows it was able to provide quantitative measurements of acetone number density and the overall mixture fraction within the test chamber. Also, the size and mass of droplets that have broken off from the main jet were determined as well, though the ability is limited to small droplets (d~100 μm). In addition, the technique was able to delineate the low diffusivity (subcritical)/high diffusivity (supercritical) interface very well.

Syngas, a gaseous mixture of H2, CO and diluents such as N2, CO2, is a clean fuel generated via gasification of coal or biomass. Syngas produced via gasification typically has low calorific values due to very high dilution levels (~60% by volume). It has been recognized as an attractive energy source for stationary power generation applications. The present work focuses on experimental and numerical investigation of syngas-air counterflow diffusion flames with varying composition of syngas. Laser-based diagnostic techniques such as Particle Imaging Velocimetry, Rayleigh thermometry and Laser-induced fluorescence have been used to obtain non-intrusive measurements of local extinction strain rates, temperature, quantitative OH and NO concentrations, respectively, for three different compositions of syngas. Complementing the experiments, numerical simulations of the counterflow diffusion flame have been performed to assess the performance of five H2/CO chemical kinetic mechanisms from the literature. The first part of the work involved determination of local extinction strain rates for six H2 /CO mixtures, with H2:CO ratio varying from 1:4 to 1:1. The extinction strain rates were observed to increase from 600 sec-1 to 2400 sec-1 with increasing H2:CO ratio owing to higher diffusivity and reactivity of the H2 molecule. Numerical simulations showed few mechanisms predicting extinction conditions within 5% of the measurements for low H2:CO ratios, however, deviations of 25% were observed for higher H2 :CO ratios. Sensitivity analyses revealed that the chain branching reactions, H+O2 <=>O+OH, O+H2 <=>H+OH and the third body reaction H+O2 +M<=>HO2 +M are the key reactions affecting extinction limits for higher H2:CO mixtures. The second phase of work involved quantitative measurement of OH species concentration in the syngas-air diffusion flames at strain rates varying from 35 sec-1 to 1180 sec-1. Non-intrusive temperature measurements using Rayleigh thermometry were made in order to provide the temperature profile necessary for full quantification of the species concentrations. The [OH] is observed to show a non-monotonous trend with increasing strain rates which is attributed to the competition between the effect of increased concentrations of H2 and O2 in the reaction zone and declining flame temperatures on the overall reaction rate. Although the kinetic mechanisms successfully captured this trend, significant deviations were observed in predictions and measurements in flames with H2:CO ratios of 1:1 and 4:1, at strain rates greater than 800 sec-1 . The key reactions affecting [OH] under these conditions were found to be the same reactions identified earlier during extinction studies, thus implying a need for the refinement of their reaction-rate parameters. Significant disagreements were observed in the predictions made using the chemical kinetic mechanisms from the literature in flames with high H2 content and high strain rate. The final phase of work focused on measurement of nitric oxide (NO) species concentrations followed by a comparison with predictions using various mechanisms. NO levels as high as ~ 48 ppm were observed for flames with moderate to high H2 content and low strain rate. Quantitative reaction pathway diagrams (QRPDs) showed thermal-NO, NNH and prompt-NO pathways to be the major contributors to NO formation at low strain rates, while the NNH pathway was the dominant route for NO formation at high strain rates. The absence of an elaborate CH chemistry in some of the mechanisms has been identified as the reason for underprediction of [NO] in the low strain rate flames. Overall, the quantitative measurements reported in this work serve as a valuable reference for validation of H2/CO chemical kinetic mechanisms, and the detailed numerical studies while providing an insight to the H2:CO kinetics and reaction pathways, have identified key reactions that need further refinement.

Comprendre la phénoménologie du mélange turbulent de fluides hétérogènes reste à ce jour l'un des défis majeurs dans le domaine de la turbulence. Contrairement à une croyance répandue, la viscosité a une influence non négligeable sur le mélange. Dans cette étude, nous comparons un jet rond à viscosité constante (CVF) avec un jet à viscosité variable (VVF). Les différences dans la naissance et la croissance de la turbulence sont mises en lumière via l'utilisation de statistiques classiques et conditionnées. Le calcul du coefficient d'entraînement nous permet de montrer que, si un processus d'entraînement classique est à l'oeuvre dans le cas CVF, le mélange est assuré par le detrainment dans le cas VVF. Augmenter le nombre de Reynolds conduit à une apparition plus tardive de ce dernier, ainsi que des effets visqueux en général. Ainsi, une meilleure compréhension du mélange à viscosité variable est nécessaire pour, par exemple, améliorer l'efficacité des systèmes de combustion existants. / Understanding the phenomenology associated with heterogeneous mixtures of gases is one of the mort persistent challenges in turbulent mixing. Contrary to what has been thought, it was previously found that viscosity variations have a non-negligible impact on mixing and should be taken into account. In this study, we carry out a comparison between Constant Viscosity Flows (CVF) and Variable Viscosity Flows (VVF), in a round jet. The disparity in the birth and growth of turbulence was highlighted using traditional and conditional statistics. The computation of the entrainment ratio allowed us to conclude that, while a classical entrainment phenomenon is observed for the CVF configuration, the VVF case exhibits a detrainment process. Increasing the Reynolds number leads to a delayed apparition of the latter and of the viscous effects. Thus, a better understanding of the heterogeneous fluid mixing phenomenology is necessary to improve the efficiency of existing combustion systems.

<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>

An experimental study of the effects of turbulent flow regime on the flame structure is conducted by using perforated-plate-stabilized hydrogen-piloted lean premixed methane/air turbulent flames. The underlying non-reacting turbulent flow field was investigated using two-dimensional three-components particle imaging velocimetry (2D3C-PIV) with and without three perforated plates. The non-reacting flow data allowed a separation of the turbulent flow regime into axial velocity dominated and vortex dominated flows. A plate with 62\% blockage ratio was used to represent the stream-dominant flow regime and another with 86\% blockage ratio was used to represent the vortex-dominant flow regime. OH laser-induced fluorescence was used to study the effects of the turbulent flow regime on the mean progress variable, flame brush thickness, flame surface density, and global consumption speed. In comparison with the stream-dominant flow, the vortex-dominant flow makes a wider and shorter flame. Also, the vortex-dominant flow has a thicker horizontal flame brush thickness and a thinner longitudinal flame brush thickness. Especially, the horizontal flame brush thickness for the vortex-dominant flow does not follow the turbulence diffusion theory. Then, the vortex-dominant flow shows a relatively constant flame surface density along the stream-wise direction, while the stream-dominant flow shows a decreasing flame surface density. Lastly, the vortex-dominant turbulent flow improves the consumption speed in comparison to the stream-dominant turbulent flow regime with the same velocity fluctuation level.

Sécuriser un approvisionnement fiable de micro-algues est récemment devenu un enjeu industriel. Pour assurer la croissance de micro-algues dans des photobioréacteurs clos, un transfert de masse optimum de l'oxygène et du dioxyde de carbone doit être assuré. Dans cette thèse, une étude du transfert de masse gaz-liquide dans les conduites horizontales a été menée. Dans les trois premiers chapitres, un modèle unidimensionnel de transfert de masse dans le photobioréacteur a été développé. Tout d'abord, le transfert de masse entre une bulle de gaz allongée et un écoulement liquide turbulent a été 'étudié expérimentalement. En considérant l'interface comme étant plane, les coefficients de transfert de masse mesurés sont proches d'une corrélation de Lamourelle (1972). Le modèle de Taitel pour les écoulements stratifiés a été comparé à des modèles plus complets pour la prédiction de l'interface des bulles allongées. Une approche analytique basée sur un bilan de masse et utilisant les modèles testés a ensuite été développée et adaptée à un photobioréacteur afin de prédire l'efficacité de la conversion du CO2 en biomasse en fonction des paramètres d'exploitation. Les deux derniers chapitres visent à développer une simulation numérique du transfert de masse gaz-liquide. Une mesure de la concentration en CO2 dans le sillage d'une bulle de gaz ascendante a été effectuée à l'aide d'une méthode améliorée de traitement des données de Fluorescence Induite par Plan Laser (FIPL). Enfin, une simulation numérique a été réalisée sous COMSOL / Securing a reliable supply of microalgae has recently become an industrial stake. To ensure successful growing of microalgae in enclosed, tubular photobioreactor systems as in Microphyt, an optimum mass transfer of oxygen and carbon dioxide should be secured. In this thesis an investigation of the gas-liquid mass transfer in horizontal pipes has been conducted. In the first three chapters, a one dimensional mass transfer model in horizontal gas-liquid flows will be developed and applied to horizontal photobioreactors. Firstly, a study of mass transfer between an elongated gas bubble under a turbulent liquid flow immobilized in a duct has shown that under the hypothesis considering the interface as a flat plane estimated, the measured mass transfer coefficients appear to be well fitted by a correlation from Lamourelle (1972). The interface prediction for stratified flows have been compared to more complete unit-cell models for intermittent flow interface and found to be a good first estimate. The photobioreactor’s conversion efficiency of CO2 into biomass as a function of operating parameters is investigated using an analytical approach to complete the mass balance and classical two-phase flow approach from Taitel (1976). The last two chapters aim at developing a numerical simulation of gas-liquid mass transfer. A measurement of CO2 wake structures behind free rising bubbles have realized using an improved data treatment method for Planar laser-induced fluorescence (PLIF) and pH sensitivity of fluorescein sodium. Finally, an implementation of the experimental measurements under COMSOL has been realised

Dans beaucoup de situations réelles, qui vont du domaine des transports (moteurs fusée, avions et automobiles), en passant par la production d'électricité (centrale thermique) ou encore de la thermique (chaudière, fours industriels, foyers domestiques...), le prémélange turbulent entre l'air et la combustible conduit généralement à une hétérogénéité du mélange. En conséquence, la richesse du mélange est variable à la fois dans le temps et dans l'éspace, produisant une combustion sous la forme de flamme partiellement prémélangées, loin des cas classiques prémélangé et de diffusion. Dans cette thèse, on s'intéresse plus particulièrement aux effets des hétérogénéités de richesse sur la structure locale (en terme de courbure, d'épaisseur, d'étirement, de plissement, de fraction molaire de combustible) de ce type de flamme. Deux configurations expérimentales, stationnaire et instationnaire (présentant chacune leurs avantages) sont alors étudiées. Une méthode expérimentale est proposée afin de mesurer simultanément la température de la flamme et la fraction molaire, dont son originalité repose sur sa capacité à corriger l'influence de la température sur l'efficacité des mesures de PLIF sur acétone. L'étude des flammes laminaires montre que la flamme est fortement influencée par la stratification, notamment en terme de configuration géométrique pour la flamme instationnaire. Mais aussi du point de vue de l'épaisseur pour la flamme stationnaire. L'évolution de l'épaisseur en fonction de l'étirement présente une modification notable avec un changement de pente en fonction du nombre de Lewis. De plus, il est montré que la flamme se propage dans un milieu très pauvre (en dessous des limites d'inflammabilités) du fait du soutien de la flamme dû au réservoir chimique et thermique induit.<br />L'étude du comportement est poursuivie dans la seconde partie, pour deux turbulences de grilles données, où la comparaison est effectuée entre des flammes de prémélanges homogènes et hétérogènes, et ce pour différentes fractions molaires de combustible et niveaux de fluctuations. Elle montre qu'il existe une modification de la structure de la flamme non seulement par l'étirement local mais aussi par les variations de la vitesse de flamme avec la composition du mélange. Ceci introduit un étirement additionnel Kpp non négligeable notamment lorsque la richesse le long du front de flamme est faible.

Natural turbulent convection studies encompass a wide range of flows occurring in nature, for example, atmospheric and oceanic flows, con-vection in the Earth’s mantle, convection in the stars and also in many engineering applications. Rayleigh-Benard convection (RBC), i.e. con-vection in a horizontal fluid layer confined between two plates with a temperature differential maintained across them, has been a proto-type problem in the studies of turbulent natural convection. Many small scale and global features of the flow in the turbulent regime of RBC are known, yet the flow dynamics is not fully understood, es-pecially at high Rayleigh numbers (Ra). Present work comprises of experimental investigations of a different type of flow, high Rayleigh number turbulent convection in a long vertical tube (abbreviated as tube convection or TC). The tube of aspect ratio (length to diameter) of about 10, open at both the ends interconnects two large tanks. The flow driven by an unstable density difference created between the two tanks, has some unique features, different from RBC. The net flow at any tube cross-section is zero and the time averages of the velocities, the Reynolds shear stress and the mean shear are also zero. Turbu-lent energy production is therefore solely due to buoyancy. The flow is axially homogeneous and axisymmetric. In the homogeneous region, the mean density gradient is linear. Rayleigh number in TC is conve-niently defined based on the mean (linear) density gradient (denoted by Rag). Two sets of experiments are carried out. In one set of experiments, the density difference is created using brine and fresh water and in another set, it is created using heat. The ranges of Rag achieved are 3 × 108 < Rag < 8 × 109 in the experiments using salt (Schmidt number, Sc ≈ 600) and 5 × 104 < Rag < 5 × 106 in the experiments using heat (Prandtl number, P r ≈ 6). From the measured salt and heat fluxes in both the sets of experiments, the non dimensional flux 1 1 scaling above a certain value of Rag is obtained as N ug ∼ Rag2 P r 2 and from the velocity measurements in the experiments using salt, the 1 Reynolds number scaling is obtained as Re ∼ Rag2 P r− 12 . Both these are as per the predicted scalings by the mixing length model proposed by Arakeri et al. (2000) for high Rag convection in the vertical tube. The flux scaling N u ∼ (RaP r)2 , also known as the ‘ultimate regime’ of convection, expected at very high Ra but not yet observed in the experiments in classical RBC, is easily achieved in TC at relatively lower values of Ra. The fluxes and Reynolds numbers in TC are orders of magnitude higher as compared to those obtained in RBC for similar values of Ra and P r. In the lower range of Rag values for P r ≈ 6, a transition to a new flux scaling, N u ∼ (RaP r)0.29 is found. Similar transitions are also found to be present in the results of Tovar (2002) for Sc ≈ 600 and in the DNS results of Schmidt et al. (2012) for P r = 1, at different values of Rag. Collecting all these data, it is shown that the transition occurs at a fixed Grashof number of 1.6 × 105, independent of P r. Velocity measurements are carried out using particle image velocime-try (PIV) in the salt experiments. Kinetic energy spectra computed from the velocity fields are presented for the locations from the tube axis to the wall, for the lowest and the highest values of Rag achieved in the experiments. The spatial energy spectrum of lateral velocity at the tube axis follows Kolmogorov-Obukhov (KO) scaling (−5/3 scaling exponent) while the spatial spectrum of longitudinal velocity shows a scaling slightly higher than −5/3 but lower than −11/5 (the Bolgiano-Obukhov (BO) scaling). The scalar spectra is computed from the concentration fields obtained from planar laser induced fluorescence (PLIF) in the experiments using salt, and also from the temperature measurements from the experiments using heat. Both the concentra-tion and temperature fluctuations spectra show some evidence of dual scaling - BO scaling (−7/5 scaling exponent) in the inertial subrange followed by Obukhov-Corrsin (OC) scaling (−5/3 scaling exponent) over a narrow range of scales. Light propagation through the buoyancy driven turbulent flow in TC has also been experimentally investigated. Light propagation through convective turbulence is encountered in many situations. In some cases e.g. in observational astronomy it is undesirable, while in some other cases it is useful, e.g. in remote sensing of meteorological parameters. In the present study, light intensity and angle of arrival fluctuations in a parallel beam of light are measured. Laser shadowgraphy is used in the intensity measurements while the angle of arrival is obtained by measuring deflections of narrow laser beams, created by passing collimated laser light through a mask having equispaced grid of holes. Background oriented schlieren (BOS) measurements have also been carried out to obtain the displacements, which are proportional to the angle of arrivals. The equations for frequency spectrum of intensity and angle of arrival from the literature, developed for isotropic, ho-mogeneous turbulent media, are modified for the flow in the present case and the asymptotic scalings for high and low frequency ranges are obtained. The scalings in the frequency spectra computed from the measurements of intensity and angle of arrival fluctuations are com-pared with the obtained asymptotic scalings. The results from the present work are also compared with results from studies in the atmo-sphere and lab experiments.

The influence of stratification on the structure of turbulent methane/air combustion is investigated using experimental data from laboratory scale burners: a weakly turbulent slot burner, and a higher turbulence co-annular swirl burner. The degree of stratification can be controlled independently of the overall fuel/air flow rate. The resulting measurements of scalar and velocity fields provide detailed test cases for existing and emerging turbulent flame models, covering a range of u'/sL from 1 to 10, turbulence intensities from 5% to 60%, and stratification ratios from 1 to 3. Simultaneous Rayleigh/Raman/CO-LIF measurements of temperature and major species concentrations - CH4, CO2, CO, H2, H2O and O2 - along a line are used to investigate the structure of a series of flames in both the slot and swirl burners. Concurrent cross-planar OH-PLIF allows thermal gradients to be angle corrected to their three-dimensional values. Finally, non-reacting and reacting velocity fields complete the flame database. The behavior of major species concentrations in the slot and swirl burner with respect to temperature is found to agree well on the mean with unstrained premixed laminar flame calculations. Scalar means conditioned on stoichiometry also show good agreement, aside from hydrogen which is enhanced under stratified conditions. Surface density function and scalar dissipation are lower than calculated values in all cases, suggesting that turbulence-induced thickening dominates the effect of increased strain. Metrics commonly used to derive flame surface density (FSD) were investigated. FSD may be determined using a statistical method based on measurements of temperature and its gradient, or a geometric method based on 2D temperature or LIF imaging. A third metric, an extension of the geometric method, is proposed. Good agreement is observed between the three metrics. The current database provides the first detailed high resolution scalar measurements for premixed and stratified flames. The data analysis provides insight into the physics of stratification: for the flames considered, the effects of stratification appear to be surprisingly small compared to those of turbulence, even at significant stratification ratios. The datasets provide a means of validating current and future computational turbulent combustion models.