Cinematographic stereoscopic PIV, combined with Taylor's frozen flow hypothesis, is used to generate three-dimensional (3D) quasi-instantaneous pseudo volumes of the three-component (3C) velocity field in the far field of turbulent nonpremixed jet flames at jet exit Reynolds number Reδ in the range 8,000-15,300. The effect of heat release, however, lowers the local (i.e., based on local properties) Reynolds number to the range 1,500-2,500. The 3D data enable computation of all nine components of the velocity gradient tensor ∇u from which the major 3D kinematic quantities, such as strain rate, vorticity, dissipation and dilatation, are computed. The volumetric PIV is combined with simultaneously acquired 10 Hz OH planar laser-induced fluorescence (PLIF). A single plane of the OH distribution is imaged on the center-plane of the volume and provides an approximate planar representation of the instantaneous reaction zone. The pseudo-volumes are reconstructed from temporally and spatially resolved kilohertz-rate 3C velocity field measurements on an end-view plane (perpendicular to the jet flame axis) invoking Taylor's hypothesis. The interpretation of the measurements is therefore twofold: the measurements provide a time-series representation of all nine velocity gradients on a single end-view plane or, after volumetric reconstruction, they offer a volumetric representation, albeit approximate, of the spatial structure of the flow. The combined datasets enable investigation of the fine-scale spatial structure of turbulence, the effect of the reaction zone on these structures and the relationship between the jet kinematics and the reaction zone. Emphasis is placed on the energy dissipation field and on the presence and role of dilatation. Statistics of the components of the velocity gradient tensor and its derived quantities show that these jet flames exhibit strong similarities to incompressible turbulent flows, such as in the distribution of the principal strain rates and strain-vorticity alignment. However, the velocity-gradient statistics show that these jet flames do not exhibit small-scale isotropy but exhibit a strong preference for high-magnitude radial gradients, which are attributed to regions of strong shear induced by the reaction zone. The pseudo volumes reveal that the intense-vorticity field is organized in two major classes of structures: tube-like away from the reaction zone (the classical worms observed in incompressible turbulence) and sheet-like in the vicinity of the local reaction zone. Sheet-like structures are, however, the dominant ones. Moreover, unlike incompressible turbulence where sheet-like dissipative structures enfold, but don't coincide with, clusters of tube-like vortical structures, it is observed that the sheet-like intense-vorticity structures tend to closely correspond to sheet-like structures of high dissipation. The primary reason for these features is believed to be due to the stabilizing effect of heat release on these relatively low local Reynolds number jet flames. It is further observed that regions of both positive and negative dilatation are present and tend to be associated with the oxidizer and fuel sides of the OH zones, respectively. These dilatation features are mostly organized in small-scale, short-lived blobby structures that are believed to be mainly due to convection of regions of varying density rather than to instantaneous heat release rate. A model of the dilatation field developed by previous researchers using a flamelet approximation of the reaction zone was used to provide insights into the observed features of the dilatation field. Measurements in an unsteady laminar nonpremixed jet flame where dilatation is expected to be absent support the simplified model and indicate that the observed structure of dilatation is not just a result of residual noise in the measurements, although resolution effects might mask some of the features of the dilatation field. The field of kinetic energy dissipation is further investigated by decomposing the instantaneous dissipation field into the solenoidal, dilatational and inhomogeneous components. Analysis of the current measurements reveals that the effect of dilatation on dissipation is minimal at all times (it contributes to the mean kinetic energy dissipation only by about 5-10%). Most of the mean dissipation arises from the solenoidal component. On average, the inhomogeneous component is nearly zero, although instantaneously it can be the dominant component. Two mechanisms are believed to be important for energy dissipation. Near the reaction zone, where the stabilizing effect of heat release generates layers of laminar-like shear and hence high vorticity, solenoidal dissipation (which is proportional to the enstrophy) dominates. In the rest of the ow the inhomogeneous component dominates in regions subjected to complex systems of nested vortical structures where the mutual interaction of interwoven vortical structures in intervening regions generates intense dissipation. / text

Combustion instability is a process which involves unsteady chemical kinetic, fluid mechanic, and acoustic processes. It can lead to unstable behavior and be detrimental in ways ranging from faster part fatigue to catastrophic system failure. In terms of combustion methodology, combustion instability has been a key issue for lean premixed combustion. The primary objective of this work is to improve understanding of combustion dynamics through an experimental study of lean premixed combustion using a low swirl combustor. This special burner was developed at the Lawrence Berkeley National Laboratory and has recently received significant interest from the gas turbine industry. In these experiments, acoustic perturbations (chamber modes) are imposed on a low swirl stabilized methane-air flame using loudspeakers. The flame response is examined and quantified with OH planar laser induced fluorescence. Rayleigh index maps of the flame are computed for each frequency and operating condition. Examining the structures in the Rayleigh maps, it is evident that, while the flame shows no significant response in some cases, acoustic forcing in the 70-150 Hz frequency range induces vortex shedding in the flame shear layer. These vortices distort the flame front and generate locally compact and sparse flame areas. This information about the flow field shows that, besides illuminating the combustion dynamics, the Rayleigh index is a useful way to reveal interesting aspects of the underlying flow. The experiments also revealed other interesting aspects of this flame system. It was found that the flame becomes unstable when the perturbation amplitude reaches 0.7% of the mean pressure. Decreasing the swirl number makes the flame shape more jet-like, but does little to alter the shear-layer coupling. In a similar fashion, increasing the pressure was found to alter the flame shape and flame extent, but the thermo-acoustic coupling and induced large scale structure persisted to 0.34MPa, the highest pressure tested.

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.

Ce travail s’inscrit dans le cadre de nos recherches sur les écoulements à bulles. C’est l’étude expérimentale du mélange d’un traceur très peu diffusif (fluoresceine dans l’eau) dans l’écoulement instationnaire engendré par le passage d’un écoulement uniforme au travers d’un réseau d’obstacles sphériques (2% de fraction volumique) répartis aléatoirement dans l’espace. Cet écoulement reproduit correctement les caractéristiques de l’agitation dans un essaim de bulles en ascension. La vitesse du fluide est mesurée par PIV de manière assez classique. Le traceur est injecté en amont du réseau et l’´evolution de sa concentration est mesurée par PLIF. L’utilisation de la PLIF pour mesurer des champs de grande extension (15 cm) et avec une grande dynamique d’intensité lumineuse fluorescée constitue une contribution importante de ce travail. Les résultats ainsi obtenus montrent que, à petit nombre de Reynolds, le mélange est régi par les forts gradients de vitesse qui existent dans le voisinage des sphères. A grand nombre de Reynolds, il est maîtrisé par la turbulence qui se développe alors. L’analyse des résultats comporte deux parties principales : (1) une analyse statistique des profils de concentration aboutissant à la détermination d’un coefficient de diffusion effectif et (2) une description de la distribution spectrale des fluctuations de vitesse et de concentration. / This research is a part of our research about bubbly flows. Experiments are performed about mixing of a high Schmidt scalar component (fluorescein in water) by the agitation generated by the flow through a random array of fixed spheres (at high Re and with a volume fraction of solid equal to 2%). This flow mimics for a great part the agitation in the liquid phase of a bubble swarm rising in a liquid otherwise at rest. The velocity of the liquid is estimated from PIV measurements. The scalar is injected through a point source in the array and the evolution of its concentration is estimated by PLIF method. An important part of this research is the measurement of large fields of concentration (15 cm) with a good precision by PLIF. The results show that, at moderate Reynolds number (100), mixing is controled by the steep velocity gradients located near the spheres, while, at large Reynolds number, it is controled by the turbulence that develops. The analysis of the results is composed of two parts : (1) the statistical analysis of the spatial distribution of scalar concentration, and the determination of an effective diffusion coefficient, (2) a spectral analysis of the velocity and concentration fields.

Les méthodes de stérilisation à basse température prennent une importance croissante pour la décontamination de matériaux thermosensibles utilisés dans les appareils exposés aux risques microbiologiques tels que les endoscopes dans les hôpitaux, les emballages dans l'industrie agroalimentaire, ou les équipements soumis aux agents microbiologiques de guerre dans les zones de conflits. Les méthodes de stérilisation standards non-thermiques souffrent de limitations liées à leur toxicité, leurs coûts élevés, leurs faibles compatibilités avec les matériaux, et/ou leurs longs cycles de stérilisation (quelques heures). Une approche alternative consiste à utiliser des plasmas hors équilibre à pression atmosphérique produits par décharges électriques. Les plasmas permettent des cycles de stérilisation pluscourts car les surfaces traitées sont exposées à de nombreux agents biocides, notamment à du rayonnement, à des espèces réactives oxygénées et azotées (RONS), et à des espèces chargées. Cependant, à pression atmosphérique, le volume du plasma est généralement faible. Les traitements en post-décharge permettent d'augmenter la surface de traitement, tout en réduisant la dégradation du matériau par les espèces chargées. Dans la post-décharge, les principaux agents biocides sont les RONS. L'objectif de cette thèse est d'étudier la production et le transport des RONS générés par des décharges pulsées non-thermiques dans l'air et l'azote à pression atmosphérique au moyen de diagnositcs de spectroscopie d'absorption UV et mid-IR (QCLAS), de fluorescence / Low-temperature sterilization methods are of increasing importance for the decontamination of heat-sensitive materials in devices exposed to biohazards, such as endoscopes in hospitals, containers in the food industry, or contaminated equipment in areas exposed to war acts. Standard non-thermal sterilization methods suffer from limitations related to their toxicity, high cost, low material compatibility, and/or long sterilization cycles (several hours). An alternative approach consists in using atmospheric pressure nonequilibrium plasmas produced by electric discharges. Plasmas provide shorter sterilization cycles because they combine various biocidal agents including radiation,reactive oxygen and nitrogen species (RONS), and charged species. However, at atmospheric pressure the plasma volume is usually small. Post-discharge treatment allows to increase the treated surface area, and in addition to reduce surface degradation by charged species. In post-discharge treatment, the main biocidal agents are the RONS. The objective of this thesis is to study the production and transport of RONS generated by non-thermal pulsed discharges in nitrogen and air at atmospheric pressure by means of UV and mid-IR (QCLAS) absorption spectroscopy, planar laser induced fluorescence (PLIF), and absolute emission spectroscopy.

L’écoulement de Taylor-Couette entre deux cylindres concentriques (cylindre interne en rotation et cylindre externe fixe) est actuellement mis à profit au CEA pour étudier les performances d’extraction d’une colonne liquide/liquide pour le retraitement du combustible nucléaire. Ces performances étant fortement liées au mélange, il est important de le quantifier. En monophasique, les propriétés de mélange ont été étudiées dans une thèse précédente, à la fois expérimentalement et numériquement, et ont été reliées aux paramètres hydrodynamiques de l’écoulement. L’effet du nombre de Reynolds, du régime d’écoulement et de la taille des rouleaux (longueur d’onde axiale) ont notamment été prouvés. Le but de ce travail est d’étendre les précédentes études aux écoulements de Taylor-Couette diphasiques. Pour des raisons pratiques et afin de s’affranchir des phénomènes de coalescence et de rupture, des billes de PMMA de diamètres 800 µm à 3 mm sont choisies pour simuler la phase dispersée, en suspension dans une solution aqueuse de Dimethylsulfoxyde (DMSO) et de Thiocyanate de Potassium (KSCN). Le montage expérimental couple les méthodes de PIV et de PLIF afin d’obtenir en simultané les informations concernant l’hydrodynamique de l’écoulement et le mélange. Cependant la mise en place du diphasique impose un certain nombre de contraintes qui doivent être prises en compte. Bien que les deux phases soient soigneusement choisies afin d’être adaptées en indice et en densité, le recours à une deuxième chaine d’acquisition PLIF est nécessaire afin d’améliorer la qualité des mesures. Ainsi, une première voie de PLIF classique suit l’évolution au cours du temps de la concentration de Rhodamine WT, injectée au centre de la colonne au début de l’expérience. La voie supplémentaire visualise un autre fluorophore, de la Fluorescéine répartie de manière homogène dans la colonne, permettant ainsi de créer un masque dynamique des billes. Grâce à ce montage expérimental, une étude paramétrique (taille, rétention des billes) a été menée. Un double effet des billes sur le mélange a ainsi été observé. D’une part, la présence d’une phase dispersée modifie les propriétés hydrodynamiques de l’écoulement : les régimes (Couette, Taylor Vortex Flow et Wavy Vortex Flow) sont d’autant plus déstabilisés que la rétention ou la taille des billes augmente. De plus un régime supplémentaire, inhabituel dans le cas du cylindre externe fixe, apparait, forcé par la phase dispersée : le régime Spiral Vortex Flow, dans lequel le mélange est très efficace. D’autre part, une influence propre des billes sur le mélange a été mise en évidence, en fonction de leur taille et de leur concentration. Ces deux effets se combinent pour expliquer une forte augmentation du mélange en présence de la phase dispersée. Les mécanismes physiques liés à ces résultats sont ensuite discutés, et leur influence relative est comparée. Enfin, le rôle du mélange local sur le coefficient de dispersion global, paramètre classiquement utilisé en génie chimique afin de prédire les performances des colonnes d’extraction, est discuté. / In the scope of the nuclear fuel reprocessing, Taylor-Couette flows between two concentric cylinders (the inner one in rotation and the outer one at rest) are used at laboratory scale to study the performances of new liquid/liquid extraction processes. Separation performances are strongly related to the mixing efficiency, the quantification of the latter is therefore of prime importance. A previous Ph.D. work has related the mixing properties to the hydrodynamics parameters in single-phase flow, using both experimental and numerical investigations. The Reynolds number, flow state and vortices height (axial wavelength) impacts were thus highlighted. This Ph.D. work extends the previous study to two-phase configurations. For experimental simplification, and to avoid droplets coalescence or breakage, spherical solid particles of PMMA from 800 µm to 1500 µm diameter are used to model rigid droplets. These beads are suspended in an aqueous solution of dimethyl sulfoxide (DMSO) and potassium Thiocyanate (KSCN). The experimental setup uses coupled Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence (PLIF) to access simultaneously the hydrodynamic and the mixing properties. Although the two phases are carefully chosen to match in density and refractive index, these precautions are not sufficient to ensure a good measurement quality, and a second PLIF channel is added to increase the precision of the mixing quantification. The classical PLIF channel monitors the evolution of Rhodamine WT concentration, while the additional PLIF channel is used to map a Fluorescein dye, which is homogeneously concentrated inside the gap. This way, a dynamic mask of the bead positions can be created and used to correct the Rhodamine WT raw images. Thanks to this experimental setup, a parametric study of the particles size and concentration is achieved. A double effect of the dispersed phase is evidenced. On one hand, the particles affect the flow hydrodynamic properties : the more the particles size and concentration grows, the more the studied flow regimes (Couette, Taylor Vortex Flow and Wavy Vortex Flow) are destabilized. In addition, a new flow state appears in presence of a dispersed phase, that is unusual in the configuration we use where the outer cylinder is at rest. This Spiral Vortex Flow is characterized by an enhanced mixing. On the other hand, for given hydrodynamic properties and depending on the particles size and concentration, a specific effect of the particles on mixing is highlighted. Both the “hydrodynamic” and “intrinsic“ effects are responsible for the significant increase of the global mixing observed in two-phase configuration. Possible physical mechanisms are proposed to analyze these results, and their relative influence is compared. At last, an attempt is made to relate the local mixing properties to a global dispersion coefficient of the flow, data commonly used in chemical engineering to predict the performances of extraction columns.

Une étude expérimentale du développement de jets turbulents à masse volumique variable est menée. Les comportements axiaux et radiaux des propriétés statistiques moyennes et fluctuantes obtenues sur les champs de vitesse et de concentration sont analysés. Ces résultats apportent une meilleure compréhension des phénomènes de mélange et de l’effet de la variation de densité dans le but d’apporter des données complémentaires pour le développement des calculs numériques et accroître leurs précisions. Les cas étudiés dans cette thèse sont deux jets d’hélium marqués par de la vapeur d’acétone pour des nombres de Reynolds Re = 7000 et 11000 permettant d’explorer l’effet de fortes variations de masse volumique (S = 0,39 et 0,41, respectivement) par rapport à l’air ambiant. Le cas d’un jet d’air (Re = 16000) également porteur de vapeur d’acétone est utilisé comme cas de référence comparable au cas d’un contaminant passif avec S = 1,17. L’étude porte sur la région de proche sortie de jet, jusqu’à une distance de 40 fois le diamètre Dj de sortie du jet. La configuration adoptée est celle d’un jet de tube rond, libre, axisymétrique, vertical et ascendant dont le diamètre intérieur du tube est Dj = 3,5mm se développant dans l’air ambiant. La particularité de cette étude est la mise en œuvre d’un système de mesure par diagnostic optique qui permet un couplage spatial et temporel des mesures de vitesse et de concentration sur une région de l’écoulement de quelques cm2 et non sur un seul point. Pour ce faire, le couplage des mesures PIV, pour la mesure du champ de vitesse, et PLIF basée sur la fluorescence de la vapeur d’acétone, pour la mesure du champ de concentration, a été étudié, conçu et testé. Il nous a permis d’obtenir une base de données, encore trop rares aujourd’hui, sur l’évolution des flux turbulents croisés de vitesse et concentration. Ces données ont mis en évidence une évolution plus rapide du jet le plus léger. Cependant, les coefficients de corrélation semblent identiques lorsque l’on atteint la zone autosimilaire du jet. Une approche basée sur les probabilités de densité conjointes vitesse-concentration a permis de mettre en évidence des différences dans la région extérieure des jets, où l’intermittence de frontière donne son empreinte sur les propriétés du mélange. / An experimental study of the development of turbulent jets with variable density is presented. The axial and radial behaviours of the mean and fluctuating statistical properties obtained on the velocity and concentration fields are analysed. These results provide a better understanding of mixing phenomena and the effect of density variation in order to provide complementary data for the development of numerical calculations and to increase their precision. The cases studied in this thesis are two helium jets carrying acetone vapor for Reynolds numbers Re = 7000 and 11000 to explore the effect of large density variations (S = 0.39 and 0.41, respectively) relative to the ambient air. The case of an air jet (Re = 16000) also carrying acetone vapor is used as a reference case comparable to the case of a passive contaminant with S = 1.17. The study focuses on the region of near jet exit, up to a distance of 40 times the jet outlet diameter Dj. The adopted configuration is a round jet tube, free, axisymmetric, vertical and ascending whose internal diameter of the tube is Dj = 3.5 mm developing in the ambient air. The particularity of this study is the implementation of an optical diagnostic measurement system that allows a spatial and temporal coupling of speed and concentration measurements over a region of the flow of a few cm2. To do this, the coupling of the PIV measurements for the measurement of the velocity field and the PLIF based on the fluorescence of the acetone vapor for the measurement of the concentration field has been studied, designed and tested. It allowed us to obtain a database, still too rare today, on the evolution of the turbulent flow of speed and concentration. These data have shown a faster evolution of the lightest jet. However, the correlation coefficients appear to be identical when the self-similar zone of the jet is reached. An approach based on joint speed-concentration density probabilities has revealed differences in the outer region of the jets, where the intermittent boundary gives its imprint on the properties of the mixture.

The probability of a fire occurring in space vehicles and facilities is amplified by the amounts of electrical equipment used. Additionally, the lack of egress for space personnel and irreplaceable resources used aboard space vehicles and facilities require a rapid response of a suppression system and quick extinguishment. Current experimental means that exist to gather data in space vehicles and facilities are limited by both size of the experiment and cost. Thus, more economical solutions must be considered. The aim of this research was to develop a reliable and inexpensive methodology for the prediction of flame extinction and suppression in any three-dimensional environment. This project was split into two parts. Part one included the identification and validation of a computational model for the prediction of gas dispersion. Part two involved the development of an analytical parameter for predicting flame extinction. For model validation, an experimental apparatus was constructed. The experimental apparatus was one-eighth of the volume of electronics racks found aboard typical space facilities. The experimental apparatus allowed for the addition of parallel plates to increase the complexity of the geometry. Data acquisition consisted of gas concentration measurements through planar laser induced fluorescence (PLIF) of nitrogen dioxide and velocity field measurements through particle image velocimetry (PIV). A theoretical framework for a generalized Damköhler number for the prediction of local flame extinction was also developed. Based on complexities in this parameter, the computational code FLUENT was determined to be the ideal means for predicting this quantity. The concentration and velocity field measurements provided validation data for the modelling analysis. Comparison of the modelling analysis with experimental data demonstrated that the FLUENT code adequately predicted the transport of gas to a remote location. The 5 FLUENT code was also used to predict gas transport at microgravity conditions. The model demonstrated that buoyancy decreases the time to achieve higher gas concentrations between the parallel plates. As an example of the use of this methodology for a combustion scenario, the model was used to predict flame extinction in a blow-off case (i.e., rapid increase in strain rate) and localized flame extinction (i.e., flame shrinking) in a low-strain dilution case with carbon dioxide over time. The model predictions demonstrated the potential of this methodology with a Damköhler number for the prediction of extinction in three-dimensional environments.

The laser-induced fluorescence (LIF) technique is used to visualize and quantify the concentration field around a conductivity probe. The LIF data are compared to the signal collected by the conductivity probe. The objective is to compare the signal of the contact-sensor to the "ground-truth" measurement of the LIF data. Detailed comparison of the temporal response and the peak detection are presented. In addition, a proof-of-concept of a two-color LIF technique is presented using Rhodamine 6G paired with an Argon-ion laser and simultaneously Oxazine 725 paired with a Krypton-ion laser. Optical filters on two digital cameras isolate the emitted light from these respective laser/dye combinations. The objective is to provide detailed quantitation of two over-lapping (non-reactive)chemical plumes.