La compréhension du processus de stabilisation des flammes Diesel constitue un défi majeur en raison de son effet sur les émissions de polluants. En effet, la relation étroite entre la distance de lift-off (distance entre la flamme et l’injecteur) et la production de suie est maintenant bien établie. Cependant, différents mécanismes de stabilisation ont été proposés mais sont toujours sujets à discussion. L'objectif de cette thèse est de fournir une contribution expérimentale et numérique pour identifier les mécanismes de stabilisation majeurs.La combustion d'un spray n-dodécane issu d'un injecteur mono-trou a été étudiée dans une cellule à volume constant en utilisant une combinaison de diagnostics optiques : mesures hautes cadences et simultanées de schlieren, LIF à 355 nm, chimiluminescence haute température ou de chimiluminescence OH *. Des expériences complémentaires sont effectuées au cours desquelles le mélange est allumé entre l’injecteur et le lift-off par plasma induit par laser. L’évolution du lift-off jusqu’à son retour à une position d’équilibre plus en aval est ensuite étudiée pour différentes conditions opératoires. L'analyse de l'évolution du lift-off sans allumage laser révèle deux types principaux de comportement : des sauts brusques en amont et un déplacement plus progressif en aval. Alors que le premier comportement est attribué à des événements d'auto-inflammation, le second est analysé grâce aux résultats obtenus par allumage laser. Il a été constaté que l'emplacement du formaldéhyde avait un impact important sur la vitesse de retour du lift-off.Une simulation numérique directe (DNS en anglais) bidimensionnelle d'une flamme liftée turbulente se développant spatialement dans les mêmes conditions opératoires que les expériences et reproduisant l'évolution temporelle de la distance de lift-off est proposée. Du fait que les expériences montrent que la flamme se stabilise en aval du spray liquide, la DNS ne couvre qu'une région en aval où l’écoulement est réduit à un jet gazeux. La chimie de l’n-dodécane est modélisée à l'aide d'un schéma cinétique (28 espèces transportées) prenant en compte les chemins réactionnels basse et haute température. Comme observé expérimentalement, la stabilisation de la flamme est intermittente : des auto-inflammations se produisent tout d'abord puis se font convecter en aval jusqu'à ce qu'une nouvelle auto-inflammation se produise. Le mécanisme principal de stabilisation est l'auto-inflammation. Toutefois, on observe également à la périphérie du jet diverses topologies de flammes, telles que des flammes triples, qui aident la flamme à se stabiliser en remplissant des réservoirs de gaz brûlés à haute température localisés à la périphérie, ce qui déclenche des auto-inflammations. Toutes ces observations sont résumées dans un modèle conceptuel décrivant la stabilisation de la flamme.Enfin, un modèle prédisant les fluctuations de la distance du lift-off autour de sa valeur moyenne temporelle est proposé. Ce modèle a été développé sur la base d’observations faites dans l’étude expérimentale et numérique : premièrement, le suivi temporel du lift-off a été décomposé en une succession d’auto-inflammations et d’évolutions en aval. Deuxièmement, la période entre deux auto-inflammations et la vitesse d'évolution en aval ont été modélisées à l'aide de corrélations expérimentales disponibles dans la littérature. Troisièmement, le modèle a été adapté afin de prendre en compte l’effet des réservoirs à haute température sur les fluctuations de la flamme. Et enfin, le modèle a été comparé aux données expérimentales, au cours desquelles des variations de la température ambiante, de la concentration en oxygène et de la pression d'injection ont été effectuées. Dès lors que le modèle a montré une bonne correspondance avec les données expérimentales, il peut être utilisé en complément du modèle prédisant la distance du lift-off moyen afin de mieux décrire la stabilisation d’une flamme Diesel. / The understanding of the stabilization process of Diesel spray flames is a key challenge because of its effect on pollutant emissions. In particular, the close relationship between lift-off length and soot production is now well established. However, different stabilization mechanisms have been proposed and are still under debate. The objective of this PhD is to provide an experimental and numerical contribution to the investigation of these governing mechanisms.Combustion of an n-dodecane spray issued from a single-hole nozzle was studied in a constant-volume precombustion vessel using a combination of optical diagnostic techniques. Simultaneous high frame rate schlieren, 355LIF (laser-induced fluorescence) and high-temperature chemiluminescence or OH* chemiluminescence are respectively used to follow the evolution of the gaseous jet envelope, formaldehyde location and lift-off position. Additional experiments are performed where the ignition of the mixture is forced at a location upstream of the natural lift-off position by laser-induced plasma ignition. The analysis of the evolution of the lift off position without laser ignition reveals two main types of behaviors: sudden jumps in the upstream direction and more progressive displacement towards the downstream direction. While the former is attributed to auto-ignition events, the latter is studied through the forced laser ignition results. It is found that the location of formaldehyde greatly impacts the return velocity of the lift-off position.A two-dimensional Direct Numerical Simulation (DNS) of a spatially developing turbulent lifted flame at the same operating conditions than the experiments and reproducing the temporal evolution of the lift-off length is proposed to provide a better understanding of the flame stabilization mechanisms. The DNS only covers a downstream region where the flow can be reduced to a gaseous jet, since experimental observations have shown that the flame stabilized downstream of the liquid spray. N-dodecane chemistry is modeled using a reduced chemical kinetics scheme (28 species transported) accounting for the low- and high temperature reaction pathways. Similar to what has been observed in the experiments, the flame stabilization is intermittent: flame elements first auto-ignite before being convected downstream until another sudden auto-ignition event occurs closer to the fuel injector. The flame topologies, associated to such events, are discussed in detail, using the DNS results, and a conceptual model summarizing the observations made is proposed. Results show that the main flame stabilization mechanism is auto-ignition. However, multiple reaction zone topologies, such as triple flames, are also observed at the jet periphery of the fuel jet helping the flame to stabilize by filling high-temperature burnt gases reservoirs localized at the periphery, which trigger in its turn auto-ignitions.Finally, a model predicting the fluctuations of the lift-off length around its time-averaged value is proposed. This model has been developed based on observations made in the experimental and numerical study: first, the lift-off length time-evolution was decomposed into a succession of auto-ignition events and downstream evolutions. Second, the period between two auto-ignition and the velocity of the downstream evolution was modeled using experimental correlations available in the literature. Third, the model has been adapted to take into account the effect of the high-temperature reservoirs on the flame fluctuations. Last, the model was compared to experimental data, where the ambient temperature, oxygen concentration and injection pressure were varied. Since the model showed good agreement with the experimental data, it can be used in addition to the model predicting the time-averaged lift-off length to better describe the Diesel flame stabilization.

In current afterburners combustion is stabilized by the high temperature, recirculating region behind bluff body flame holders, such as V-gutters. Blocking the high speed flow with bluff bodies causes a significant pressure drop, and heating the flame holder by the hot combustion product causes a thermal signature, which is a critical problem in a military jet. To reduce these problems, ignition methods using a high frequency (HF) spark discharge, or a radical jet generator (RJG) were developed. The HF discharge ignited and stabilized a flame successfully in a premixed methane-air flow. The electrical power consumption was very small compared to the combustion heat release, as long as the operating velocity was relatively low. However, a theoretical study showed that the ratio of the electrical power consumption to the heat generation by the stabilized flame increases rapidly with increasing flow velocity. For flame stabilization in a high velocity flow, the developed RJG showed much better performance than direct exposure to a plasma. The present study investigated the characteristics of a radical jet produced in a RJG and injected into a main combustor. The limits of flame stabilization by this jet was measured experimentally, and compared to those of bluff body flame holders. The flame holding performance of the radical jet was also experimentally compared to that of a thermal jet. The effect of radicals on flame stabilization was examined using CHEMKIN, and the limit of flame stabilization by the radical jet was estimated for a simple flow configuration using an approximate solution. The results suggest that the reduction of local spontaneous ignition delay time by active species in the radical jet and the longer length of a typical radical jet compared to the dimension of the recirculation zone behind a bluff body increases the maximum velocity at which a flame can be stabilized.

No description available.

Autoignition characteristics of pre-vaporized iso-octane, primary reference fuels, gasolines, and dimethyl ether (DME) have been investigated experimentally in a coflow with elevated temperature of air. With the coflow air at relatively low initial temperatures below autoignition temperature Tauto, an external ignition source was required to stabilize the flame. Non-autoignited lifted flames had tribrachial edge structures and their liftoff heights correlated well with the jet velocity scaled by the stoichiometric laminar burning velocity, indicating the importance of the edge propagation speed on flame stabilization balanced with local flow velocity. At high initial temperatures over Tauto, the autoignited flames were stabilized without requiring an external ignition source. The autoignited lifted flames exhibited either tribrachial edge structures or Mild combustion behaviors depending on the level of fuel dilution. For the iso-octane and n-heptane fuels, two distinct transition behaviors were observed in the autoignition regime from a nozzle-attached flame to a lifted tribrachial-edge flame and then a sudden transition to lifted Mild combustion as the jet velocity increased at a certain fuel dilution level. The liftoff data of the autoignited flames with tribrachial edges were analyzed based on calculated ignition delay times for the pre-vaporized fuels. Analysis of the experimental data suggested that ignition delay time may be much less sensitive to initial temperature under atmospheric pressure conditions as compared with predictions. For the gasoline fuels for advanced combustion engines (FACEs), and primary reference fuels (PRFs), autoignited liftoff data were correlated with Research Octane Number and Cetane Number. For the DME fuel, planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and CH* chemiluminescence were visualized qualitatively. In the autoignition regime for both tribrachial structure and mild combustion, formaldehyde were found mainly between the fuel nozzle and the lifted flame edge. On the other hand, they were formed just prior to the flame edge for the non-autoignited lifted flames. The effect of fuel pyrolysis and partial oxidation were found to be important in explaining autoignited liftoff heights, especially in the Mild combustion regime. Flame structures of autoignited flames were investigated numerically for syngas (CO/H2) and methane fuels. The simulations of syngas fuel accounting for the differential diffusion have been performed by adopting several kinetic mechanisms to test the models ability in predicting the flame behaviors observed previously. The results agreed well with the observed nozzle-attached flame characteristics in case of non-autoignited flames. For autoignited lifted flames in high temperature regime, a unique autoignition behavior can be predicted having HO2 and H2O2 radicals in a broad region between the nozzle and stabilized lifted flame edge. Autoignition characteristics of laminar nonpremixed methane jet flames in high- temperature coflow air were studied numerically. Several flame configurations were investigated by varying the initial temperature and fuel mole fraction. Characteristics of chemical kinetics structures for autoignited lifted flames were discussed based on the kinetic structures of homogeneous autoignition and flame propagation of premixed mixtures. Results showed that for autoignited lifted flame with tribrachial structure, a transition from autoignition to flame propagation modes occurs for reasonably stoichiometric mixtures. Characteristics of Mild combustion can be treated as an autoignited lean premixed lifted flame. Transition behavior from Mild combustion to a nozzle-attached flame was also investigated by increasing the fuel mole fraction.

The probability density function (PDF) approach is a powerful technique for large eddy simulation (LES) based modeling of turbulent reacting flows. In this approach, the joint-PDF of all reacting scalars is estimated by solving a PDF transport equation, thus providing detailed information about small-scale correlations between these quantities. The objective of this work is to further develop the LES/PDF approach for studying flame stabilization in supersonic combustors, and for soot modeling in turbulent flames. Supersonic combustors are characterized by strong shock-turbulence interactions which preclude the application of conventional Lagrangian stochastic methods for solving the PDF transport equation. A viable alternative is provided by quadrature based methods which are deterministic and Eulerian. In this work, it is first demonstrated that the numerical errors associated with LES require special care in the development of PDF solution algorithms. The direct quadrature method of moments (DQMOM) is one quadrature-based approach developed for supersonic combustion modeling. This approach is shown to generate inconsistent evolution of the scalar moments. Further, gradient-based source terms that appear in the DQMOM transport equations are severely underpredicted in LES leading to artificial mixing of fuel and oxidizer. To overcome these numerical issues, a new approach called semi-discrete quadrature method of moments (SeQMOM) is formulated. The performance of the new technique is compared with the DQMOM approach in canonical flow configurations as well as a three-dimensional supersonic cavity stabilized flame configuration. The SeQMOM approach is shown to predict subfilter statistics accurately compared to the DQMOM approach. For soot modeling in turbulent flows, an LES/PDF approach is integrated with detailed models for soot formation and growth. The PDF approach directly evolves the joint statistics of the gas-phase scalars and a set of moments of the soot number density function. This LES/PDF approach is then used to simulate a turbulent natural gas flame. A Lagrangian method formulated in cylindrical coordinates solves the high dimensional PDF transport equation and is coupled to an Eulerian LES solver. The LES/PDF simulations show that soot formation is highly intermittent and is always restricted to the fuel-rich region of the flow. The PDF of soot moments has a wide spread leading to a large subfilter variance. Further, the conditional statistics of soot moments conditioned on mixture fraction and reaction progress variable show strong correlation between the gas phase composition and soot moments. / text

Cette thèse est une contribution à l’étude des flammes de méthane turbulentes non prémélangées en rotation, dites swirlées, avec ou sans enrichissement en oxygène de l’oxydant. L’étude se focalise sur la stabilité de la flamme, les émissions polluantes et la dynamique du jet en non réactif et réactif. Notre dispositif expérimental se compose d’un brûleur à swirler coaxial avec injection radiale de méthane au voisinage de la sortie du brûleur. Ce dernier est confiné dans une chambre de combustion. La teneur en oxygène dans l’oxydant, le nombre de swirl géométrique et la richesse globale à l’injection sont les principaux paramètres qui peuvent être précisément contrôlés. La stabilité de la flamme est caractérisée par chimiluminescence OH*. Les émissions polluantes sont mesurées par des analyseurs en ligne via un prélèvement dans les gaz brûlés. La dynamique du jet est caractérisée principalement par PIV stéréoscopique dans un plan longitudinal et plusieurs plans transverses. La diffusion du méthane dans le jet swirlé est abordée qualitativement par fluorescence induite par laser de l’acétone dans un plan. À ce jour, peu de travaux portent sur la caractérisation notamment dynamique de ces flammes swirlées avec enrichissement en O2. La mise en rotation du jet est à l’origine d’une zone de recirculation centrale qui favorise la stabilisation de la flamme en régime pauvre et à grand nombre de Reynolds. L’étude des émissions polluantes montre que les régimes de combustion à l’air pour lesquels la flamme est liftée stable sont aussi ceux qui produisent du CO et du CH4 résiduel en des quantités non négligeables. L’enrichissement en oxygène permet alors de convertir les imbrûlés et ce pour de faibles enrichissements tout en améliorant la stabilité de flamme via une diminution de la hauteur d’accrochage et des fluctuations associées comme le confirment de précédentes études. L’augmentation des NOx par la voie thermique a été quantifiée pour des enrichissements en oxygène inférieurs à 30 % vol. L’étude systématique en non réactif et réactif apporte des détails sur la topologie tridimensionnelle du jet swirlé suivant les paramètres de l’étude. L’étude de la décroissance des vitesses et de la décroissance du nombre de swirl dans la direction de l’écoulement permetde mettre en évidence l’effet de la flamme sur le jet swirlé. Un couplage entre l’évolution du taux d’entraînement par la recirculation externe et les émissions polluantes est mis en évidence pour expliquer l’évolution des NOx suivant la richesse globale à l’injection. Nous avons proposé une modélisation des écoulements swirlés qui repose sur les écoulements à vorticité hélicoïdale afin d’identifier les caractéristiques principales des structures hélicoïdales au sein de l’écoulement. / This thesis is a contribution to the study of turbulent non-premixed swirling methane flames with or without oxygen addition in the oxidizer. The study deals with the flame stability, the pollutant emissions and the jet dynamic behaviour in non-reacting and reacting conditions. The burner, operating in a combustion chamber, consists of two concentric tubes with a swirler placed in an annular arrangement, which supplied the oxidant flow (air or oxygen-enriched air). The central pipe delivers fuel (methane) radially just below the burner exit plane. The oxygen content in the oxidizer, the geometric swirl number and the global equivalence ratio are the main parameters, which can be precisely set. OH* chemiluminescence imaging is used to characterize flame stability. Multi-gas analyzers are used to measure pollutant emissions in the exhaust gas. The flow is characterized using stereoscopic PIV measurements in different longitudinal and transverse planes. A qualitative study dealing with the methane diffusion imaging is also conducted by use of acetone planar laser-induced fluorescence. Up to now only few studies have examined the dynamic behavior of this type of swirled flames with oxygen addition. Introducing swirl allows creating a central recirculation zone which favors lean flame stabilization at higher Reynolds numbers. The mapping of the combustion regimes combined with the pollutant emission results show that the stable lifted flames are related to high CO and residual CH4 emission levels in the exhaust gas. Oxygen addition, even by a few percent, allows improving CO and unburned hydrocarbons conversion and increasing flame stability at the same time via a decrease of liftoff heights and the related fluctuations. The NOx emissions increase via the thermal pathway with increasing the oxygen-enrichment rate up to 30 % vol. A comparative study in non-reacting and reacting conditions is conducted to give insight into the tridimensional flow field topology varying the above-mentioned parameters. Mean streamwise velocity and swirl number decay rates show the flame effects on the flow dynamics. A coupling mechanism between the entrainment rate of the surroundings via the external recirculation and the pollutant emissions is proposed to explain the NOx emission trend with the global equivalence ratio. A model is also proposed based on the helical vortices to identify the main features of helix structures in the jet in non-reacting and reacting conditions.