Combustion dynamics of annular systems.

New results on the dynamics of annular combustors during ignition and combustion instabilities will be reviewed. Ignition dynamics is consid- ered first by examining experiments carried out in a system comprising a plenum feeding premixed gaseous reactants through multiple swir- ling injectors and an annular combustor formed by two concentric transparent quartz walls allowing full optical access to the flame. The analysis focuses on the “light-round” process during which the flame spreads from one injector to the next eventually leading to established flames on each injector. The transparent lateral walls allow a full view of the flame propagation from a spark igniter located in the neighborhood of one injector. High speed imaging is used to examine flame displace- ment and deduce the ignition delay yielding a full light around of the annular combustor. Changes associated to operation with spray flames are then discussed. The second part of this article is concerned with combustion instabilities of annular systems coupled by azimuthal modes. This type of oscillation has received considerable attention in recent years because the underlying coupling is often observed in the advanced premixed combustion architectures used in modern gas tur- bines. Recent studies have allowed a detailed examination of the dynamics of annular devices comprising multiple swirling injectors. Experiments on annular systems and single sector configurations pro- vide new insight on the coupling process between acoustics and unsteady combustion. Results for self-sustained combustion oscillations coupled by azimuthal modes are presented for operation with gaseous premixed reactants and with spray flames

Flame and spray dynamics during the light-round process in an annular system equipped with multiple swirl spray injectors

A successful ignition in an annular multi-injector combustor follows a sequence of steps. The first injector is ignited; two arch-shaped flame branches nearly perpendicular to the combustor backplane form; they propagate, igniting each injection unit; they merge. In this paper, characterization of the propagation phase is performed in an annular combus- tor with spray flames fed with liquid n-hepane. The velocity and the direction of the arch- like flame branch are investigated. Near the backplane, the flame is moving in a purely azi- muthal direction. Higher up in the chamber, it is also moving in the axial direction due to the volumetric expansion of the burnt gases. Time-resolved particle image velocimetry (PIV) measurements are used to investigate the evaporating fuel droplets dynamics. A new result is that, during the light-round, the incoming flame front pushes the fuel droplets in the azimuthal direction well before its leading point. This leads to a decrease in the local droplet concentration and local mixture composition over not yet lit injectors. For the first time, the behavior of an individual injector ignited by the passing flame front is examined. The swirling flame structure formed by each injection unit evolves in time. From the igni- tion of an individual injector to the stabilization of its flame in its final shape, approxi- mately 50ms elapse. After the passage of the traveling flame, the newly ignited flame flashbacks into the injector during a few milliseconds, for example, 5 ms for the conditions that are tested. This could be detrimental to the service life of the unit. Then, the flame exits from the injection unit, and its external branch detaches under the action of cooled burnt gases in the outer recirculation zone (ORZ

Dynamique de l’injection et des flammes de spray swirlées et couplage azimutal dans les foyers annulaires

The deployment of low-emission technologies in aero-engines does not only require that new designs produce reduced amounts of pollutants, but also that their dynamical behavior (ignition, blow-off, and combustion instabilities) be compatible with the high safety standards prevailing in aeronautics. This research is focused on the latter aspect of combustor design. A transparent annular combustor equipped with 16 swirled spray injectors is used to represent at the laboratory scale the combustion chamber of a jet engine. This system is used in conjunction with a single sector rig to investigate dynamical issues.Theory, experimentation and large eddy simulation are combined to examine a range of items pertaining to the injector dynamics, flow structure, swirl number determination, spray characteristics, and coupling between injector flow and acoustic field. A database of injectors is introduced to investigate the impact of injection parameters on combustion dynamics. These injectors are examined under steady and unsteady conditions by combining laser diagnostics and high-fidelity simulations which allows the characterization of spray-specific behaviors of relevance to the dynamics of injection systems. One important result is that the presence of a liquid film formed on the wall of the injection unit gives rise to a multi-modal distribution of droplet velocities. A novel method is introduced to examine the space-time behavior of the flow and flame of a swirling injector submitted to axial modulations. A detailed investigation of the process by which convective perturbations couple with the acoustic field allows to examine the time lags that control combustion instability and sort out the respective roles of convection and droplet spray evaporation. High speed tomography relying on SnO2 particles provides major results on the Precessing Vortex Core structure and show its behavior under acoustic forcing. The impact of injector head loss on combustion instabilities is examined using several injection systems with similar levels of swirl. The head loss is shown to play a major role in the coupling between the flame and the upstream plenum. The previous results obtained in a single sector rig are used to guide experiments on the annular combustor. The focus is placed on high amplitude combustion oscillations coupled by a standing azimuthal mode inducing flame blow-off near the pressure nodal line. The deformation of the acoustic distribution is tracked using a novel expansion on azimuthal harmonics allowing the determination of the critical conditions leading to this phenomenon. New results are also presented about the transient dynamics of an injector during ignition and about lean blow out with a proof of concept extension of the LBO limit by nanosecond plasma discharges.Le déploiement de technologies à faibles émissions dans les moteurs d’avion ne nécessite pas seulement que les nouvelles conceptions émettent des quantités réduites de polluants, mais également que leur comportement dynamique (allumage, extinction et instabilités de combustion) soit compatible avec les normes de sécurité élevées en vigueur dans l’aéronautique. Ce travail de recherche se concentre sur ces derniers aspects. Une chambre annulaire transparente équipée de 16 injecteurs swirlés représentant à échelle réduite le foyer d’un moteur d’hélicoptère est utilisée conjointement avec un système à un seul secteur pour étudier les problèmes dynamiques. Théorie, expérimentation et simulation aux grandes échelles sont combinées pour examiner une gamme de questions ayant trait à la dynamique de l’injecteur, la structure de l’écoulement, la détermination du niveau de rotation, les caractéristiques du spray, le couplage entre l’injection et le champ acoustique. Une base de données d’injecteurs est introduite pour étudier l’impact des paramètres d’injection sur la dynamique de la combustion. Ces injecteurs sont examinés dans des conditions stables et instables en combinant des diagnostics laser et des simulations permettant la caractérisation de comportements spécifiques à la dynamique du spray et du système d’injection. Un résultat important est que la présence d’un film liquide formé sur la paroi de l’injecteur induit une distribution multimodale des vitesses des gouttelettes. Une nouvelle méthode est introduite pour examiner le comportement spatio-temporel de l’écoulement et de la flamme lorsque l’injecteur est soumis à des modulations axiales. Une étude du processus par lequel les perturbations convectives se couplent au champ acoustique permet d’examiner les délais qui contrôlent l’instabilité de combustion et d’identifier les rôles respectifs de la convection et de l’évaporation des gouttes. La tomographie à grande vitesse reposant sur des particules de SnO2 fournit des résultats majeurs sur la structure du noyau tourbillonnaire en précession et montrent son comportement sous forçage acoustique. L’impact de la perte de charge de l’injecteur sur les instabilités de combustion est examiné à l’aide de plusieurs systèmes d’injection avec des niveaux de rotation semblables. Il est démontré que ce paramètre joue un rôle majeur dans le couplage entre la flamme et le plénum. Les résultats précédents sont utilisés pour guider les expériences sur la chambre de combustion annulaire. L’accent est mis sur les oscillations de combustion de grande amplitude couplées par un mode azimutal stationnaire induisant une extinction de flamme au voisinage de la ligne nodale de pression. La déformation du champ acoustique est suivie à l’aide d’une développement en série d’harmoniques azimutaux permettant la détermination des conditions critiques conduisant à ce phénomène. De nouveaux résultats sont également présentés sur la dynamique transitoire d’un injecteur lors de l’allumage et sur l’extinction pauvre avec une preuve de concept de la possibilité d’étendre la limite d’extinction par des décharges plasma nanoseconde

Appareil de fixation d'un référentiel optique sur une personne

L'invention concerne un appareil de fixation d'un référentiel optique sur une personne, cet appareil comprenant un corps muni de moyens de fixation à la personne et de moyens de maintien du référentiel optique

The suitability of different swirl number definitions for describing swirl flows: Accurate, common and (over-) simplified formulations

International audienceSwirling flows are of considerable practical importance. They are used for example to increase heat transfer in pipes or to stabilize flames at a distance from the injector unit by means of a central reverse flow that establishes an inner recirculation zone of hot combustion products. The level of swirl is governed by a dimensionless parameter designated as the swirl number, which essentially quantifies the ratio of the flow rate of the angular momentum axial component to the flow rate of axial momentum. This ratio controls to a great extent the structure of the swirling flow and its value determines whether an inner recirculation zone is established and whether a precessing vortex core (PVC) is formed in the flow. However, a major difficulty resides in calculating the swirl number from experimental measurements and over the last 50 years, several simplified formulas have been proposed to overcome this difficulty. The present study is aimed at using velocity and pressure profiles obtained by a large eddy simulation in a generic configuration to examine these simplified expressions and determine the conditions under which they may be applicable. The geometry comprises a cylindrical swirling injector, flush mounted in the back plane of a cylindrical cavity. Although the swirl number is in principle constant when the flow is established in a duct with a constant cross-section, provided that viscous forces at the wall are negligible, one finds that this quantity varies substantially if inadequate approximations are made. Among the many possibilities one concludes that two swirl numbers should be distinguished. The first corresponding to the original definition features conservation properties, but is difficult to properly calculate from experimental data. The second is a highly simplified formulation that is commonly used today but does not share the conservation properties of the first formulation. Recommended practices are provided on how each of these swirl numbers should be calculated. It is also shown that the other formulations yield values that notably differ from those provided by the original definition

Appareil de fixation d'un référentiel optique sur une personne

L'invention concerne un appareil de fixation d'un référentiel optique sur une personne, cet appareil comprenant un corps muni de moyens de fixation à la personne et de moyens de maintien du référentiel optique

Methoology of Knowledge Library Management for Concurrent Cooperative Design

International audienc