Réponse de la dynamique d’une flamme prémélangée à des modes acoustiques transverses

This study concerns combustion instabilities. It analyzes the dynamics of flames submited to an azimuthal acoustic perturbation and its driving mechanisms. For that purpose, an inverted conical laminar premixed flame is placed at different positions in a standing transverse acoustic field from middle to high frequencies f0.The fluidic system behavior has been classified in the basins of influence of three antinodes : acoustic pressure (VP), acoustic intensity (VI) and acoustic velocity (VV). The acoustic pressure of the field imposes a mode conversion from the transverse wave to a longitudinal wave at the burner exit, the amplitude of which, slowly decreasing from VP to VI, strongly diminishes from VI to VV where pressure effects are residual. The jet adaptation to the fluctuating pressure environment produces vortices at the burner exit and at the rear of the stabilisation rod. At any points of the field, the flow as it develops, is characterized by an aerodynamic field resulting from the superimposition of perturbations of acoustic and convective natures. Flame responses to the perturbed jet present CH* emission fluctuations, representative of heat release rate fluctuations q˙′ related to its area variation under vortical actions. The flame stabilization is piloted by its foot dynamics, the displacement frequency of which is always f0. The foot periodic movement, from a simple vertical displacement at VP, evolves towards a complex movement combining the vertical displacement with the lateral one which is maximum at VV.In the basin of VP, the flame shows nonlinear behaviors related to hydrodynamic nonlinearities or inherent nonlinearities to combustion. Scenarios of frequency bifurcations of CH* emission between f0, its subharmonic f0~2 and/or an aperiodic state have been identified.In the basin of VI, the pressure gradient along the acoustic axis induces asymmetry of flow (velocity, vorticity, jet area, reverse flows,...) and flame (wrinkling, area, CH* emission,...) properties according to the acoustic plane containing the burner axis. Asymmetry is maximum at the point of maximum volumic acoustic potentiel energy density, i.e. VI. In the basin of VV, the acoustic velocity imposes a fluctuating lateral displacement to the fluidic system, which does not impose any fluctuation q˙′ contrary to the residual acoustic pressure whose impact on q˙′ is however weak. A steady deviation of hot gases and the flame towards VP results from a nonlinear acoustic action involving radiation pressure effects.In a context where several flames would be present along the acoustic axis, the spatial non-uniformity of the tranverse field leads to the simultaneous presence of various flames (unpertubed, perturbed at f0, at f0~2, aperiodic and extinguished). In particular, a locallyextinguished flame could be re-ignited by hot gases of a flame in its vicinity. Then, the acoustic field non-uniformity would allow a succession of local ignition and exctintion. The fluidic system asymmetry in the basin of VI and the steady deviation of the hot gases and the flame in VV would create a topology of the fluidic system which converges towards a point located between VV and VI, which could modify flame dynamics by front interactions.It has been shown the possibility to imply these flame behaviors in complex coupling loops of self-excited systems, in particular via mode hanges. The jump from one kind of perturbation to another, may lead to the appearance and disappearance of thermoacoustic instabilities.Cette étude concerne les instabilités de combustion. Elle analyse la dynamique de flammes soumises à une perturbation acoustique azimutale et les mécanismes la pilotant. Pour cela, une flamme laminaire prémélangée en cône inversé est placée en différentes positions d’un champ acoustique stationnaire transverse de fréquence f0 moyenne à haute. Le comportement du système fluide (jet et flamme) a été classé suivant les bassins d’influence des ventres de pression (VP), intensité (VI) et vitesse acoustiques (VV). La pression acoustique du champ impose un mécanisme de conversion du mode transverse en un mode longitudinal en sortie de brûleur dont l’amplitude, lentement décroissante de VP vers VI, décroît fortement de VI à VV où les effets de pression sont résiduels. L’adaptation du jet à l’environnement de pression fluctuante produit des vortex en sortie de brûleur et à l’arrière de la tige de stabilisation. En tout point du champ, l’écoulement, lors de son développement, est caractérisé par un champ aérodynamique résultant de la superposition de perturbations de natures acoustique et convective. La flamme en réponse au jet perturbé présente des fluctuations d’émission CH*, représentatives des fluctuations du taux de dégagement de chaleur q˙′, dues à la variation de son aire par l’action des vortex. La stabilisation de la flamme est pilotée par la dynamique du pied dont la fréquence de déplacement est toujours à f0. Le mouvement périodique du pied, d’un simple déplacement vertical en VP, évolue vers un mouvement complexe, combinant avec ce dernier un déplacement latéral qui est maximal en ventre de vitesse. Dans le bassin de VP, la flamme admet des comportements non-linéaires issus des non-linéarités hydrodynamiques ou inhérentes à la combustion. Des scénarios de bifurcations fréquentielles de l’émission CH* entre f0, son sous-harmonique f0~2 et/ou un état apériodique ont été identifiés.Dans le bassin de VI, le gradient de pression suivant l’axe acoustique provoque la dissymétrisation des propriétés de l’écoulement (vitesse, vorticité, aire du jet, écoulement de retour...) et de la flamme (plissements, aire, émission CH*,...) par rapport au plan acoustique contenant l’axe du brûleur. Le maximum de dissymétrie est observé au point de densité volumique d’énergie acoustique potentielle maximale, soit VI.Dans le bassin de VV, la vitesse acoustique provoque une fluctuation latérale du système fluide qui n’induit pas de fluctuation q˙′ contrairement à la pression acoustique résiduelle dont l’impact sur q˙′ est néanmoins faible. Une déviation stationnaire des gaz chauds et de la flamme vers un VP résulte d’une action d’acoustique non-linéaire impliquant des effets de pression de radiation.Dans un contexte où plusieurs flammes seraient allumées le long de l’axe acoustique, la non-uniformité spatiale du champ transverse conduit à la présence simultanée de flammes de types différents (non perturbées, perturbées à f0, à f0~2, apériodique et soufflées). En particulier, une flamme localement éteinte pourrait être rallumée par les gaz chauds d’une flamme voisine. La non-uniformité du champ acoustique rendrait possible une succession d’allumages et d’extinctions locaux. La dissymétrie du système fluide dans le bassin de VI et la déviation des gaz chauds et de la flamme dans celui de VV conduiraient à une topologie du système fluide convergente entre VV et VI qui pourrait modifier la dynamique des flammes par interactions de fronts.Il a été montré la possibilité d’implication de ces comportements de flammes dans des boucles de couplage complexes de systèmes auto-oscillants, notamment via des changements de mode. Le saut d’un type de perturbation à un autre peut conduire successivement à l’apparition et à la disparition des instabilités thermoacoustiques

Subharmonic frequency bifurcation of a flame in response to jet dynamics modulated by a transverse acoustic field

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Response of a laminar premixed V-flame to a high-frequency transverse acoustic field

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Responses of V-flames placed in an HF transverse acoustic field from a velocity to pressure antinode

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Volumetric velocity measurements (V3V) on turbulent swirling flows

International audienceThis paper presents some results of Volumetric V3V 3D3C velocity measurements on a turbulent flow from a swirl burner. The flow out from the burner used is highly three-dimensional. The study aims at using a system of instantaneous 3D velocity measurements in order to characterize the turbulent swirling flow. The burner used consists of two coaxial tubes with a swirler placed in an annular part supplying the oxidant flow. The central pipe delivers the fuel radially (in the case of reacting flow) through eight holes symmetrically distributed on the periphery of the tube. The burner is placed at the bottom of a combustion chamber and the flow develops vertically along the confinement. The Volumetric 3-component Velocimetry V3V® technique commercialized by TSI is used for 3D velocity measurements in a non reacting flow. The measurement volume above the burner is located at 1.3Db (=49.4 mm) and is 50×50×22 mm3. The operating conditions considered in this study are 4.67 m/s of bulk velocity, Re=7531 of Reynolds number and Sn=1.4 of swirl number. There are very limited works on the application V3V technique on fluid flows. This paper presents the first results concerning an isothermal non-reacting gas flow. Instantaneous and mean velocity 3D3C volumes are measured and shown. Streamlines and velocity iso-surfaces are also analyzed together with different velocity profiles. The results are compared to previous SPIV (Stereoscopic Particle Image Velocimetry) measurements performed by the authors. A good agreement is found between the results of both techniques; the discrepancy did not exceed 10%. V3V results allowed a fine description of 3D aspects of the flow including the recirculation zone and the annular zone with swirling jet effects. The swirling part of the flow and the central recirculation zone are clearly identified by 3D fields of velocities and streamlines. Velocity volume indicates the presence of a central zone with a negative longitudinal velocity, which can reach −1 m/s at the burner center. Under the swirl effect, the tangential velocity is relatively high, particularly in the annular zone of the burner. Indeed, this velocity is varied between 3 and −3 m/s for a bulk velocity of 4.7 m/s

Comparative study between transverse acoustic modes and longitudinal acoustic modes on a premixed V-shape flame

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Réponse non-linéaire de flamme de prémélange dans un champ acoustique transverse

Une flamme en V est étudiée expérimentalement en ventre de pression d'un champ acoustique transverse stationnaire. Les réponses du brûleur et de l'enceinte sont mesurées par microphone. La dynamique de flamme est analysée par caméra rapide intensifiée et photomultiplicateur calé sur l'émission CH* reliée au taux de dégagement de chaleur. L'écoulement est quantifié par Anémométrie Doppler Laser et visualisations tomographiques rapides. On montre que le jet de réactants répond suivant un mode longitudinal à la fréquence imposée f0 tandis que la flamme peut répondre à f0/2

Burning characteristics of aluminum-air flames

International audienceMetal particles present an interesting potential as a carbon-free energy carrier. Indeed, they are highly energetic, and its combustion in atmospheric air could provide the specific energy and power density that metal-air batteries are still struggling to achieve. In order to better evaluate this potential, it is necessary to know the metal-air aerosol laminar burning velocity, which is a fundamental property describing the mixture’s reactivity, heat production, and heat transfer. However, experimental data on the burning velocity of metal-air aerosols are still scarce and scattered. Moreover, metal flames are subjected to a series of particular effects such as the presence of a particle size distribution, implying different burning rates for particles of different sizes in the powder, and possibly scale-dependent influence of radiation [1]. These effects raise even the question of whether a non-stretched laminar burning velocity can be properly defined for metal dust clouds [2]. Therefore, in order to reduce such uncertainties on the burning velocity measurements, it is necessary to work under well-controlled conditions. Among the different possible metal choices for an energy carrier, this work focuses on aluminum, since it presents satisfying energy density and specific energy. Furthermore, the development of a laminar dust burner inspired by previous works in the literature [3, 4] is presented. The experimental setup is capable of stabilizing aluminum-air flames through the generation of well-controlled dust aerosols. Various optical diagnostics, such as high-speed tomography, direct visualization of AlO(g) emissions and spectroscopy, were conducted in order to measure the flame properties. The results are then compared with the existing literature, and analyzed in a context of zero-carbon power generation

Mushroom-shaped bubble plume under bubble crown

International audienceWe study free-rising bubble plumes produced by gas injection in pressurized water to characterize the subsea gas release conditions. Time evolution illustrations of a bubble plume under the chaining regime at atmospheric pressure given above were obtained employing a diffused backlight illumination technique. First, a continuous jet, four times larger than the nozzle diameter, and a large bubble, ten times larger than the nozzle diameter, merge in a mushroom-shaped bubble plume at the jet interruption. Then, the apparition of a new jet enlarges the bubble plume length. The process ends with the mushroom cap detachment while fragmenting into a bubble crown. Question: What is the impact of subsea pressure conditions on the bubble plume evolution up to the sea level surface? The authors gratefully acknowledge France Relance and SEGULA Technologies for funding the project

Horizontal Planar Angular Light Scattering (HPALS) characterization of soot produced in a laminar axisymmetric coflow ethylene diffusion flame

International audienceIn-flame optical characterization of soot is of vital importance to understand soot formation mechanisms as well as to develop and validate accurate soot models. The present work introduces an unconventional methodology adapted to laminar axisymmetric flames that avoids the issue of variable measurement vol- ume with varying scattering angle in the existing light scattering techniques and thus enables the deter- mination of aggregate size with a higher spatial resolution. Coupled with multi-wavelength line-of-sight attenuation measurements, the proposed Horizontal Planar Angular Light Scattering at 532 nm was found able to provide radial profiles of aggregate size, number and diameter of primary spheres, soot volume fraction, and number density in a laminar axisymmetric coflow ethylene/air diffusion flame established over a Gülder burner. The spatial variation of soot optical properties associated with soot maturity was considered in data analysis