Liquid jet and droplet deformation induced by non-uniform acoustics radiation pressure distribution

International audienceThe present work focuses on non linear acoustic effects on an elliptic cylinder or an ellipsoid. These effects are encountered in acoustic levitation, ultrasonic standing wave atomization or two-phase flow combustion instabili-ties. Theoretical approaches mainly paid attention on the total radiation force, but a modeling of the distribution of acoustic radiation pressure around the object is needed to predict liquid object deformation. In the present study, a semi-analytical model is presented in order to compute the local radiation pressure as the only reason for liquid jet or droplet deformation. The method used here imposes an incident field to, a posteriori, compute the scattered field as a function of the object geometrical properties. A partial wave decomposition(PWD) model is developed to express incident and scattered fields by and immovable object with rigid boundary conditions. Radiation pressure is computed for progressive and standing wave fields. Validation of our method is done by comparing with the radiation force results from the literature. Results show that the larger the deformation, the higher the acoustic effects in a direction perpendicular to the acoustic wave axis. Introduction Non linear effects of acoustics are encountered in applications such as acoustic levitation, ultrasonic standing wave atomization or two-phase flow combustion instabilities occurring in rocket engines [1-4]. Most of the studies dealing with interaction of acoustics and spherical [5-9] or cylindrical [10-14] objects focused on the stationary radiation force. The main objective was there to determine the displacement of these objects. However, their deformation is also of a great interest in applications dealing with liquid objects. In studies on acoustically levitating droplets, some authors considered the radiation pressure distribution as the source of the stationnary deformation of the free surfaces [2, 15, 16]. They showed that spherical droplets became oblate when exposed to the radiation pressure. For cylindrical objects, it was experimentally proven that cylindrical liquid jets subjected to a low frequency standing wave were susceptible to be deformed into elliptic cylinders [17]. Thus, by relying on those results it appeared that knowing radiation pressure distribution around elliptic objects was necessary to correctly analyze the interaction between acoustics and deformed objects. Hasheminejad et al. [18, 19] developed an approach based on elliptic functions, namely Mathieu functions, to describe the acoustic scattered field. This is a powerful method, but limited in its applications due to the occurrence of Mathieu polynomials instability. Other authors considered a theoretical approach based on the expression of the incident and scattered waves by means of the formal cylindrical or spherical functions [17, 20-22]. All the studies cited above focused only on the modeling of the radiation force computed with the far field assumption avoiding the computation of radiation pressure distribution. To tackle the problem of object deformation induced by acoustics, it is needed to model the radiation pressure distribution. This is done here for elliptic cylinders and ellipsoids. The two-way coupling between incident acoustic harmonic plane waves and these objects is explored by computing the radiation pressure field and resulting radiation force. In the first section is presented the method used to compute the acoustic velocity potential field scattered by elliptic objects and the consequent computation of the radiation pressure and radiation force. Results showing the convergence of the method, its validation and the radiation pressure distribution are presented in the second section. Finally, the last section is dedicated to some conclusions

High amplitude/high frequency acoustic field effects on coaxial inkection

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Flammes minces et interface

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Comportement transitionnel et stabilisation de flammes-jets non-prémélangés de méthane dans un coflow d'air dilué en CO2

Ce travail s'intéresse à la compréhension du comportement des flammes non-prémélangées issues d'un jet de méthane assisté par un coflow d'air dilué avec du CO2, ou d'autres gaz chimiquement inertes pour discriminer les différents phénomènes impliqués dans la dilution. Les phénomènes transitionnels, décrochage et extinction, quantifiés par des limites de stabilité, sont analysés à l'aide de grandeurs physiques représentatives. Le domaine de stabilité de flamme est limité par des surfaces 3D dans le domaine physique ( Qdiluant/Qair (taux de dilution), Uair (vitesse d'air), UCH4 (vitesse de méthane)), révélant un effet compétitif entre l'aérodynamique et la dilution. Des cartographies génériques de décrochage et d'extinction communes à tous ces diluants sont proposées. Des grandeurs liées à la stabilisation sont toutes soumises à des lois d'évolution auto-simlilaires. Il en ressort que la vitesse de propagation de flamme est l'élément clé du mécanisme de stabilisation lors de la dilution.This work focuses on the understanding of the behaviours of non-premixed methane flame inside an air coflow diluted by carbon dyoxide (CO2) or by other chemically inert diluents in order to discriminate different phenomena involved in dilution. Transitional phenomena (liftoff and extinction) quantified trough the stability limits, are analyzed trough representative physical quantities. The flame stability domain is limited by 3D-surfaces (liftoff and extinction) in the physical domain (Qdiluant/Qair (dilution level), Uair (air velocity), UCH4 (methane velocity)) revealing a competitive effect between aerodynamics and dilution. Generic diagrams of flame liftoff and extinction are proposed for all the diluents. Physical quantities related to flame stabilization process are all submitted to, regardless of diluent, self-similar laws. This is explained by flame burning velocity which is considered as the key element in the flame stabilization mechanism with air-side dilution.ROUEN-INSA Madrillet (765752301) / SudocSudocFranceF

Experimental study of the lifting characteristics of the leading-edge of an attached non-premixed jet-flame: Air-side or fuel-side dilution

WOS:000382513000025International audienceThe impact of air-side and methane-side dilution (CO2, N-2 and Ar) on the lifting process of attached non-premixed methane air coaxial jet flames is studied over a wide range of aerodynamic conditions. The study of the competition between aerodynamics and dilution has allowed to discriminate and quan. tify the different phenomena involved in the lifting process. First, two effects, only dependent on the amount of the added diluent, contribute to promoting flame detachment (liftoff): a fluid mechanical effect that causes the bulk velocity of the reactants to increase; a mixing effect that changes the mixture fraction spatial distribution. They are significant for the methane-side dilution but negligible for the air side dilution. Then, the main mechanism and the dimensionless numbers characterizing flame liftoff are identified. The attached-flame stability is analyzed on the basis of the lifting limits measured by the critical flow rate ratios, (Q(d)/Q(f))(lift) and (Q(d)/Q(ox))(lift) when the diluent is added either to the methane or the air stream. These limits follow self-similarity relationships based on the fuel and oxidant Peclet numbers of the diluted streams, satisfied whatever the diluents. Results using PIV and CH* measurements are interpreted through a flame-leading-edge approach, where CH4/air/diluent are mixed locally at the flame base. The flame propagation velocity St, which balances the incoming gas velocity, is shown to be described by self-similarity relationships based on the molar fraction at the leading-edge reduced by values at liftoff, X-d/X-ilft(d). To confirm the leading-edge propagation characteristics, the flame attachment height Ha and radius R-a are investigated at the attached-flame base. R-a is representative of the mixing and mass effects induced by the pure dilution. Contrary to R-a, H-a is piloted by S-L, and evolves according to a unique law dictated by X-d/X-lift(d). X-lift(d) is a self-similar parameter highlighting the propagation nature of the leading edge. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Equations de bilan d'interface pour les flammes de prémélangeC.R. Acad. Sc., t. 304, II, n° 17, pp. 1047-1050.

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Equations de bilan d'interface pour les flammes de prémélangeC.R. Acad. Sc., t. 304, II, n° 17, pp. 1047-1050.

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Etude de l atomisation d un jet liquide assisté par air soumis à une onde acoustique stationnaire transverse haute fréquence

Les instabilités haute fréquence dans les moteurs fusées impliquent un couplage entre l acoustique du foyer et la combustion. Cette étude porte sur l influence d un champ traverse sur le processus d atomisation d un jet liquide assisté par air. Un injecteur coaxial est placé à un ventre de pression (VP) ou de vitesse (VV) d une onde plane stationnaire limitée à des fluctuations de 3600 Pa. En VP, l onde impose à l écoulement gazeux annulaire une modulation pouvant conduire à l émission d intenses vortex, permettant au jet liquide de développer une instabilité précoce de type cisaillement. En VV, des effets non linéaires de pression de radiation provoquent l aplatissement en nappe du corps continu liquide du jet. Un critère, basé sur un nombre de Bond de radiation, détermine son apparition. Elle s atomise sous l action d instabilités intrinsèques de nappe, de Faraday et de rupture de membranes. Enfin, l acoustique impose à certaines gouttes de rester ou revenir au ventre de vitesse.High frequency combustion instabilities ( few kHz) in rocket engines imply a coupling between chamber acoustics and combustion. This study debates on the influence of a transverse acoustic field on the atomization of an air-assisted jet. A coaxial injector is placed at a pressure anti-node (PA) or velocity anti-node (VA) of a stationary plane acoustic wave with maximum fluctuation of 3600 Pa. At hte Pa, the acoustic wave modulates the annular gaseous flow and can induce the emission of intense vortices, which leads to a early shear-stress instability. Placed at a VA, non linear effects due to radiation pressure flattened the jet under the form of a sheet. A criterion, established from a radiation acoustic Bond number, determines its apparition. The sheet is atomised under the action of intrinsic sheet instabilities, Faraday instability and break-up of membranes. Acoustic effects organize the spray: amass and low speed droplets stay or go to the velocity anti-node.ROUEN-BU Sciences (764512102) / SudocROUEN-BU Sciences Madrillet (765752101) / SudocSudocFranceF

Lois de comportement d'interface (Actes).

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Stabilisation d'une flamme non-prémélangée dans un écoulement de jets coaxiaux (effets d'un champ acoustique)

Ce travail s'intéresse aux phénomènes instationnaires en combustion dont la maîtrise est un point clé dans le fonctionnement efficace des foyers et des moteurs. Les limites de stabilité et les mécanismes de stabilisation sont analysés pour des flammes de jets coaxiaux (méthane/air) non-prémélangés, accrochées au brûleur ou suspendues. La transition entre flamme accrochée et suspendue est caractérisée à l'aide d'un critère thermique original pour les deux processus de décrochage identifiés au cours desquels la base de flamme passe d'une extrémité essentiellement propagative à une essentiellement diffusive. Les propriétés de la flamme, suspendue en zone d'hystérésis ou de lift pur, et celles des structures tourbillonnaires du jet, détectées par une technique d'identification automatique, sont quantifiées par des diagnostics d'imagerie et de vélocimétrie. L'organisation de la couche de mélange méthane/air régit la réponse de la flamme qui passe d'une forme laminarisée à base lobée à une forme turbulente lorsque la vitesse de l'air (Uo) croît. La flamme est stabilisée par la dynamique des tourbillons contra-rotatifs issus des instabilités secondaires, initialement influencés par les vortex de Kelvin-Helmholtz. Elle s'adapte aux conditions imposées par Uo dont le rapport avec la vitesse de flamme laminaire (Sl) pilote son comportement laminarisé ou turbulent. Dans les foyers, les instabilités de combustion sont accrues par les ondes acoustiques créant divers modes en interaction avec les modes propres de l'écoulement. L'expérience est donc ajustée pour étudier la flamme soumise à un forçage sinusoïdal du méthane. Ses réponses sont discriminées en fonction des fréquences et amplitudes du forçage; augmenter Uo déplace les limites des zones identifiées, certaines pouvant même disparaître. Quel que soit Uo, forcé au voisinage : - de sa fréquence naturelle (1200 Hz), le jet présente une organisation accrue ; - de sa première surharmonique (2600 Hz), le jet se caractérise par l'apparition de modes en interaction non linéaire. Pour ces deux hautes fréquences, le forçage conduit à un mélange plus efficace, donc à une réduction de la hauteur de suspension de flamme (Hl) pour les faibles Uo. Mais cet effet est freiné avec Uo croissant, voire inversé à 2600 Hz pour Uo élevé. Aux moyennes fréquences (200 Hz), le comportement est unifié vers une flamme turbulente dont la hauteur pivote autour de celle obtenue pour Uo ~ Sl (Hl croît (diminue) pour Uo faible (élevé)). Pour toute condition (Uo, fréquence, amplitude), les mécanismes d'interaction de modes régissent la réponse de flamme à travers les structures tourbillonnaires.This work focuses on unsteady combustion phenomena whose control is a key point in the efficient operation of engines and furnaces. Stability limits and stabilization mechanisms are analyzed for flames of non-premixed coaxial jets (methane/air), anchored or lifted above the burner. The transition from attachment to liftoff is characterized by an original thermal criterion for both identified lifting processes during which the flame base passes from a mainly propagative extremity to a mainly diffusive one. The properties of the flame, lifted either in its hysteresis zone or in the liftoff zone, and those of vortical structures of the jet detected by an automatic identification technique, are quantified by imagery and velocimetry diagnostics. The organization of the methane/air mixing layer governs the flame response from a laminarized aspect with a lobed base to a turbulent one when the air velocity (Uo) is increased. The flame is stabilized by the dynamics of the counter-rotating vortices issued from secondary instabilities, influenced when they are formed by the Kelvin-Helmholtz vortices. The flame adapts to conditions imposed by Uo whose ratio with the laminar flame speed (Sl) pilots its laminarized or turbulent behavior. Inside the chambers, combustion instabilities are increased by acoustic waves creating various modes in interaction with the own modes of the stream. So, our experiment is adjusted to study the flame submitted to a sinewave forcing of the methane. Its responses are discriminated according to the forcing frequencies and amplitudes; increasing Uo shifts the limits of the identified zones, even some of them can disappear. For all Uo, forced : - near its natural frequency (1200 Hz), the jet is more ordered ; - near its first harmonic frequency (2600 Hz), the jet shows several non linearly interacting modes. For both high frequencies, forcing contributes to a better mixing, and so to a reduction of the liftoff height (Hl) for small Uo. But, this effect is weakened with Uo increase, or even reversed at 2600 Hz for high Uo. For medium frequencies (200 Hz), the behavior tends to a turbulent flame whose liftoff height pivots around a data obtained for Uo ~ Sl (Hl grows (reduces) for small (high) Uo). For any condition (Uo, frequency, amplitude), the mode interaction mechanisms govern the flame response through the vortical structures.ROUEN-BU Sciences Madrillet (765752101) / SudocSudocFranceF