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Instabilité de flapping : origine et effets sur la structure et le spray d'un jet atomisé

Abstract : Jet or sheet atomized by a fast coaxial gas jet is currently used in industry, like aeronautical propulsion (turbofan) or spatial propulsion (cryotechnic rocket engine). Many physical processes allows liquid coherent structure fragmentation into drops. Stripping, which appears downstream near injector, has been largely studied (Marmottant et Villermaux 2004, Hong & al 2004), mecanisms has been correctly described.However, the origin of large scale - or 'flapping' instabilities - intervening further downstream, instabilities that are causing the production of large drops, remains poorly understood. This is particularly true for cylindrical jets which, unlike the case of sheets, have been the subject of very few studies. We are therefore committed to understand the origin of the "flapping", to analyze its relationship with interfacial shear instabilities, and to quantify its impact on the structure of the jet as well as on the drops produced. For this, experiments were carried out in water/air on wide set of parameters, both in terms of phasic speed than the dimensions of the gas gap and liquid diameter. Special care were made to the internal flow control.For all the geometries, we showed that the length of the liquid cone is driven by the large scale displacements and not by the stripping process. Furthermore, the length of brokenness jet presents a decline marked with the gas speed, then remains constant beyond a critical gas speed. A model was proposed for this asymptotic behavior in which the break-up length is driven by the report of the liquid injection speed to a capillary speed built on the liquid diameter.Measurement of the frequency of large scale displacement technology has been implemented from images acquired by shadowgraphy proved operational over the gas velocity range considered. This frequency, which varies not spatially, present two behaviors: a first where it increases with the speed of the gas, and a second where it remains independent of the gas speed. This second scheme is not mentioned in the literature. For the original plan, the link between flapping and shear instability has been demonstrated based on analyses of stability. The associated Strouhal number is controlled by the shear gas side. The dependence of the frequency of heartbeat to the thickness of vorticity gas side is thus established when shear instability is driven by an inviscide mechanism. For the second scheme, the opportunistic nature of the flapping has been demonstrated using forcing experience: the flapping amplifies liquid structures of wavelength greater than those associated with shear instability. A Strouhal number built on liquid jet diameter and the speed of the liquid jet at break distance has been proposed. Finally, the ratio of the diameter of the liquid jet at the wavelength of the shear instability seems relevant to define the border between these two regimes.Sizes drops produced on the symmetry axis were measured using an optical probe. It appears that granulometric distribution is evolving strongly with speed gas, and it is multi-modal, reflecting the presence of several mechanisms of brokenness. The average size of the drops decreases overall as UG - 2, in the limit of strong numbers of aerodynamic Weber. This medium size is also very sensitive to geometry: it decreases when the thickness of the gas increases until it reaches a floor value, and it grows with the liquid diameter. Finally, by forcing large amplitude lateral displacement, the average radial distribution of sizes of drops has been made much more homogeneous, and the average size of the drops on the axis has been reduced by a factor of 2. These results therefore open opportunities in terms of control of atomization.
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Antoine Delon. Instabilité de flapping : origine et effets sur la structure et le spray d'un jet atomisé. Mécanique des fluides [physics.class-ph]. Université Grenoble Alpes, 2016. Français. ⟨NNT : 2016GREAI074⟩. ⟨tel-01689732⟩

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