Experimental exploration of membranes in liquid jet atomization
CNRS UMR 6614 – CORIA, Site Universitaire du Madrillet, 76801 Saint Etienne du Rouvray, TASC Department (Turbulence, Atomization& Sprays, Chaos), Atomization & Spray Group
Multiphase flows, Primary atomization, image analysis, interfaces.
Oct. 2018- Oct. 2021
Around 1400€ net
The objective of this PhD thesis is to characterize the dynamics of liquid jet atomization in which membrane fragmentation occurs. To this end, an experimental work will be undertaken with the aim of better quantifying the geometrical properties of liquid membrane when visualized by backlight illumination. A second task of the thesis will be devoted to the characterization of the physical mechanisms at play through a scale-by-scale analysis of the fragmentation of the membranes, contraction into ligaments and formation of droplets.
The characteristics of liquid-dispersed two-phase flows condition the evolution of many natural phenomena and the efficiency of a wide variety of industrial applications. A simple observation of such two-phase flows generally reveals the early formation of wrinkled thin liquid sheets or membranes which perforate and contract in ligaments and droplets. This scenario is notably encountered in liquid flows with large transversal momentum as produced by e.g. triple-disc-geometry injectors [1, 2] or in air-assisted atomization processes (e.g. coaxial jets). Liquid membranes can also be promoted in the presence of cavitation or by adding polymers to the liquid phase. Consequently, the processes related to the liquid sheets appear as the initial condition for the creation of ligaments and for the resulting drop size distribution. In the last few years, the Atomization & Spray group at CORIA laboratory has focused mainly on the ultimate step of the atomization processes, i.e. the stretching of ligaments and formation of droplets. To complete the picture, the objective of this PhD thesis is to shed light onto the prior step, providing quantitative data and deep analysis of the physical mechanisms at play during 1/ the liquid-jet injection and destabilization, 2/ the formation of liquid-sheets and membranes 3/ their perforation and 4/ contraction into ligaments. On the basis of our research team expertise, the applicant will first develop some experimental diagnostics for the morphological analysis of liquid structures and will propose pragmatic solutions to help distinguishing thin liquid membranes in backlight images. Experimental images will then be explored in the light of recent scale-by-scale analysis with the aim of highlighting and characterizing the multi-scale features of the fragmentation of liquid-sheets and formation of ligaments. The main difficulty when visualizing liquid sheet is due the refraction and diffraction of light interacting with liquid structures. Generally, the liquid (gas) phase on backlight illumination images is associated with low (high) gray scale levels . However, a thin liquid sheet is a transparent medium which leads to gray levels similar to those of the background. Consequently, the distinction between the gas and the liquid phase cannot be carried out on the basis of the gray level solely and another information is needed for the segmentation of images. One solution then consists in calculating the local textural (e.g. spectral) content of the images in order to discriminate between the light transmitted directly to the camera (which have very low textural content) to the one which has travelled through the liquid (which have a larger textural content). The applicability of such corrections will be validated by comparing corrected images to that inferred from the ballistic imaging technique which allow optically separating ballistic photons from that which have passed through the liquid . Two flow configurations will be studied, a wrinkled liquid-sheet as formed by two obliquely impacting jets  and by the triple disc injector .
An ensuing task for the PhD work will be devoted to the characterization of the fragmentation scenario of such liquid sheets and membranes. In this context, one particular objective is to describe the multiscale content of the liquid sheet fragmentation and contraction and of the ligament formation processes. This will be undertaken on the basis of observables such as the surface based scale distribution  or the volume-of-fluid spatial increments by identifying the predominant terms in their respective transport equations.
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 Blaisot, J.B. & Yon, J. (2005). Exp. Fluids, 39, pp. 977–994
 Idlahcen, S., Rozé, C., Méès, L., Girasole, T., & Blaisot, J. B. (2012). Exp. Fluids, 52(2), 289-298
 Bremond, N., Clanet, C., & Villermaux, E. (2007). Journ. Fluid Mech., 585, pp. 421-456.
Funding for the PhD thesis is conditioned by the quality of the applicant. The candidate’s experience must align with some of the project thematic fields, namely optics, multiphase flows or image processing. A strong background in fluid mechanics is required and knowledge of C and python coding is mandatory. A master degree or in progress is required. Applicants will send a CV and a motivation letter.
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