Analysis of ligamentary atomization of highly perturbed liquid sheets

International audienceLigamentary structures are often encountered in liquid atomization processes. They appear for instance during the atomization of liquid sheets issuing from triple-disk nozzles. Because of the development of turbulence along the internal wall of the discharge orifice, these sheets are highly perturbed and exhibit the formation of ligaments along their sides. The present investigation addresses the question of the origin of size dispersion of the droplets emanating from the atomization of these ligaments. The adopted strategy consists in describing the atomization process reported by experimental images by using a multi-scale tool and in conducting an analysis that concentrates on the dynamic of the small structures carried by the ligaments that might be the main small droplets providers. The advantages of the tool are its ease of application as well as its capacity to bring an information that incorporate the shape of the analyzed interface. The statistical scale analysis demonstrates that the small structures carried by the ligaments are subject to an elongation mechanism whose strength varies from one liquid to another. This mechanism is not dominant in the production of the small drops from the breakup of the ligaments. The dispersion of the drop size distribution, represented by the order of a Gamma distribution, is found to strongly correlate with the initial deformation of the ligaments. An interesting result here is the ease of characterizing the average ligament deformation with the concept of scale distribution. The fact that a single parameter is sufficient to represent the drop size dispersion and that this dispersion correlates with the ligaments deformation suggests the dominance of a single mechanism in the ligament breakup, i.e., the capillary mechanism. As demonstrated in a previous investigation, capillary mechanism is associated with elongation mechanism at small scales. All these results are strengthened by the fact that the explored experimental conditions are uncorrelated

A New Theoretical Framework for Characterizing the Transport of Liquid in Turbulent Two-Phase Flows

International audienceWhen a liquid stream is injected into a gaseous atmosphere, it destabilizes and continuously passes through different states characterized by different morphologies. Throughout this process, the flow dynamics may be different depending on the region of the flow and the scales of the involved liquid structures. Exploring this multi-scale, multi-dimensional phenomenon requires some new theoretical tools, some of which need yet to be elaborated. In the present study, an innovative general framework is established by transposing the machinery of two-point statistical analysis to a relevant metric of liquid-gas flows (the liquid volume fraction). This allows distinguishing the transport of liquid which occurs in geometrical space (i.e. from one position in the flow to the other) and the one occurring in scale space (e.g. from large to small scales). These equations are exact and do not rely on any particular assumptions. The notion of scale is explicit and unambiguously defined. They further apply to the entire flow field, from the injection to the spray dispersion zone and irrespectively of the flow configuration or regime. This new set of equations is here invoked to characterize the air-assisted atomization of a planar liquid layer simulated by means of Direct Numerical Simulation using the ARCHER code

Behaviour of free falling viscoelastic liquid jets

[EN] In a recent work, a protocol to measure the relaxation time of dilute polymer solutions, known to be challenging, has been established [1]. This protocol is based on a 2D multi-scale description of free-falling low velocity viscoelastic liquid jets. Although the relaxation time reached an asymptotic value for high jet velocities, a significant dependence with the jet velocity is observed for low velocities. The present work reconsiders these previous experimental data using a 3D multi-scale analysis in order to identify the origin of the dependence between the relaxation time and the jet velocity. The 3D analysis demonstrates the importance of a velocity–dependent coalescence mechanism in the jet behaviour. Thanks to a simple model of jet deformation it is demonstrated that this coalescence mechanism prevents the elasto-capillary contraction of the smallest scales from occurring when the jet velocity is reduced.The authors acknowledge the financial support from the Frend National Research Agency (ANR) through the program Investissement d’Avenir (ANR-10 LABX-09-01), LABEX EMC3Tirel, C.; Renoult, M.; Dumouchel, C.; Blaisot, J. (2017). Behaviour of free falling viscoelastic liquid jets. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 241-248. https://doi.org/10.4995/ILASS2017.2017.4700OCS24124

Entropy-Based Cavitation and Primary Atomization Analysis with a 2D Transparent Injector

International audienceA transparent scale-up injector with asymmetric incoming flow direction was designed to promote cavitation on mainly one-side of the orifice. This orifice has a rectangular cross-section that provides straightforward optical access to the internal flow. This geometry was designed to study the role of the internal flow, and particularly of the development of cavitation, on the modification of the primary atomization process occurring as soon as the liquid emanates from the injector. The internal flow is classified into four regimes based on the extent of the cavitation zone; 1-no-cavitation, 2-developing cavitation, 3-super cavitation and 4-semi-hydraulic flip cavitation. Image series of 500 images was recorded for different flow rates belonging to regimes 2-4. Image segmentation is applied to each individual image to identify liquid and vapour phase regions. Based on these segmented images, the mean and rms values of the cavitation extent are determined for each cavitating regime. Furthermore, a more detailed statistical analysis of the cavitation is obtained with the computation of an entropy image, bringing indication on interface between vapour and liquid and on the shed cavitation bubbles. The liquid jet fragmentation is qualified from the entropy analysis also. The primary atomization is associated with the region in the image where the liquid core is fragmented in detached ligaments and drops. This region is easily identified from the entropy analysis and the primary atomization is quantified through the computation of the area of this region. In the presence of cavitation, the primary atomization process is altered. The way liquid fragmentation is modified by cavitation is shown to be correlated to the extent of the cavitation zone in the orifice. In this paper we give a quantification of these modifications, clearly visible to the naked eye on the images

Multi-scale analysis of a viscoelastic liquid jet

WOS:000403628200001International audienceA multi-scale analyzing tool is now available to investigate the temporal evolution of two phase flows such as liquid systems experiencing an atomization process. Thanks to its multi-scale and global nature, it allows identifying all dynamics simultaneously involved in the process with no restriction of the liquid system shape. In the present work this multi-scale tool is applied on 2D visualizations of free falling jets of a low-viscosity viscoelastic solution. The jets are produced from a cylindrical discharge orifice and the liquid is a very dilute polymer solution containing 5 ppm of Poly(ethylene oxide). High spatial resolution images of the free falling jets are performed as a function of the velocity and at several distances from the discharge orifice. For every operating condition, the liquid jet remains cylindrical first, then shows the development of a sinusoidal perturbation and finally adopts a beads-on-a-string pattern before breakup occurs. The multi-scale analysis is performed on a high number of images and at several spatial positions in order to return statistical and temporal information, respectively. The results of this analysis show that during the sinusoidal perturbation stage, the large-scale region follows an exponential increase as predicted by the linear stability theory and during the beads-on-a-string stage, the small-scale region follows an exponential decrease similar to an elasto-capillary regime from which the relaxation time of the polymer solution can be extracted. This work positions the multi-scale approach as a promising and complementary tool to the currently used techniques in order to probe complex liquid rheology, especially in the case of mobile viscoelastic solutions

Liquid transport in scale space

When a liquid stream is injected into a gaseous atmosphere, it destabilizes and continuously passes through different states characterized by different morphologies. Throughout this process, the flow dynamics may be different depending on the region of the flow and the scales of the involved liquid structures. Exploring this multi-scale, multi-dimensional phenomenon requires some new theoretical tools, some of which need yet to be elaborated. Here, a new analytical framework is proposed on the basis of two-point statistical equations of the liquid volume fraction. This tool, which originates from single phase turbulence, allows notably to decompose the fluxes of liquid in flow-position space and scale space. Direct Numerical Simulations of liquid-gas turbulence decaying in a triply periodic domain are then used to characterize the time and scale evolution of the liquid volume fraction. It is emphasized that two-point statistics of the liquid volume fraction depend explicitly on the geometrical properties of the liquid-gas interface and in particular its surface density. The stretch rate of the liquid-gas interface is further shown to be the equivalent for the liquid volume fraction (a non diffusive scalar) of the scalar dissipation rate. Finally, a decomposition of the transport of liquid in scale space highlights that non-local interactions between non adjacent scales play a significant role

Where does the drop size distribution come from?

[EN] This study employs DNS of two-phase flows to enhance primary atomization understanding and modelling to be used in numerical simulation in RANS or LES framework. In particular, the work has been aimed at improving the information on the liquid-gas interface evolution available inside the Eulerian-Lagrangian Spray Atomization (ELSA) framework. Even though this approach has been successful to describe the complete liquid atomization process from the primary region to the dilute spray, major improvements are expected on the establishment of the drop size distribution (DSD). Indeed, the DSD is easily defined once the spray is formed, but its appearance and even the mathematical framework to describe its creation during the initial breakup of the continuous liquid phase in a set of individual liquid parcels is missing. This is the main aim of the present work to review proposals to achieve a continuous description of the DSD formation during the atomization process. The attention is here focused on the extraction from DNS data of the behaviour of geometrical variable of the liquidgas interface, such as the mean and Gauss surface curvatures. A DNS database on curvature evolution has been generated. A Rayleigh-Plateau instability along a column of liquid is considered to analyse and to verify the capabilities of the code in correctly predicting the curvature distribution. A statistical analysis on the curvatures data, in terms of probability density function, was performed in order to determine the physical parameters that control the curvatures on this test case. Two different methods are presented to compute the curvature distribution and in addition, the probability to be at a given distance of the interface is studied. This approach finally links the new tools proposed to follow the formation of the spray with the pioneering work done on scale distribution analysis.Canu, R.; Dumouchel, C.; Duret, B.; Essadki, M.; Massot, M.; Ménard, T.; Puggelli, S.... (2017). Where does the drop size distribution come from?. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 605-612. https://doi.org/10.4995/ILASS2017.2017.4706OCS60561

Droplet oscillations in a turbulent flow

The oscillations of an initially unperturbed spherical droplet immersed in a homogeneous and isotropic turbulent background flow are investigated through spherical harmonic decomposition. As suggested in the literature, the shape oscillations under turbulent conditions are related to the frequency of droplets oscillating in a fluid without background flow. A series of direct numerical simulations (DNS) of droplets with single deformation modes in a fluid at rest are first performed. The frequency and damping rate are compared with weakly viscous linear theory. Then, a database of 220 droplets deformed under turbulent conditions for a single Weber and Reynolds number is generated with an identical numerical set-up. Each spherical harmonic coefficient shows an oscillatory motion with comparable frequency to the single deformation mode simulations. The power spectrum of the coefficients provides the amount of surface of each mode. After a transient regime, the surface area reaches a stationary saturation level. The saturation level of each mode is linked to the turbulence and the energy stored at the interface. Droplets after a high deformation are studied with and without background flow. As expected, the physics of relaxation is driven by capillary forces

A New Formulation of the Maximum Entropy Formalism to Model Liquid Spray Drop-Size Distribution

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