Where does the droplet size distribution come from?

2019

The purpose of the present article is to present a dynamic multi-scale approach for turbulent liquid jet atomization in dense flow (primary atomization), together with the possibility to recover Interface Capturing Method (ICM) / Direct Numerical Simulation (DNS) features for well resolved liquid-gas interface. A full ICM-DNS approach should give the best comparison with experimental data, but it is not industrially affordable for the time being, therefore models are mandatory. A numerical representation based on full ICM-DNS, for the initial destabilization of the complex turbulent liquid jet, going up to the spray formation, for which well established numerical models can be used, is appealing but has not yet been applied. Indeed such an approach requires the ICM-DNS to be applied up to the formation of each individual droplet. Hence, in many situation models have to be applied to the dense, unresolved and turbulent liquid-gas flow. To achieve this goal, the most important unresol...

Mathematical and Computer Modelling

The aim of this paper is the evaluation and validation of the Eulerian-Lagrangian Spray Atomization (ELSA) model implemented in a CFD code by Renault. ELSA is an integrated model for capturing the whole spray evolution, in particular including primary break-up and secondary atomization. Two-dimensional simulations have been performed during the study, which is in fact enough to capture some of the main features of the spray, such as the spray penetration and the axial velocity. A mesh independence study has also been carried out in order to characterize the lowest mesh size that can be used to correctly characterize the spray. Furthermore, the two-equation k-ε turbulence model has been adjusted by changing some of the parameters of the dissipation rate transport equation in order to accurately characterize the spray. Finally some analyses of the results obtained, in terms of penetration, liquid mass fraction and droplet number and size, are presented in the last section of the paper.

2008

It is well known that both combustion e±ciency in diesel engines, and the the quenching of ¯res, is conditioned by the surface to volume ratio of fuel/water droplets. This requires a deeper understanding of the droplet break- up process within liquid sprays. The break-up of individual droplets follows well known behaviour although how nearby droplets in the spray in°uence this process is not well understood. By numerically simulating the break-up behaviour of two equally sized droplets in two distinct geometrical con¯gurations it is shown that the break up of each droplet is strongly in°uenced by the presence of the other.

A detailed model for secondary atomization of liquid droplets by aerodynamic forces is presented. As an empirical extension of dynamic droplet deformation models, it accounts for temporal variations of the relative velocity between droplet and gas phase during the deformation and breakup process and describes the characteristic features of different breakup mechanisms (deformation kinetics, aerodynamics and product properties). Computed droplet trajectories and Sauter mean diameters have been compared using the proposed deformation-based model and a model using a breakup criterion based on the local instantaneous Weber number. It is concluded, that a deformation-based criterion should be used for droplets exposed to complex aerodynamic loading and that more experimental research is required to identify the accuracy of the presented detailed breakup product model. Breakup model Prior to the description of the breakup model a short introduction of the breakup process is given. The temporal evolution of droplet deformation and breakup is depicted in Figure 2 for impulsive aerodynamic loading. The

International Journal of Multiphase Flow, 2003

A stochastic subgrid model for large-eddy simulation of atomizing spray is developed. Following KolmogorovÕs concept of viewing solid particle-breakup as a discrete random process, atomization of liquid blobs at high relative liquid-to-gas velocity is considered in the framework of uncorrelated breakup events, independent of the initial droplet size. KolmogorovÕs discrete model of breakup is rewritten in the form of differential Fokker-Planck equation for the PDF of droplet radii. Along with the Lagrangian tracking of spray dynamics, the size and number density of the newly produced droplets is governed by the evolution of this PDF in the space of droplet-radius. The parameters of the model are obtained dynamically by relating them to the local Weber number with two-way coupling between the gas and liquid phases. Computations of spray are performed for the representative conditions encountered in idealized diesel and gas-turbine engine configurations. A broad spectrum of droplet sizes is obtained at each location with coexistence of large and small droplets. A novel numerical algorithm capable of simultaneously simulating individual droplets as well as a group of droplets with similar properties commonly known as parcels is proposed and compared with standard parcels-approach usually employed in the computations of multiphase flows. The present approach is shown to be computationally efficient and captures the complex fragmentary process of liquid atomization.

2017

A multi-scale approach to investigate liquid atomization processes is introduced. It describes the liquid system by the scale distribution whose determination is inspired from the Euclidean Distance Mapping used to measure the fractal dimension of a contour. The scale distribution is introduced in 2D and in 3D and applications from previous investigations are presented. The 2D applications are performed on experimental images and the 3D applications are performed on results obtained from Direct Numerical Simulation. The multi-scale analysis allows identifying and quantifying the mechanisms responsible for the interface evolution according to the scale. Among other results, the analyses presented here demonstrate the improvement of the atomization process when an elongation mechanism contributes to the thinning of the small structures. The multi-scale tool also provides new metrics that may be used to validate simulation results. An example of this is presented and discussed. Finally...

International Journal of Vehicle Systems Modelling and Testing, 2011

The aim of this paper is the evaluation and validation of the Eulerian-Lagrangian Spray Atomization (ELSA) model implemented in a CFD code by Renault. ELSA is an integrated model for capturing the whole spray evolution, in particular including primary break-up and secondary atomization. Two-dimensional simulations have been performed during the study, which is in fact enough to capture some of the main features of the spray, such as the spray penetration and the axial velocity. A mesh independence study has also been carried out in order to characterize the lowest mesh size that can be used to correctly characterize the spray. Furthermore, the two-equation k-ε turbulence model has been adjusted by changing some of the parameters of the dissipation rate transport equation in order to accurately characterize the spray. Finally some analyses of the results obtained, in terms of penetration, liquid mass fraction and droplet number and size, are presented in the last section of the paper.

Proceedings ILASS–Europe 2017. 28th Conference on Liquid Atomization and Spray Systems, 2017

Numerical simulations are often used to understand spray atomisation and estimate the size of the liquid fragments.Several techniques (Level Set, Volume of Fluid, Smooth Particle Hydrodynamics, among others) exist to compute multiphase flows and potentially represent liquid-break-up. However, the complexity of the breakup process and the wide range of scales prevents the use of an unified approach to simulate the complete spray. Numerical techniques face different challenges depending on the spray characteristics. The incorrect representation of surface forces in capillary dominated flows, creates large parasitic currents that distort and in some cases destroy the interface. Methods that perform well in the capillary regime aim to capture the interface directly and the surface radius cur- vature is therefore larger than the mesh size. However, this creates large constrains on the mesh resolution and limits its applications to low Weber number flows, when there is no extensive atomiz...

Proceedings ILASS–Europe 2017. 28th Conference on Liquid Atomization and Spray Systems

This study employs DNS of two-phase flows to enhance primary atomization understanding and modelling to beused 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 f...

Recent studies have shown that the assisted atomization of a liquid layer by a high speed gas is controlled by two successive instabilities: an axial Kelvin-Helmholtz type instability leading to the formation of waves, followed by a transverse Rayleigh-Taylor instability leading to the formation of ligaments that subsequently break into drops. A phenomenological model based on these instabilities has been developed for the mean drop size. In that paper, that model is confirmed by new experiments spanning a wider range of flow parameters. In addition, a specific behaviour at low gas velocities has been identified which explains why larger drops are produced in such conditions. Finally, a few investigations performed at a lower dynamic pressure ratio have clearly shown a modification of the axial instability that deserves to be further investigated.