Where does the drop size distribution come from?

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...

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.

We have investigated the atomisation of a thick liquid film by high speed co-flowing gas jet. The study is focused on the mechanisms controlling the drop spatial distribution downstream injection. These results are needed for properly initializing two-fluid euler-euler [1] or lagrangian [2] simulations. Measurements of the drop characteristics (concentration, velocity, size),available downstream injection, indicate that the spatial dispersion of the drops is related to their size, the smallest drops being more widely spread than the larger ones. However, the standard dispersion model based on turbulent dispersion is unable to describe such a behaviour up to a downtream distance about 6 Hg. According to the experimental results presented, the drawback may originate from the initial ejection conditions. Another origin could be a change in the gas structure (jet expansion, turbulent intensity) at short distances. The relatives contributions of these mechanisms needs to be quantified.

Progress in Energy and Combustion Science, 2010

This review attempts to summarize the physical models and advanced methods used in computational studies of gasÀliquid two-phase jet flows encountered in atomization and spray processes. In traditional computational fluid dynamics (CFD) based on Reynolds-averaged NavierÀStokes (RANS) approach, physical modelling of atomization and sprays is an essential part of the two-phase flow computation. In more advanced CFD such as direct numerical simulation (DNS) and large-eddy simulation (LES), physical modelling of atomization and sprays is still inevitable. For multiphase flows, there is no model-free DNS since the interactions between different phases need to be modelled. DNS of multiphase flows based on the one-fluid formalism coupled with interface tracking algorithms seems to be a promising way forward, due to the advantageous lower costs compared with a multi-fluid approach. In LES of gasÀliquid two-phase jet flows, subgrid-scale (SGS) models for complex multiphase flows are very immature. There is a lack of well-established SGS models to account for the interactions between the different phases. In this paper, physical modelling of atomization and sprays in the context of CFD is reviewed with modelling assumptions and limitations discussed. In addition, numerical methods used in advanced CFD of atomization and sprays are discussed, including high-order numerical schemes. Other relevant issues of modelling and simulation of atomization and sprays such as nozzle internal flow, dense spray, and multiscale modelling are also briefly reviewed.

SAE Technical Paper Series, 2011

During the last fifteen years Computational Fluid Dynamics (CFD) has become one of the most important tools to both understand and improve the Diesel spray development in Internal Combustion Engine (ICE). Most of the approaches and models used pure Eulerian or Lagrangian descriptions to simulate the spray behavior. However, each one of them has both advantages and disadvantages in different regions of the spray, it can be the dense zone or the downstream dilute zone. One of the most promising techniques, which has been in development since ten years ago, is the Eulerian-Lagrangian Spray Atomization (ELSA) model. This is an integrated model for capturing the whole spray evolution, including primary break-up and secondary atomization. In this paper, the ELSA numerical modeling of Diesel sprays implementation in Star-CD (2010) is studied, and simulated in comparison with the Diesel spray which has been experimentally studied in our institute, CMT-Motores Térmicos. Since many of the most important characteristics of the spray development, as the penetration or the axial velocity, can be captured using 2D simulations, in this preliminary validation of ELSA model only two-dimensional simulations have been performed. Moreover, the main objective of the paper is to: firstly, obtain mesh independency for further analysis and secondly, improve the classic k-ε RANS model for ELSA model. Apart from this, several characteristics of the spray as can be the droplet formation of the liquid penetration are also showed.

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.

International Conference on Liquid Atomization and Spray Systems (ICLASS)

Motivated by industrial applications, the spray assessment of twin-fluid atomizers is paramount for improving their design and performance. Although several studies have addressed the spray characteristics, the coupled analysis of the droplet sizes and velocities at high flow rates is still not sufficiently understood. Therefore, the present study investigates the spray instabilities from a specific variance of a Y-jet atomizer correlated to droplet size and axial velocity distribution along the spray centerline. The atomizer was operated at Reynolds numbers in the order of 10 4 , resulting in different air-to-liquid mass ratios. For that, an experimental rig operated with air and water is available for spray analysis. Data obtained with a phase Doppler anemometer showed that the mass flow rate of both fluids is directly proportional to the velocities and inversely proportional to droplet diameters. The size-velocity correlations showed that closer to the spray, the smaller droplets had higher velocities than the bigger ones. As they flowed downstream the nozzle, the impingement between the droplets and their interaction with the surrounding air decelerated them and increased their size.

Calculations of a transient atomization process are presented, which simulates fuel injection of sprays in gasoline direct injection engines. Only non-reacting sprays are considered with the focus on the atomization process. The FIRE code, developed by AVL, is used as the platform to test three different atomization models: (i) Taylor Analogy Breakup (TAB) model; (ii) surface wave instability (WAVE) model; and the more recent (iii) FIPA (Fractionnement Induit Par Acceleration) model. Comparisons of calculations with experimental data reveal significant discrepancies regardless of the atomization model used. It is acknowledged that, in this study, only the standard model constants are adopted and that may be further optimised to improve the calculations. However, the fact remains that all the atomization models start with an initial distribution of spherical droplets at the injector tip. An assumption that is not supported by recent measurements which show that fluid elements rather than spherical droplets dominate this early zone.

International Journal of Multiphase Flow, 2018

Traditional Discrete Particle Methods (DPM) such as the Euler-Lagrange approaches for modeling atomization, even if widely used in technical literature, are not suitable in the near injector region. Indeed, the first step of atomization process is to separate the continuous liquid phase in a set of individual liquid parcels, the so-called primary break-up. Describing two-phase flow by DPM is to define a carrier phase and a discrete phase, hence they cannot be used for primary breakup. On the other hand, full scale simulations (direct simulation of the dynamic DNS, and interface capturing method ICM) are powerful numerical tools to study atomization, however, computational costs limit their application to academic cases for understanding and complementing partial experimental data. In an industrial environment, models that are computationally less demanding and still accurate enough are required to meet new challenges of fuel consumption and pollutant reduction. Application of DNS-ICM methods without fairly enough resolution to solve all length scales are currently used for industrial purposes. Nevertheless, effects of unresolved scales are generally cast aside. The Euler-Lagrange Spray Atomization model family (namely, ELSA, also called, Σ − Y or Ω − Y) developed by Vallet and Borghi pioneering work [1], and [2], at the contrary aims to model those unresolved scales. This approach is actually complementary to DNS-ICM method since the importance of the unresolved term depends directly on mesh resolution. For full interface

1987

A multi-dimensional computer model is used to study atomization and vaporization of a liquid jet injected from a round hole into a compressed gas. Atomization is described using a new method whereby 'blobs' are injected (with sizes equal to the nozzle exit diameter), and breakup of the blobs and the resulting drops is modeled using a stability analysis for liquid jets. This method can also predict various regimes of breakup which result from the action of different combinations of liquid inertia, surface tension and aerodynamic forces on the jet. The product drops are distinguished from the parent drop by having different dropsizes (previous drop breakup models have lumped the parent and product drops together). This has a significant effect on the fuel vapor distribution in a high-pressure spray because the small product drops vaporize rapidly. Like existing models, the model accounts for drop collision and coalescence, and the effect of drops on the gas turbulence. These effects are important in high-pressure sprays where breakup of the liquid yields a core region near the nozzle containing large drops. Fuel vaporization in the core is found to depend strongly on the atomization details near the nozzle. Downstream of the core, fuel-air mixing is found to be determined by a competition between local drop breakup, coalescence and vaporization rates.

International Conference on Liquid Atomization and Spray Systems (ICLASS)

At high Reynolds and Weber numbers typical of diesel-like sprays, the finest turbulent structures are highly intermittent in nature, resulting in intense fluctuations in gas phase velocities and can significantly contribute to the atomization process. In order to account for their influence on breakup of spray droplets, we introduce a new stochastic breakup model to be used in conjunction with large eddy simulation (LES) for the gaseous flow. The model is based on a stochastic parent-to-child relaxation of droplets, whose parameters are linked to the viscous dissipation rate on residual scales "seen" by a droplet along its trajectory. In order to introduce the intermittency effects on the droplet breakup, this dissipation rate is simulated stochastically, in the framework of log-normal process. The non-reacting "Spray-A" experiment from Engine Combustion Network (ECN) is used to assess the performance of the new stochastic breakup model in comparison to the standard hybrid KH-RT breakup model in terms of evolution of liquid penetration length and droplet size statistics. The results clearly show that in comparison to the hybrid KH-RT model the stochastic breakup model gives a more accurate prediction of different parameters with relatively less sensitivity to the grid resolution.

International Journal of Multiphase Flow, 2015

International Conference on Liquid Atomization and Spray Systems (ICLASS)

Understanding the process of primary and secondary atomization in liquid jets is crucial in describing spray distribution and droplet geometry for industrial applications and is essential in the development of physics-based low-fidelity atomization models. Significant advances in numerical modelling and computational resources allows research groups to conduct detailed numerical simulations of these flows. However, the large size of the datasets produced by highfidelity simulations limit researchers' ability to analyze them. Consequently, the process of a coherent liquid core breaking into droplets has not been analyzed in simulation results even though a complete description of the jet dynamics exists. The present work applies a droplet physics extraction technique to high-fidelity simulations to track breakup events and data associated with the local flow. The data on the atomization process is stored in a Neo4j graphical database providing an easily accessible format. Results will provide a robust, quantitative description of the process of atomization and the details on the local flow field will be useful in the development of low-fidelity atomization models.

Atomization and Sprays, 2013

Atomization involves complex physical processes and gas-liquid interaction. Primary atomization on diesel spray is not well understood due to the difficulties to perform experimental measurements in the near nozzle field. Hence computational fluid dynamics (CFD) has been used as a key element to understand and improve diesel spray. A recent new code for incompressible multiphase flow with adaptive octree mesh refinement has been used to perform simulations of atomization at low injection pressure conditions. The multiphase flow strategy to manage different flows is the volume of fluid (VOF) method. The adaptive mesh allows to locally refine the mesh at each time step where a better resolution is needed to capture important gradients instead of using a static mesh with a fixed and high number of cells which, in turn, would lead to an unaffordable computational cost. Even with this approach, the cell number is very high to achieve a Direct Numerical Simulation (DNS) at reasonable computational cost. To reduce the computational cost, an idea has been explored, the possibility of setting a maximum number of cells of the domain. Following this idea, the code has been tested with different configurations to understand their effects on numerical stability, the change in different spray parameters and the benefits achieved in terms of execution time. The outcomes have been validated against a theoretical model.

SAE International Journal of Fuels and Lubricants, 2018

In order to improve understanding of the primary atomization process for diesel-like sprays, a collaborative experimental and computational study was focused on the near-nozzle spray structure for the Engine Combustion Network Spray D single-hole injector. These results were presented at the 5th Workshop of the Engine Combustion Network in Detroit, Michigan. Application of x-ray diagnostics to the Spray D standard cold condition enabled quantification of distributions of mass, phase interfacial area, and droplet size in the near-nozzle region from 0.1 to 14 mm from the nozzle exit. Using these data, several modeling frameworks, from Lagrangian-Eulerian to Eulerian-Eulerian and from Reynolds-Averaged Navier Stokes (RANS) to Direct Numerical Simulation (DNS), were assessed in their ability to capture and explain experimentally observed spray details. Due to its computational efficiency, the Lagrangian-Eulerian approach was able to provide spray predictions across a broad range of conditions. In general, this "engineering-level" simulation was able to reproduce the details of the droplet size distribution throughout the spray after calibration of the spray breakup model constants against the experimental data. Complementary to this approach, higher fidelity modeling techniques were able to provide detailed insight into the experimental trends. For example, interface-capturing multiphase simulations were able to capture the experimentally observed bi-modal behavior in the transverse interfacial area distributions in the near-nozzle region. Further analysis of the spray predictions suggests that peaks in the interfacial area distribution may coincide with regions of finely atomized droplets, whereas local minima may coincide with regions of continuous liquid structures. The results from this study highlight the potential of x-ray diagnostics to reveal salient details of the near-nozzle spray structure, and to guide improvements to existing primary atomization modeling approaches.