An Evaluation of Atomization Models for Dense Sprays (original) (raw)
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Spray Atomization Models in Engine Applications, from Correlations to Direct Numerical Simulations
Oil & Gas Science and Technology – Revue d’IFP Energies nouvelles, 2011
Modèles de spray dans les applications moteur, des corrélations aux simulations numériques directes -Les sprays sont parmi les principaux facteurs de qualité, dans la formation du mélange et la combustion, dans un grand nombre de moteurs (à combustion interne). Ils sont de toute première importance dans la formation de polluants et l'efficacité énergétique, bien qu'une modélisation adéquate soit encore en développement. Pour un grand nombre d'applications, la validation et la calibration de ces modèles demeurent une question ouverte. Aussi, présentons-nous un aperçu des modèles existants et proposons quelques voies d'amélioration. Les modèles sont classés en nondimensionnels et dimensionnels allant de formules simples dédiées à des applications proches du temps réel à des descriptions détaillées des premiers stades de l'atomisation.
A computationally efficient spray model is presented for the simulation of transient vaporizing engine sprays. It is applied to simulate high-pressure fuel injections in a constant volume chamber and in mixture preparation experiments in a light-duty internal combustion engine. The model is based on the Lagrangian-Particle/Eulerian-Fluid approach, and an improved blob injection model is used that removes numerical dependency on the injected number of computational parcels. Atomization is modeled with the hybrid Kelvin-Helmholtz/Rayleigh-Taylor scheme, in combination with a drop drag model that includes Mach number and Knudsen number effects. A computationally efficient drop collision scheme is presented, tailored for large numbers of parcels, using a deterministic collision impact definition and kd-tree data search structure to perform radius-of-influence based, grid-independent collision probability estimations. A near-nozzle sub-grid scale flow-field representation is introduced to reduce numerical grid dependency, which uses a turbulent transient gas-jet model with a Stokes-Strouhal analogy assumption. An implicit coupling method was developed for the Arbitrary Lagrangian-Eulerian (ALE) turbulent flow solver. A multi-objective genetic algorithm was used to study the interactions of the various model constants, and to provide an optimal calibration. The optimal set showed similar values of the primary breakup constants as values used in the literature. However, different values were seen for the gas-jet model constants for accurate simulations of the initial spray transient. The results show that there is a direct correlation between the predicted initial liquid-phase transient and the global gas-phase jet penetration. Model validation was also performed in engine simulations with the same set of constants. The model captured mixture preparation well in all cases, proving its suitability for simulations of transient spray injection in engines.
Gasoline direct injection spray simulation
International Journal of Thermal Sciences, 2006
In this paper the problems related to mixture formation in a GDI engine are analyzed. The atomization of a hollow cone fuel spray generated by a high pressure swirl injector is studied by means of a numerical technique. The model distinguishes between primary atomization and secondary breakup. The latter was modeled, as done in a previous work on Diesel atomization, using different mechanisms as the droplet Weber number changes. At first the spray atomization in a quiescent chamber, at ambient pressure and temperature, was considered. The validation of the model was made comparing the numerical penetration and spray morphology with experimental results. Combustion simulations were also performed comparing numerical results with experimental data of a GDI (Gasoline Direct Injection), 4 stroke, 4 cylinder, 4 valves per cylinder engine. Such simulations were made to analyze and understand the mixture formation mechanism in both stoichiometric and stratified operation mode. The results show how, the interaction between the air motion and the fuel spray, leading factor in spray atomization, is fundamental to realize an efficient mixture formation and combustion locally very lean, typical of stratified charge combustion. 2005 Elsevier SAS. All rights reserved.
SAE Technical Paper Series, 2006
Modern diesel engines operate under injection pressures varying from 30 to 200 MPa and employ combinations of very early and conventional injection timings to achieve partially homogeneous mixtures. The variety of injection and cylinder pressures results in droplet atomization under a wide range of Weber numbers. The high injection velocities lead to fast jet disintegration and secondary droplet atomization under shear and catastrophic breakup mechanisms. The primary atomization of the liquid jet is modeled considering the effects of both infinitesimal wave growth on the jet surface and jet turbulence. Modeling of the secondary atomization is based on a combination of a drop fragmentation analysis and a boundary layer stripping mechanism of the resulting fragments for high Weber numbers. The drop fragmentation process is predicted from instability considerations on the surface of the liquid drop. Validation of the model has been performed by comparing the computational results with experimental measurements from isolated drops in shock tube experiments as well as with observations from fully developed diesel sprays.
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.
Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 2007
A multi-component fuel vaporization model has been developed and implemented into an in-house multi-phase computational fluid dynamics flow solver simulating the flow, spray, and air-fuel mixing processes taking place in gasoline direct injection (GDI) engines. Multi-component fuel properties are modelled assuming a specified composition of pure hydrocarbons. High-pressure and -temperature effects, as well as gas solubility and compressibility, are considered. Remote droplet vaporization is initially investigated in order to quantify and validate the most appropriate vaporization model for conditions relevant to those realized with GDI engines. Phenomena related to the fuel injection system and pressure-swirl atomizer flow as well as the subsequent spray development are considered using an onedimensional fuel injection equipment model predicting the wave dynamics inside the injection system, a Eulerian volume of fluid-based two-phase flow model simulating the liquid film formation process inside the injection hole of the swirl atomizer and a Lagrangian spray model simulating the subsequent spray development, respectively. The computational results are validated against experimental data obtained in an optical engine and include laser Doppler velocimetry measurements of the charge air motion in the absence of spray injection and charge coupled device images of the fuel spray injected during the induction stroke. The results confirm that fuel composition affects the overall fuel spray vaporization rate, but not significantly relative to other flow and heat transfer processes taking place during the engine operation.
Modeling atomization process in high-pressure vaporizing sprays
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.
Journal of Engineering for Gas Turbines and Power, 2000
The Low Pressure spray Breakup (LPB) model of Papageorgakis and Assanis (1996) has been implemented in the multi-dimensional code KIVA-3V as an alternative to the standard Taylor Analogy Breakup (TAB) model. Comparison of spray predictions with measurements shows that the LPB model, in conjunction with the standard k-ε turbulence model, has the potential for simulating the evolution of hollow cone sprays with acceptable fidelity, both from qualitative and quantitative standpoints. After validating the LPB model, illustrative studies of mixture stratification are conducted for a Direct Injection Gasoline (DIG) combustion chamber resembling the Mitsubishi design. The effects of reverse tumble strength and injection timing on mixture quality in the vicinity of the spark plug are explored. Overall, the study demonstrates how the KIVA-3V code with the LPB model can contribute to the optimization and control of mixing in DIG engines. SPRAY BREAKUP MODELING The Taylor Analogy Breakup (TAB) model (O'Rourke and Amsden, 1987) is standard in the KIVA-3V code. The model is based on an analogy between an oscillating and distorting drop and a damped, forced harmonic oscillator. The model considers the effects of the liquid viscosity on the oscillations of small droplets, and predicts that there is no unique critical Weber number of droplet breakup. Nevertheless, the TAB model can only track one oscillation mode, whereas in reality there are many such modes. Several investigators have shown that the TAB model is best suited for spray calculations under high injection pressures where aerodynamic effects are predominant, such as with diesel engines (Reitz, 1987). The LPB model developed by Papageorgakis and Assanis (1996) has shown promise in capturing the large-ligament droplet breakups that occur due to Rayleigh-Taylor instabilities within the droplet flow field. The LPB model is based on the fundamental work of Harper et al. (1972) who solved the distortion problem of a single droplet moving at constant acceleration. The analysis addressed the interior flow within the droplet and its interaction with the exterior domain. A force balance was
Non-reacting fuel spray simulations under direct diesel engine conditions
Issn 0282 1990, 2012
Because of the increasing demand of low polluting and efficient diesel engines, numerous studies related to the different processes that take place in them are being carried out. This study treats the direct injection of fuel at high pressures (c.a. 1500 bar). Ignition is not simulated in this work since this is intended to be a previous step for future studies which include combustion. A combination of LES for turbulence modeling and LPT for liquid phase modeling, which is a rather new approach, has been used and its advantages and limitations are reviewed herein. Six different simulations with different conditions as: mesh resolution, number of injected parcels, droplet size distribution and initial turbulence field; have been performed, analyzed and compared to ECN Sandia Spray A experimental results. OpenFOAM, which is an open source flow solver, has been employed in this study. Vapor penetration results for the best case have a good agreement with experimental results, with a maximum relative error of 5.77%. However, liquid length predictions are highly inaccurate in every case, as a consequence of the usage of a droplet size distribution in the nozzle outlet instead of implementing an atomization model In diesel engines, a very accurate description of the liquid zone is not required, but a good description of the vapor distribution is preferred. This makes this work useful in this area. Furthermore, LES-LPT involves a low computational effort compared to Eulerian techniques, becoming it a feasible approach to be used along with reaction schemes.