Atomization Model Research Articles

Coupled/decoupled spray simulation comparison of the ECN spray a condition with the Σ-Y Eulerian atomization model

This work evaluates the performance of the Σ-Y Eulerian atomization model at reproducing the internal structure of a diesel spray in the near-field. In the study, three different computational domains have been used in order to perform 3D and 2D coupled simulations, where the internal nozzle flow and external spray are modeled in one continuous domain, and 2D decoupled simulations, where only the external spray is modeled. While the 3D simulation did the best job of capturing the dense zone of the spray, the 2D simulations also performed well, with the coupled 2D simulation slightly outperforming the decoupled simulation. The similarity in results between the coupled and the decoupled simulation show that internal and external flow calculations can be performed independently. In addition, the use of spatially averaged nozzle outlet conditions, in the case of an axisymmetric (single-hole) convergent nozzle, leads to a slightly worse near-field spray predictions but to an accurate far-field ones. Finally, a novel constraint on turbulent driven mixing multiphase flows is introduced which prevents the slip velocity from exceeding the magnitude of the turbulent fluctuations through a realizable Schmidt number. This constraint increased model stability, allowing for a 4x increase in Courant number.

A new phenomenological model to predict drop size distribution in Large-Eddy Simulations of airblast atomizers

A new atomization model for prefilming airblast atomizers is presented and applied in the Large-Eddy Simulation of an academic experiment. The model, named PAMELA, expresses the drop size Probability Density Function of the spray in the form of a Rosin–Rammler distribution whose parameters depend on flow conditions. A mechanism of liquid fragmentation is proposed where a Rayleigh–Taylor instability develops in the transverse direction. The wavelength of this instability (i) is assumed to be proportional to the Sauter Mean Diameter of the spray, and (ii) scales with a Weber number based on the atomizing edge thickness, providing a first link between flow conditions and the Rosin–Rammler parameters. The second link is found by introducing a second Weber number based on the thickness of the boundary layer developing on the prefilmer. A first comparison with academic experiments shows that the model assumptions are valid and allows to calibrate the model constants. PAMELA is then implemented in a LES solver to perform the numerical simulation of an academic airblast atomizer. The obtained drop size distribution and spatial structure of the spray are in good agreement with measurements, demonstrating the validity of the proposed approach in the context of LES, and that the proposed PAMELA model may now be used to describe the liquid spray in LES of industrial nozzles.

Improved atomization, collision and sub-grid scale momentum coupling models for transient vaporizing engine sprays

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.

A comprehensive Two-Fluid Model for Cavitation and Primary Atomization Modelling of liquid jets - Application to a large marine Diesel injector

In this paper, a comprehensive two-fluid model is suggested in order to compute the in-nozzle cavitating flow and the primary atomization of liquid jets, simultaneously. This model has been applied to the computation of a typical large marine Diesel injector. The numerical results have shown a strong correlation between the in-nozzle cavitating flow and the ensuing spray orientation and atomization. Indeed, the results have confirmed the existence of an off-axis liquid core. This asymmetry is likely to be at the origin of the spray deviation observed experimentally. In addition, the primary atomization begins very close to the orifice exit as in the experiments, and the smallest droplets are generated due to cavitation pocket shape oscillations located at the same side, inside the orifice.

The impact of fuel evaporation on the gas-phase kinetics in the mixing region of a diesel autothermal reformer

Autothermal reforming is a promising technology to produce syngas from diesel fuel. However, the mixing of liquid diesel fuel with high temperature streams of steam and oxygen presents a challenge: how to avoid formation of ethylene, a likely deposit precursor, in the region upstream of the catalyst bed. This work describes a coupled CFD-kinetics study in the mixing region of a diesel autothermal reformer that considers: (1) an atomizer model to explicitly account for fuel evaporation, (2) n-dodecane as a surrogate diesel fuel, (3) oxygen as the oxidant instead of air. The predictions indicate unacceptable levels of ethylene (>0.1 mol%) will be present at the mixer exit if the mixer gas temperature is greater than ∼350 °C. This temperature is likely to be too low for proper catalyst performance. Thus this analysis suggests that either improved mixer designs or a different choice of catalyst might be required to achieve suitable diesel autothermal reforming performance.

Investigation of the flow field for a double-outlet nozzle during minimum quantity lubrication grinding

Efficient application of minimum quantity lubrication (MQL) in grinding is not only related to grinding conditions and delivery parameters but also affected by spraying atomization characteristics. In this study, a double-outlet nozzle is proposed and the flow field of the MQL grinding is investigated by two-stage atomization model. The side-mixing structure of double-outlet nozzle indicates that the grinding fluid is atomized at the windward side and the flow rate of grinding fluid for a single radial hole is smaller than that for the liquid pipe. Therefore, more excellent atomization performances, in terms of liquid droplet size, uniformity, and velocity of the liquid droplets, are obtained for double-outlet nozzle in comparison with single-outlet nozzle. The liquid droplets sprayed from auxiliary outlet of double-outlet nozzle impact on the grinding wheel and change the airflow direction around the grinding wheel. Thus, the air barrier around the grinding wheel is disturbed and the liquid droplets sprayed from main outlet can be injected into the grinding zone easily. Experimental results indicate that two-stage atomization model is reliable.

Multiscale simulation of atomization with small droplets represented by a Lagrangian point-particle model

Modeling and simulation of atomization is challenging due to the existence of a wide range of length scales. This multiscale nature of atomization introduces a fundamental challenge to numerical simulation. A pathway to comprehensive modeling is still to be found. The present study proposes a multiscale multiphase flow model for atomization simulations, where the large-scale interfaces are resolved by the Volume-of-Fluid (VOF) method and the small droplets by the Lagrangian point-particle (LPP) model. Particular attention is focused on the momentum coupling between LPP and resolved flow and the conversion between droplets represented by VOF and LPP. A series of multiphase flow problems are considered to validate the model. The results obtained by a number of simulations are compared against direct numerical simulation (DNS) results and experimental data. In particular, the model is applied to simulate the gas-assisted atomization experiment, and the numerical results are compared to the experimental measurements for a quantitative validation.

Modeling the atomization of high-pressure fuel spray by using a new breakup model

The main objective of this research is to develop a new breakup model and investigate the influence of cavitation, turbulence on processes of high-pressure diesel sprays. The model distinguishes between jet primary atomization and droplet secondary atomization. The primary atomization was simulated based on a modified turbulence induced atomization model taken into account the coupling effects of the relaxation of the velocity profile, cavitation and turbulent fluctuation based on the principle of conservation of energy. The growth time scale of surface waves was provided by Kelvin–Helmholtz (K–H) instability theory on an infinite length cylinder for an inviscid liquid jet. The time scale of initial surface waves based on K–H instability theory on an infinite plane jet is much larger than that based on infinite length cylindrical jet. Based on the present modified model, the weighting coefficients of turbulence and cavitation on the overall atomization can be distinguished clearly. There was remarkable variation in simulated spray shape with present models. It could be seen that the original turbulence induced atomization model results in a shorter spray penetration and smaller drops near the spray core than the modified model. When applying the turbulent weighting coefficient Ct in the determination of the spray angle, the resultant value of spray angle gradually drops due to the reduction of Ct. However, the spray angle increases with increasing the cavitation weighting coefficient Ccav. Comparing the experimental results (such as spray angle, tip penetration and spray shape) with the theoretical ones for different injection pressure, gives a reasonable agreement.

Multi-scale analysis of atomizing liquid ligaments

The atomization of individual liquid ligaments appearing during the disintegration of liquid sheets issuing from a triple-disk injector is investigated. High-speed visualizations report a temporal evolution of the ligaments from their production to their disintegration into drops. The parameter of the experiments is the surface tension of the liquid. A multi-scale analysis consisting in describing the temporal evolution of the ligament shape by measuring the scale-distribution is performed. This analysis introduces the notion of scale-diameter whose temporal variation leads to the following conclusion. The ligaments are subject to elongation, capillary deformation and break up and relaxation mechanisms. These mechanisms appear concomitantly on different scales. This concomitancy depends on the surface tension, i.e., on a Weber number based on the ligament initial elongation rate and initial size: the decrease of the surface tension favor the propensity of the atomization process to cascade in the scale space towards the small scale region. A mathematical scale-distribution of the final drops is satisfactorily derived from an atomization model of the literature. Furthermore, the parameters introduced by this model are well correlated to initial ligament Weber number and size.

SLIMP: Strong laser interaction model package for atoms and molecules

We present the SLIMP package, which provides an efficient way for the calculation of strong-field ionization rate and high-order harmonic spectra based on the single active electron approximation. The initial states are taken as single-particle orbitals directly from output files of the general purpose quantum chemistry programs GAMESS, Firefly and Gaussian. For ionization, the molecular Ammosov–Delone–Krainov theory, and both the length gauge and velocity gauge Keldysh–Faisal–Reiss theories are implemented, while the Lewenstein model is used for harmonic spectra. Furthermore, it is also efficient for the evaluation of orbital coordinates wavefunction, momentum wavefunction, orbital dipole moment and calculation of orbital integrations. This package can be applied to quite large basis sets and complex molecules with many atoms, and is implemented to allow easy extensions for additional capabilities. Program summaryProgram title: SLIMPCatalogue identifier: AEWE_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEWE_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 90277No. of bytes in distributed program, including test data, etc.: 927942Distribution format: tar.gzProgramming language: Fortran 90.Computer: All computers with a Fortran compiler supporting at least Fortran 90.Operating system: All operating systems with such a compiler. The makefile depends on a Unix-like system and needs modification under Windows.RAM: Memory requirement depends on the size of the molecules and the number of basis functions. For instance, about 2.8MB RAM is required for N2 molecules with the acc-pvqz basis set (210 basis functions). However, the SF6 molecules require 22MB RAM with the same basis set (739 basis functions).Classification: 16.1, 16.10, 2.7.Nature of problem: In modeling of atoms and molecules subjected to intense laser pulses, there is a general need for the calculation of theoretical models. However, the application of these models requires the effective evaluation of orbital coordinates wavefunction, momentum wavefunction, orbital dipole moment or calculation of orbital integrations. This package provides an efficient way for these calculations directly from the output files of the widely used GAMESS, Firefly and Gaussian programs.Solution method: The molecular orbital is constructed from a linear combination of atomic orbitals, which is further expanded with Cartesian gaussian type orbitals (GTO). The orbital momentum wavefunction and dipole moment can be constructed with the Fourier transformed GTOs. Orbital integrations rely on the elementary integrations of GTOs. These results are used directly as input in the subsequent models calculations.Restrictions: Current implementation supports the S, P L, D, F and G types, of GTOs. Here L indicates the diffuse basis. However, the extension to higher order basis is straight-forward. This package supports the versions of GAMESS (US, 2013), Firefly (8.1), and Gaussian (both 09 and 03). For older versions, mild modifications on the reading formats may be necessary in the modules.Running time: The running time depends on the size of the molecules, the number of basis functions, and most importantly, on the problem to which this package has been employed. Typically for a single-processor PC-type computer it can vary between a few seconds and days.

Eulerian–Lagrangian RANS Model Simulations of the NIST Turbulent Methanol Spray Flame

A methanol spray flame in a combustion chamber of the NIST was simulated using an Eulerian–Lagrangian RANS model. Experimental data and previous numerical investigations by other researchers on this flame were analyzed to develop methods for more comprehensive model validation. The inlet boundary conditions of the spray were generated using semi-empirical models representing atomization, collision, coalescence, and secondary breakup. Experimental information on the trajectory of the spray was used to optimize the parameters of the pressure-swirl atomizer model. The standard k-ϵ turbulence model was used with enhanced wall treatment. A detailed reaction mechanism of gaseous combustion of methanol was used in the frame of the steady laminar flamelet model. The radiative transfer equations were solved using the discrete ordinates method. In general, the predicted mean velocity components of the gaseous flow and the droplets, the droplet number density, and the Sauter mean diameter (SMD) of the droplets at various heights in the present study show good agreement with the experimental data. Special attention is paid to the relative merits of the employed method to set inlet boundary conditions compared to the alternative method of using a measured droplet size and velocity distribution.

ATOMIZATION MODELLING OF LIQUID JETS USING A TWO-SURFACE-DENSITY APPROACH

In internal combustion engines, the liquid fuel injection is an essential step for the air/fuel mixture preparation and the combustion process. Indeed, the structure of the liquid jet coming out from the injector plays a key role in the proper mixing of the fuel with the gas in the combustion chamber. The present work focuses on the liquid jet atomization phenomena under diesel engine conditions. Under these conditions, liquid jet morphology is assumed, including a separate liquid phase (i.e., a liquid core) and a dispersed liquid phase (i.e., a spray). This article describes the developmental stages of a new atomization model, for a high-speed liquid jet, based on an Eulerian-Eulerian two-fluid approach. The atomization phenomena are modelled by defining different surface density equations for the liquid core and the spray droplets. This new model has been coupled with a turbulent and highly compressible two-fluid system of equations. The process of ligament breakup and its subsequent breakup into droplets are handled by acquiring knowledge from the available high-fidelity experiments and numerical simulations. In the dense region of the liquid jet, the atomization is modelled as a dispersion process due to the stretching of the interface, from the liquid side in addition to the gas side. The model results have been compared to a recently published DNS results under typical direct injection diesel engine conditions. In particular, it has been shown that the interface instabilities and the turbulence in the leading tip of the liquid core play a major role in the primary atomization of diesel jets.

Effect of nozzle hole size coupling with exhaust gas re-circulation on the engine emission perfomance based on KH-ACT spray model

To research an effective measure of reducing the Soot and NOx in engine at the same time, different nozzle hole diameters coupled with exhaust gas recirculation (EGR) were adopted in this study based on KH-ACT spray breakup model, which takes the aerodynamic-induced ,cavitation-induced and turbulence-induced breakup into account. The SAGE detailed chemistry combustion and the new atomization model used in the simulation have been verified with the experiment data from a YN4100QBZL engine. Different diesel nozzles was adopted in the study combined with different EGR rates ranging from 0% to 40%. The simulation results show that the NOx emission could be reduced effectively for both small(0.1mm) and large(0.15mm) diesel nozzle when increasing EGR ratio. The soot emission increases for the small nozzle hole size as the EGR increasing. However, when it comes to the large diesel nozzle, the emission increases slightly first and decrease quickly when the EGR rate above 20%.

Spray Combustion Modeling in Lean Direct Injection Combustors, Part I: Single-Element LDI

Lean direct injection (LDI) is one of the most promising low NOx combustion concepts for aero-engines. This work reports turbulent reacting spray modeling in a single-element LDI combustor to determine a validated numerical framework that can be used to model such complex combustion systems involving highly swirling flows, droplet breakup, and combustion. Different sub-models are employed within a numerical framework to characterize the spray processes involved in the primary atomization and secondary breakup regimes. The sensitivity of the associated breakup model parameters to the predicted behavior is investigated, thereby fine tuning the sub-models for turbulent spray combustion in LDI. The two-way coupling accomplishes the interactions between the liquid and gas phases and the respective phase properties are analyzed. The size and distribution of drops are computed at various locations and the results are compared with the measurements. The work demonstrates that modeling of primary atomization improves the overall predictions of both gas phase and spray. The validation with the measurements reflects that the numerical framework implemented in this study is able to characterize the effects of highly swirling flows with strong turbulence on spray velocity and size distribution, and strong momentum exchange between the gas and liquid phases.

Predicting Droplet Size Distribution by Image Processing Technique for an Air Blast Atomizer

An image processing technique was used to predict the size distribution of the high speed, fine droplets at downstream of an air blast atomizer. The spray visualization setup consisted of UV lamps as light source, a stroboscope for slowing down the droplet motion, and a digital camera to capture the droplet images. The experiments were carried out at different liquid flow rates with various nozzle diameters. Two key unknown parameters (spray half angle and dispersion angle) of the air blast atomizer model in Fluent were obtained from these experiments. Using the obtained parameters and other structural parameters, the spray modeling was performed, and the Rosin–Rammler distribution was obtained and compared with those obtained from image processing technique through a diagnostic matrix. Regarding the kappa value, the agreement between predictions of the Fluent model and the image processing technique was moderate.

Effect of turbulence model and inlet boundary condition on the Diesel spray behavior simulated by an Eulerian Spray Atomization (ESA) model

Simulating liquid spray first and second atomization is not an easy task. Many models have been developed over the past years, but Eulerian ones have proved their better performance for the dense zone of the spray. In this work a new compressible Eulerian model is used to compute the internal flow together with the spray. Up to five two-equation turbulence models have been tested and its influence is remarkable in terms of spray behavior, but also greatly affects the mass flow rate and the momentum flux. At the end, SST k–ω model proves to be best than the others. Additionally, different types of inlet boundary conditions have been also tested and analyzed. Results when compared with previously obtained experimental data show that the commonly used for external flow time-varying velocity boundary condition gives also good performance for the internal flow.

Session details: Atomicity and memory models

Session details: Atomicity and memory models

A RECENT EULERIAN–LAGRANGIAN CFD METHODOLOGY FOR MODELING DIRECT INJECTION DIESEL SPRAYS

The global objective of this work is to show the capabilities of the Eulerian–Lagrangian spray atomization (ELSA) model for the simulation of Diesel sprays in cold starting conditions. Our main topic is to focus in the analysis of spray formation and its evolution at low temperature 255 K (-18°C) and nonevaporative conditions. Spray behavior and several macroscopic properties, included the liquid spray penetration, and cone angle are also characterized. This study has been carried out using different ambient temperature and chamber pressure conditions. Additionally, the variations of several technical quantities, as the area coefficient and effective diameter are also studied. The results are compared with the latest experimental results in this field obtained in our institute. In the meantime, we also compare with the normal ambient temperature at 298 K (25°C) where the numerical validation of the model has shown a good agreement.

MULTIDIMENSIONAL MODELING OF COMPRESSED BIO GAS (CBG) ENGINE FOR ULTRA LOW EMISSION

The purpose of this study is to find out in-cylinder flow characteristics of a four-cylinder, 2,3 liter diesel engine before the fuel injection and also properly simulate the atomization process in order to improve the combustion efficiency and emission parameters. A complete intake stroke with Compressed Bio Gas (CBG) port injection, compression stroke, pilot fuel injection at the end of the compression and exhaust stroke simulated in 3D. Both naturally aspirated and turbo-charged modes of operations were studied. In this study, cold flow structure, fuel atomization for different spray models and dual fuel combustion are investigated. A comprehensive model for atomization of liquid sprays under high injection pressures has been employed. Finally, a calculation on an engine configuration with compression, spray injection and CBG-diesel combustion in a direct injection dual fuel diesel engine is presented. In 3D CFD study, selected proper cases modeled using full geometry of diesel engine. Fine grids were tested with 1,700,000 hexahedral cells. In this paper, fine grid results are evaluated. CBG fuel induced during intake process in dual fuel diesel combustion engine and then mixed with air before the premixed gas enters the cylinder. It is ignited by the injection of pilot diesel fuel near the Top Dead Center (TDC). This lean combustion and diluted gas mixture cause low in-cylinder temperature and resulting in a NOx emission. The results are in agreement qualitatively with the previous studies in the literature.

Clustering Rule Bases Using Ontology-Based Similarity Measures

Rules are increasingly becoming an important form of knowledge representation on the Semantic Web. There are currently few methods that can ensure that the acquisition and management of rules can scale to the size of the Web. We previously developed methods to help manage large rule bases using syntactical analyses of rules. This approach did not incorporate semantics. As a result, rule categorization based on syntactic features may not be effective. In this paper, we present a novel approach for grouping rules based on whether the rule elements share relationships within a domain ontology. We have developed our method for rules specified in the Semantic Web Rule Language (SWRL), which is based on the Web Ontology Language (OWL) and shares its formal underpinnings. Our method uses vector space modeling of rule atoms and an ontology based semantic similarity measure. We apply a clustering method to detect rule relatedness, and we use a statistical model selection method to find the optimal number of clusters within a rule base. Using three different SWRL rule bases, we evaluated the results of our semantic clustering method against those of our syntactic approach. We have found that our new approach creates clusters that better match the rule bases’ logical structures. Semantic clustering of rule bases may help users to more rapidly comprehend, acquire, and manage the growing numbers of rules on the Semantic Web.