Atomization Model Research Articles

Multiscale modeling of turbulent atomization: Droplet-size sampling

With an interest on atomization in turbulent flows, and modeling alternatives to classic atomization models and Direct Numerical Simulations (DNS), the present work derives and tests two droplet-size distributions parametrized by local information about the flow. One distribution is heuristic and another one is based on the linear-eddy model. Predictions from these distributions are tested with numerical simulations in which the unresolved droplet-mass production is closed by assuming that turbulence is the rate controlling mechanism and by using a correction step. Moreover, the testing considers a round jet with a Reynolds number of 5000 from which DNS data is available. Overall, it is found that the present atomization model provides an enhancement of accuracy of the droplet-mass production and droplet-diameter distribution in comparison with using no model. Furthermore, the predicted droplet-diameter distribution is more accurate than that using a previous assumption of taking droplet diameters equal to local Taylor microscales. These results are obtained with simulations whose cost is orders of magnitude less than DNS.

Numerical and experimental study of twin-fluid two-phase internal-mixing atomizer to develop maximum entropy method

This paper presents an analytical, numerical, and experimental study on atomization characteristics and droplet distribution of a twin-fluid two-phase internal mixing atomizer to develop a Maximum Entropy Method (MEM). A two-phase Eulerian-Lagrangian method is utilized for atomization modeling of the inside and outside atomizer. In order to modify energy and momentum sources in the MEM, parametric studies are performed, and experimental tests are carried out to verify the results by applying the shadowgraph method. An advanced test stand is developed to prepare a wide range of changes in atomization characteristics and mixing ratios. A high degree of consistency is found between numerical results from the developed MEM and experimental tests with different gas-phase pressures and liquid flow rates. The droplet diameter and velocity distribution are reviewed based on various Weber numbers, sources of energy, and momentum. Turbulence modeling assists to estimate the breakup length and time scale precisely in the developed MEM, and distribution ranges with mean values are achieved. With reference to a strong correlation between upstream turbulence flow and the developed MEM verified by experimental tests, an ideal droplet size and velocity distribution prediction is observed.

Subgrid-scale Capillary Breakup Model for Liquid Jet Atomization

ABSTRACT A subgrid-scale capillary breakup model has been developed by coupling a Eulerian interface-capturing approach to a Lagrangian point particle method. At the fully resolved scale, the evolution of phase interface is captured by the Refined Level Set Grid method (RLSG), while the under-resolved, small-scale liquid structures are represented by Lagrangian point particles. To couple the level-set method with the Lagrangian approach, a numerical algorithm is developed to identify the under-resolved structures and compute their shape metric. The capillary instability theory is then applied to predict the sizes and number of post-breakup drops needed to replace them with Lagrangian drops. The secondary atomization of the Lagrangian drops is modeled by the stochastic breakup model. To validate the present model, numerical simulations are performed for the breakups of a single liquid filament and a round liquid coaxial jet. Grid-convergence studies for the drop-size distribution of the resulting spray are conducted and compared to the experimental data. The numerical results show good agreement with the experimental data. The present subgrid-scale capillary breakup model reduces the computational cost by relaxing the stringent grid resolution required for multi-scale atomization simulations.

3D-CFD Simulation of a GDI Injector Under Standard and Flashing Conditions

In the optimization of GDI engines, fuel injection plays a crucial role since it can affect the combustion process and, thus, fuel efficiency and pollutant emissions. The challenging task is to obtain the required fuel distribution and atomization inside the combustion chamber over a wide range of engine operating conditions. To achieve such goals, flash-boiling can be exploited. Flash-boiling is a phenomenon occurring when fuel temperature exceeds saturation temperature or, similarly, when ambient pressure is lower than saturation one. Under these conditions, which can occur inside the injector or directly in the combustion chamber, the fuel undergoes extremely accelerated breakup and quickly evaporates. The proposed manuscript shows the application of an alternative flashboiling model, recently implemented by Siemens-PLM in STAR-CD V.2019.1, to be applied in 3D-CFD Lagrangian simulations of GDI sprays. Results are validated against experimental data, provided by the SprayLAB of the University of Perugia, on a single-hole research injector. The new flash-boiling model consists of three main parts: an atomization model able to compute droplet initial conditions and the overall spray cone angle; an evaporation model and, finally, a droplet break-up model; the last two models are designed to simulate all the physical events occurring when droplets are injected into the combustion chamber. As for the investigated operating condition, vessel pressure and temperature are 40 kPa and 293K, respectively; as for the fuel (n-Heptane) temperature, it ranges from 303.15 K to 393.15 K, on equal injection pressure (10 MPa). The numerical-experimental comparison is carried out in terms of liquid penetration, imaging, and droplet sizing.

Atomization performance of Tiantaishan tunnel spray dust reduction

In order to further improve the efficiency of spray dust removal in tunnel and obtain favorable equipment parameters, the dust removal performance of high pressure spray atomization particle size was deeply studied. Taking the dust generated in Tiantai Mountain tunnel as the research object and combining with the numerical simulation method. Based on FLUENT pressure-swirl atomization model, in this paper, the mechanism of high pressure spray dust reduction is analyzed. The pressure nozzle is simulated at 4 pressures (2, 3, 4, 5 Mpa) and 3 nozzles (2.0, 1.5, 1.0 mm in diameter). The simulation results show that when the nozzle diameter is constant, the droplet size decreases with the increase of pressure, and the atomization effect is better. When the spray pressure is constant, the droplet size increases with the increase of nozzle diameter. According to the measured results of dust in Tiantai Mountains, it is suggested that the nozzle diameter should be 1.5 mm and the working pressure should be 4 Mpa, which can reasonably and effectively settle the fine particles of dust in the tunnel.

The Impact of Magnetic Dipole Radiation and Decay on the Variation of Period and Inclination Angle of Pulsars

IJPSR emphasizes on various innovative, experimental and hypothetical studies related to all aspects of physics. The scope of the journal covers a large range from difficult atoms and diatomic interactions to modelling of atoms and molecules using electromagnetic waves.

Assessment of varying primary/secondary breakup mechanism of diesel spray on performance characteristics of HSDI engine

Fuel spraying process and hydrodynamic interaction of fuel jet-air plays a crucial role in the mixture formation pattern. A baseline diesel engine is considered and based on the operational values ​​of the experimental engine, the geometric and spraying model is designed and meshed. The values ​​of the primary and secondary breakup coefficients are adjustable according to the tested engine. By fixing the geometrical features, different atomization models with related tuning constants are selected to determine the effect of these factors on the spray quality and engine output in terms of combustion and emissions, and to provide a general framework for model performance for the users. In the present study, both the primary breakup and atomization models, as well as the secondary breakup model are considered emphasizing the modification and identification of the functions of these parameters. In the second breakup of the WAVE model is adopted and the C2 coefficient or constant in the range of 10–60 in the 10 steps is launched by the software for each mode, then in the secondary breakup model, KH-RT with a coefficient of C2 = 12 is investigated. In the next step, the primary breakup model of “Core Injection” is evaluated with C1 coefficients between 10 and 50, and its C2 coefficient will be analyzed for the values 15, 25, and 35. It has been understood that there is a linear correlation between equivalence ratio and C2 of secondary breakup with the determination coefficient of R2 = 0.954. Increasing C2 of WAVE results in deceleration in spray droplet strip (dR/dt) such that C2 = 60 gives the biggest SMD, the lowest pressure, heat release rate, and soot amount.

Modelling of hydrogen atoms in crystallography – an overview

Modelling of hydrogen atoms in crystallography – an overview

Impact of the Primary Break-Up Strategy on the Morphology of GDI Sprays in 3D-CFD Simulations of Multi-Hole Injectors

The scientific literature focusing on the numerical simulation of fuel sprays is rich in atomization and secondary break-up models. However, it is well known that the predictive capability of even the most diffused models is affected by the combination of injection parameters and operating conditions, especially backpressure. In this paper, an alternative atomization strategy is proposed for the 3D-Computational Fluid Dynamics (CFD) simulation of Gasoline Direct Injection (GDI) sprays, aiming at extending simulation predictive capabilities over a wider range of operating conditions. In particular, attention is focused on the effects of back pressure, which has a remarkable impact on both the morphology and the sizing of GDI sprays. 3D-CFD Lagrangian simulations of two different multi-hole injectors are presented. The first injector is a 5-hole GDI prototype unit operated at ambient conditions. The second one is the well-known Spray G, characterized by a higher back pressure (up to 0.6 MPa). Numerical results are compared against experiments in terms of liquid penetration and Phase Doppler Anemometry (PDA) data of droplet sizing/velocity and imaging. CFD results are demonstrated to be highly sensitive to spray vessel pressure, mainly because of the atomization strategy. The proposed alternative approach proves to strongly reduce such dependency. Moreover, in order to further validate the alternative primary break-up strategy adopted for the initialization of the droplets, an internal nozzle flow simulation is carried out on the Spray G injector, able to provide information on the characteristic diameter of the liquid column exiting from the nozzle.

Spray dynamics simulations for pulsatile injection at different ambient pressure and temperature conditions

A numerical study on the transient characteristics of a pulsatile, iso-octane spray issuing from a pressure-swirl atomizer is presented. The effects of system pressure and temperature, as well as the initial fuel temperature on spray dispersion and evaporation, are highlighted. The computations were carried out using ANSYS FLUENT-15.0, assuming the spray dispersion to be axisymmetric. Gas phase turbulence is simulated using the renormalized group k- ε model, while the discrete phase model is used for tracking fuel droplets. The linear instability sheet atomization model is adopted for the primary breakup of the liquid sheet, and the Taylor Analogy Breakup and Wave Breakup models are adopted for the secondary breakup, depending upon the operating conditions. The drag force on the droplet is evaluated, after incorporating the effects of evaporation and neighbouring droplets, along with droplet shape distortion. The significance of droplet collision on the evolution of droplet size distribution is examined. The local mean drop sizes and spray penetration length are in agreement with the experimental results of the literature. The predicted results indicate that the spray is narrower and penetrates less at higher ambient pressure. In this respect, the additional force on droplets due to local static pressure gradient is examined in detail. The effect of ambient conditions on the spray evaporation process is studied based on the spatio-temporal evolution of the equivalence ratio of the mixture of fuel vapour and air.

Close Nozzle Spray Characteristics of a Prefilming Airblast Atomizer

The formation of pollutant emissions in jet engines is closely related to the fuel distribution inside the combustor. Hence, the characteristics of the spray formed during primary breakup are of major importance for an accurate prediction of the pollutant emissions. Currently, an Euler–Lagrangian approach for droplet transport in combination with combustion and pollutant formation models is used to predict the pollutant emissions. The missing element for predicting these emissions more accurately is well defined starting conditions for the liquid fuel droplets as they emerge from the fuel nozzle. Recently, it was demonstrated that the primary breakup can be predicted from first principles by the Lagrangian, mesh-free, Smoothed Particle Hydrodynamics (SPH) method. In the present work, 2D Direct Numerical Simulations (DNS) of a planar prefilming airblast atomizer using the SPH method are presented, which capture most of the breakup phenomena known from experiments. Strong links between the ligament breakup and the resulting spray in terms of droplet size, trajectory and velocity are demonstrated. The SPH predictions at elevated pressure conditions resemble quite well the effects observed in experiments. Significant interdependencies between droplet diameter, position and velocity are observed. This encourages to employ such multidimensional interdependence relations as a base for the development of primary atomization models.

Experimental and numerical study on the effect of dimensionless parameters on the characteristics of droplet atomization caused by periodic inertial force

Fuel atomization will seriously affect the combustion and emission characteristics of IC engine. It is important to study the atomization characteristics of single droplet under dynamic inertial force for improving the high precision secondary atomization model in cylinder. Firstly, the process of droplet atomization under sinusoidal inertial force was studied experimentally. Then, the effects of dimensionless parameters, including Bond number (Bo), gas-liquid density ratio (ρG/L), gas-liquid viscous ratio (μG/L), Weber number (We) and Reynolds number (Re), on the average wavelength and atomization time of droplet surface wave were numerically investigated. The results show that with the development of time, the droplet surface appears zonal standing wave, radial standing wave and volcanic standing wave in turn. When the droplet atomizes, the sub droplet sprays first from the top of the droplet, and the atomization intensity at the top is stronger than that at both sides. Bo, ρG/L and Re have little influence on the average wavelength of droplet surface wave, μG/L has no effect, while We has a great influence. As Bo, We and Re increase, the atomization time decreases rapidly at first, and then converges. When We > 104 or Re > 104, the effect of surface tension and viscous force on atomization time can be neglected. As ρG/L increases, the atomization time first remains unchanged and then increases rapidly. When ρG/L < 0.1, its influence can be neglected. μG/L has no effect on the atomization time. Furthermore, according to the variation of the average wavelength and We number, the dimensionless form of the empirical formula for the average diameter of droplets is obtained: dm∗=(1.0±0.1)∙We-1/3. Finally, the critical conditions of droplet atomization are determined based on the actual atomization time of IC engine.

Development and assessment of an improved droplet breakup model for numerical simulation of spray in a turbulent flow field

The present article proposes an improved spray breakup model considering the effect of carrier phase turbulence on both the critical Weber number and the breakup rate. For this aim, the effective surface tension as a new concept was introduced to combine the effects of turbulent and aerodynamic induced breakup. A simple analytical expression has been developed for calculating the effective surface tension according to the local turbulence characteristics of the carrier phase. Afterwards, the proposed effective surface tension was used for calculating the critical Weber number in turbulent flow fields and an excellent agreement was shown in comparison with the available experimental data. In the following, an improved secondary atomization model was developed by implementation of the effective surface tension in the classic Pilch-Erdman model. To assess the capabilities of the new breakup model, it was implemented into a CFD code and the results compared with experimental data and those obtained by the previous breakup models. The present model showed a better agreement with experimental data in predicting the spray tip penetration length and local Sauter mean diameter (SMD). It is concluded that the improved model is effectively more reliable for simulation of spray droplet breakup in turbulent gaseous environments.

Comparison of secondary breakup models for droplet-laden compressor flows

Different secondary breakup models are assessed to simulate gas-droplet flows that involve large velocity gradients and yield relative velocities between both phases (e.g. wet compression flows). For the simulation of such two-phase flows, the Euler-Lagrange approach is employed, using a commercial flow solver and an in-house FORTRAN source code. The droplet deformation models: Taylor Analogy Breakup (TAB) and a simplified implementation of the Non-linear TAB (NLTAB3) are each tested with two different breakup criteria to evaluate the adequacy for considered droplet-laden compressor flows. A detailed atomization model is applied to account for a temporal fragmentation process. By contrast with experimental results good agreements are found for Weber number distributions in the inter blade flow. Though, different sensitivities and locations for secondary breakup occurrence are observed in dependence of applied droplet deformation model and breakup criterion. In spite of incisive model simplifications to reduce its computational effort, the non-linear droplet deformation model (NLTAB3) in combination with a physically substantiated breakup criterion showed good agreements with qualitative experimental data.

Modelling of Nickel Atoms Interacting with Single-Walled Nanotubes

Herein, the encapsulation mechanism of nickel atoms into carbon and boron nitride nanotubes is investigated to determine the interaction energies between the nickel atom and a nanotube. Classical modelling procedures, together with the 6-12 Lennard-Jones potential function and the hybrid discrete-continuous approach, are used to calculate the interaction of a nickel atoms with(i,i)armchair and(i,0)zigzag single-walled nanotubes. Analytical expressions for the interaction energies are obtained to determine the optimal radii of the tubes to enclose the nickel atom by determining the radii that give the minimum interaction energies. We first investigate the suction energy of the nickel atom entering the nanotube. The atom is assumed to be placed on the axis and near an open end of a semi-infinite, single-walled nanotube. Moreover, the equilibrium offset positions of the nickel atoms are found with reference to the cross-section of the nanotubes. The results may further the understanding of the encapsulation of Ni atoms inside defective nanotubes. Furthermore, the results may also aid in the design of nanotube-based materials and increase the understanding of their nanomagnetic applications and potential uses in other areas of nanotechnology.

Fluid dynamic and autoignition characteristics of early fuel sprays using hybrid atomization LES

Early fuel spray development and subsequent autoignition are investigated using our turbulent atomization model that has enabled physically-closed primary atomization modeling. Thanks to its modeling methodology, no parameter tuning or adjustment is needed to simulate a turbulent spray. Due to turbulent atomization, a dense droplet layer is formed over the injected liquid core. With evaporation, this region has a triple layer structure, which is composed of the inner layer with many droplets, the vaporization layer and the outer layer exposed to the hot air. With combustion, hot spots are generated in the outer layer, while the inner layer is too cold and hinders the hot gas to penetrate inside. The group combustion number indicates that this is sheath evaporation/combustion mode and its structure is preserved down to the liquid core length. The liquid core thinning is determined by atomization characteristics and under evaporative/reactive conditions where the outer gas temperature is high and thus the kinematic viscosity is also high, the flow motion is larger and the core is unsteadily affected by this motion. Due to this, the Rayleigh–Taylor (RT) atomization mode is further excited and core breakup occurs relatively faster. It is also observed that the unsteady core head flapping motion leads to non-uniform distributions of droplets and vapor. Due to this non-uniformity, the initial autoignition in the outer layer does not occur uniformly. After core breakup, the droplet layer rolls up in the front region and the group combustion number becomes lower, where external group combustion behavior is observed. Multistep chemistry improves the ignition delay estimation compared with the global chemistry, but the spray dynamics is mostly determined by the above fluid dynamic effects. Therefore, the present study indicates that the accurate simulation capability of fluid dynamics, i.e. primary atomization and early spray development, is important in predicting spray combustion behavior.

Selection of model liquid with refractive index matching for visualization of internal flow in a scaled atomizer model

A scaled transparent modular model of pressure-swirl (PS) atomizer was prepared from cast PMMA (Poly(methyl methacrylate), Perspex™, Plexiglas™) with the aim to achieve a better understanding of internal flow and subsequent spray formation. Because of use of high-speed imaging and Laser Doppler Anemometry (LDA) the working liquid had to be selected with respect of a refractive index matching (RIM) with the atomizer material. The liquid should be colourless and chemically non-aggressive to the model material with suitable viscosity to achieve the Reynolds number of the internal flow of the original atomizer. Froude number should be high enough to neglect the influence of gravity on the flow. An extensive search for transparent liquids and materials of enlarged models was made with a focus on RIM in performed experiments. Several liquids were chosen, and their chemical effect on PMMA was tested. Despite the successful tests that proved the liquid suit the case, the model material was damaged and the tests proved to be insufficient. For this reason, the tests were modified to better involve the stress of the bolted model. It turned out that a force effect (bolt in the thread, pre-stressed bolt connection) on the material has a significant influence on the acceleration of the chemical effect. The internal flow was examined using a high-speed camera with several liquids.

Effect of liquid preheating on high-velocity airblast atomization: From water to crude rapeseed oil

Airblast atomization is a suitable model platform to understand atomization physics since the atomizer geometry has an insignificant influence on the spray formation. Besides its theoretical relevance, this configuration is used in several practical applications ranging from healthcare to combustion. Presently, a plain-jet airblast atomizer has been investigated experimentally under atmospheric conditions at various atomizing pressures and liquid preheating temperatures. To cover a wide range of liquids by viscosity and surface tension, water, diesel oil, light heating oil, and crude rapeseed oil were atomized to evaluate the droplet size-velocity correlations when the spray is fully developed. Increasing the temperature of high-viscosity liquids prior to atomization improves the spray characteristics until their kinematic viscosity decreases to a certain value that is newly introduced as a limiting viscosity. Further preheating has a marginal effect on droplet size-velocity plots, and the spray becomes more homogeneous. Several SMD-estimating formulae were analyzed and improved to consider the effect of liquid preheating and to extend their range of validity. When the kinematic viscosity exceeded the limiting viscosity, the part containing the Weber number was corrected linearly by the preheating temperature. The coefficient of the Ohnesorge number was corrected by the inverse of the kinematic viscosity, without considering the limiting viscosity. The above results help to correct the SMD of atmospheric measurements to elevated liquid temperatures and to contribute to advanced atomization models for numerical software.

Electroelastic investigation of drying rate in the direct contact ultrasonic fabric dewatering process

Ultrasonic vibrations, used to atomize liquids into a fine mist, are a promising solution for the future of efficient clothes drying technology. The world’s first ultrasonic dryer—demonstrated by researchers at Oak Ridge National Laboratory—successfully applies the scientific principles behind ultrasonic drying, and several working prototypes have been demonstrated. This technology is based on direct mechanical coupling between mesh piezoelectric transducers and wet fabric. During the atomization process, vertical oscillations of a contained liquid, called Faraday excitations, result in the formation of standing waves on the liquid surface. At increasing amplitudes and frequencies of oscillation, wave peaks become extended and form “necks” connecting small secondary droplets to the bulk liquid. When the oscillation reaches an acceleration threshold, the droplet momentum is sufficient to break the surface tension of the neck and enable the droplets to travel away from the liquid. In this work, we investigate the atomization process using an ultrasonic transducer as it pertains to moisture retained within a fabric. An experimentally validated electromechanical analytical-numerical model is proposed. This model bridges the vibrations of a piezoelectric mesh transducer to the critical acceleration needed for fabric drying to occur. Then, the drying rate model is developed, consisting of an initial nonlinear region due to atomization, followed by a linear thermal evaporation region. The models developed identify the influence of key parameters on ultrasonic drying and will aid in improving atomizer design for efficient, timely fabric drying. This study is the first proposed model for the ultrasonic atomization of fabrics saturated with water, applicable to any type of transducer. The results present a non-dimensional equation for the ultrasonic dewatering of fabrics, dependent only on transducer acceleration and the surface area of the cloth. The development of this technology using the proposed physical models will allow for global reductions in electrical demand related to clothes drying.

Recent Advances in Computational Modeling of Primary Atomization of Liquid Fuel Sprays

Recent advances in modeling primary atomization in order to enable accurate practical-scale jet spray simulation are reviewed. Since the Eulerian–Lagrangian method is most widely used in academic studies and industrial applications, in which the continuous gas phase is treated in the Eulerian manner and droplets are calculated as Lagrangian point particles, the main focus is placed on improvement within this framework, especially focusing on primary atomization where modeling is the weakest. First, limitations of the conventional methods are described and then novel modeling proposals intended to tackle these issues are covered. These new modeling proposals include the Eulerian surface density approach, and the hybrid Eulerian surface/Lagrangian subgrid droplet generation approach. Compared to conventional simple yet sometimes non-physical models, recent models try to include more physical findings in primary atomization which have been obtained through experiments or direct numerical simulation (DNS). Model accuracy ranges from one that still needs some adjustment using experimental or DNS data to one which is totally self-closed so that no parameter tuning is necessary. These models have the potential to overcome the long-recognized bottleneck in primary atomization modeling and thus to improve the accuracy of whole spray simulation, and may greatly help to improve the spray design for higher combustion efficiency.