Spray Simulation Research Articles

Numerical Simulation of Protective Spraying by Helicopter-Type Unmanned Aerial Vehicles

Numerical Simulation of Protective Spraying by Helicopter-Type Unmanned Aerial Vehicles

Multiscale simulation of spray and mixture formation for a coaxial atomizer

Coaxial atomization is an established atomization strategy for many stationary combustion systems. While modeling spray formation in coaxial atomization is challenging due to the existence of a wide range of length and time scales, typical models introduce a substantial uncertainty for Euler–Lagrange simulations of the actual application, e.g., a spray flame. To reduce uncertainties, a recently proposed multiscale approach is adopted for simulations of realistic applications in this work. The multiscale approach uses three one-way coupled simulation domainss that cover the internal nozzle flow, the interfacial flow of the near-field, and the dispersed flow of the far-field. The capabilities of the approach are explored by applying it to a standardized non-reacting experiment from the flame spray pyrolysis research community. In order to assess the relevance for application simulations, results are discussed in the context of mixture formation. The results are compared against shadowgraphy images of the near-field and measured droplet statistics in the far-field. It is found that the multiscale approach is capable of providing similarly accurate droplet statistics as experiments or models derived from them. In addition, it is found that the breakup dynamics in the near-field introduce substantial mixture fraction fluctuations. These fluctuations are only included because of the deterministic coupling of the multiscale approach and are typically neglected in conventional approaches.

On the question whether surface roughness can explain the absence of a prominent single-collision peak in keV heavy-ion scattering off a polycrystalline Ru surface

In a joint experimental and modeling effort, we have studied 15 keV Ar2+, Kr2+, and Xe2+ projectiles scattering off a polycrystalline Ru surface to assess the possible role of surface roughness in the apparent near-absence of the single-collision (SC) peak for Xe, the heaviest of the ions. The surface roughness of the Ru sample was determined by atomic force microscopy (AFM). The AFM image is used as input to simulations by means of the ray tracing code SPRAY. The observed spectra display a significant SC peak in the energy distributions of reflected Ar and Kr ions, which is not the case for Xe. The energy spectrum produced by the reflection of Xe and recoiled Ru ions closely corresponds to the results obtained from our SPRAY simulations. It is evident that surface roughness plays a crucial role in the visibility of the SC peak. From simulations for different target roughnesses, it is clear that a concentrated distribution of inclination angles centered around 2° effectively reduces the intensity of the single-collision peak for Xe ion scattering. The presence of a distinct SC peak for Ar and Kr ions, unlike Xe ions, supports the notion that the absence of a prominent SC peak in Xe ion scattering is not primarily due to geometric blocking of trajectories by surface roughness. This suggests that for slow, heavy ions like Xe, scattering effects arising from the nearest neighbors to the binary collision partner are significant and should be carefully examined.

Numerical simulation of agricultural unmanned helicopter corn canopy spraying

Plant protection by unmanned aerial vehicle (UAV) is a highly complicated process, especially considering the vast and diversified crop canopy. Furthermore, the droplet dispersal mechanism above and inside the crop canopy remains unclear. Owing to this, the authors used the lattice Boltzmann method (LBM) to simulate the application of plant protection UAV. The LBM is suitable not only to simulate the rotor tip vortex of UAV but also to model the crop canopy with detailed geometry. It is found that the streamwise and spanwise speed of the downwash flow achieve an extreme value both on top of the corn canopy and near the ground. When the droplet diameter decreased from 400 to 50 μm, its deposit region width doubled and its floating time increased from 3 s to more than 12 s. Small droplets are more likely to be lifted by the secondary flow near the ground and drift for a longer distance, while larger droplets are deposited directly. The increase in application height weakened the secondary spanwise flow near the ground and caused less droplet re-lifting and spanwise movement. The best point to initiate and terminate spraying is 2.5 m after passing the target corn canopy region. The droplet penetration rate in the canopy decreased with an increase in droplet size. This study used the LBM to model the unmanned helicopter application on a full corn canopy. Droplet deposition and distribution correspond to droplet size and canopy structure. The simulation results establish a foundation to optimize agricultural aerial spraying in an efficient and methodical manner.

Dynamic coupling effects of injection on spray and mixture formation in an asymmetrically vibrating combustion engine

Enhanced methods of energy conversion present a viable solution for addressing energy load challenges. This study introduces a vibrating combustion engine designed to enhance energy conversion efficiency by harnessing flexible dynamics and variable asymmetric motion. However, the consideration of its unique dynamic effects has emerged as a pivotal challenge in fuel-air mixing and combustion research. This study introduces a fuel spray and mixing simulation method that facilitates the reciprocal exchange of coupling parameters between the combustion and dynamics models. It iterates the simulation outcomes of motion and combustion to predict mixture formation. Subsequently, the method is employed to explore the impact of injection position on fuel spray and mixture formation. Results show that there is an optimum injection position for the operation frequency, and retarding or advancing leads to slow compression and weak gas motion for fuel spray and mixing, although retarded injection provides high in-cylinder gas pressure and temperature. The early injection generally causes long penetration, small droplet diameter, slight impingement, and fast evaporation, unless it is too early. The results further indicate that when IAP = 6 mm, a more uniform mixture can be attained at the beginning of combustion, leading to a thermal efficiency of up to 42.9% for the engine.

Orifice section velocity fitting method and its application in flash spray research

Although spray technology is widely used, research on spray flow is limited by the complexity of its flow field. To simplify the spray simulation, this work proposed an orifice section velocity fitting method, which can simplify the calculation of the flow field inside the nozzle by using a set of special velocity fitting equations as boundary conditions. For the application and verification of the method, the characteristics of methyl nonafluorobutyl ether (HFE7100) flash spray under temperature influence were experimentally studied in this paper using phase Doppler particle analyzer equipment and compared the results of simulation and experiment. The comparison results show that the simulation and experimental results of the spray axial velocity distribution have good consistency at different temperatures. The simulation results show that the swirling flow in the orifice is stronger when the temperature is lower than the boiling point and the spray velocity isosurface is conical. The swirling flow in the nozzle is attenuated by flash evaporation when the temperature increases above the boiling point, while the spray velocity isosurface changes to a bell shape. The experimental results show that the spray velocity increases with increasing evaporation caused by increasing temperature and that the spray axial velocity distribution also changes from a saddle shape to a single peak shape. The spray droplet size increased significantly under the influence of flash evaporation, but the spatial distribution maintained a saddle shape. This study can provide a reference for spray simulation analysis and the study of flash spray characteristics.

Spray formation and spray-wall-flow interaction in a gasoline direct-injection (GDI) engine under early-injection conditions: A flow bench study

The work presented in this study aims to understand the spray-wall-flow interaction within a gasoline direct-injection (GDI) engine flow bench under simulated early-injection conditions. The Engine Combustion Network (ECN) Spray G injector is installed in the Darmstadt optically accessible engine flow bench. Under simulated early-injection conditions, the formation of a multi-hole spray and the interaction with characteristic intake flows, such as the intake jet and central tumble flow, are extensively discussed. By reducing the complexity in the number of variables inherent in engine flow and whole-engine simulation, an engine flow bench operating under various mass flow rates is applied in this study. The numerical simulation is carried out using Large Eddy Simulation (LES) under the Eulerian-Lagrangian framework for spray simulation. Experimental data, acquired through particle image velocimetry (PIV) measurements, provides 2-D flow fields on both the central tumble and valve planes, facilitating the validation of in-cylinder flow fields. Furthermore, experimental data obtained through Mie scattering is utilized to investigate spray formation and evolution within the GDI engine, providing the liquid penetration length and liquid spray angle. Comparison between the numerical and experimental data demonstrates several agreements. Moreover, the variation of different spray plumes under different mass flow rates is observed in the case of both experimental and numerical data. Increasing the mass flow rate distorts the overall plume shape and shifts it away from the intake port. This phenomenon is examined by extracting the liquid volume fraction and vapor fields of each plume. Spray plumes encounter different convective disturbances and evaporation due to their local characteristic in-cylinder flow. Furthermore, spray-wall-flow interaction and wall film deposits are observed during the injection. Lastly, the influence of the spray-induced turbulence is analyzed under different mass flow rates.

CFD-EDEM simulations of droplets in an airless rotary spray coating process

Numerical simulations of airless rotary spray coating process are presented using CFD-EDEM with JKR contact model. The JKR model parameters are suggested according to the virtual test of accumulation angles in the funnel device. The reasonable droplet-wall surface energy is recommended from a relation of accumulation angles. The gas turbulence model is evaluated based on the comparisons of velocities and droplet number fractions using RNG k-ε, realizable k-ε and transition SST models. The simulated film thicknesses agree with measurements in literature. The correlation of exit droplet velocity of sprayer holes is proposed with a discharge coefficient of 2.27. The velocities and film thickness are predicted with changing rotation speeds. The normal force is an order of magnitude larger than the tangential force of droplet-wall collisions, and the ratio of the normal to tangential energy losses of droplet-wall collisions varies from 2.0 to 5.0 in airless rotary spray coating processes.

Research on Scavenging Flow Dynamics of Marine Two-Stroke Engines With a CFD-Derived Quasi-Dimensional Model

The complex in-cylinder gas state and flow significantly affect fuel-air mixture, combustion efficiency and emissions. However, the scavenging CFD model of large bore marine engines in digital twin system requires substantial computational resources. Therefore, a fast-run phenomenological model of the scavenging process is built in this study. The model simulates the thermal states and flow dynamics of inlet and exhaust gas based on the energy and momentum conservation principles, considering the ideal in-cylinder swirl velocity profile with effects of air mass loss, wall friction, and swirl shear. The model’s accuracy is confirmed by comparing it with CFD simulations on different engines, showing an average relative error less than 3.5%. It also analyzes the impacts of intake pressure. The model provides accurate boundary conditions for subsequent fuel spray, combustion, and emission simulations and can be combined with these models in the future, thereby applied to engine design, diagnostics, and control.

A one-way coupling approach for simulating in-nozzle flow and spray characteristics of a pressure-swirl atomizer

This article proposes a model framework coupling in-nozzle flow and external spray and presents its application to the simulation of a commercial pressure-swirl atomizer, focusing on the transient characteristics of the internal flow and subsequently the impact on the spray characteristics. High-fidelity in-nozzle simulation of the liquid–gas interactions is performed using the volume-of-fluid (VOF) method. Then, a corresponding Lagrangian simulation of sprays is performed where the parcels are injected using the information from the VOF predictions instead of phenomenological models. Both the internal flow and the spray are compared to the experimental data that are available in the literature, and satisfactory agreement is obtained in terms of the in-nozzle velocity, film thickness, and Sauter mean diameter. The effect of the different liquid properties and geometric features on the air–core formation, and consequently, on the spray characteristics have been obtained directly through spray simulation coupled with nozzle flow. As indicated by the Eulerian simulation results, the viscosity plays a key role in the formation of the air core, as the hollow-cone shape can degenerate into a solid cylindrical liquid jet under high viscosity conditions. Additionally, significantly distinct spray characteristics in terms of droplet velocity, mean diameter, and penetration were predicted depending on the formation of air core. Even if there is no stable air core in the nozzle, the spray is still discharged in a swirling motion. As opposed to the converging angle and orifice length, the nozzle diameter has a direct correlation with the formation of air core and spray atomization. This study implies that the in-nozzle flow field, which is usually ignored in fuel spray simulation, has a substantial impact on the spray characteristics and should be taken into account for design optimization by applying the developed one-way coupling approach.

Dynamic Numerical Simulation of Curved Surface Coating Trajectory Based on STL Slicing Algorithm

The thickness of the coating on the surface of a workpiece is an important factor in determining the quality of spraying. However, it is challenging to estimate the distribution of film thickness accurately before the actual spraying process. This lack of estimation hinders the optimization of spraying process parameters and trajectory. To overcome this, a numerical simulation of surface spray coating thickness was conducted to provide guidance for the actual coating process. The research consists of three main parts. Firstly, the spray trajectory of the spray gun is determined using the proposed Stereo Lithography (STL) model slicing algorithm. Secondly, a two-phase flow spray model and collision adhesion model are established to construct the spray film model. The surface mesh is determined, and the spraying process parameters are set. Finally, numerical simulation is conducted to analyze the dynamic spraying trajectory and the distribution of coating thickness. The results show that the coating thickness distribution on an arc surface is thicker in the middle and thinner on the edges. The distribution is symmetric with respect to both the transverse and longitudinal directions of the arc surface. The coating thickness distribution at both ends is not as uniform as in the middle section. The concave part of the free surface has the largest coating thickness, while the coating thickness distribution on the convex part is not as uniform as on the relatively flat part. This method of simulating the coating thickness distribution on complex surfaces provides a solid foundation for further optimization of spraying process parameters and trajectory, ultimately improving the qualification rate of workpiece spraying processing.

Sea spray emissions from the Baltic Sea: comparison of aerosol eddy covariance fluxes and chamber-simulated sea spray emissions

Abstract. To compare in situ and laboratory estimates of sea spray aerosol (SSA) production fluxes, we conducted two research campaigns in the vicinity of an eddy covariance (EC) flux tower on the island of Östergarnsholm in the Baltic Sea during May and August 2021. To accomplish this, we performed EC flux measurements for particles with diameters between 0.25 and 2.5 µm simultaneously with laboratory measurements using a plunging jet sea spray simulation chamber containing local seawater sampled close to the footprint of the flux tower. We observed a log-linear relationship between wind speed and EC-derived SSA emission fluxes, a power-law relationship between significant wave height and EC-derived SSA emission fluxes, and a linear relationship between wave Reynolds number and EC-derived SSA emission fluxes, all of which are consistent with earlier studies. Although we observed a weak negative relationship between particle production in the sea spray simulation chamber and seawater chlorophyll-α concentration and a weak positive relationship with the concentration of fluorescent dissolved organic matter in seawater, we did not observe any significant impact of dissolved oxygen on particle production in the chamber. To obtain an estimate of the size-resolved emission spectrum for particles with dry diameters between 0.015 and 10 µm, we combined the estimates of SSA particle production fluxes obtained using the EC measurements and the chamber measurements in three different ways: (1) using the traditional continuous whitecap method, (2) using air entrainment measurements, and (3) simply scaling the chamber data to the EC fluxes. In doing so, we observed that the magnitude of the EC-derived emission fluxes compared relatively well to the magnitude of the fluxes obtained using the chamber air entrainment method as well as the previous flux measurements of Nilsson et al. (2021) and the parameterizations of Mårtensson et al. (2003) and Salter et al. (2015). As a result of these measurements, we have derived a wind-speed-dependent and wave-state-dependent SSA parameterization for particles with dry diameters between 0.015 and 10 µm for low-salinity waters such as the Baltic Sea, thus providing a more accurate estimation of SSA production fluxes.

Large-Eddy Simulation Study of Injector Geometry and Parcel Injection Location on Spray Simulation of the Engine Combustion Network Spray G Injector

Abstract Recent improvements in computing power and numerical techniques have enabled us resolve minute details of spray plume behavior and its wall boundary interactions, and made detailed spray simulations using large-eddy simulation (LES) turbulence models available to many more engineers. However, guidelines for parcel-based spray simulation boundary and initial conditions are still based on results from lower-resolution and Reynolds averaged Navier–Stokes (RANS) simulations. Hence, it is necessary to critically examine those assumptions and compare their impact with results using a RANS turbulence model. Three different parameters, including whether a simulation includes a detailed injector tip geometry or a flat surface, whether parcels are initialized at the counterbore exit, which is more common, or at the nozzle exit, and whether to use an experimentally derived rate of injection or one-way coupling with a separate internal nozzle volume of fluid simulation, were examined with an LES turbulence model. Both local data close to the injector and global penetration results were used to compare simulations. Local data, such as the local liquid volume fraction, showed greater variation between the conditions, which may have an impact on mixing and combustion predictions in engine applications. Spray penetration and other global measures demonstrated limited sensitivity to the boundary conditions/initialization procedure. Results were also compared with prior results that used a RANS turbulence model. RANS simulations had overall smoother responses to the changes, as would be expected, but LES simulations showed similar trends in the effects for the measured variables.

A grid-independent dispersion approach for large-eddy simulation of spray

This study proposes a subgrid-scale (SGS) dispersion approach for large-eddy simulation (LES) of spray. Rather than associating the turbulence correlation timescale used in the dispersion approach with the computational grid, we put forward an approximation of this timescale proportional to the Kolmogorov timescale. The Kolmogorov scale kinetic energy is added to the variance of the stochastic part of the SGS dispersion velocity to account for the effects of micro-scale flows on droplets. The proposed approach supports the notion of grid-independent LES and is illustrated for two benchmark Engine Combustion Network's Spray-A cases, including a vaporizing spray case and a non-vaporizing spray case. We investigate the performance of the approach in LESs of two cases by comparing it with other dispersion approaches. In particular, the investigation mainly covers spray structures, vapor and liquid penetrations, and mass fraction profiles. Results obtained using the proposed approach shows the statistics of the simulated spray cases that exhibit both good agreement with measurements and a noticeable improvement over simulations based on other tested approaches. The approach also yields more consistent results under different grid resolutions, which shows its ability to achieve LESs in non-uniform grids.

Derivation of a universal constant set in the Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) breakup model for spray simulations of various fuels

Derivation of a universal constant set in the Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) breakup model for spray simulations of various fuels

Ethanol Inhomogeneous Lean Combustion Under Different Ambient Temperatures and Pressures with Hydrogen Enrichment

ABSTRACT The effects of different ambient temperatures and pressures on the combustion of hydrogen-enriched liquid ethanol spray were studied in a constant-volume vessel, using experimental and numerical methods. The inhomogeneous combustion experiment was conducted under ultra-lean conditions at an overall equivalence ratio of 0.3 (33% spray ethanol/67% premixed hydrogen). The spray and flame images and combustion pressures were obtained. The spray simulation models used were the RANS k-ε turbulence model, KH-RT model, and wall film interaction model. The experimental results showed that the inhomogeneous combustion of the liquid ethanol spray can be divided into two stages: a rapid-burning and a slow-burning stage. The slow-burning stage was sensitive to changes in ambient temperature and pressure. Combustion duration decreased with elevated pressure and declined temperature. Under low pressure and medium temperature conditions, the flame reached its fastest propagation speed. The simulation results showed that the fuel concentration and concentration gradient around the ignition position are the key factors that affect inhomogeneous lean combustion. When the equivalence ratio was over 0.8 around the ignition position and the gradient of the equivalence ratio increased no less than 0.1/mm, the optimal combustion characteristics were achieved.

Numerical simulation of multi-array spray cooling for hot rolled seamless steel pipes

Spray cooling technology is an efficient cooling method. In the industrial application, the large area cooling requires the multi-nozzle spray design. Reasonable selection of nozzle array form is particularly important. Therefore, the simulation is applied in this study to investigate the effect of the nozzle array on the spray cooling of steel pipes after the model is validated. A steel pipe with a diameter of 140 mm, a wall thickness of 20 mm, and a length of 350 mm is employed as the research object, with an initial temperature of 900 °C. The scheme has four options for double spray spacing: OLR = −15 %, 0 %, 15 %, and 30 %. The results show that when the OLR grows, the cooling area gradually diminishes, and the cooling area's average temperature and average surface heat flux fall and subsequently increase. When OLR=30 %, the best cooling zone shape is obtained. The minimum average temperature, 231 °C, is obtained when OLR = 15 %. After comprehensive analysis, the ideal spray spacing range is found at OLR = 15 %∼30 %. In order to study the nozzle array, spray nozzles are arranged in multiple rows according to OLR = 30 %. This study also explores the strategies of unidirectional inclined spray and opposite inclined spray for lessening the area of the hot spot. There are six nozzle array arrangements in total. By comparing the results, it is discovered that the inclined spray increases the cooling area and accelerates the surface cooling of steel pipes. Under the side-by-side unidirectional inclined spray and the staggered unidirectional inclined spray, the cooling areas are 146,868 mm2 and 142,914 mm2 respectively, the average temperatures are 157.09 °C and 154.28 °C respectively. It is indicated that side-by-side inclined spray and staggered inclined spray are the better nozzle array arrangement to obtain the maximum cooling efficiency.

Numerical study on spray cooling of aircraft equipment cabin

With the increase in the number of aircraft onboard equipment and the sharp increase in heat dissipation power, the thermal control of cabin equipment has become one of the important issues restricting the development of high-speed aircraft technology. Spray cooling has the advantages of high cooling efficiency, simple cooling system structure, easy adjustment and control, etc. It is one of the most effective methods for thermal control of the aircraft equipment cabin. This work builds a simplified physical model of the aircraft equipment cabin, and improves the simulation method of the spray cooling of the equipment cabin. To test the accuracy of the simulation method, an experimental validation study was carried out. In the three-dimensional numerical simulation of cabin spray cooling, the structure characteristics of multi-nozzle spray droplet field, temperature field and flow field are addressed. It’s found that using multi-nozzle spray cooling for large-area and multi-surface equipment cabins is feasible, which can effectively control the temperature of most surfaces. However, for some slit areas, such as between adjacent equipment and between the equipment and the cabin, the temperature is relatively high due to the ineffective coverage of spray cooling. An improved method was applied to investigate the variation of temperature and pressure of the cabin during spray cooling. The study found that the pressure in the cabin can be flexibly controlled by adjusting the size of the exhaust hole of the cabin. The matching of the heating power of the cabin and the spray cooling capacity is an important guarantee condition for realizing the temperature and pressure control of the cabin. This study provides valuable reference data for engineers to design aircraft cabin cooling schemes.

Physics-based reduced-order modeling of flash-boiling sprays in the context of internal combustion engines

Flash-boiling injection is one of the most effective ways to accomplish improved atomization compared to the high-pressure injection strategy. The tiny droplets formed via flash-boiling lead to fast fuel–air mixing and can subsequently improve combustion performance in engines. While most of the previous studies related to the topic focused on modeling flash-boiling sprays using three-dimensional (3D) computational fluid dynamics (CFD) techniques such as direct numerical simulations (DNS), large-eddy simulations (LES), and Reynolds-averaged Navier–Stokes (RANS) simulations, the present work introduces a reduced-order cross-sectionally averaged spray (CAS) model with significantly reduced computational cost. The proposed CAS model incorporates several physical submodels in flash-boiling sprays such as those for air entrainment, drag, superheated droplet evaporation, flash-boiling induced breakup, and aerodynamic breakup models. The CAS model is then applied to different fuels to investigate macroscopic spray characteristics such as liquid and vapor penetration lengths under flash-boiling conditions. It is found that the newly developed CAS model captures the trends in global flash-boiling spray characteristics reasonably well for different operating conditions and fuels. Moreover, the CAS model is shown to be faster by up to four orders of magnitude compared with simulations of 3D flash-boiling sprays. The novelty of the present work lies in providing a reduced-order flash-boiling spray model that offers cost-effective computational representations of complex phenomena. The proposed model can be very valuable for applications, such as experimental design, fuel screening, and creating digital twins, thus playing a crucial role in the advancement of research and development in the field of spray combustion.

Large eddy simulation investigation of ammonia spray characteristics under flash and non-flash boiling conditions

Ammonia is an ideal zero-carbon fuel for energy systems, and it can be used directly in a liquid state. However, liquid ammonia is susceptible to flash boiling due to its unique physical and chemical properties, which brings challenges for liquid ammonia spray and combustion simulations. In the present study, high-pressure liquid ammonia injection under flash boiling and non-flash boiling conditions was investigated. Large eddy simulations were conducted in an Eulerian–Lagrangian framework. The aim is to explore in detail the influence of the critical parameters on the characteristics of liquid ammonia spray. Under the flash boiling condition, the effects of ambient pressure were examined. Comparisons of the measured and predicted spray penetration and morphology demonstrate that the present simulations can reproduce liquid ammonia spray characteristics well. Varying ambient pressures causes the ammonia spray to be at different superheat levels, resulting in significant changes in spray features. Under the non-flash boiling condition, the effects of injection pressure and ambient pressure were investigated. The higher injection pressure feature a higher gas velocity and improves the mixing of ammonia and air. The ammonia spray propagates more rapidly and the spray width becomes wider at lower ambient pressure.