Spray Model Research Articles

Numerical modelling of droplet breakup for flash-boiling fuel spray predictions

Flash-boiling of fuel sprays can occur under injection of superheated fuel into ambient pressure that is lower than the saturation pressure of the fuel and can dramatically alter spray formation due to complex two-phase flow effects and rapid droplet evaporation phenomena. Such phenomena exist in-cylinder at low-load in-city driving conditions where strict engine emission regulations apply, hence the need for faithful flash-boiling fuel spray models by engine designers. To enhance the current modelling capability of superheated fuel sprays, with focus on near-nozzle plume expansion, a flash-boiling breakup modelling approach was developed to introduce the thermal breakup mechanism of droplets caused by nucleation and bubble growth. This model was particularly aimed at sprays where levels of superheat introduced noticeable radial expansion of the plumes upon discharge from the nozzle orifice. The model was able to simulate droplet shattering by introducing Lagrangian child parcels at breakup sites with additional radial velocity components instigated by rapid bubble growth and surface instabilities. Combination of the flash-boiling droplet breakup model with a flash-boiling effective nozzle model that was used as boundary condition for the spray plumes offered a more complete modelling approach, where both in-nozzle phase change effects and near-nozzle flashing through droplet shattering were incorporated into the Eulerian-Lagrangian two-phase computational framework. Sensitivity studies were carried out to investigate important parameters which are inherently difficult to measure experimentally and offered valuable insight into modelling superheated sprays. The model was able to capture important flash-boiling spray characteristics and quantitative validation was achieved through comparison to experimental data in the form of penetration lengths and droplet sizes with a good level of agreement.

New perspectives on capturing particle agglomerates in CFD modeling of spray dryers

Current computational fluid dynamic (CFD) models of spray dryers lack the capability to predict the structure of the agglomerates formed; loose or compact agglomerates. This is mainly due to the conventional simplistic approach in numerically “fusing” of the colliding particles forming the agglomerate. A new theoretical treatment is introduced in this work, suitable for implementation in CFD simulations, which numerically fuses the particles and yet retain information on the structure of the agglomerate. This new theoretical treatment is based on tracking the reduction of the agglomerate surface area as the agglomerate is progressively formed. Analysis revealed that the reduction in the agglomerate surface area exhibits a unified correlation with the degree of compactness of the agglomerate. Further analysis comparing this new approach to the conventional numerical fusing of the particles revealed inherent numerical discrepancies, which has not been noted in the literature before. Understanding these discrepancies will provide clarity to the interpretation of the modelling and simulation of spray drying particle agglomeration in CFD. Moreover, this work lays the groundwork for a more comprehensive CFD model for agglomeration which can be potentially utilized to predict final powder properties.

Biocidal spray product exposure: Measured gas, particle, and surface concentrations compared with spray model simulations

The purpose of the study was to compare measured air and surface concentrations after application of biocidal spray products with concentrations simulated with the ConsExpo Web spray simulation tool. Three different biocidal spray products were applied in a 20 m3 climate test chamber with well-controlled environmental conditions (22 ± 1 °C, 50 ± 2% relative humidity, and air exchange rate of 0.5 h−1). The products included an insect spray in a pressurized spray can, another insect spray product, and a disinfectant, the latter two applied separately with the same pumped spray device. The measurements included released particles, airborne organic compounds in both gas and particle phase, and surface concentrations of organic compounds on the wall and floor in front of the spraying position and on the most remote wall. Spraying time was a few seconds and the air concentrations were measured by sampling on adsorbent tubes at 9–13 times points during 4 hr after spraying. The full chamber experiment was repeated 2–3 times for each product. Due to sedimentation the concentrations of the particles in air decayed faster than explained by the air exchange rate. In spite of that, the non-volatile benzalkonium chlorides in the disinfectant could be measured in the air more than 30 min after spraying. ConsExpo Web simulated concentrations that were about half of the measured concentrations of the active substances when as many as possible of the default simulation parameters were replaced by the experimental values. ConsExpo Web was unable to simulate the observed faster decay of the airborne concentrations of the active substances, which might be due to underestimation of the gravitational particle deposition rates. There was a relatively good agreement between measured surface concentrations on the floor and calculated values based on the dislodgeable amount given in the selected ConsExpo Web scenarios. It is suggested to always supplement simulation tool results with practical measurements when assessing the exposure to a spray product.

Investigation of the process parameters and restitution coefficient of ductile materials during cold gas dynamic spray (CGDS) using finite element analysis

Cold gas dynamic spray is a cold spray technique for obtaining solid-state surface coating. Several materials such as metal, metal alloys, composite materials, and polymer have been deposited successfully through cold spray onto a substrate material. A number of industrial applications for cold spray have been developed worldwide in the field of aerospace, energy, automobile, biotechnology, and military applications. In the current study, effects of various processing parameter such as impact velocity, substrate preheating temperature, a combination of different materials and coefficient of friction were used to describe the impact behaviour of ductile materials (copper, Cu, and aluminium, Al) after deposition to find a way of addressing high-strain-rate dynamic problems. The parameters were also used to verify the deposition process for the modelling of cold gas dynamic spray (CGDS) by the Lagrangian approach of finite element analysis. The results of the analysis (simulation) and that of the published experimental results in the literature correlated well. The understanding of the impact behaviour using different parameters was evident by the analysis of temperature and equivalent plastic strain (PEEQ). It was discovered that the deposition process and deformation are largely affected by particle material as compared to the substrate. A lower restitution coefficient was obtained when different materials of varying properties were combined compared to the combination of the same material. Also, the parameters under investigation do not affect the CGDS process individually, as their effects are interrelated.

Design and performance evaluation of a variable-rate orchard sprayer based on a laser-scanning sensor

To improve the precision spraying strategy and reduce excessive pesticide application in orchards, an air-assisted sprayer integrated with a laser-scanning system was developed to realize the toward-target variable-rate spraying. In the spray control system, a method of calculating canopy gridding volumes was designed to ensure the canopy was divided into a uniform grid size, a variable-rate spray model was used in the flow rate decision software to control the spray output according to the canopy gridding volumes and travel speed, and a method of saving and accessing spray data was used to control the spray delay. The effects of different grid sizes and travel speeds on the spray performance were evaluated by quantifying spray coverage uniformity inside tree canopies. The results showed that spray coverage uniformity declined with increasing grid width from 0.14 to 0.28 m although the mean spray coverage on each target location showed no significant differences. Additionally, there were no significant variations in mean spray coverage at speeds of 1.0, 1.2 and 1.4 m/s for a tested tree of 1.6 m width and at any experimental speeds for a tested tree of 1.3 m width, which indicated that the variable-rate sprayer could provide good spray coverage uniformity under various travel speeds with a canopy size limitation. Compared with the same sprayer without the variable-rate spray function, the intelligent sprayer prototype realized effective toward-target spraying and avoided overspraying while providing sufficient spray coverage. Keywords: laser-scanning sensor, variable-rate spray, orchard, precision sprayer, pesticide application efficiency DOI: 10.25165/j.ijabe.20191206.4174 Citation: Cai J C, Wang X, Gao Y Y, Yang S, Zhao C J. Design and performance evaluation of a variable-rate orchard sprayer based on a laser-scanning sensor. Int J Agric & Biol Eng, 2019; 12(6): 51–57.

Improved urea-water solution spray model for simulations of selective catalytic reduction systems

Successive diesel engine emissions’ regulations impose increasingly tighter limits on the automotive industry. A well-established method to meet nitrogen oxides emissions’ requirements is selective catalytic reduction with a urea-water solution injection. State-of-the-art designs are based on close-coupled exhaust architecture which suffers from a lack of space for a proper decomposition of a urea-water solution. The optimization of such systems is very challenging and is always intensively supported by numerical simulations. Within this work a two-zone spray representation for a urea-water solution injection is proposed. The two-zone approach captures a non-uniform droplet distribution inside the spray cone while maintaining a reasonable calculation time, since it is still based on a Lagrangian method. The proposed model was applied into exhaust system simulations and tested against a standard spray representation. The results revealed that uniform spray representation led to a smaller wall film formation and different mass balance. Ammonia distribution at the catalyst inlet was not directly affected significantly; however, in real systems one could expect catalyst clogging due to wall film formation, followed by a decrease of ammonia distribution uniformity. The results clearly show that uniform spray representation may lead to misestimation of the selective catalytic reduction system’s performance and the novel two-zone approach should be used instead.

Extended validation and verification of XPS/AVL-Fire™, a computational CFD-DEM software platform

The goal of this work is an comprehensive experimental validation and verification of a computational CFD-DEM software platform. The software platform's modules and performance requirements specifically address the needs of pharmaceutical industry in terms of batch size (i.e., number of particles) and related phenomena (i.e., physical complexity). Moreover, we critically assessed the numerical models implemented. In addition to the review of relevant literature and selection of validation experiments, novel analytical solutions for the spray class in question are presented. As the presented method is independent of the spray model, the solution can be used to prove the correct implementation of various available spray models. The final outcome of this work is a validated software framework, which can be further used to investigate large-scale processes by resolving single particle trajectories in a coupled environment, including heat, mass and momentum transfer.

Impact of Spray Droplet Distribution on the Performances of a Kerosene Lean/Premixed Injector

Large-Eddy Simulation (LES) of a novel lean-premixed (LP) injection system for aeronautical burners is performed at elevated pressure / temperature operating conditions. This LP injector was designed to operate at high-pressure and high-temperature and it was experimentally investigated at DLR Cologne and at CORIA laboratory in Rouen. At DLR Cologne, the purpose was to measure droplets sizes and velocities thanks to the Phase Doppler Anemometry (PDA) technique. At CORIA, combustion under realistic pressure conditions was investigated on the HERON test rig with advanced and combined optical diagnostics, such as OH and Kerosene Planar Laser-Induced Fluorescence (PLIF). It was observed that the flame topology strongly depends on the pressure. This configuration is therefore an interesting validation database for LES. In this LP injector, liquid kerosene is injected far upstream of the injector in order to provide a gaseous fuel distribution as homogeneous as possible. In the LES, the full geometry of the injector is taken into account. One reference operating point at 8.33bar and 669.3K is simulated. A Lagrangian point-particle approach is chosen for the spray modeling with a prescribed Rosin-Rammler diameter distribution at injection. The two-phase combustion model relies on a tabulated chemistry approach. The flame/turbulence interactions are included via a presumed-PDF closure (PCM-FPI model). The impact of spray characteristics on the flame is analyzed through a dedicated parametric study. Four sets of parameters each with a different Sauter Mean Diameter are used to describe the spray droplet size distribution. An analysis of the characteristic evaporation time and its influence on the flame has been carried out. Numerical results are then compared with experimental data in order to bring detailed insights into the flame topology. This parametric study shows that the quality of the atomization strongly influences the flame topology and the fuel distribution inside the combustion chamber.

Investigation on an Injection Strategy Optimization for Diesel Engines Using a One-Dimensional Spray Model

Common rail systems have been widely used in diesel engines due to the stricter emission regulations. The advances in injector technology and ultrahigh injection pressure greatly promote the development of multiple-injection strategy, leading to the shorter injection duration and more variable injection rate shape, which makes the mixing process more significant for the formation of pollutant emission. In order to study the mixing process of diesel sprays under variable injection rate shapes and find the optimized injection strategy, a one-dimensional spray model was modified in this paper. The model was validated by the measured spray penetrations based on shadowgraphy experiments with the varying injection rate. The simulations were performed with five injection rate shapes, triangle, ramping-up, ramping-down, rectangle and trapezoid. Their spray penetrations, entrainment rates and equivalence ratios along spray axial distance are compared. The potentials of multiple-injection and gas-jet after end-of-injection (EOI) to improve mixing process and emission reduction are discussed finally. The results indicated that ramping-up injection rate obtains the highest entrainment rate after EOI, and it needs 1.5 times of injection duration for the entrainment wave to arrive at the spray tip. For the other four injection rates, the sprays can be treated as a steady-like state, needing twice of injection duration from EOI to the time the entrainment wave reaches the spray tip. The multiple-injection with proper injection rate shape enhanced the entrainment rate, and the gas-jet after EOI affected the mixture distribution and entrainment rate in spray tail under ramping-down injection rate.

Modelling of sprays: simple solutions of complex problems

Some new approaches to the solution of complex problems, focused on spray modelling, using relatively simple mathematical tools are briefly summarised. The following problems are considered: modelling of spray drying with pharmaceutical applications, modelling of heating and evaporation of suspended droplets with applications to water sprays for fire suppression, modelling of micro-explosions with automotive applications, and parameterisations of slow invariant manifolds with applications to spray ignition modelling. The modelling of spray drying is based on the assumption that the array of small suspended solid particles inside droplets can be treated as a non-evaporating liquid component. The modelling of the effect of a support rod on droplet heating and evaporation is based on the assumption that this heat is homogeneously distributed inside the whole volume of a droplet. In the modelling of a puffing/micro-explosion it is assumed that a water sub-droplet is located exactly at the centre of a fuel droplet and the temperature at the surface of the fuel droplet is fixed. In the presentation of slow invariant manifolds in parametric form these manifolds are found using asymptotic expansions.

Eulerian CFD modeling of nozzle geometry effects on ECN Sprays A and D: assessment and analysis

Diesel spray modeling is a multi-scale problem with complex interactions between different flow regions, that is, internal nozzle flow, near-nozzle region and developed spray, including evaporation and combustion. There are several modeling approaches that have proven particularly useful for some spray regions although they have struggled at other areas, while Eulerian modeling has shown promise in dealing with all characteristics at a reasonable computational effort for engineering calculations. In this work, the [Formula: see text]–Y single-fluid diffuse-interface model, based on scale separation assumptions at high Reynolds and Weber numbers, is used to simulate the engine combustion network Sprays A and D within a Reynolds-averaged Navier–Stokes turbulence modeling approach. The study is divided into two parts. First of all, the larger diameter Spray D is modeled from the nozzle flow till evaporative spray conditions, obtaining successful prediction of numerous spray metrics, paying special attention to the near-nozzle region where spray dispersion and interfacial surface area can be validated against measurements conducted at the Advanced Photon Source at Argonne National Laboratory, including both the ultra-small-angle X-ray scattering and the X-ray radiography. Afterwards, an analysis of the modeling predictions is made in comparison with previous results obtained for Spray A, considering the nozzle geometry effects in the modeling behavior.

Three-Dimensional Modeling of Cold Spray for Additive Manufacturing

The cold spray process can be used to form three-dimensional objects following the layer-by-layer approach which is called cold gas dynamic manufacturing or cold spray additive manufacturing (CSAM). In this study, a three-dimensional simulation of CSAM for a high-pressure nozzle with axial powder injection is investigated. During the manufacturing of a desirable object by CSAM, formation of the sharp edges, inclined surfaces, stagnation points, and corners will inevitably influence the trajectories of the particles. This leads to dispersion and lack of particle deposition in these areas which can eventually reduce the precision and efficiency of the deposition process. Three different objects of a cylinder and two frustums with different angles have been numerically simulated on a substrate to represent typical additively manufactured parts. Particle trajectories and impact conditions, i.e., size and velocity distributions, with and without these objects, have been compared. The results of numerical modeling provided useful information for understanding the limitations and challenges associated with CSAM, which can help us to improve the quality and precision of particle deposition.

Polydispersity Effects in Low-order Ignition Modeling of Jet Fuel Sprays

ABSTRACT Low-order ignition models are important tools in the design of aviation gas turbines. In this paper, a stochastic model that predicts the ignition probability in a combustor based on a time-averaged cold-flow solution is extended to include local fuel concentration fluctuations due to the polydisperse nature of the spray. For this, a stochastic approach to modeling such fluctuations is considered, and the effects of the flow and mixture parameters on the resulting equivalence ratio pdfs are investigated. The concentration of fuel in large droplets results in a high variation of the local equivalence ratio, hence affecting the local flammability factor at the model’s cell scale. The extinction criterion of the ignition model based on a critical Karlovitz number is calibrated based on ignition probability data from canonical experiments using jet fuel, suggesting critical Karlovitz values of spray flames between 0.2 and 0.6, which is to be contrasted with values of 1.5 for gaseous fuels.

Consideration on aircraft tire spray when running on wet runways

The tire spray produced by aircraft running on wet runways may enter the intake of engines, which may lead to the compressor stall, surge, or even combustion flameout. Therefore, it is necessary to do some research work on the tire spray to help to solve this problem. Firstly, the mechanism of tire spray is analyzed, and some parameters are defined to describe the spray pattern. Secondly, the numerical model of tire spray is established by coupled Smoothed Particle Hydrodynamics (SPH) and Finite Element Method (FEM) in LS-DYNA software, and the model is validated by a simplified water spray experiment. Then some influence factors on the spray pattern are studied by numerical simulation. Finally, some suggestions are proposed on different types of aircraft to avoid too much tire spray being ingested into the engine intakes.

Toward predictive and computationally affordable Lagrangian–Eulerian modeling of spray–wall interaction

The work presented in this article focuses on improving the understanding of spray–wall interaction phenomena and the predictivity of computational models for modern internal combustion engine–like conditions. Previous work from the authors highlighted some of the limitations of the currently available spray–wall interaction sub-models, especially those based on experimental observations carried out on single droplet impingement of non-hydrocarbon fluids. Previous experimental observations and direct numerical simulations provided unique qualitative and quantitative details that were leveraged to improve Lagrangian–Eulerian simulations of spray–wall interaction. In this work, Yarin and Weiss’ theory for droplet train impingements, as interpreted by Stanton and Rutland, was implemented in the CONVERGE software. A novel and mathematically rigorous calculation of the impingement frequency for Lagrangian parcels was proposed to replace the zero-droplet-spacing assumption present in the original model and improve the prediction of droplet splash. Preliminary testing and validation of the new formulation against experiments carried out in a constant volume vessel showed encouraging results. The feedback loop between experiments, Lagrangian–Eulerian simulations, and direct numerical simulations also motivated the investigation of the surface roughness model available in the Lagrangian–Eulerian framework. This part of the study pointed out the importance of correctly predicting the interaction between liquid and surrounding gas in the near-wall region. Overall, the results shown indicate that correctly capturing the pre-impingement physics and liquid–gas interaction, together with improving splash predictions, was key for improving the accuracy of Lagrangian–Eulerian computational fluid dynamics simulations of spray–wall impingement.

Numerical simulation of the influence of fuel temperature and injection parameters on biodiesel spray characteristics

AbstractThe purpose of this study is to investigate the effects of fuel temperature and nozzle length‐diameter ratio (L/D) on biodiesel spray characteristics, under high injection pressure and high ambient pressure, by numerical method. The analysis of spray characteristics is carried out in conjunction with the transient flow inside nozzle. To achieve this, three‐dimensional calculation grids of spray nozzle and spray domain were setup, and the needle movement was achieved by dynamic mesh technique. The reliability of spray models was validated by experimental results. It was established that the variation in spray tip penetration (STP) and Sauter mean diameter (SMD) is similar at different initial conditions. The STP increases at a faster rate initially and then slows down, whereas the SMD gradually decreases with time after the injection. In addition, a sensitivity analysis of the effects of injection parameters on biodiesel spray characteristics was conducted. Compared to the nozzle L/D, injection pressure and fuel temperature have a greater impact on biodiesel spray characteristics. The increase in injection pressure has a significant effect on the velocity distribution, STP and SMD. When the injection pressure is increased from 100 to 200 MPa, the maximum velocity of the spray core zone increases by 33.96%, the STP increases by 27.17%, and the SMD reduces by 14.81%. Furthermore, an increase in fuel temperature mainly affects the concentration distribution and the SMD of atomized droplets. When the fuel temperature is increased from 300 to 350 K, the maximum concentration of axial spray center lowers by 25.41%, and the SMD decreases by 17.19%. However, the increase of L/D chiefly impacts the concentration of axial spray center, but has little effect on other spray parameters. When the nozzle L/D is increased from 4 to 8, the maximum concentration of axial spray center increases by 20.29%.

Application of a one‐dimensional spray model to teach diffusion flame fundamentals for engineering students

AbstractThis study presents the application of an existing interactive application for teaching spray dynamics in engineering degrees. The model is based on spray momentum conservation and can be used to evaluate both fuel‐air mixing characteristics in inert conditions as well as diffusion flame performance once combustion takes place. During a dedicated computer‐lab session, the students perform parametric studies regarding the influence of the nozzle outlet diameter, the combustion chamber density and the spray cone opening angle on the mixing process, characterized by the maximum stoichiometric length. Later on, the effect of the combustion reaction on the mixing field is evaluated. The results are analyzed taking as a reference to the theoretical development made by Spalding and Schlichting for diffusion gas jets. The outcomes of several years using this technique are reported.

Macroscopic non-reacting spray characterization of gasoline compression ignition fuels in a constant volume chamber

Recently, gasoline compression ignition (GCI) engines have become a topic of interest due to its benefits in high thermal efficiency and low emissions. Combustion in GCI engines is highly governed by the fuel stratification which is strongly associated with the spray characteristics like penetration length. Researchers have proposed both theoretical, and regression models for liquid penetration length of diesel and gasoline direct injection (GDI) spray at non-evaporating conditions. However, there are no models for gasoline sprays at elevated ambient gas temperatures, pressures, and injection pressures in literature. This research gap needs to be bridged, as it is crucial for GCI engine technology. In this study, penetration length was investigated using a high-pressure custom-made multi-hole gasoline injector. High reactivity low carbon fuel (RON 77), designed explicitly for GCI engines, and E10 certification fuel (RON 91) were used and compared. Diffused back illumination (DBI) and shadowgraph were implemented for liquid and vapor phase penetration measurement, respectively. It was found that the spray characteristics of high reactivity fuel were similar to E10 certification fuel under non-reacting conditions. Statistical analysis was performed for both liquid and vapor phase penetration lengths, and empirical models were developed with good agreements to the experimental data under high ambient gas pressure and temperature relevant to GCI engine operating conditions using GCI fuels at higher injection pressures. A ‘separation point’ was defined for the liquid phase after which it reaches a steady state, and it was demonstrated to be different from the ‘breakup time’ found in the literature.

Parallelization Performances of PMSS Flow and Dispersion Modeling System over a Huge Urban Area

The use of modeling as a support tool for crisis management and decision planning requires fast simulations in complex built-up areas. The Parallel Micro SWIFT SPRAY (PMSS) modeling system offers a tradeoff between accuracy and fast calculations, while retaining the capability to model buildings at high resolution in three dimensions. PMSS has been applied to actual areas of responsibilities of emergency teams during the EMERGENCIES (very high rEsolution eMERGEncy simulatioN for citIES) and EMED (Emergencies for the MEDiterranean sea) projects: these areas cover several thousands of square kilometers. Usage of metric meshes on such large areas requires domain decomposition parallel algorithms within PMSS. Sensitivity and performance of the domain decomposition has been evaluated both for the flow and dispersion models, using from 341 up to 8052 computing cores. Efficiency of the Parallel SWIFT (PSWIFT) flow model on the EMED domain remains above 50% for up to 4700 cores. Influence of domain decomposition on the Parallel SPRAY (PSPRAY) Lagrangian dispersion model is less straightforward to evaluate due to the complex load balancing process. Due to load balancing, better performance is achieved with the finest domain decomposition. PMSS is able to simulate accidental or malevolent airborne release at high resolution on very large areas, consistent with emergency team responsibility constrains, and with computation time compatible with operational use. This demonstrates that PMSS is an important asset for emergency response applications.

Evaluation of a Scale-Resolving Methodology for the Multidimensional Simulation of GDI Sprays

The introduction of new emissions tests in real driving conditions (Real Driving Emissions—RDE) as well as of improved harmonized laboratory tests (World Harmonised Light Vehicle Test Procedure—WLTP) is going to dramatically cut down NOx and particulate matter emissions for new car models that are intended to be fully Euro 6d compliant from 2020 onwards. Due to the technical challenges related to exhaust gases’ aftertreatment in small-size diesel engines, the current powertrain development trend for light passenger cars is shifted towards the application of different degrees of electrification to highly optimized gasoline direct injection (GDI) engines. As such, the importance of reliable multidimensional computational tools for GDI engine optimization is rapidly increasing. In the present paper, we assess a hybrid scale-resolving turbulence modeling technique for GDI fuel spray simulation, based on the Engine Combustion Network “Spray G” standard test case. Aspects such as the comparison with Reynolds-averaged methods and the sensitivity to the spray model parameters are discussed, and strengths and uncertainties of the analyzed hybrid approach are pointed out. The outcomes of this study serve as a basis for the evaluation of scale-resolving turbulence modeling options for the development of next-generation directly injected thermal engines.