Multiscale simulations and experiments on water jet atomization

Numerical simulation of primary atomization at high Reynolds number is still a challenging problem. In this work a multiscale approach for the numerical simulation of liquid jet primary atomization is applied, using an Eulerian-Lagrangian coupling. In this approach, an Eulerian volume of fluid (VOF) method, where the Reynolds stresses are closed by a Reynolds stress model is applied to model the global spreading of the liquid jet. The formation of the micro-scale droplets, which are usually smaller than the grid spacing in the computational domain, is modelled by an energy-based sub-grid model. Where the disruptive forces (turbulence and surface pressure) of turbulent eddies near the surface of the jet overcome the capillary forces, droplets are released with the local properties of the corresponding eddies. The dynamics of the generated droplets are modelled using Lagrangian particle tracking (LPT). A numerical coupling between the Eulerian and Lagrangian frames is then established via source terms in conservation equations. As a follow-up study to our investigation in Saeedipour et al. (2016a), the present paper aims at modelling drop formation from liquid jets at high Reynolds numbers in the atomization regime and validating the simulation results against in-house experiments. For this purpose, phase-Doppler anemometry (PDA) was used to measure the droplet size and velocity distributions in sprays produced by water jet breakup at different Reynolds numbers in the atomization regime. The spray properties, such as droplet size spectra, local and global Sauter-mean drop sizes and velocity distributions obtained from the simulations are compared with experiment at various locations with very good agreement.     

The atomization of water–oil emulsions

The paper presents the results of experimental studies on atomization of the emulsions flowing through twin-fluid atomizers obtained by the use of the digital microphotography method. The main elements of the test installation were: nozzle, reservoir, pump and measurement units of liquid flow. The photographs were taken by a digital camera with automatic flash at exposure time of 1/8000 s and subsequently analyzed using Image Pro-Plus. The oils used were mineral oils 20–90, 20–70, 20–50 and 20–30. The studies were performed at flow rates of liquid phase changed from 0.0014 to 0.011 (dm³/s) and gas phase changed from 0.28 to 1.4 (dm³/s), respectively. The analysis of photos shows that the droplets being formed during the liquid atomization have very different sizes. The smallest droplets have diameters of the order of 10 μm. The experimental results showed that the changes in physical properties of a liquid phase lead to the significant changes in the spray characteristics. The analysis of the photos of water and emulsions atomization process showed that the droplet sizes are dependent on gas and liquid flow rates, construction of nozzle and properties of liquid. The differences between characteristics of atomization for water and emulsions have been observed. Analysis of photos on forming the droplets in air–water and air-emulsions systems showed that droplets are bigger in air-emulsion system (at the same value of gas to liquid mass ratio). The values of Sauter mean diameter (SMD) increased with increase of volume fraction of oil in emulsion. The droplet size increased with emulsion viscosity.     

Atomization of liquid droplets on surfaces exposed to moving shock waves

Many engineering applications involve the stripping of liquid droplets from surfaces, one example being the entrainment of surface fuel from the inlet valves, ports, cylinder walls and piston crowns of internal combustion engines during the induction process. This configuration is likely to exhibit differences from the more commonly studied case of suspended droplets. In order to study the atomization of liquids from surfaces, shock waves at low Mach numbers (M = 1.05 and 1.12) have been used in the present work to initiate the flow over water droplets with visualization obtained from shadowgraph photographs, high-intensity flash photography and a CCD camera. Visualization paths both normal and angled at ±45° to the flow were used in order to obtain improved examination of the atomization details. Surface wave formation and a specific pattern of droplet distortion followed by stripping, was observed. There are similarities in the processes to those of suspended droplets that are modified by the boundary layer effects. At the Weber numbers considered, a cave-like formation occurs near the wall due to surface flow around the droplet with a major liquid flow directed tangentially across the air flow towards the cave peak where bag or chaotic type break-up and stripping takes place.     

Use of Faraday instabilities to enhance fuel pulverisation in air-blast atomisers

The atomization of liquids into a spray is an important process in many industrial applications and particularly in the aero-engine sector. Conventional air-blast injectors in aircraft engines today use aerodynamic shearing effects to atomize the liquid fuel. However, at operating conditions where the air velocity is below 30 m/s (such as ground start and high altitude restart) the atomization quality is poor. Consequently combustion is less efficient with high pollutant emissions. The objective of this study is to validate a new concept of injector which couples the shearing effects with the principle of ultrasonic atomization. The latter consists of using piezoelectric actuators to generate the oscillations of a wall in contact with the liquid film. This excitation perpendicular to the liquid film surface creates Faraday instabilities at the liquid/air interface. Amplitudes higher than a defined threshold value induce the break-up of ligaments and the formation of droplets. To cite this article: M. Boukra et al., C. R. Mecanique 337 (2009).     

Unsteadiness of the internal flow in an effervescent atomizer nozzle

Liquid atomization system has been extensively applied as the most significant process in many industrial fields. In the internal combustion engine, the combustion phenomenon is strongly influenced by the spray characteristics of the fuel given by the atomization process. In order to completely understand the whole atomization process, a detail investigation of relations between the liquid jet characteristics and the breakup phenomenon is required. In this study, a non-intrusive method called as laser tagging method by photochromic dye has been developed with aim to study the breakup process of liquid sheet in detail, covering from the behavior in film until disintegrated into ligament and droplets. The laser tagging method by photochromic dye is based on a shift in the absorption spectrum of photochromic dye molecules tagged by ultraviolet laser. The shift results a color change at the tagged region of liquid containing the dye. In this study, the motions of the dye traces were analyzed as the liquid surface velocity. As a result, liquid sheet was found to keep its velocity constantly in film before suddenly increase around broken point. However, it then decreased after broken into droplets. By forming a set of four points of dye traces on the liquid sheet, the change of relative position of the set enabled the measurement of deformation and rotational motion of the liquid sheet. As a result, the normal strain of the liquid sheet parallel to the flow direction depended on the flow behavior of ligament formation.     

Simulation of liquid solvent atomization in compressed CO2

The work investigates numerically the atomization regime of a liquid injected into compressed CO₂ under subcritical conditions, i.e. below the CO₂-solvent critical pressure. To vary the conditions within the atomization regime whilst keeping up with realistic experimental background, ethanol and methylene chloride were selected as injected fluid and pressure was modified as well. Results first show that the jet indeed breaks up by atomization, which confirms the validity at high pressure of the breakup classification diagram. Aiming at evaluating the size distribution of the droplets formed by the jet atomization, two methods of interface tracking were investigated. Compared to the VOF-PLIC classical method, the novel sub mesh (VOF-SM) approach allows for determining smaller sized droplets without digital broadcasting.     

Atomized non-equilibrium two-phase flow in mesochannels: Momentum analysis

Two-phase pressure drop measurements are very difficult to make while the fluid is in non-equilibrium condition, i.e. while phase change is taking place. This is further complicated when an atomized liquid is introduced in the system at much higher velocity than other components such as liquid layer, vapor core, and entrained droplets. The purpose of this paper is to develop a model to predict the two-phase pressure characteristics in a mesochannel under various heat flux and liquid atomization conditions. This model includes the momentum effects of liquid droplets from entrainment and atomization. To verify the model, an in-house experimental setup consisting of a series of converging mesochannels, an atomization facility and a heat source was developed. The two-phase pressure of boiling PF5050 was measured along the wall of a mesochannel. The one-dimensional model shows good agreement with the experimental data. The effects of channel wall angle, droplet velocity and spray mass fraction on two-phase pressure characteristics are predicted. Numerical results show that an optimal spray cooling unit can be designed by optimizing channel wall angle and droplet velocity.     

Flow characteristics of a large-size pressure-atomized spray using DTV

The purpose of this study is to characterize the atomization of a jet of water sprayed into the air at high velocity through a commercial nozzle widely used for sprinkler irrigation. The typical diameter of the droplets present in the spray is in the range of several tens of micrometers to several millimeters. They are visualized by ombroscopy. A specific Droplet Tracking Velocimetry (DTV) technique is developed to estimate the size and velocity of these highly polydispersed droplets that are distinctly non spherical. This analysis is performed from the rupture of the liquid core region (about a distance of 550 nozzle diameters) to the dispersed zone (about a distance of 900 nozzle diameters). With this technique, we obtain joint size-velocity measurements that are rarely produced. Especially two velocity components and also a large diameter range are characterized at the same time; while with other techniques, such as Particle Doppler Anemometry (PDA), the diameter range is quite reduced and requires specific settings. Additional measurements of the liquid volume fraction are performed using a single mode fiber-optic probe. In the light of our experimental data, it appears that the turbulent droplet motion in the spray is strongly anisotropic. This anisotropy is quite unexpected because other studies on sprays (generally concerned with engine applications) show a relatively low anisotropy. We attribute this increase of anisotropy to the fact that, for this type of spray, the droplet relaxation time is long in comparison to the characteristic time of the turbulence and that biggest droplets are still submitted to atomization process. This strong anisotropy is responsible for the poor radial dispersion of the spray.     

A two-fluid model of gas-assisted atomization including flow through the nozzle,phase inversion,and spray dispersion

This paper describes a new approach to modelling compressible gas–liquid flows that undergo change of the continuous phase. The presented model includes the system of the ensemble averaged Navier–Stokes equations together with the particle number density equation for each phase. The constitutive equations that depend on the flow regime are obtained from many sub-models that have been developed alongside the main model. Droplet size is allowed to vary in the flow field but is considered constant within a control volume. Bubbles and droplets break-up and coalescence models are adapted to the flow conditions. The proposed model for atomization treats it as a catastrophic phase inversion that takes place over the surface determined by the local values of phase volume fractions. The model is applied to simulate the premixed air-assisted atomization of water in a nozzle-type device. The computational domain includes the nozzle and the surrounding area of the spray dispersion. The model performance has been verified by comparing the predicted and measured liquid flow rates in the spray as well as the pressure values along the nozzle wall. Computational results are analysed, and the main flow features are presented.     

Interpretation of atomization rates of the liquid film in gas-liquid annular flow

For vertical gas-liquid annular flow the fraction of the liquid in the gas is controlled by the rate of atomization of the liquid film flowing along the wall and the rate of deposition of droplets entrained in the gas. Measurements of the rate of atomization are interpreted by a Kelvin-Helmholtz mechanism. Small wavelets on the liquid film are visualized to be entrained when wave-induced variations in the gas pressure cannot be counterbalanced by surface tension effects.     

Atomization and spray characteristics of bioethanol and bioethanol blended gasoline fuel injected through a direct injection gasoline injector

The focus of this study was to investigate the spray characteristics and atomization performance of gasoline fuel (G100), bioethanol fuel (E100), and bioethanol blended gasoline fuel (E85) in a direct injection gasoline injector in a gasoline engine. The overall spray and atomization characteristics such as an axial spray tip penetration, spray width, and overall SMD were measured experimentally and predicted by using KIVA-3V code.The development process and the appearance timing of the vortices in the test fuels were very similar. In addition, the numerical results accurately described the experimentally observed spray development pattern and shape, the beginning position of the vortex, and the spray breakup on the spray surface. Moreover, the increased injection pressure induced the occurrence of a clear circular shape in the downstream spray and a uniform mixture between the injected spray droplets and ambient air. The axial spray tip penetrations of the test fuels were similar, while the spray width and spray cone angle of E100 were slightly larger than the other fuels. In terms of atomization performance, the E100 fuel among the tested fuels had the largest droplet size because E100 has a high kinematic viscosity and surface tension.     

An Eulerian–Lagrangian hybrid model for the coarse-grid simulation of turbulent liquid jet breakup

In this paper we present a numerical model for the coarse-grid simulation of turbulent liquid jet breakup using an Eulerian–Lagrangian coupling. To picture the unresolved droplet formation near the liquid jet interface in the case of coarse grids we considered a theoretical model to describe the unresolved flow instabilities leading to turbulent breakup. These entrained droplets are then represented by an Eulerian–Lagrangian hybrid concept. On the one hand, we used a volume of fluid method (VOF) to characterize the global spreading and the initiation of droplet formation; one the other hand, Lagrangian droplets are released at the liquid–gas interface according to the theoretical model balancing consolidating and disruptive energies. Here, a numerical coupling was required between Eulerian liquid core and Lagrangian droplets using mass and momentum source terms. The presented methodology was tested for different liquid jets in Rayleigh, wind-induced and atomization regimes and validated against literature data. This comparison reveals fairly good qualitative agreement in the cases of jet spreading, jet instability and jet breakup as well as relatively accurate size distribution and Sauter mean diameter (SMD) of the droplets. Furthermore, the model was able to capture the regime transitions from Rayleigh instability to atomization appropriately. Finally, the presented sub-grid model predicts the effect of the gas-phase pressure on the droplet sizes very well.     

Spray features in the near field of a flow-blurring injector investigated by high-speed visualization and time-resolved PIV

In a flow-blurring (FB) injector, atomizing air stagnates and bifurcates at the gap upstream of the injector orifice. A small portion of the air penetrates into the liquid supply line to create a turbulent two-phase flow. Pressure drop across the injector orifice causes air bubbles to expand and burst thereby disintegrating the surrounding liquid into a fine spray. In previous studies, we have demonstrated clean and stable combustion of alternative liquid fuels, such as biodiesel, straight vegetable oil and glycerol by using the FB injector without requiring fuel pre-processing or combustor hardware modification. In this study, high-speed visualization and time-resolved particle image velocimetry (PIV) techniques are employed to investigate the FB spray in the near field of the injector to delineate the underlying mechanisms of atomization. Experiments are performed using water as the liquid and air as the atomizing gas for air to liquid mass ratio of 2.0. Flow visualization at the injector exit focused on a field of view with physical dimensions of 2.3 mm × 1.4 mm at spatial resolution of 7.16 µm per pixel, exposure time of 1 µs, and image acquisition rate of 100 k frames per second. Image sequences illustrate mostly fine droplets indicating that the primary breakup by FB atomization likely occurs within the injector itself. A few larger droplets appearing mainly at the injector periphery undergo secondary breakup by Rayleigh–Taylor instabilities. Time-resolved PIV is applied to quantify the droplet dynamics in the injector near field. Plots of instantaneous, mean, and root-mean-square droplet velocities are presented to reveal the secondary breakup process. Results show that the secondary atomization to produce fine and stable spray is complete within a few diameters from the injector exit. These superior characteristics of the FB injector are attractive to achieve clean combustion of different fuels in practical systems.     

Producing droplets smaller than the nozzle diameter by using a pneumatic drop-on-demand droplet generator

A pneumatic droplet generator to produce water/glycerin droplets smaller than the nozzle diameter is described. The generator consists of a T-junction with a nozzle fit into one opening, the second opening connected to a gas cylinder through a solenoid valve and the third connected to a length of steel tubing. The droplet generator is filled with liquid. Opening the valve for a preset time creates a pulse of alternating negative and positive pressure in the gas above the surface of the liquid, ejecting a single droplet through the nozzle. Droplet formation was photographed and the pressure variation in the droplet generator recorded. The effect of various experimental parameters, such as nozzle size, pressure pulse width and liquid properties on droplet formation was investigated. Small droplets could not be generated when liquid viscosity was too low or too high. For pure water, droplet diameters were several times that of the nozzle. Using more viscous glycerin mixtures, droplets with diameters as small as 65% of the nozzle diameter could be produced.     

On the role of the liquid flow characteristics on low-Weber-number atomization processes

At low Weber numbers, the aerodynamic forces due to the interaction between gas and liquid do not influence liquid atomization processes. In these situations, atomization processes depend on issuing liquid flow characteristics only. According to the literature, the atomization efficiency is best when the issuing liquid flow shows a high turbulence level. Some injectors are based on this concept and promote the production of turbulence by imposing deflection of the flow inside the nozzle. However, many studies indicate that the level of turbulence does not solely control the atomization efficiency. By conducting a numerical and experimental study on the behavior of cavity nozzles, it is found that internal flow deflection to produce turbulence also produces a non-axial flow component at the nozzle exit whose effect on the atomization process is of paramount importance. Indeed, the results show that the surface energy produced during the atomization process is linearly dependent on the sum of the turbulent kinetic energy and the non-axial kinetic energy at the nozzle exit. This sum represents the energy available for the atomization process, and the influence of the injection pressure as well as of the nozzle geometry on this energy is investigated.     

镓铟锡液滴撞击泡沫金属表面的运动学特性研究

常温下为液态的镓铟锡合金以其优异的导热性能在具有特殊要求的传热领域有着重要的应用价值,与传统流动介质相比较大的表面张力使得其产生的流动现象必有所区别.本文研究镓铟锡所形成的液滴撞击泡沫金属表面后所产生的铺展、回缩及回弹现象.采用高速相机拍摄液滴投影轮廓随液滴运动的变化过程,并通过图像处理获得不同撞击速度、底板表面孔径下的液滴铺展系数、中心位置轮廓高度以及液滴回弹后在空中的振动特性.研究结果表明:具有较高表面张力的镓铟锡液滴的铺展系数随无量纲时间的变化在铺展初始阶段仍满足常规流体的1/2次幂关系,只在铺展后期与底板的无量纲孔径有关系;液滴的最大铺展系数在较小无量纲孔径底板大于在光滑镍板,且随底板无量纲孔径增大而逐渐减小;在回弹过程,由于底板孔隙结构的存在使得液滴回弹后在空中的振动呈现3种形态:规则的横向和纵向振动、带旋转的横向和纵向振动以及旋转振动;最后,通过对振动频率的拟合和分析,进一步拓展了传统振动频率理论公式在非规则振动过程预测中的应用.      

有约束、轴对称抛撒条件下液体破碎、雾化的远场研究

液体轴对称抛撒在研制云雾爆轰武器中有着重要的实际应用背景和学术研究价值，由于过去这方面大量的工作集中在野外场地中进行，对液体燃料抛撒初期及后期形成的云雾区特性缺少细致的实验研究，本文用垂直无膜激波管与液体抛撒发生器结合起来，用多根隔条形成不同宽度的间隙，利用激光粒子测量仪研究在有约束的条件下，液体破碎、雾化的远场特性，并得到了一些结论。     

液体环二次破碎所形成云雾颗粒尺寸测量和测试系统的标定

介绍了激光散射法测量颗粒尺寸系统的工作原理和标定结果,并对液体环轴对称抛撒进行了光学测量。实验结果表明,液体环二次破碎产生云雾区的液滴Ｓａｕｔｅｒ平均直径在固定点随时间的增加呈减小的趋势,而云雾区的宽度和云雾区前缘的液滴颗粒的Ｓａｕｔｅｒ平均直径则随测量的距离增加均有所增加。