Evaluation of effervescent atomizer internal design on the spray unsteadiness using a phase/Doppler particle analyzer

The purpose of this research was to investigate the dependence of effervescent spray unsteadiness on operational conditions and atomizer internal design by the ideal spray theory of Edwards and Marx. The convergent–divergent effervescent atomizer spraying water with air as atomizing medium in the “outside-in” gas injection was used in this study. Results demonstrated that droplet formation process at various air to liquid ratio (ALR) led to the spray unsteadiness and all droplet size classes exhibited unsteadiness behavior in spray. The spray unsteadiness reduced quickly at ALR of 3% and decreased moderately at ALR of other values as the axial distance increased. When the axial distance was 200 mm, the spray unsteadiness reduced dramatically with the increase in radial distance, but lower spray unsteadiness at the center of spray and higher spray unsteadiness at the edge of spray were shown as the axial distance increased. The spray unsteadiness at the center region of spray increased with the injection pressure. Low spray unsteadiness and good atomization performance can be obtained when the diameter of incline aeration holes increased at ALR of 10%. Although short mixing chamber with large discharge orifice diameter for convergent–divergent effervescent atomizer produced good atomization, the center region of spay showed high spray unsteadiness and maybe formed the droplet clustering.     

Controlled atomization using a twin-fluid swirl atomizer

This paper presents the results of an experimental study of a twin-fluid internally mixed swirl atomizer. In this type of injectors, atomization is attained by injecting a small amount of air (i.e. of the order of less than 16% of the mass flow rate of liquid) into a liquid stream within the injector and the two-phase air liquid mixture is passed through a swirling passage to impart a swirling motion to the flow. Since most of the energy for atomization is supplied to the liquid by the atomizing air, a significantly small pressure drop can produce very fine spray with a small amount of atomizing air. At low values of air–liquid mass ratio (ALR), the appreciable tangential component of velocity with respect to the axial velocity provides a hollow cone spray structure, which turns into a solid cone spray with the increase in axial momentum, through either an increase in ALR or the liquid supply pressure. The results presented in this paper suggest that the investigated injector could be used to control the flow rate and spray characteristics (e.g. spray cone angle, spray solidity, breaking distance, and the droplet diameter) independent of each other by simultaneously varying the supply pressure of the liquid and the atomizing air flow rate. The controlled atomization studied in this paper for a twin-fluid internally mixed swirl atomizer makes it attractive to be used for various commercial applications as the atomizer is capable of providing various spray characteristics depending upon the application requirement.     

Flow and atomization characteristics of a twin-fluid nozzle with internal swirling and self-priming effects

A twin-fluid nozzle was proposed for low-pressure atomization. The nozzle is featured by swirling air flows in the mixing chamber. Liquid medium is thereby inhaled due to the pressure difference. An experimental work was performed to investigate the atomization performance of the nozzle and the hydrogen peroxide solution served as the liquid medium. Droplet size and droplet velocity were measured. Effects of the diameter of the air-injection orifice and the air-injection pressure were investigated. The results show that small droplet size is achieved with the proposed nozzle. As the spray develops, Sauter mean diameter (SMD) of the droplets decreases first and then increases, irrespective of the variation of the air-injection orifice diameter and the air-injection pressure. Overall SMD varies inversely with the air-injection orifice diameter and air-injection pressure. Near the nozzle, cross-sectional velocity distribution exhibits a peak-valley pattern, which is replaced with uniformized velocity distributions away from the nozzle. Similarity of cross-sectional radial velocity distribution at different air pressures is evidenced. Furthermore, the correlation between droplet size and droplet velocity is established.     

Application of the scale entropy diffusion model to describe a liquid atomization process

Whatever the situation, liquid atomization processes show a continuous evolution of the liquid system shape. However, such a system is a multiscale object, i.e., its shape cannot be fully described by a single geometrical parameter. The present work makes use of the scale entropy function to describe this multiscale object. This function is found similar to the scale distribution previously introduced to take into account the droplet shape in liquid spray characterization. Time-averaged scale entropy is locally measured on images of atomizing liquid flows issuing from a low injection pressure single-hole triple-disk nozzle. The advantage in using this nozzle is that the atomization process and the spray are inscribed in a plane and can be fully described by 2-D visualizations. The measurements are performed from the nozzle exit down to the spray region. The operating conditions consider varying injection pressure and liquid physical properties. The temporal evolution of the scale entropy is described by the scale entropy diffusion model. Initially developed in turbulence, this model introduces new parameters such as the scale diffusivity and the local scale entropy flux sink, which characterize the diffusion dynamic of the scale entropy in the scale space. For the first time, these parameters are measured and strong correlations between them and the working conditions are evidenced. Furthermore, new parameters are introduced such as a scale viscosity and the total scale entropy flux lose. These results demonstrate the relevance of the scale entropy diffusion model to describe a liquid atomization process. This application is the first of its kind.     

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.     

A study on the fuel injection and atomization characteristics of soybean oil methyl ester (SME)

The spray atomization characteristics of an undiluted biodiesel fuel (soybean oil methyl ester, SME) in a diesel engine were investigated and compared with that of diesel fuel (ultra low sulfur diesel, ULSD). The experimental results were compared with numerical results predicted by the KIVA-3V code. The spray characteristics of the spray tip penetration, spray area, spray centroid and injection delay were analyzed using images obtained from a visualization system. The Sauter mean diameter (SMD) was analyzed using a droplet analyzer system to investigate the atomization characteristics.It was found that the peak injection rate increases and advances when the injection pressure increases due to the increase of the initial injection momentum. The injection rate of the SME, which has a higher density than diesel fuel, is higher than that of diesel fuel despite its low injection velocity. The high ambient pressure induces the shortening of spray tip penetration of the SME. Moreover, the predicted spray tip penetration pattern is similar to the pattern observed experimentally. The SMD of the SME decreases along the axial distance. The predicted local and overall SMD distribution patterns of diesel and SME fuels illustrate similar tendencies when compared with the experimental droplet size distribution patterns.     

On simulations investigating droplet diameter–charge distributions in electrostatically atomized dielectric liquid sprays

A general procedure has been developed for the simulation of charged liquid and electrostatically atomized sprays. The procedure follows a Lagrangian approach for simulation of spray droplets and a Eulerian approach for gas‐phase variables, including the electric field generated by the charge presence on droplets. Validation of the procedure was examined through simulations of previously published charged spray experiments. Results showed that for the specification of initial droplet charge, modelling the droplet charge–diameter relationship through a scaling law is as reliable a method as using a directly obtained charge–diameter relationship from experimental measurements. The normalized root‐mean‐square errors for sprays using the two methods were shown to be within 12% of one another, for the prediction of spatially averaged profiles of mean droplet diameters, mean axial velocities and mean radial droplet velocities. Results showed that the general spatial characteristics and dynamics of a charged liquid spray can successfully be reproduced, including the axial and radial dispersal pattern of droplets and the distribution of mean droplet diameters throughout the spray plume. For all sprays with droplet charges defined through a scaling law relationship, the normalized root‐mean‐square errors range from 9.0% to 31.6% for mean droplet diameters, 10.4% to 67.9% for mean axial droplet velocities and 16.8% to 38.6% for mean radial droplet velocities. Lastly, we present a brief set of general recommendations for simulating electrostatically atomized dielectric liquid sprays.Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Air flow and pressure inside a pressure-swirl spray and their effects on spray development

Air flow and pressure inside a pressure-swirl spray for direct injection (DI) gasoline engines and their effects on spray development have been analyzed at different injector operating conditions. A simulation tool was utilized and the static air pressure at the centerline of the spray was measured to investigate the static pressure and flow structure inside the swirl spray. To investigate the effect of static air pressure on swirl spray development, a liquid film model was applied and the Mie-scattered images were captured. The simulation and experiment showed that recirculation vortex and air pressure drop inside the swirl spray were observable and the air pressure drop was greater at high injection pressure. At high fuel temperature, the air pressure at the nozzle exit showed higher value compared to the atmospheric pressure and then continuously decreased up to few millimeters distance from the nozzle exit. The pressure drop at high fuel temperatures was more than that of atmospheric temperature. This reduced air pressure was recovered to the atmospheric pressure at further downstream. The results from the liquid film model and macroscopic spray images showed that the air pressure started to affect the liquid film trajectory about 3 mm from the nozzle exit and this effect was sustained until the air pressure recovered to the atmospheric pressure. However, the entrained air motion and droplet size have more significant influence on the spray development after the most of the liquid sheet is broken-up and the spray loses its initial momentum.     

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.     

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.     

Aerodynamic behavior of liquid sprays

An analysis is presented for the effect of entrained gas flows on drop trajectories and spray distributions from liquid atomizing nozzles. In particular, the effect of the pressure (or density) of the environment into which the liquid is sprayed is examined. The contraction of atomized sprays at elevated pressure which has been observed by various workers is explained, and the analysis is substantially confirmed by their data and by new data presented here. Both the data and the theory show that the amount of spray contraction increases with increasing ambient pressure and nozzle pressure drop, and decreases with increasing nozzle diameter and drop size. The theory examines the entrained gas flow around and into a spray and its subsequent effect on the trajectories of the liquid droplets comprising the spray.     

Planar drop-sizing and liquid volume fraction measurements of airblast spray in cross-flow using SLIPI-based techniques

This paper presents planar measurements of Sauter mean diameter (SMD) and liquid volume fraction distributions of airblast spray injected into cross-flow. The experiments are conducted using a combination of structured laser illumination planar imaging with laser sheet drop-sizing (SLIPI-LSD) and particle/droplet imaging analysis (PDIA) techniques. Effect of gas to liquid mass ratio (GLR) and cross-flow velocity (U_cross) is studied. Planar SMD distribution at low GLR improved with increase in U_cross due to secondary atomization of large droplets. Uniform SMD distribution in a range of 10–20 µm is observed for GLR more than 3. The distribution of liquid volume fraction at low GLR condition shows poor dispersion with most of the liquid concentrated near the injector. The liquid volume fraction distribution improves with increase in GLR and better dispersion is observed for GLR more than 3 and two-phase momentum ratio (q₂) greater than 13.06. Spatial bifurcation in liquid fraction is found for high GLR conditions. The SMD in the range of 10–20  µm and uniform distribution of liquid are observed for GLR more than 3 and q₂ > 25.6.     

Atomization of liquids by disintegrating thin liquid films using gas jets

Liquid atomization is useful in many applications, such as engineering, science, pharmaceutics, medicine, forensics and others. In the present research, an innovative methodology and a new device for atomization of liquids into mists of micron and submicron droplets have been developed. The new liquid-atomization method exploits the physical phenomenon of fragmentation of thin liquid films into fine micron and submicron droplets by gas jets. For several tested prototypes, the direct observations using a high-speed visualization technique have demonstrated that bubbles were generated within a liquid and their shells have been subsequently destroyed by applying a mechanical impulse (pressure of a compressed air) once the bubbles came over the liquid surface. The main characteristics of the generated tap water mists have been experimentally measured by means of the laser diffraction technique under various conditions for each prototype. One of the prototype devices allowed obtaining mists containing 90–99% of droplets smaller than 1 µm, with the minimum arithmetic and Sauter mean droplet diameters of 1.48 µm and 2.66 µm, and the 2.64 ml/min of droplet flow rate for 3.5 bar manometer pressure of atomizing air. The gas to liquid mass ratios (GLR) in the new device are depending on the atomizing tube length and the number of perforated orifices in the tube: more the tube length, hence more the number of perforated orifices, and therefore more liquid droplets will form for the same gas flow rate. The measured GLR values related to 1 m length of the utilized atomizing tube were in the range of 0.65–1.06, and for the specifically utilized atomizing tube of 72 mm length were among 9.07–14.67. The results of this study demonstrate that the developed method of generation of very fine droplet mists has many advantages over the existing techniques and can be perspective for many practical applications.     

Experimental analysis of liquid–gas interface at low Weber number: interface length and fractal dimension

The present paper reports an experimental investigation on atomizing liquid flows produced by simplified cavity nozzles. The Weber number being kept low, the sprays produced by these injectors depend on the liquid flow characteristics only, and more precisely, on the non-axial kinetic energy and of the turbulent kinetic energy at the nozzle exit. The investigation reported here concentrates on the characterization of liquid flows during atomization by measuring the spatial variation of the local interface length and of the local interface fractal dimension. Both parameters were found representative of the physics of atomization process: they depend on the characteristics of the flow issuing from the nozzle and they are related to the subsequent drop size distribution. The local interface length is representative of the amount of liquid–gas interface surface area, and is a function of both the non-axial and the turbulent kinetic energies at the nozzle exit. The fractal dimension is representative of the tortuosity of the liquid–gas interface and, as expected, is mainly related to the turbulent kinetic energy at the nozzle exit. As far as the drop size distribution is concerned, it is found that the local interface length at the instant of break-up determines a representative drop diameter of some kind, whereas the fractal dimension at the same instant controls the dispersion of the distribution.     

Effects of Turbulence, Evaporation and Heat Release on the Dispersion of Droplets in Dilute Spray Jets and Flames

The dispersion characteristics of a selection of non-evaporating non-reacting, evaporating non-reacting, and reacting dilute spray jets issuing in ambient air (Gounder et al, Combust Sci Technol 182:702–715, 2010; Masri and Gounder, Combust Flame 159:3372–3397, 2010) and in a hot coflow (Oloughlin and Masri, Flow Turbul Combust 89:13–35, 2012) are analysed. Other than the cases found in those contributions, two additional sprays of kerosene have been investigated in order to systematically study the effects of evaporation. The burners are well designed such that boundary conditions may be accurately measured for use in numerical simulations. The dynamics and dispersion characteristics are analysed by conditioning results on the droplet Stokes numbers and by systematically investigating changes in dispersion and dynamics as a function of carrier air velocity, liquid loading, ignition method, and location within the flame or spray jet. The tendency for droplet dispersion defined by the ratio of radial rms velocity to axial mean velocity varies significantly between reacting and non-reacting flows. However, dispersion is found to be largely unaffected by evaporation. The total particle concentration, or number density of droplets within the spray has also been used as a direct measure of spray dispersion with the effect of evaporation on a turbulent polydisperse spray being isolated by investigating acetone and kerosene sprays with similar boundary conditions. The rate of change of droplet size with radial position is almost identical for the kerosene and acetone cases. The dispersion characteristics, closely related to the ‘fan spreading’ phenomenon are dependant on the carrier air velocity and axial location within the spray.     

阶梯型加速段对旋流喷嘴雾化特性的影响

旋流内芯是压力旋流式喷嘴最主要的旋流发生构件, 其几何特征直接影响压力旋流式喷嘴的喷雾特性.目前采用平滑型加速段的旋流内芯导流效率较低.为减小高流量条件下的能量损失, 使喷嘴旋流内芯加速段对喷雾介质产生预旋效应, 增强旋流强度, 本文设计喷嘴旋流内芯加速段为阶梯型, 其下段阶梯相对上段阶梯旋转15°, 旋向与喷嘴旋流槽方向相同.利用粒子动态分析仪(particle dynamics analysis system, PDA) 和高速摄影(charge coupled device, CCD)系统实验研究了加速段结构改进前后喷嘴的喷雾流量、雾场索特尔平均直径(Sauter mean diameter, SMD)、雾滴速度以及喷雾锥角, 并分析了SMD、 雾滴速度的轴向和径向分布特性. 结果表明, 背压差0.08\mathrm{~} 0.46 MPa 范围内, 阶梯型加速段对喷雾介质具有较好的预旋效果.喷嘴的流量提高了48.0% ~ 51.8%; 喷雾的轴向速度提升了31.4% ~ 32.8%, 径向速度提升了1.6% ~ 16.8%; 喷雾锥角减小了4.21°~6.57°; 较高背压差下喷雾下游的SMD减小了9.8%.与平滑型加速段相比, 阶梯型加速段的设计有效地提高了喷嘴的雾化质量.      

Experimental and numerical analysis of spray-atomization characteristics of biodiesel fuel in various fuel and ambient temperatures conditions

The purpose of this work is to reveal the effects of fuel temperatures and ambient gas conditions on the spray-atomization behavior of soybean oil methyl ester (SME) fuel. The spray-atomization behavior was analyzed through spray parameters such as the axial distance from the nozzle tip, local and overall Sauter mean diameter (SMD). These parameters were obtained from a spray visualization system and a droplet measuring system. In addition, the experimental results were compared with the numerical results calculated by the KIVA-3V code. It was revealed that the increase of the fuel temperature (from 300 K to 360 K) little affects the spray liquid tip penetration. The increase of the ambient gas temperature (from 300 K to 450 K) caused a increase in the spray liquid tip penetration. Also, biodiesel fuel evaporation actively occurred due to the increase in the fuel temperature and the ambient gas temperature. Of special significance was that the highest vapor fuel mass concentration was observed at the center region of the spray axis. In the results of the microscopic characteristics, the detected local droplet size at the axial direction and overall droplet size at the axial and radial direction in a control volume increased when the fuel temperature increased. This is believed to be due to an increase in the number of small droplets that quickly evaporated. In addition, the increased fuel temperature caused the decrease of the number of droplets and the increase of the vapor fuel mass. The mean axial velocity of droplets decreased with increasing fuel temperature.