Evaporation and Settling Behavior of Metalworking Fluid Aerosols

Environmental and industrial hygiene issues have been receiving ever increasing attention. One important issue is metalworking fluid (MWF) mist formation since it can have a negative effect on workplace air quality. A model is presented to characterize the evaporation and settling behavior of a diluted water-soluble MWF mist under ambient conditions. The model is based on the Langmuir-Knudsen law and Stoke' law. The Langmuir-Knudsen law is employed to describe the evaporation process of each MWF droplet and allows for nonequilibrium effects associated with small droplet size. Stoke' law is applied to characterize the droplet settling mechanism at small Reynolds numbers. Experiments are performed to validate the model. Experiments show that the evaporation rate decreases to zero as the water in a droplet completely evaporates to produce a small nonvolatile oil drop. It is found that the evaporation and settling of a collection of MWF droplets can be predicted by the proposed model.     

Numerical simulation of gas-droplets mixing and spray evaporation in rotary spray desulfurization tower

Motion and evaporation of droplets significantly affect the semidry flue gas desulfurization efficiency and long-term operation. Both the flow field distribution and the heat and mass transfer in the spray towers are studied by numerical simulation, and the process of droplet motion and evaporation is analyzed in detail. Then, two indices, mixing variance and droplet mass-weighted life, are provided to quantify gas droplet mixing and the droplet group evaporation time. The simulation results show that the radial penetration distance of the droplets is longer with the diameter increase, and the appropriate swirl number improves the mixing between the flue gas and droplets. With the increase of droplet diameter and velocity, the droplet distribution in the tower is more widely, obtaining the optimum mixing variance. The droplet mass-weighted life is promoted linearly with the increase of average droplet diameter and the decrease of flue gas temperature. With flue gas temperature increase from 458 k to 488 K, the droplet mass-weighted life decreases linearly by 31%. In comparison, the initial droplet velocity and spray angle have a slight effect on the droplet mass-weighted life.     

Experimental and Theoretical Investigation on Rapid Evaporation of Ethanol Droplets and Kerosene Droplets During Depressurization

This paper reports an experimental and theoretical study of rapid evaporation of ethanol droplets and kerosene droplets during depressurization. For experimental method, an ethanol droplet or a kerosene droplet was suspended on a thermocouple, which was also used to measure the droplet center temperature transition. And the droplet shape variation was recorded by a high speed camera. A theoretical analysis was developed based on the heat balance to estimate the droplet center temperature transition, and the evaporation model proposed by Abramzon and Sirignano was used to describe the droplet vaporization. According to the experimental data and theoretical analysis, both of the environmental pressure and the initial droplet diameter have a prominent influence on the droplet temperature transition. Comparing the evaporation processes of ethanol droplets and kerosene droplets with water droplets, the ethanol droplets have the fastest evaporation rate, followed by water, and the evaporation rates of kerosene droplets are the slowest. Also it was found that a bubble can easily emerge within kerosene droplet, and its lifetime is more than 1 s.     

Review on modelling of corrosion under droplet electrolyte for predicting atmospheric corrosion rate

Atmospheric corrosion of metals is the most common type of corrosion which has a significant impact on the environment and operational safety in various situations of everyday life.Some of the common examples can be observed in land,water and air transportation systems,electronic circuit boards,urban and offshore infrastructures.The dew drops formed on metal surface due to condensation of atmospheric moisture facilitates corrosion as an electrolyte.The corrosion mechanisms under these droplets are different from classically known bulk electrolyte corrosion.Due to thin and non-uniform geometric thickness of the droplet electrolyte,the atmospheric oxygen requires a shorter diffusion path to reach the metal surface.The corrosion under a droplet is driven by the depletion of oxygen in the center of the droplet compared to the edge,known as differential aeration.In case of a larger droplet,differential aeration leads to preferential cathodic activity at the edge and is controlled by the droplet geometry.Whereas,for a smaller droplet,the oxygen concentration remains uniform and hence cathodic activity is not controlled by droplet geometry.The geometry of condensed droplets varies dynamically with changing environmental parameters,influencing corrosion mechanisms as the droplets evolve in size.In this review,various modelling approaches used to simulate the corrosion under droplet electrolytes are presented.In the efforts of developing a comprehensive model to estimate corrosion rates,it has been noted from this review that the influence of geometric evolution of the droplet due to condensation/evaporation processes on corrosion mechanisms are yet to be modelled.Dynamically varying external factors like environmental temperature,relative humidity,presence of hygroscopic salts and pollutants influence the evolution of droplet electrolyte,making it a complex phenomenon to investigate.Therefore,an overview of available dropwise condensation and evaporation models which describes the formation and the evolution of droplet geometry are also presented from an atmo s pheric corrosion viewpoint.     

Theoretical Investigation on Rapid Evaporation of a Saline Droplet During Depressurization

This paper reports a theoretical investigation on rapid evaporation of a saline droplet during depressurization. A mathematical model was developed to simulate the droplet temperature variation by considering the ambient pressure change, the heat transfers due to evaporation and convection at the droplet surface, accompanying the heat and mass transfer inside the droplet. The component diffusion and the temperature gradient inside the droplet were mainly discussed by comparing the numerical droplet temperature with the experimental data. The result shows that, the variation of internal concentration is small, while the temperature gradient within the droplet is significant during the evaporation process. In addition, the influencing factors of the droplet temperature variation were analyzed, such as: the final ambient pressure, theinitial salt concentration and the initial droplet temperature. The present model calculations help to understand the thermodynamic process of rapid evaporation of a saline droplet during depressurization.     

Delayed thermal explosion in flammable gas containing fuel droplets: Asymptotic analysis

The problem of thermal explosion in a flammable gas mixture with addition of volatile fuel droplets is studied based on the asymptotic method of integral manifolds. The model for the radiative heating of droplets takes into account the semitransparency of droplets. A simplified model for droplet heat-up is used. The results of the analysis are applied to the modelling of thermal explosion in diesel engines. Two distinct dynamical situations have been considered, depending on the initial droplet concentration. These are far zone (small initial liquid volume fraction and small droplet radii) and near zone (large initial liquid volume fraction and large droplet radii). The conditions of the first zone are typical for the areas in the combustion chamber which are far from the fuel injectors, while the conditions of the second zone are typical for the areas in the combustion chamber which are relatively close to the fuel injectors. It has been pointed out that small droplets heating and evaporation time in the far zone is smaller than the chemical ignition delay of the fuel vapor/air mixture. The total ignition delay decreases with increasing initial gas temperature. In the near zone for large droplets, the process starts with the initial gas cooling and slight heating of droplets. This is followed by a relatively slow heating of gas due to the chemical reaction, and further droplet heating. The total ignition delay in the near zone is larger than in the far zone. It is expected that before thermal explosion in the near zone takes place, the droplets break up and are removed from this zone. In optically thick gas effects of thermal radiation are negligible for small droplets but are noticeable for large droplets.     

Evaporation and discharge dynamics of highly charged droplets of heptane,octane, and p-xylene generated by electrospray ionization

We report studies of the evaporation and discharge dynamics of highly charged droplets generated by electrospray ionization from n-heptane, n-octane, and p-xylene doped with Stadis-450, a conductivity-enhancing agent. A phase Doppler anemometer (PDA) characterizes individual droplets moving through the uniform electric field within an ion mobility cell according to size, velocity, and charge. Repeated reversal of the electric field allows multiple PDA measurements on selected droplets with diameters ranging from 3 to 60 microm and up to 10(7) elementary positive charges. This "ping-pong" technique provides individual droplet histories from which we determine the dynamics of solvent evaporation and charge loss. On average, n-heptane discharges at 101% of the Rayleigh limit of charge, while n-octane and p-xylene droplets discharge at 87% and 89% of their respective limits. Discharge events release an average of 19% of the charge in n-heptane and 17% of the charge in both n-octane and p-xylene. Within the limits of the measurements, no detectable change in droplet diameter accompanies observed discharge events, indicating the loss of a relatively small fraction of the total volume. We compare these results to previous experiments, theoretical models for droplet evaporation and discharge, and predictions from the Rayleigh model. We report both Stadis-450 and triethylamine mass spectra in octane and discuss issues regarding the use of hydrocarbon solvents in electrospray mass spectrometry.     

Amplitude Equations for Large-Scale Marangoni Convection in a Liquid Layer with Insoluble Surfactant on Deformable Free Surface

The numerical simulation has been carried out to investigate the motion of a droplet initially near a wall under gravity and confirm the existence of the wall repulsive force on the droplet. The numerical model is developed based on a mass conservation level set algorithm to capture the surface deformation of the droplet. The results show that the wall repulsive force on the droplet initially near the wall plays an important role in the droplet falling process, and the viscosity force affects the oscillatory trajectory of the falling droplet. In addition, the mutual repulsive effect between two droplets is also studied by settling symmetrically two droplets, and the oscillatory mechanism of droplet motion is discussed as well.     

Theoretical study of stable dropwise condensation on an inclined micro/nano-structured tube

In this paper, a numerical model for dropwise condensation on a micro/nano-structured inclined tube is developed based on previous theoretical and experimental studies for flat plates. Thermal resistances consisting of conduction through a droplet, vapor–liquid interfacial resistance, droplet curvature, promoter layer, and micro/nano-pillars are included into the model and incorporated in calculating the heat transfer rate across an individual droplet. Droplet morphologies (Wenzel, Cassie, and partially wetting) are considered in the model. Effects of various parameters are investigated on single droplets behaviors and total heat transfer. Results show that average droplets radii on a horizontal tube are higher than on a vertical tube; consequently, vertical tubes have better heat transfer rate. For example, vertical tubes have five times higher overall heat flux than horizontal tubes in some cases. In addition, partially wetting droplets have higher heat transfer rates in comparison with Wenzel or Cassie state in all configurations.     

Numerical and experimental investigations on the use of mist flow process in refrigerated display cabinets

This study concerns the use of mist flow whereby fine water droplets are injected into the air curtain to improve the performance of Refrigerated Display Cabinets (RDCs). The deposition and evaporation of droplets on the surface of products partially compensate the radiative heat gained by the products by removing from it the amount of latent heat of the evaporated droplets.The experiments were carried out on an actual display cabinet. Numerical modelling was performed using Fluent Computational Fluid Dynamics (CFD) software. In two-phase flow, an Euler–Lagrange approach was adopted to predict the transport of droplets by the air curtain and their spatial distribution on the product surface of the RDC. An original numerical procedure was built in the CFD model in order to compute the deposited droplets while taking into account the evaporative flux of droplets on the product surface.The two-phase flow model was used to analyse the performance of the mist cooling process in terms of surface temperature decrease and the homogeneity of droplet deposition on the product surface of the RDC as a function of inlet droplet injection configurations.     

Effects of ambient pressure and precursors on soot formation in spray flames

The effects of ambient pressure on soot formation in spray jet flames are investigated by means of a two-dimensional direct numerical simulation (DNS). In addition, the effects of precursors on soot formation are also discussed. The inception models considering acetylene and polycyclic aromatic hydrocarbon (PAH) precursors are employed and compared in this study. The extended flamelet/progress-variable approach (EFPV) in which heat transfer between a droplet and the surrounding fluid can be considered is employed as a combustion model. Jet A is used as a liquid fuel with considering the detailed chemistry including 274 chemical species and 1537 elemental reactions by the flamelet library. The evaporating droplets’ motions are tracked by the Lagrangian method and the non-equilibrium Langmuir–Knudsen model is used as an evaporation model. Results show that the spray jet flame structure and the soot formation characteristics are considerably affected by the ambient pressure condition. The soot volume fraction increases with increasing the ambient pressure. It is also revealed that the dominant process to promote the soot formation is different between the acetylene and PAH precursor models. It is essential to model a soot inception process with appropriate precursors in order to understand a detail mechanism of soot formation, since the precursor which plays the important role depends on the type of fuel and the combustion conditions.     

脱硫废水烟气喷射蒸发流动特性实验研究

确保喷雾液滴在接触烟道壁面前完全蒸发,是保障电站脱硫废水在锅炉尾部烟道内蒸发处理安全运行的关键。喷雾液滴的破碎、聚并等动力学行为,以及液滴群的粒径分布和速度等因素的影响机制,是喷雾蒸发的主要特性。设计搭建了热态风洞实验台,利用激光粒度分析仪和粒子图像测速仪（particle image velocimeter,PIV）,在不同的引射空气压力、喷嘴水流量,以及风速、加热空气温度等条件下,对喷雾液滴群的粒径变化和速度变化进行了测量和分析。实验结果表明：以大液滴形态离开喷嘴的射流在引射气流的携带作用下,因破碎而形成小液滴,而后液滴间聚并效果会显现出来。液滴初始粒径仅与引射气体压力和水流量有关;风速的提高一定程度上会促进液滴间的聚并。提高高压气体压力、温度、风速以及减小水流量均有助于提高液滴群速度,其中提高风速对液滴群的增速效果最为明显。研究结果为喷雾的数值模拟及工程应用改进方向提供了参考。     

Investigation of laser-induced destruction of droplets by acoustic methods

Experimental investigations of acoustic signals generated by individual laser-irradiated water droplets are reported. The dependence of droplet destruction thresholds on droplet radius and radiative heating rate is determined. A theoretical explanation of our experimental results is provided in terms of a model that includes the processes of droplet evaporation and fragmentation in response to intense laser heating.     

Heat and Mass Transfer in a Two‐Component Developed Turbulent Gas‐Vapor‐Droplet Flow

A calculation model is developed and a numerical study is made of the heat and mass transfer characteristics in a turbulent gas–vapor–droplet flow moving in a round tube. The model takes into account the evaporation of droplets, the diffusion of vapor into air, and the acceleration of a carrier flow. Distributions of the parameters of the twophase flow are obtained with respect to the tube radius for different initial concentrations of the gas phase. Heat and masstransfer calculations are compared to the experimental and numerical works. On the whole, the evaporation of the droplets in the vaporgas flow leads to the intensification of heat transfer as compared to a onecomponent vapordroplet flow and singlephase flow of vapor.     

Experimental Investigation on Flash Evaporation of Saltwater Droplets Released into Vacuum

In this paper, the flash evaporation process of saltwater droplets released into vacuum is experimentally investigated. During the experiment, a saltwater (NaCl) droplet was suspended on a thermocouple junction, which was used to measure the temperature evolution. The droplet surface temperature was captured by an infrared thermal imager, and the shape variation was recorded by a high speed camera. According to the experimental results, the component and solution concentration has great influence on the evaporation process. With a rise of salt concentration in water, the evaporation rate decreases. The shape of temperature transition curve also depends on the salt concentration in solution, no matter whether it is higher or lower than the eutectic point (22.4%). The effects of environmental pressure, initial droplet temperature and initial droplet diameter on the temperature transition of droplets were also summarized based on the experimental data.     

Influence of the liquid layer thickness on the growth of droplets controlled by the thermal action of laser radiation

We have studied the growth of droplets in a thin layer of aqueous ethanol solution under conditions of the solutocapillary convective flow controlled by a laser beam. It is established that, with increasing layer thickness, the time necessary for the formation of a droplet separated from the layer increases, while the time required for the droplet to reach its maximum possible diameter (determined by the criteria of stability) decreases. In the initial stage of droplet formation, the diameter exhibits some decrease related to a short-term increase in the solvent evaporation rate.     

Experimental estimation of the influence of the droplet evaporation process on the conditions of movement in an oncoming high-temperature gas flow

By means of high-speed video registration, the cross-correlation system, and panoramic optical methods of trace visualization, experimental estimation of the influence of liquid (water) droplet evaporation on the conditions of droplet movement (acceleration and deceleration) through the high-temperature (about 1100 K) gases was made. The experiments were conducted with droplets about 1–6 mm in diameter at start velocities of 1–5 m/s. We compare the integral characteristics of the droplet movement in the air at a temperature of about 300 K (in the ongoing flow and through the steady gas medium) and in the combustion product flow at a temperature of about 1100 K. The gas and the air flow velocities were about 1.5 m/s. The typical difference in the droplet velocities under essentially different ambient temperatures was discovered. The contribution of water evaporation and the ongoing gas movement into droplet deceleration was discovered.     

低压环境固着盐水液滴蒸发析晶过程实验研究

实验研究了低压环境下固着盐水液滴在不同基底表面(铜、载玻片和聚四氟乙烯)的蒸发析晶过程,分析了表面性质和环境压力的影响。结果表明,低压环境下易在接触线处析出白色盐晶。铜表面由于表面能较大,接触面上覆盖盐晶体,液滴蒸发过程接触直径几乎不变,接触角逐渐减小。在载玻片表面,当环境压力较高时,液滴蒸发造成接触线收缩,伴随盐晶体的生长和移动接触角波动。在聚四氟乙烯表面,接触面处易产生气泡,气泡的生长和爆裂导致接触角明显波动。Pe数可以揭示液滴蒸发过程外部传质扩散和内部离子扩散的相对大小。研究成果有助于指导海水淡化的工业应用。     

The Image Processing of Droplet for Evaporation Experiment in SJ-10

We have completed an experiment for droplet evaporation processing using Young-Laplace fitting, exponent fitting, polynomial fitting and ellipse fitting, which could be used for multiple shapes of droplets. The droplet evaporation experiment test was an important science experiment in SJ-10. In order to get the change process of the physical parameter, such as the touching edges and the droplet evaporation rate, we had gained the contour edge image of the droplet and used mathematic method to do the fitting analysis. The accuracy of the physical parameter was depended on the accuracy of the mathematic fitting. Using the original Young-Laplace fitting method could not process all the images of evaporation and liquid interface from the space experiment facility of SJ-10, especially the smaller droplet images. We could get more accurate contour fitting and result using the new method described in this article. This article proposes a complete solution, including edge detecting and contour fitting. In edge detecting, Canny detector was applied to extract droplet edge. In contour fitting, Young-Laplace fitting, exponent fitting, polynomial fitting and ellipse fitting are designed to fit the contour of droplets, which make the solution apply to all of droplets in SJ-10.     

Atmospheric glories: simulations and observations

Mie theory can be used to provide full-color simulations of atmospheric glories. Comparison of such simulations with images of real glories suggests that most glories are caused by spherical water droplets with radii between 4 and 25 microm. This paper also examines the appearance of glories taking into account the size of the droplets and the width of the droplet size distributions. Simulations of glories viewed through a linear polarizer compare well with the few available pictures, but they show some features that need corroboration by more observations.