Water Droplets In Air Research Articles

Acoustic streaming outside and inside a fluid particle undergoing monopole and dipole oscillations.

An analytical theory is developed for acoustic streaming induced by an acoustic wave field inside and outside a spherical fluid particle, which can be a liquid droplet or a gas bubble. The particle is assumed to undergo the monopole (pulsation) and the dipole (translation) oscillation modes. The dispersed phase and the carrier medium are considered to be immiscible, compressible, and viscous. The developed theory allows one to calculate the acoustic streaming both outside and inside the fluid particle. In contrast to earlier works, no restrictions are imposed on the thickness of the outer and inner viscous boundary layers with respect to the particle radius. A numerical implementation of the obtained analytical results is used to evaluate the acoustic streaming for different experimentally relevant configurations, such as an air bubble in water, a water droplet in oil, and a water droplet in air, considering both traveling and standing acoustic waves. The results show the richness of streaming pattern variations that arise in bubbles and droplets.

Electrostatic Effects of Corona Discharge on the Spectrum and Density Evolution of Water Droplets in Air

As one of the emerging application areas of the corona discharge technique, rain enhancement induced by atmospheric air ionization has made a great breakthrough. Although natural rainfall modified by artificial ionization was successfully observed via the statistical data of field experiments, deficient efforts have been devoted to the research on the near-field effect of corona discharge on the evolution of water droplets around the discharge device. In this article, we studied the electrostatic effects of corona discharge on the spectrum and density evolution of water droplets in a 96-m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> cloud chamber. An interesting phenomenon was found that two different electrostatic effects appeared when various high voltages were applied on the corona electrode. In detail, the growth of droplets larger than 10 μm was detected once the applied high voltage exceeds the critical value (e.g. -80 kV in our discharge device), and this is attributed to the electrostatic induction effect. By contrast, a narrow droplet size spectrum and reduced density caused by the strong electrostatic precipitation effect were shown below this critical voltage. To figure out the mechanisms underlying these phenomena, the charging status of water droplets with various sizes, forces of charged droplets under various electric field strength, and collision kernels between droplets, are analyzed through theoretical calculations. Results show that the electric field plays a dominant role in the charging process of droplets and the acceleration of collision-coalescence between charged and neutral droplets, which results in the different electrostatic effects on the evolution of droplets. This article may provide some enlightenment to fog elimination in industrial application and cloud-seeding in atmospheric water resources comprehensive utilization.

Multifunctional Magnetocontrollable Superwettable-Microcilia Surface for Directional Droplet Manipulation.

In nature, fluid manipulations are ubiquitous in organisms, and they are crucial for many of their vital activities. Therefore, this process has also attracted widescale research attention. However, despite significant advances in fluid transportation research over the past few decades, it is still hugely challenging to achieve efficient and nondestructive droplet transportation owing to contamination effects and controllability problems in liquid transportation applications. To this end, inspired by the motile microcilia of micro‐organisms, the superhydrophobicity of lotus leaves, the underwater superoleophobicity of filefish skin, and pigeons' migration behavior, a novel manipulation strategy is developed for droplets motion. Specifically, herein, a superwettable magnetic microcilia array surface with a structure that is switchable by an external magnetic field is constructed for droplet manipulation. It is found that under external magnetic fields, the superhydrophobic magnetic microcilia array surface can continuously and directionally manipulate the water droplets in air and that the underwater superoleophobic magnetic microcilia array surface can control the oil droplets underwater. This work demonstrates that the nondestructive droplet transportation mechanism can be used for liquid transportation, droplet reactions, and micropipeline transmission, thus opening up an avenue for practical applications of droplet manipulation using intelligent microstructure surfaces.

Magnetocontrollable Droplet and Bubble Manipulation on a Stable Amphibious Slippery Gel Surface

AbstractSmart manipulation of liquid/bubble transport has garnered widespread attention due to its potential applications in many fields. Designing a responsive surface has emerged as an effective strategy for achieving controllable transport of liquids/bubbles. However, it is still challenging to fabricate stable amphibious responsive surfaces that can be used for the smart manipulation of liquid in air and bubbles underwater. Here, amphibious slippery surfaces are fabricated using magnetically responsive soft poly(dimethylsiloxane) doped with iron powder and silicone oil. The slippery gel surface retains its magnetic responsiveness and demonstrates strong affinity for bubbles underwater but shows small and switching resistance forces with the water droplets in air and bubbles underwater, which is the key factor for achieving the controllable transport of liquids/bubbles. On the slippery gel surface, the sliding behaviors of water droplets and bubbles can be reversibly controlled by alternately applying/removing an external magnetic field. Notably, compared with slippery liquid‐infused porous surfaces, the slippery gel surface demonstrates outstanding stability, whether in air or underwater, even after 100 cycles of alternately applying/removing the magnetic field. This surface shows potential applications in gas/liquid microreactors, gas–liquid mixed fluid transportation, bubble/droplet manipulation, etc.

A Note on Dispersion Analysis in Two Phase Flows

In present article a theoretical attempt is made to present the variation of attenuation by water droplets in air for various values of particle volume fraction and Prandtl number. It is concluded that as particle volume fraction increases dispersion has a decreasing trend while attenuation increases and attend maximum value and then decreases continuously. Similarly for fixed values of and for different value of Prandtl number dispersion decreases and reached minimum and then increases while attenuation increase and attend the maximum value and then decreases. In preparation of graphs MATLAB is used .

Energy efficiency analysis of wet compression systems through thermo-fluid dynamic considerations

Wet compression systems are deployed in gas turbine power plants for energy savings in the compressor. The system works by injecting fine water droplets in the form of mist into the air stream ahead of the compressor. The water droplets evaporate both before entering the compressor and within the compressor and reduce the air temperature by absorbing latent heat of evaporation. In this study, an analysis is done on two-phase compression of air-water droplet mixture. The work is focused on understanding the fundamental concepts associated with two-phase compression. Hence, a simple piston-cylinder system is used as the domain to simulate the compression of air-water droplet mixture. During the processes, the pressure, temperature and relative humidity inside the cylinder have been monitored. Parametric studies are then performed for various values of starting relative humidity of the air, overspray percentage of water droplets, droplet diameter, and compression speed of the piston. In these studies, the deviation of PV and TV curves from the dry air compression curves, resulting from latent heat of vaporizing droplets are investigated. The studies show that smaller droplets, higher overspray and slower compression speeds significantly reduces the compressor work requirement, thereby increasing cost savings and environmental benefits of power generation. Interestingly, it is seen that the effect of initial relative humidity of the working fluid on energy savings is minimal. The corresponding efficiencies achieved through wet compression process are determined and tabulated.

Numerical simulation of thermal water delivery in the human nasal cavity

This work describes an extensive numerical investigation of thermal water delivery for the treatment of inflammatory disorders in the human nasal cavity. The numerical simulation of the multiphase air-droplets flow is based upon the Large Eddy Simulation (LES) technique, with droplets of thermal water described via a Lagrangian approach. Droplet deposition is studied for different sizes of water droplets, corresponding to two different thermal treatments, i.e. aerosol and inhalation. Numerical simulations are conducted on a patient-specific anatomy, employing two different grid sizes, under steady inspiration at two breathing intensities.The results are compared with published in vivo and in vitro data. The effectiveness of the various thermal treatments is then assessed qualitatively and quantitatively, by a detailed analysis of the deposition patterns of the droplets. Discretization effects on the deposition dynamics are addressed. The level of detail of the present work, together with the accuracy afforded by the LES approach, leads to an improved understanding of how the mixture of air-water droplets is distributed within the nose and the paranasal sinuses.

On comparison of the sharp-interface and diffuse-interface level set methods for 2D capillary or/and gravity induced flows

The present work is on comparison of numerical-methodology as well as performance of the two types of level set methods (LSMs): sharp-interface and diffused-interface. The numerical-methodology along with mathematical-formulation are presented in-detail for relatively recent ghost fluid method based sharp-interface level-set-method (SI-LSM); and compared with the methodology for the traditional diffuse-interface level-set-method (DI-LSM) as well as an improved diffuse-interface dual-grid-level-set-method (DI-DGLSM). Two different types of surface models are considered: continuum surface force (CSF) and sharp surface force (SSF) model; CSF model for the DI-LSM and SSF model for the SI-LSM. The SI-LSM considers the physically realistic sudden variation of the thermo-physical property across the sharp-interface while DI-LSM considers a smoothened value of the properties across the numerically diffused-interface. For solving the pressure Poisson equation in the SI-LSM, a finite volume method based generic formulation is proposed and its implementation-details are presented. For the SI-LSM as compared to DI-LSM and DI-DGLSM, the relative performance study is presented on four reasonably different two-phase problems: surface-tension model induced unphysical flow for a static water droplet, collapse of a water-column in air, falling of a water-droplet in air, and coalescence of an ethanol-droplet over a pool of ethanol in air. For the performance study on various problems, a comparison of the results obtained by the three types of LSMs (SI-LSM, DI-LSM and DI-DGLSM) and various other numerical methods in the literature are presented. SI-LSM as compared to DI-LSM and DI-DGLSM is shown to substantially reduce the unphysical spurious velocity and results in better accuracy on the same grid size.

Ice Nucleation in Periodic Arrays of Spherical Nanocages

A silicious material containing massive array of spherical nanocages connected to each other by small micropores was used to study ice nucleation in confined water under conditions of well-defined pore geometry. By purposefully selecting small size of the interconnecting pores below 2 nm, ice nucleation and growth were limited to occur only within the nanocages. By exploitation of nuclear magnetic resonance, ice nucleation rates at different temperatures were accurately measured. These rates were obtained to be substantially higher than those typically observed for micrometer-sized water droplets in air. In addition, the occurrence of correlations between ice nucleation in one nanocage with the phase state in the adjacent cages were observed. These results have important implication for a deeper understanding of ice nucleation, especially in confined geometries.

Surface modification of melamine sponges for pH-responsive oil absorption and desorption

Inspired by the development of smart oil/water separation materials, a pH responsive melamine sponge has been obtained by grafting poly (4-vinylpyridine) on the skeleton surface through atom transfer radical polymerization. Through scanning electron microscopy and x-ray photoelectron spectroscopy, the successful grafting of poly (4-vinylprridine) onto the melamine sponge has been confirmed. When contacting with different pH water droplets in air, the as-prepared product shows excellent switchable wettability between super-hydrophilicity (0°) and highly hydrophobicity (135°). Meanwhile, this responsive sponge also exhibits super-hydrophilic/oleophobic property underwater at pH=1.0, and highly hydrophobic/super-oleophilic property in neutral solution at pH=7.0. Furthermore, the excellent responsiveness is remained after five cycle water contact angle tests between two different pH stages at pH 1.0 and 7.0. The modified melamine sponges could not only absorb the oil from the oily water at pH=7.0, but also quickly release the absorbed oil underwater at pH=1.0 without leaving any residues and hurting the environment nearly, showing a good potential in controlled oil/water separation and oil recovery.

Cascaded lattice Boltzmann method with improved forcing scheme for large-density-ratio multiphase flow at high Reynolds and Weber numbers.

A recently developed forcing scheme has allowed the pseudopotential multiphase lattice Boltzmann method to correctly reproduce coexistence curves, while expanding its range to lower surface tensions and arbitrarily high density ratios [Lycett-Brown and Luo, Phys. Rev. E 91, 023305 (2015)PLEEE81539-375510.1103/PhysRevE.91.023305]. Here, a third-order Chapman-Enskog analysis is used to extend this result from the single-relaxation-time collision operator, to a multiple-relaxation-time cascaded collision operator, whose additional relaxation rates allow a significant increase in stability. Numerical results confirm that the proposed scheme enables almost independent control of density ratio, surface tension, interface width, viscosity, and the additional relaxation rates of the cascaded collision operator. This allows simulation of large density ratio flows at simultaneously high Reynolds and Weber numbers, which is demonstrated through binary collisions of water droplets in air (with density ratio up to 1000, Reynolds number 6200 and Weber number 440). This model represents a significant improvement in multiphase flow simulation by the pseudopotential lattice Boltzmann method in which real-world parameters are finally achievable.

Study of falling water drop in stagnant air

The objective of this work is to propose a mathematical expression for the instantaneous velocity of falling water droplets in air and traveled height versus time. In addition, changes in terminal velocity depending on terminal height are studied. For very small spherical drops, having very low Reynolds number and obeying Stokes law for the drag force, the instantaneous velocity u of the drop with time is a well-known result. From the expression of that speed, it is easy to get the drop height h versus time. By eliminating the time, the displacement h of the droplet as a function of the velocity u is obtained. In the present work, the latter was extended to larger drops. The values predicted by the mathematical equation obtained from the combination of theory and experiment (semi empirical equation) are compared with well known experimental data. The results show good agreement between the values predicted and the experimental data.

Numerical simulation of two-fluid flow of electrolyte solution with charged deforming interfaces

The authors’ volume-of-fluid based electrokinetic flow model for multiphase flow with fluid–fluid interfaces, recently extended by them to allow for interfacial charge, is refined and more comprehensively validated under dynamic conditions. Validation is performed using (i) an analytical solution for drop relaxation in the presence of ions and interfacial charge, and (ii) published experimental observations of the shape evolution of a critically charged water droplet in air, undergoing Rayleigh instability. Predictions based on two methods of representing the electrical force, in combination with two different numerical treatments of the interfacial charge, are compared. It is shown that the most accurate predictions, when there is interfacial charge, are obtained by using a “modified pressure” formulation (in which part of the Maxwell stress is incorporated into the pressure) in combination with a published treatment for solving the Poisson equation for the electrical potential that is similar to the Ghost Fluid Method; this is therefore the recommended model combination. The choice of method (modified pressure or standard formulation) is found to be unimportant when there is no interfacial charge. Finally, the recommended model is used to predict the steady state deformation of a hexadecane drop in an electrolyte solution during pressure-driven flow through a cylindrical microchannel, when both the channel wall and the drop interface carry electric charge. It is shown that, for low interfacial tension, electrokinetic forces can significantly enhance or reduce the drop deformation, compared with that of a drop with no interfacial charge, when the charges on the drop interface and channel wall are of the same or opposite sign, respectively.

Impact of air and water vapor environments on the hydrophobicity of surfaces

HypothesisDroplet wettability and mobility play an important role in dropwise condensation heat transfer. Heat exchangers and heat pipes operate at liquid–vapor saturation. We hypothesize that the wetting behavior of liquid water on microstructures surrounded by pure water vapor differs from that for water droplets in air. ExperimentsThe static and dynamic contact angles and contact angle hysteresis of water droplets were measured in air and pure water vapor environments inside a pressure vessel. Pressures ranged from 60 to 1000mbar, with corresponding saturation temperatures between 36 and 100°C. The wetting behavior was studied on four hydrophobic surfaces: flat Teflon-coated, micropillars, micro-scale meshes, and nanoparticle-coated with hierarchical micro- and nanoscale roughness. FindingsStatic advancing contact angles are 9° lower in the water vapor environment than in air on a flat surface. One explanation for this reduction in contact angles is water vapor adsorption to the Teflon. On microstructured surfaces, the vapor environment has little effect on the static contact angles. In all cases, variations in pressure and temperature do not influence the wettability and mobility of the water droplets. In most cases, advancing contact angles increase and contact angle hysteresis decreases when the droplets are sliding or rolling down an inclined surface.

A Laser Trapping-Spectroscopy Study on Mass Transfer Processes Across a Single Micro-Droplet/Air Interface

Clouds regulate the earth's energy balance by reflecting and scattering solar radiation and by absorbing the earth's infrared radiation. The fundamental knowledge about mass transfer processes across a micro-droplet/air interface is very important to give mathematical equations that describe the growth process of clouds for climate models. So far, experimental studies on the condensation growth of water droplets have been conducted by using either an aerosol flow tube or a vibrating orifice aerosol generator. However, the mass accommodation coefficients evaluated by such techniques are very scattered and, therefore, the detailed mechanisms of condensation growth of micrometer-sized water droplets are still controversial. The primary reason for this is difficulties in observing the growth processes of single water droplets in air. In this study, we demonstrate a novel approach for in situ observation of the evaporation and condensation processes of single water droplets levitated in air by means of a laser trapping technique.

Reversible control of the equilibrium size of a single aerosol droplet by change in relative humidity.

Noncontact levitation of single micrometer-sized water droplets in air can be achieved by a laser trapping technique. The equilibrium size of a water droplet is quite sensitive to relative humidity in the surrounding gas phase. In order to investigate the physical and chemical properties of single water droplets in air as a function of the droplet size or solute concentration, laser trapping experiments were conducted under controlled humidity conditions. In this study, we developed a trapping chamber equipped with a relative humidity controller and demonstrated the reversible control of the equilibrium size of a single droplet levitated in air through a change in relative humidity. Furthermore, relative humidity was successfully evaluated by means of cavity enhanced Raman spectroscopy of a trapped water droplet.

Depolarization coefficients of light in multiply scattering media.

The depolarization coefficients are calculated for multiply scattered linearly and circularly polarized light. For a number of media (aqueous suspension of polystyrene particles, water droplets in air), the calculations are carried out both numerically, with solving the vector radiative transfer equation and analytically, within the polarization mode approximation. In the latter case the depolarization coefficients are expressed explicitly in terms of the scattering and absorption coefficients, and the scattering matrix elements of the medium. The range of applicability of the polarization mode approximation is established. For most practically important cases, this method is shown to provide a satisfactory degree of accuracy. We also find the fundamental values of the depolarization coefficients for a Rayleigh medium.

Effects of geometric parameters and electric indexes on performance of a vertical wet electrostatic precipitator

Abstract A single-stage, single-wire vertical wet electrostatic precipitator was designed and operated in air–water droplets flow to investigate its performance. The efficiency was compared with a glass micro fiber filter and proposed semi-empirical efficiency model, which was in good accuracy while considering the vapor content. Effects of geometric parameters on efficiency under different charge conditions were discussed. Due to evaporation mechanism, the corona current decreases for high flow rates at the same applied voltage. Findings indicated while developing flow is created inside the ESP, there exists an optimum wire-to-flow inlet spacing that provides maximum droplet collection efficiency.

A fast pressure-correction method for incompressible two-fluid flows

We have developed a new pressure-correction method for simulating incompressible two-fluid flows with large density and viscosity ratios. The method's main advantage is that the variable coefficient Poisson equation that arises in solving the incompressible Navier–Stokes equations for two-fluid flows is reduced to a constant coefficient equation, which can be solved with an FFT-based, fast Poisson solver. This reduction is achieved by splitting the variable density pressure gradient term in the governing equations. The validity of this splitting is demonstrated from our numerical tests, and it is explained from a physical viewpoint.In this paper, the new pressure-correction method is coupled with a mass-conserving volume-of-fluid method to capture the motion of the interface between the two fluids but, in general, it could be coupled with other interface advection methods such as level-set, phase-field, or front-tracking. First, we verified the new pressure-correction method using the capillary wave test-case up to density and viscosity ratios of 10,000. Then, we validated the method by simulating the motion of a falling water droplet in air and comparing the droplet terminal velocity with an experimental value. Next, the method is shown to be second-order accurate in space and time independent of the VoF method, and it conserves mass, momentum, and kinetic energy in the inviscid limit. Also, we show that for solving the two-fluid Navier–Stokes equations, the method is 10–40 times faster than the standard pressure-correction method, which uses multigrid to solve the variable coefficient Poisson equation. Finally, we show that the method is capable of performing fully-resolved direct numerical simulation (DNS) of droplet-laden isotropic turbulence with thousands of droplets using a computational mesh of 10243 points.

Numerical modeling of the drift and deposition of droplets emitted by mechanical cooling towers on buildings and its experimental validation

A CFD numerical modeling for simulating the drift emitted by mechanical cooling towers located in built-up urban areas is presented. The averaged Navier–Stokes equations for the air motion are discretized through a finite volume method, with the k–ɛ model to simulate the turbulent flow. An appropriate set of boundary conditions is employed, in order to avoid the non-homogeneity problem in the Atmospheric Boundary Layer (ABL). A Eulerian–Lagrangian model is used to simulate the air–water droplet motion, taking into account the effects of evaporation. The results are validated with experimental data of the deposition of droplets emitted by a pilot cooling tower, obtained through a technique based on water sensitive papers. Local meteorological data are measured through a meteorological tower adjacent to the cooling tower. The present study shows the influence of variables such as the ambient dry bulb temperature, the ambient specific humidity and the droplet exit temperature from cooling tower on the water droplet deposition and on the size of the affected area for mechanical cooling towers. The numerical procedure has proved useful for obtaining the outlined objectives.