Evaporation Of Liquid Droplets Research Articles

The Effect of the Substrate Temperature and the Height of the Mini-Channel on the Evaporation of Liquid Droplet Blown by the Gas Flow

The experimental setup has been designed and fabricated to study the evaporation of liquid droplets, blown by the gas flow in a mini-channel with a height of 6 and 9 mm. Substrates were removable, and the surface temperature of the substrate was maintained at a constant level. The shadow method was the main method of measurement. A series of experiments with a water drop of 100 μl volume settled on a polished stainless substrate were carried out. As a result the dependences for the droplet evaporation rate were obtained in the range of temperatures 25–70 °C and Reynolds numbers of gas 0–2000.

Peculiarities of heat transfer in water droplets with a solid inclusion during heating in a high-temperature gas medium

A physical model and a mathematical model of heat transfer in the conditions of inhomogeneous (with a solid inclusion—a carbon particle) liquid droplet evaporation while moving through high-temperature (800–1500 K) gases are formulated. Numerical investigations were performed using, as an example, a spherical inhomogeneous water droplet during heating in the air medium. The most probable mechanism of phase transitions in a water–carbon particle–heated air system is considered (the initial droplet size, radius, varied in the range from 0.5 mm to 1.5 mm, the inclusion radius was 0.1–1 mm). It has been found that in certain conditions, besides water evaporation from the outer (free) droplet surface, intensive vaporization is possible at the liquid–solid inclusion interface. Conditions of realization of these phase transitions in inhomogeneous water droplet–high-temperature gas medium systems are identified.

Experimental investigation of evaporation enhancement for water droplet containing solid particles in flaming combustion area

The experimental study of integral characteristics of extinguishing liquid (water) droplet evaporation in flaming combustion area has been held. Optical methods of two-phase and heterogeneous mixtures diagnostics (?Particle Image Velocimetry? and ?Interferometric Particle Imaging?) have been used for heat and mass transfer process investigation. It was established that small-size solid particles (for example, carbon particles) in droplet structure can enhance water evaporation in flame area. It was shown that the rate of evaporation process depends on concentration and sizes of solid particles in a water droplet. The correlations have been determined between the sizes of solid particles and water droplets for maximum efficiency of fire extinguishing. The physical aspects of the problem have been discussed.

Numerical study of droplet evaporation in contact with hot porous surface using lattice Boltzmann method

This paper presents a numerical simulation of a liquid droplet evaporation during penetration in a porous medium based on lattice Boltzmann method. The binary-fluid model of Lee, which is applicable to high density ratio, is combined with a passive scalar LB thermal model to simulate two-phase flow. A proper source term is incorporated at the phase interface, and the classical convective Cahn–Hilliard equation in the presence of phase change is employed in the multiphase LB framework of Lee. Effects of different non-dimensional parameters including the Archimedes number, the Bond number, the Stefan number, the Prandtl number, the porosity, contact angle and density ratio on the penetration of liquid inside the porous media and the droplet evaporation are investigated and temperature profiles, flow fields, mass source contours are analyzed. Increasing the Bond number, the Archimedes number and density ratio will decrease the penetration rate, and accelerates the evaporation. Also, increasing the Stefan number and porosity accelerate the evaporation, while increasing the Prandtl number leads reduction in evaporation rate.

Molecular origin of contact line stick-slip motion during droplet evaporation

Understanding and controlling the motion of the contact line is of critical importance for surface science studies as well as many industrial engineering applications. In this work, we elucidate the molecular origin of contact line stick-slip motion during the evaporation of liquid droplets on flexible nano-pillared surfaces using molecular dynamics simulations. We demonstrate that the evaporation-induced stick-slip motion of the contact line is a consequence of competition between pinning and depinning forces. Furthermore, the tangential force exerted by the pillared substrate on the contact line was observed to have a sawtooth-like oscillation. Our analysis also establishes that variations in the pinning force are accomplished through the self-adaptation of solid-liquid intermolecular distances, especially for liquid molecules sitting directly on top of the solid pillar. Consistent with our theoretical analysis, molecular dynamics simulations also show that the maximum pinning force is quantitatively related to both solid-liquid adhesion strength and liquid-vapor surface tension. These observations provide a fundamental understanding of contact line stick-slip motion on pillared substrates and also give insight into the microscopic interpretations of contact angle hysteresis, wetting transitions and dynamic spreading.

Analysis of the Water Droplet Evaporation Features with a Solid Nontransparent Inclusion in High-Temperature Gas Environment as a Part of University Research Work

This paper examines the features of the heating and evaporation of heterogeneous liquid (water) droplets (with the initial radius from 1.5 to 2.5mm) in the combustion products of typical liquid combustible substances (acetone, technical ethanol, butane-propane gas mixture) and heated air explored as a part of university research work. The investigations have been performed by means of high-speed video recording and software “TEMA Automotive” and “Phantom Camera Control”. The gas temperature varied from 300 to 850K in the experiments. The characteristic times of droplet existence (complete evaporation) have been determined. The influence of radiative, convective and conductive heat transfer has been defined.

Investigations of Droplet‐Plasma Interaction using Multi‐Dimensional Coupled Model

AbstractRecently developed multi‐dimensional coupled fluid‐droplet model is used to investigate the behavior of complex interaction between the liquid precursor droplets and atmospheric pressure plasma (APP). The significance of this droplet‐plasma interaction is not well understood under diverse realm of working conditions in two‐phase flow. In this study, we explain the implication of vaporization of liquid droplets in APP which are subsequently responsible to control major characteristics of surface coating depositions. Coalescence of water droplets is more dominant than Hexamethyldisiloxane (HMDSO) droplets because of its sluggish rate of evaporation. A disparity in the performance of evaporation is identified in two independent mediums, such as gas mixture and discharge plasma using HMDSO precursor. The length of evaporation of droplets is amplified by an increment of gas flow rate indicating with a reduction in the gas temperature and electron mean energy. In particular, the spatio‐temporal density distributions of charged particles show a clear pattern in which the typical nitrogen impurity ions are primarily effective as compared to other helium ionic species along the pulse of droplets in APP. Finally, we contrast the behavior of discharge species in the pure helium and He‐N2 gas mixtures revealing the importance of stepwise and Penning ionization processes. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical measurements from single levitated particles using a linear electrodynamic quadrupole trap.

We have recently made advancements in a linear electrodynamic quadrupole (LEQ) device for capturing and levitating either single or multiple micro-particles that provides significant improvements in capture efficiency, reliability, and optical measurement access. We have used our LEQ to trap particles ranging from 30 to less than 0.5 μm in size and provide a controlled environment to study particle physical/chemical dependencies on temperature, relative humidity, and gas constituents. To demonstrate this approach, we present data and analysis of liquid-droplet evaporation rates for two materials: glycerol and dibutyl sebacate. Droplet size was monitored as a function of time by two independent optical methods: direct imaging and fixed-angle light scattering. This new approach provides a means to rapidly characterize a wide range of aerosol particle properties and a platform for development of new aerosol optical-diagnostic measurements.

Modelling of the process of fragmentation and vaporization of non-reacting liquid droplets in high-enthalpy gas flows

The intensification of the fragmentation and vaporization of liquid droplets in two-phase flows with the gas stagnation temperature Тg = 800−2500 K is an important scientific and technological problem. One should note that despite a high practical importance the mechanism of the vaporization of droplets with their preliminary gas-dynamic fragmentation in high-enthalpy flows has been studied insufficiently completely and requires additional research. The paper presents a mathematical model and the results of the computations of the fragmentation and vaporization of liquid droplets in subsonic and supersonic flows with a high stagnation temperature. A comparison of the obtained data with the experiments of other authors has been done. The extension of the regions of the gas-dynamic fragmentation and droplet vaporization in flow ducts with a variable distribution of parameters has been estimated. The found peculiarities may be used at the design of energy installations of the promising samples of the aerospace technology and gas-dynamic pipes.

Sedimentation of harmful dust by means of ultrasonic waves and additional disperse phase

The harmful effect on the health of superfine dust in industry is a serious problem. An especially important and complex task is the development of new methods for sedimentation of dust particles with a characteristic size of less than 10 μm. The solution offered by the authors involves a combined method of impact on industrial dust using high-frequency acoustic fields and spraying of a superfine water aerosol. Acoustic radiation sources are successfully applied to cleaning rooms from dust having fairly large particle sizes (10 μm and larger). The sedimentation of fine aerosols (particle diameter of 1–10 μm) is a more complicated challenge. The paper is devoted to substantiation of the acoustic sedimentation method for such aerosols. Good results were achieved when additional disperse phase was applied together with the ultrasonic source. Calculation results according to the mathematical model for coagulation and sedimentation, based on Smoluchowski’s equation, taking into account evaporation of liquid droplets, are given. Results of the experimental and theoretical study of the processes of acoustic sedimentation of superfine aerosols are presented. Recommendations are offered for the use of sources of ultrasonic radiation and sprayers of water aerosol for optimum removal of dust from workplace air.

Computational fluid dynamic modeling of the flame spray pyrolysis process for silica nanopowder synthesis

A computational fluid dynamic model that couples the fluid dynamics with various processes involving precursor droplets and product particles during the flame spray pyrolysis (FSP) synthesis of silica nanopowder from volatile precursors is presented. The synthesis of silica nanopowder from tetraethylorthosilicate and tetramethylorthosilicate in bench- and pilot-scale FSP reactors, with the ultimate purpose of industrial-scale production, was simulated. The transport and evaporation of liquid droplets are simulated from the Lagrangian viewpoint. The quadrature method of moments is used to solve the population balance equation for particles undergoing homogeneous nucleation and Brownian collision. The nucleation rate is computed based on the rates of thermal decomposition and oxidation of the precursor with no adjustable parameters. The computed results show that the model is capable of reproducing the magnitude as well as the variations of the average particle diameter with different experimental conditions using a single value of the collision efficiency factor α for a given reactor size.

Pinning of the Contact Line during Evaporation on Heterogeneous Surfaces: Slowdown or Temporary Immobilization? Insights from a Nanoscale Study.

The question of the effect of surface heterogeneities on the evaporation of liquid droplets from solid surfaces is addressed through nonequilibrium molecular dynamics simulations. The mechanism behind contact line pinning which is still unclear is discussed in detail on the nanoscale. Model systems with the Lennard-Jones interaction potential were employed to study the evaporation of nanometer-sized cylindrical droplets from a flat surface. The heterogeneity of the surface was modeled through alternating stripes of equal width but two chemical types. The first type leads to a contact angle of 67°, and the other leads to a contact angle of 115°. The stripe width was varied between 2 and 20 liquid-particle diameters. On the surface with the narrowest stripes, evaporation occurred at constant contact angle as if the surface was homogeneous, with a value of the contact angle as predicted by the regular Cassie-Baxter equation. When the width was increased, the contact angle oscillated during evaporation between two boundaries whose values depend on the stripe width. The evaporation behavior was thus found to be a direct signature of the typical size of the surface heterogeneity domains. The contact angle both at equilibrium and during evaporation could be predicted from a local Cassie-Baxter equation in which the surface composition within a distance of seven fluid-particle diameters around the contact line was considered, confirming the local nature of the interactions that drive the wetting behavior of droplets. More importantly, we propose a nanoscale explanation of pinning during evaporation. Pinning should be interpreted as a drastic slowdown of the contact line dynamics rather than a complete immobilization of it during a transition between two contact angle boundaries.

Analytical modeling and assessment of overspray fogging process in gas turbine systems

In this study, analysis of the overspray fogging process, which consists of inlet fogging and wet compression processes in gas turbine cycle, is carried out with a non-equilibrium analytical modeling based on liquid droplet evaporation. The transient behaviors of droplet diameter and air temperature are examined. Special attention is paid to the effects of system parameters such as pressure ratio, water injection ratio, and initial droplet diameter on the process performances, including evaporation time of droplets, compressor outlet temperature, and compression work. Process performances are also estimated under the condition of critical water injection ratio above which complete evaporation of injected water droplets within a compressor is not possible. The analytical results indicate that the overspray fogging process has a potential of lowering compressor outlet temperature and saving significant compression work when compared to dry compression, and this technique is more effective for a higher pressure ratio and smaller size of injected water droplet.

Evaporation of Liquid Droplet in Nano and Micro Scales from Statistical Rate Theory.

The statistical rate theory (SRT) is applied to predict the average evaporation flux of liquid droplet after the approach is validated in the sessile droplet experiments of the water and heavy water. The steady-state experiments show a temperature discontinuity at the evaporating interface. The average evaporation flux is evaluated by individually changing the measurement at a liquid-vapor interface, including the interfacial liquid temperature, the interfacial vapor temperature, the vapor-phase pressure, and the droplet size. The parameter study shows that a higher temperature jump would reduce the average evaporation flux. The average evaporation flux can significantly be influenced by the interfacial liquid temperature and the vapor-phase pressure. The variation can switch the evaporation into condensation. The evaporation flux is found to remain relative constant if the droplet is larger than a micro scale, while the smaller diameters in nano scale can produce a much higher evaporation flux. In addition, a smaller diameter of droplets with the same liquid volume has a larger surface area. It is suggested that the evaporation rate increases dramatically as the droplet shrinks into nano size.

Analysis of the effects of evaporative cooling on the evaporation of liquid droplets using a combined field approach.

During liquid evaporation, the equations for the vapor concentration in the atmosphere and for the temperature in the liquid are coupled and must be solved in an iterative manner. In the present paper, a combined field approach which unifies the coupled fields into one single hybrid field and thus makes the iteration unnecessary is proposed. By using this approach, the influences of the evaporative cooling on the evaporation of pinned sessile droplets are investigated, and its predictions are found in good agreement with the previous theoretical and experimental results. A dimensionless number Ec which can evaluate the strength of the evaporative cooling is then introduced, and the results show that both the evaporation flux along the droplet surface and the total evaporation rate of the droplet decrease as the evaporative cooling number Ec increases. For drying droplets, there exists a critical value EcCrit below which the evaporative cooling effect can be neglected and above which the significance of the effect increases dramatically. The present work may also have more general applications to coupled field problems in which all the fields have the same governing equation.

Liquid droplet evaporation from buckypaper: On the fundamental properties of the evaporation profile

Buckypaper is a mesoporous, self-supporting, electrically conducting mat of multiwall carbon nanotubes prepared by filtering a nanotube suspension through a submicron pore diameter membrane and drying the filter cake. Multiple phenomena take place simultaneously when a liquid droplet contacts the buckypaper surface: wetting, spreading, adsorption, capillary filling, evaporation from the surface, evaporation from the pores, infiltration of nanotube–nanotube junctions, gravity induced convective flow, etc. The electrical resistance of the buckypaper as a function of time exhibits a maximum curve during these processes. The fine details of the shape of this curve are slightly different for every solvent. We refer to this characteristic function as the evaporation profile of the studied liquid. In this paper we combine simultaneous electrical resistivity measurement, weight measurement and visible image processing methods to reveal the basic properties of the evaporation profile. A good correlation between the changes in buckypaper mass and electrical resistance was found and the distribution of the liquid penetrating the buckypaper matrix followed specific patterns and was correlated with the shape of the measured evaporation profile. Results are compared with fluid dynamic simulations. The evaporation profile emerges as a reproducible and inexpensive qualitative analytical tool for liquid identification. Alternatively, recording the evaporation profile of a chosen test liquid could become a feasible characterization method for mesoporous materials.

Combined effects of underlying substrate and evaporative cooling on the evaporation of sessile liquid droplets

The evaporation of pinned, sessile droplets resting on finite thickness substrates was investigated numerically by extending the combined field approach to include the thermal properties of the substrate. By this approach, the combined effects of the underlying substrate and the evaporative cooling were characterized. The results show that the influence of the substrate on the droplet evaporation depends largely on the strength of the evaporative cooling. When the evaporative cooling is weak, the influence of substrate is also weak. As the strength of evaporative cooling increases, the influence of the substrate becomes more and more pronounced. Further analyses indicated that it is the cooling at the droplet surface and the temperature dependence of the saturation vapor concentration that relate the droplet evaporation to the underlying substrate. This indicates that the evaporative cooling number, Ec, can be used to identify the influence of the substrate on the droplet evaporation. The theoretical predictions by the present model are compared and found to be in good agreement with the experimental measurements. The present work may contribute to the body of knowledge concerning droplet evaporation and may have applications in a wide range of industrial and scientific processes.

Stability and Two-phase Dynamics of Evaporating Marangoni-driven Flows in Laterally-heated Liquid Layers and Sessile Droplets

This paper investigates the linear and non-linear instabilities during evaporation of liquid layers and droplets by means of two-phase 3D direct numerical simulations. The interface is open to the atmosphere under the consideration that vapour diffusion is the rate- limiting mechanism for evaporation. In both configurations, the vapour-liquid interface is prone to travelling thermal instabilities, i.e., hydrothermal waves (HTWs), due to the presence of temperature gradients along the interface. We have already shown in our recent work 7 that under saturated conditions (negligible evaporation) the HTWs additionally give rise to interface deformations of similar features, i.e., physical waves. We have also demonstrated 8 that phase change plays a dual role through its effect on these instabilities: the latent energy required during the evaporation process tends to inhibit the HTWs while the accompanying level reduction enhances the physical waves by minimizing the role of gravity. The dynamics of the gas phase are also discussed. We have also established that the HTW-induced convective patterns in the gas along with the travelling nature of the instabilities have a significant impact on the local evaporation flux and the vapour distribution above the interface. The Marangoni effect plays a major role in the vapour distribution generating a vacuum effect in the warm region and vapour accumulations at the cold boundary capable of inverting the phase change, i.e., the capillary flow can lead to local condensation. These results provide evidence of the inefficiencies of the traditional phase change models based on pure vapour diffusion to capture the dynamics of thermocapillary flows. To conclude, we also present results of a parallel investigation focusing on three-dimensional phenomena on evaporating sessile drops placed on heated substrates.

Steady‐state behavior of liquid fuel hydrazine decomposition in packed bed

A general theoretical model is presented to analyze the steady‐state decomposition process of liquid monopropellants in packed beds for thruster systems. Additionally, an experiment studying the decomposition of liquid hydrazine in a packed bed is used to validate this model. The liquid droplet evaporation rate is determined through calculating the gas‐liquid mass transfer for the mixture temperatures lower than the liquid propellant boiling point and solving the gas‐liquid or liquid‐solid heat transfer equations at the temperature exceeding the boiling point. The process of liquid propellant decomposition in packed beds are simulated based on the Naive–Stokes equation for the mixture model integrated with the developed liquid evaporation rate, in which both the heterogeneous catalytic reaction coupled with the diffusion of reactants in the pore of catalyst, and the homogenous decomposition reactions are considered. The calculated results for the axial distribution of the temperature are in good agreement with the experimental data. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1064–1080, 2015

Measurement of Liquid Droplet Evaporation Rate under the Conditions of Intense Heating

This paper presents an experimental study of the evaporation of a sessile water-sodium chlorides solution drop to open atmosphere on the solid substrate (anodized aluminum) under the varying heat flux. The main parameters defining drop profile, i. e., contact diameter, contact angle, and height of the drop have been obtained. Specific evaporation rate has been calculated. According to the data analysis it was found, that the sessile water-sodium chlorides solution drop with the highest concentration (16.7%) evaporates in the "reverse depinning" mode. So, there is movement of the contact line in the direction of increasing the surface occupied by the drop. The sessile water and water-sodium chlorides solution drop with 4.8% and 9.1% concentration evaporates in the contact line pinning mode. The influence of the initial concentration of the evaporated solution on the contact angle and the specific evaporation rate was found out.