Numerical simulation of water droplet evaporation into vapor–gas medium

Numerical simulation has been employed to consider water droplet evaporation into a vapor–gas medium. An approximate approach has been proposed that makes it possible to take into account the effect of a noncondensable component on the character of variations in the droplet temperature during evaporation. The results of the calculations have been compared with the published experimental data.     

Evaporation of ethanol/water droplets: examining the temporal evolution of droplet size, composition and temperature

The evolving size, composition, and temperature of evaporating ethanol/water aerosol droplets 25-57 microm in radius are probed by cavity enhanced Raman scattering (CERS) and laser induced fluorescence. This represents the first study in which the evolving composition of volatile droplets has been probed with spatial selectivity on the millisecond time scale, providing a new strategy for exploring mass and heat transfer in aerosols. The Raman scattering intensity is shown to depend exponentially on species concentration due to the stimulated nature of the CERS technique, providing a sensitive measure of the concentration of the volatile ethanol component. The accuracy with which we can determine droplet size, composition, and temperature is discussed. We demonstrate that the CERS measurements of evolving size and composition of droplets falling in a train can be used to characterize, and thus avoid, droplet coagulation. By varying the surrounding gas pressure (7-77 kPa), we investigate the dependence of the rate of evaporation on the rate of gas diffusion, and behavior consistent with gas diffusion-limited evaporation is observed. We suggest that such measurements can allow the determination of the vapor pressures of components within the droplet and can allow the determination of activity coefficients of volatile species.     

Dynamics of variations in size and composition of a binary droplet in a mixture of two condensing vapors and a passive gas under arbitrary initial conditions

The dynamics of variations in the size and composition of a droplet in a mixture of two vapors and a noncondensable carrier gas has been studied at an arbitrary initial droplet size and two limiting initial concentrations of the solution in the droplet corresponding to a pure first or pure second component. The conditions for nonmonotonic variations in the droplet radius with time have been analyzed. The physical situation has been investigated for the course of binary condensation, in which, at an initial stage, before the droplet begins to grow and the stationary concentration of the solution in it is established, the droplet size markedly decreases. The opposite situation is also considered, in which the droplet grows at the initial stage and then passes to the regime of monotonic evaporation.     

Determination of Thermodynamic Parameters from Evaporation of Binary Microdroplets of Volatile Constituents

An experimental technique based on a modified vibrating orifice aerosol generator has been employed to study unsteady evaporation of linear streams of highly monodisperse binary microdroplets of volatile constituents over short time periods (i.e., <1 ms), such that the droplet composition remains nearly constant. The droplet size and temperature (i.e., refractive index) have been determined with high temporal resolution from the resonances observed in the simultaneous elastic and Raman light scattering spectra obtained by varying the droplet size through modulation of droplet generation frequency. By using this technique we show that thermodynamic parameters of binary systems, such as activity coefficients as well as vapor pressures of the constituents as functions of temperature, can be determined. We have applied the procedure to study unsteady evaporation rates of pure ethanol and methanol droplets as well as binary droplets containing various ratios of ethanol and methanol. We have obtained vapor pressures of ethanol and methanol as functions of temperature as well as activity coefficients of ethanol and methanol as functions of composition, and the results show excellent agreements with the values reported in the literature. The technique presented in this paper is applicable to any binary system containing at least one volatile constituent. Copyright 2000 Academic Press.     

Postnucleation droplet growth in supersaturated gas with arbitrary vapor concentration

This work concerns the reexamination and extension of the current theory of phase transition dynamics for liquid droplets growing on soluble aerosols from a supersaturated gas mixture for the general case of arbitrary value of vapor concentration. We found that the inconsistency in the common treatment of the vapor diffusion, due to an implicit assumption of the constancy of gas density in the vicinity of a droplet by neglecting its dependency on temperature and vapor concentration, leads to the obvious discrepancy in the Maxwell expression for the growth rate regarding droplets of near critical size. Restoring the correct treatment of the vapor diffusion in terms of the mass concentration of water vapor and taking into the consideration variations of gas density in the vicinity of a droplet in compliance with the equation of state of moist air, we have obtained a new expression for the droplet growth rate valid for an arbitrary value of vapor concentration. The limitations imposed by the molecular kinetic fluxes to postnucleation diffusional growth of small droplets with a large Knudsen number are also reevaluated to include previously neglected physical effects. In particular, the essential contribution of the vapor molecular energy flux into the total kinetic molecular heat flux as well as the temperature variations of mean thermal velocities of air and vapor molecules in the vicinity of the droplet interface have been taken into consideration. Surprisingly significant differences have been found in new expressions derived for the droplet growth rate and droplet temperature, even in the limit of small vapor concentration, if comparing with commonly used results. These findings could help with better interpretation of experimental measurements to infer more reliable data for the mass and thermal accommodations coefficients.     

On the evolution of a multicomponent droplet during nonisothermal diffusion growth or evaporation

A set of equations has been derived for the size, composition, and temperature of a multicomponent droplet of a nonideal solution during its diffusion nonisothermal condensation growth or evaporation in a multicomponent mixture of vapors with an incondensable carrier gas. In addition to complete equations for material and heat transfer in the vapor-gas medium surrounding the droplet, the derived set, in the general case, describes the nonstationary growth or evaporation of the droplet under arbitrary initial conditions (initial size and temperature of the droplet and the concentrations of the nonideal multicomponent solution in it) and the establishment of the stationary values of the composition, temperature, and the rate of variations in the size of the droplet with allowance for heat effects and diffusion and thermodiffusion material transfer, Stefan flux, motion of the droplet surface, and the nonideality of the solution in the droplet. A simplified set of equations obtained without taking into account the contributions from the flow, cross effects, and thermal expansion in the equations of the material and heat transfer in the vapor-gas medium has been considered. Equations describing growth/evaporation in the stationary regime have been analyzed for droplets of ideal multicomponent solutions.     

A comparative study of the mass and heat transfer dynamics of evaporating ethanol/water, methanol/water, and 1-propanol/water aerosol droplets

The mass and heat transfer dynamics of evaporating multicomponent alcohol/water droplets have been probed experimentally by examining changes in the near surface droplet composition and average droplet temperature using cavity-enhanced Raman scattering (CERS) and laser-induced fluorescence (LIF). The CERS technique provides a sensitive measure of the concentration of the volatile alcohol component in the outer shell of the droplet, due to the exponential relationship between CERS intensity and species concentration. Such volatile droplets, which are probed on a millisecond time scale, evaporate nonisothermally, resulting in both temperature and concentration gradients, as confirmed by comparisons between experimental measurements and quasi-steady state model calculations. An excellent agreement between the experimental evaporation trends and quasi-steady state model predictions is observed. An unexpectedly slow evaporation rate is observed for the evaporation of 1-propanol from a multicomponent droplet when compared to the model; possible explanations for this observation are discussed. In addition, the propagation depth of the CERS signal, and, therefore, the region of the droplet from which compositional measurements are made, can be estimated. Such measurements, when considered in conjunction with quasi-steady state theory, can allow droplet temperature gradients to be measured and vapor pressures and activity coefficients of components within the droplet to be determined.     

Stationary concentration establishment in the growing or evaporating droplet of ideal binary solution

The establishment of stationary solution concentration in a growing or evaporating droplet of an ideal binary solution (binary droplet) placed in a vapor mixture of constituting substances and passive gas is described analytically. Relations defining time dependences of solution concentration in a droplet, the number of molecules of each constituting component, and droplet radius are derived at known parameters of the vapor-gas mixture and the initial composition of a binary droplet. The results of calculations of time dependences of aforementioned values are reported for several variants of the initial composition of a droplet and a vapor mixture.     

Regimes of water droplet evaporation on copper substrates

Distilled water droplet evaporation has been studied on copper substrate surfaces with different degrees of roughness. Data on variations in the contact diameter have been employed to distinguish between the regimes of distilled water droplet spreading over the copper surfaces that proceed after the viscous regime. For each isolated regime, the duration has been determined as a fraction of the total evaporation time and the main physical processes have been described. Variations in contact angles have been analyzed as depending on copper surface temperature. It has been established that, as the substrate temperature is elevated, wetting becomes better, while the adhesion work remains almost unchanged, thereby indicating the absence of chemical and structural transformations at the liquid–substrate interface.     

Equations for the evolution of a growing or evaporating free microdroplet under nonstationary conditions of diffusion and heat transfer in a multicomponent vapor–gas medium

A set of equations has been derived for the nonstationary composition, size, and temperature of a growing or evaporating multicomponent microdroplet of a nonideal solution under arbitrary initial conditions. Equations for local nonstationary diffusion molecular and heat fluxes in a mixture of a multicomponent vapor with a noncondensable carrier gas have been obtained within the framework of nonequilibrium thermodynamics with allowance for hydrodynamic flow of the medium. The derived closed set of equations takes into account the nonstationarity of the diffusion and heat transfer, effect of thermodiffusion and other cross effects in the multicomponent vapor–gas medium, the Stefan flow, and droplet boundary motion, as well as the nonideality of the solution in the droplet. The general approach has been illustrated by the consideration of the multicomponent medium at low concentrations of vapors taking into account its thermal expansion due to the Stefan flow in the case of a nonstationary diffusion regime of the nonisothermal condensation growth of a one-component droplet.     

Electrocatalytic Activity of Pt/C Electrodes for Ethanol Oxidation in Vapor Phase

Introduction The selective oxidation of ethanol to acetaldehydeis of special interest in the areas where biomass-basedeconomics are under development since acetaldehydecan be used for the syntheses of other basic chemicalssuch as acetic acid, acetic anhydride,n-butanol,etc.[1—6]. With the advancement of fuel cell technologyand electrocatalysis stimulated by the world energy andraw material situation, some special opportunities havebeen provided for investigating electrosynthesis of or-ganic c…     

Regularities of binary condensation of vapors when one of them is undersaturated

Simple analytical expressions are derived for the stationary concentration of a binary solution in a markedly supercritical droplet growing exothermically in diffusion or free-molecular regimes in mixed vapors when one of condensing vapors is supersaturated and present in a small amount and another vapor is slightly undersaturated and present in a large amount. The condensation of sulfuric acid and water vapors on a droplet under the conditions of Earth atmosphere is considered as an example of practical importance. Under isothermic conditions, analytical expressions are obtained for the time of establishing a power law for variations in a droplet radius with time under the diffusion and free-molecular regimes of the droplet growth. The power laws are derived in an explicit form under these regimes, which describe the rapid establishment of a stationary concentration of a solution in a growing droplet.     

On the evaporation and motion of a volatile droplet near a volatile liquid surface

A problem concerning the free evaporation or condensation growth of a droplet near an infinite planar surface of the same liquid is solved. The behavior of the droplet is considered at vapor temperature and concentration gradients preset at an infinite distance from it. The boundary conditions take into account effects that are linear with respect to the Knudsen number. Equations are derived for the rate of variations in the radius of the droplet and the velocity of its steady motion induced by nonuniform temperature and concentration of the vapor. Dependences of the rate of variations in the radius and the velocity of the steady motion of the droplet on the distance from the planar surface are presented for a droplet 1 ??m in radius suspended in air.     

Permeation of nitrogen and water vapor through sulfonated polyetherethersulfone membrane

The novel polyetherethersulfone (PES-C) prepared from phenol-phthalein in our institute is an amorphous, rigid, tough material with good mechanical properties over a wide temperature range. To improve its water vapor permeability for the application of gas drying, the PES-C was sulfonated with concentrated sulfuric acid and transferred in sodium, cupric, and ferric salt forms. The sulfonation degree can be regulated by controlling the temperature and reaction time. Characterization of sulfonated PES-C in sodium form was made by IR. Some properties of the sulfonated PES-C, such as solubility, glass transition temperature, thermal stability, mechanical properties, and transport properties to nitrogen and water vapor have also been discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2133–2140, 1997     

Supersaturated vapor recondensation: Analytical theories and numerical calculation

The time dependences of droplet size within the period from the end of nucleation until the onset of asymptotic regime of recondensation are determined by the numerical integration of equations describing droplet growth in a supersaturated vapor. The obtained dependences are compared with those derived in terms of the analytical theories of condensation. The character of variations in the final self-similar size distribution of droplets is studied as depending on variations in their initial distribution function and in the system parameters.     

Droplet evaporation near a flat wall

The problem of the free evaporation of a droplet of moderately large size occurring near an infinite flat wall is solved. The cases in which the wall surface is impenetrable to an evaporating substance and vapor concentration remains unchanged at the surface are considered. The temperature of the wall surface is assumed to be constant and equal to the gas temperature at a large distance from the droplet. A set of algebraic equations is derived for molecular fluxes and the temperature and the concentration of gaseous components. Dependences of the evaporation rate of a water droplet suspended in air on its radius and distance from a wall are determined.     

Effect of additives in a carrier gas on the behavior of ions in an ion-mobility increment spectrometer

The effect of additives (ethanol, acetic acid, water, chloroform, and acetone) in air used as the carrier gas on the changes in the spectra of the H⁺(C₆H₅NH₂) ion was studied for an ion-mobility increment spectrometer (IMIS) operating in tandem with a mass spectrometer (MS). It was shown that the presence of additives could improve both the resolution of the instrument and the sensitivity of the method. The resolving power was increased fourfold using air saturated with an ethanol vapor, while the sensitivity was increased 11-fold upon adding a mixture of ethanol and acetic acid to the carrier gas. The dependences on the concentration of the additives indicate that the formation of the clusters is the reason for these effects. The presence of an ethanol vapor in the carrier gas almost completely eliminates the effect of the present water vapor; therefore, air saturated with ethanol vapors can be used in the IMIS-MS instruments without drying.     

Evaporation of ethanol and ethanol-water mixtures studied by time-resolved infrared spectroscopy

The knowledge of the physics and the chemistry behind the evaporation of solvents is very important for the development of several technologies, especially in the fabrication of thin films from liquid phase and the organization of nanostructures by evaporation-induced self-assembly. Ethanol, in particular, is one of the most common solvents in sol-gel and evaporation-induced self-assembly processing of thin films, and a detailed understanding of its role during these processes is of fundamental importance. Rapid scan time-resolved infrared spectroscopy has been applied to study in situ the evaporation of ethanol and ethanol-water droplets on a ZnSe substrate. Whereas the evaporation rate of ethanol remains constant during the process, water is adsorbed by the ethanol droplet from the external environment and evaporates in three stages that are characterized by different evaporation rates. The adsorption and evaporation process of water in an ethanol droplet has been observed to follow a complex behavior: due to this reason, it has been analyzed by two-dimensional infrared correlation. Three different components in the water bending band have been resolved.     

Evaporation dynamics of a binary sessile droplet: Theory and comparison with experimental data on a droplet of a sulfuric-acid solution

Diffusion evaporation of a sessile binary droplet in an atmosphere of a noncondensable carrier gas has been considered. For a droplet consisting of two infinitely miscible liquids, a relation between the current values of solution concentration and volume of the droplet has been derived in an explicit form under the ideal solution approximation. It has been shown that the volume of a sessile binary droplet may, as well as the volume of a free binary droplet, vary nonmonotonically with time. The evaporation of a droplet of an aqueous sulfuric-acid solution has been considered in detail taking into account the nonideality of the solution. Time variations in the volume, base area, and contact angle have been experimentally measured for the sessile droplet of an aqueous sulfuric-acid solution on a hydrophobized substrate. The experimental data obtained at different initial humidities of water-vapor and droplet-solution concentrations have been analyzed within the theory of the stationary isothermal diffusion evaporation of a sessile binary droplet.