Evaporation Rate Of Particles Research Articles

Evaporation kinetics of a polydisperse ensemble of drops.

A mathematical model of the evaporation of a polydisperse ensemble of drops, with allowance for a nonlinear 'diffusion' term in the kinetic equation for the population density distribution function, is developed. The model describes the interaction of a gas phase with vaporizing drops: it has great potential for application in condensed matter physics, thermophysics and engineering devices (e.g. spray drying, cooling, power engineering). The kinetics of heat transfer between phases is theoretically studied. An analytical solution to the integro-differential equations of the process of droplet evaporation is found in a parametric form. Analytical solutions in the presence and absence of the 'diffusion' term are compared. It is shown that the fluctuations in particle evaporation rates ('diffusion' term in the Fokker-Planck equation) play a decisive role in the evolutionary behaviour of a polydisperse ensemble of vaporizing liquid drops. This article is part of the theme issue 'Transport phenomena in complex systems (part 1)'.

Transient heating and evaporation of metallic particles under plasma conditions

The synthesis of metallic nanopowders through the vaporization and condensation of metallic precursors under plasma condition is increasingly accepted for a wide range of applications in the electronic and electrical industry. The present study aims at the computer simulation of the basic processes involved. Specific attention is given to the particle evaporation rate and the associated energy balance. Computation carried out for the vaporization of 50 µm iron and copper particles in an atmospheric pressure argon plasma at 9000 K provide means of quantifying the different energy requirements of the process including;• energy needed for the initial heating of the particle to its vaporization temperature,• energy required for particle vaporization,• energy lost by surface radiation from the particle during the evaporation process, and• energy radiated from the plasma/metal vapor cloud.The results show that energy lost by volume radiation for the plasma/metal vapor cloud is by far the most important energy requirement of the process that deserves special attention in terms of reactor engineering design in order to reduce its value and consequently increase process productivity. As expected, the results are to a large extent material dependent, varying widely with the radiative properties of the metal involved.

Particle evaporation and hydrodynamics in a shock driven multiphase instability

This paper presents results from 3D numerical simulations of a shock-driven multiphase instability of a gas-particle system with a spherical interface. Two cases, one with an evaporating particle cloud and another with a gas only approximation were run in the hydrodynamics code FLASH. Both cases had an incident Mach number of 1.65 and an effective Atwood number of 0.046. It is shown that the gas only approximation, a classical Richtmyer–Meshkov instability, cannot replicate effects from particles like, lag, clustering, and evaporation. Instead, both gas hydrodynamics and particle properties influence one another and are coupled. Qualitative and quantitative differences in the Richtmyer–Meshkov instability and shock-driven multiphase instability are briefly presented. Coupling between the interface hydrodynamic evolution and particle evaporation is explored further by examination of the multiphase case. Hydrodynamic driven particle clustering is measured through particle spatial distribution statistics and particle-vorticity correlations. It is found that the small particles form vorticity driven small scale clusters quickly while hydrodynamics act to reorganize the particles into larger scale features at later times. Particle evaporation rates are found to vary greatly, even among similar sizes, and show poor agreement with existing 1D evaporation models. The role of hydrodynamic organization in evaporation is shown by examining the spatial distribution of variations in evaporation rate. Small to medium sized particles initially located at the outside of the sphere, in the equatorial region (taking the sphere to have poles aligned with the shock transit direction) are most affected by the hydrodynamic development and show higher evaporation rates, similar to those found with a 1D, one-way coupled, single particle evaporation model.

Evaporation rate of particles in the vaporizer of the Aerodyne aerosol mass spectrometer

ABSTRACTWe present calculations for evaporation rates of particles collected on the vaporizer of the Aerodyne aerosol mass spectrometer (AMS). These calculations provide insight on certain observed phenomena associated with the size-resolved mass spectrum (MS), because the time width of the MS signal from a particle can be limited by its evaporation rate upon contact with the vaporizer. We show that the counterintuitive weak dependence of observed MS signal widths (evaporation rates) on particle volatility is due to suppression of evaporation rates induced by latent heat release, which is more prominent at high volatilities. The same physics is responsible for the observed diminishing returns associated with increasing the vaporizer temperature to achieve narrower single particle pulses. We also show that the vaporizer typical operating temperature of 600°C is sufficient to evaporate extremely low volatility organic compounds (ELVOCs) rapidly enough to obtain reliable measurements for particles smaller th...

Seasonal differences of the atmospheric particle size distribution in a metropolitan area in Japan

We compared the effect of ambient temperature observed in two different seasons on the size distribution and particle number concentration (PNC) as a function of distance (up to ~250m) from a major traffic road (25% of the vehicles are heavy-duty diesel vehicles). The modal particle diameter was found between 10 and 30nm at the roadside in the winter. However, there was no peak for this size range in the summer, even at the roadside. Ambient temperature affects both the atmospheric dilution ratio (DR) and the evaporation rate of particles, thus it affects the decay rate of PNC. We corrected the DR effect in order to focus on the effect of particle evaporation on PNC decay. The decay rate of PNC with DR was found to depend on the season and particle diameter. During the winter, the decay rate for smaller particles (<30nm) was much higher (i.e., the concentration decreased significantly against DR), whereas it was low during the summer. In contrast, for particles >30nm in diameter, the decay rate was nearly the same during both seasons. This distinction between particles less than or greater than 30nm in diameter reflects differences in particle volatility properties. Mass-transfer theory was used to estimate evaporation rates of C20–C36 n-alkane particles, which are the major n-alkanes in diesel exhaust particles. The C20–C28 n-alkanes of 30-nm particles completely evaporate at 31.2°C (summer), and their lifetime is shorter than the transport time of air masses in our region of interest. Absence of the peak at 10–30nm and the low decay rate of PNC <30nm in diameter in the summer were likely due to the evaporation of compounds of similar volatilities comparable to the C20–C36 n-alkanes from particles near the exhaust pipes of vehicles, and complete evaporation of semivolatile materials before they reached the roadside. These results suggest that the lifetime of particles <30nm in diameter depends on the ambient temperature, which differs between seasons. This leads us to conclude that these particles show distinctly different spatial distributions depending on the season.

Equilibration time scales of organic aerosol inside thermodenuders: Evaporation kinetics versus thermodynamics

Abstract The interpretation of thermodenuder (TD) data often relies on the assumption that thermodynamic equilibrium is reached inside the instrument. We modeled the evaporation of three organic aerosol types (adipic acid, α-pinene SOA and aged OA) inside a thermodenuder with a mass transfer model, and calculated equilibration time scales for these systems at realistic conditions. The equilibrium times varied from less than a second to several hours, decreasing with increasing aerosol concentrations, decreasing particle sizes, decreasing volatilities and increasing mass accommodation coefficients. The results indicate that generally TDs measure particle evaporation rates rather than equilibria, and time-dependent modeling of the evaporation is usually needed to interpret the data. Measurements at varying residence times and temperatures, on the other hand, are desirable to investigate the equilibration of the studied aerosol and decouple the kinetic effects from the effects caused by the thermodynamic properties of the aerosol. Organic aerosol is likely to be further from equilibrium under typical field conditions compared with laboratory data. When determining the aerosol properties from TD data, assuming incorrectly equilibrium results in under-prediction of the vaporization enthalpy of the evaporating species. Similar under-estimation is predicted if multicomponent aerosols are approximated with single-component properties.

Determination of evaporation coefficients of semi-volatile organic aerosols using an integrated volume—tandem differential mobility analysis (IV-TDMA) method

We present the integrated volume—tandem differential mobility analysis method for determining evaporation coefficients of semi-volatile aerosols. This thermodenuder-based method allows separate determination of the three parameters governing aerosol evaporation, namely, saturation pressure, surface free energy and evaporation coefficient. Saturation pressure is determined by measuring particle volume changes as the aerosol passes through equilibrium states, while evaporation coefficient and surface free energy are determined by fitting particle evaporation rates measured under non-equilibrium conditions to a numerical model of the evaporation process. Evaporation coefficient is determined in a size range where surface free energy effects are negligible, allowing for single parameter optimization. We demonstrate the technique by applying it to dicarboxylic acid aerosols which are pertinent to atmospheric chemistry problems. We obtained evaporation coefficients and surface free energy values of 0.07 ± 0.02 and 0.15 ± 0.07 , 0.08 ± 0.02 and 0.17 ± 0.12 and 0.24 ± 0.04 and 0.23 ± 0.08 J / m 2 for succinic, adipic, and pimelic acids, respectively.

Retardation of particle evaporation from excited nuclear systems due to thermal expansion

Particle evaporation rates from excited nuclear systems at equilibrium matter density are studied within the harmonic-interaction Fermi gas model (HIFGM) combined with Weisskopf's detailed balance approach. It is found that thermal expansion of a hot nucleus, as described quantitatively by HIFGM, leads to a significant retardation of particle emission, greatly extending the validity of Weisskopf's approach. The decay of such highly excited nuclei is strongly influenced by surface instabilities.

Aerosol-cirrus interactions: a number based phenomenon at all?

Abstract. In situ measurements of the partitioning of aerosol particles within cirrus clouds were used to investigate aerosol-cloud interactions in ice clouds. The number density of interstitial aerosol particles (non-activated particles in between the cirrus crystals) was compared to the number density of cirrus crystal residuals. The data was obtained during the two INCA (Interhemispheric Differences in Cirrus Properties from Anthropogenic Emissions) campaigns, performed in the Southern Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes. Different aerosol-cirrus interactions can be linked to the different stages of the cirrus lifecycle. Cloud formation is linked to positive correlations between the number density of interstitial aerosol (Nint) and crystal residuals (Ncvi), whereas the correlations are smaller or even negative in a dissolving cloud. Unlike warm clouds, where the number density of cloud droplets is positively related to the aerosol number density, we observed a rather complex relationship when expressing Ncvi as a function of Nint for forming clouds. The data sets are similar in that they both show local maxima in the Nint range 100 to 200cm, where the SH- maximum is shifted towards the higher value. For lower number densities Nint and Ncvi are positively related. The slopes emerging from the data suggest that a tenfold increase in the aerosol number density corresponds to a 3 to 4 times increase in the crystal number density. As Nint increases beyond the ca. 100 to 200cm, the mean crystal number density decreases at about the same rate for both data sets. For much higher aerosol number densities, only present in the NH data set, the mean Ncvi remains low. The situation for dissolving clouds allows us to offer two possible, but at this point only speculative, alternative interactions between aerosols and cirrus: evaporating clouds might be associated with a source of aerosol particles, or air pollution (high aerosol number density) might retard ice particle evaporation rates.

Characteristics of multi-phase flow with particle evaporation in a combustor with counter-flow injection

Seed particles are added to a high temperature reacting flow to achieve the desired electric conductivity for magnetohydrodynamic (MHD) power generation. The evaporation rate of the see particles and the uniformity of the seed vapor distribution in the reacting flow are major factors affecting the performance of the MHD system. An integral reacting flow (IRF) computer code was used to calculate the multi-phase flow characteristics in an MHD combustor and predict the optimum operating conditions of the combustor. The IRF code is a general hydrodynamics computer code for multi-phase, two dimensional, steady state, turbulent, and reacting flows, based on mass, momentum, and energy conservation laws. A unique integral reaction submodel in the IRF code enhances numerical convergence while preserving the major physical effects of the complex combustion processes. The combustor is a rectangular channel in which a mixture of fuel gas and seed particles enters from one end and exits at the other, while the oxidizer is injected from the sides. The major findings of a parametric study included: (1) the seed particle evaporation rate varies for different oxidizer injection angles; (2) the optimal evaporation rate was found at an injection angle around 130°; (3) the seed particle size had a major effect on particle dispersion and vapor mixing; (4) seed particles with a diameter of 34 μm or larger were only partially vaporized in the combustor, and the seed vapor distribution at the combustor exit was highly non-uniform; and (5) significant particle deposition was found on the side walls upstream of the oxidizer injection slots. Theoretical predictions were in general agreement with experimental trends.

High‐Temperature Evaporation Rates of Solid KBr and NH4Cl aerosol particles

AbstractThe evaporation rates of particles of solid KBr or NH4Cl suspended in argon were studied under high‐temperature conditions behind incident shock waves. The mass‐transfer process during particle evaporation was observed by two optical techniques allowing the measurement of scattered light from a particle ensemble and also from individual particles. The scattered light flux signals were interpreted on the basis of the Mie theory, resulting in values for the time‐dependent particle size and for the refractive index of particle materials. The experiments were performed in the gas‐phase temperature ranges TG = 1070–1300K (KBr) and 525–650K (NH4Cl). The initial size of the suspended particles ranged between 0.6 and 1.0 μm. From the measured decrease in the particle size during the mass‐transfer process the evaporation coefficients of both materials were determined as a function of the gas‐phase temperature. They depend on the vapour pressure and the diffusion coefficient for vapour into gas. Typical parameters (àKBr, m) describing the diffusion coefficient of KBr vapour in argon and the vapour pressure of NH4Cl could be determined.

The hot fermi gas model

Ample production of fast particles has been observed in intermediate heavy ion reactions [1]. These particles have often been related to sources having velocities close to half that of the beam and temperatures ranging between a few and several tens MeV. For such temperatures neither the low temperature Fermi gas model nor the Boltzmann gas model are valid. A more correct treatment is necessary in order to understand the relationship between the incident energy per nucleon, the excitation energy of the source and its temperature. In this short paper we give simple closed expressions allowing to interpolate between the Fermi and the Boltzmann regimes.In the following we consider a gas of fermions (Nucleons) at a temperatureT trapped in a square potential well of depthU. We shall not deal with the dynamics of the expansion of the gas except through the calculation of the particle evaporation rate. Likewise we do not consider the implications of a possible liquid gas transition [2].We first approximate the variation of the chemical potential as a function of the temperature. Using that, we are able to compute and find approximations to the variations of the excitation energy with temperature and vice-versa. Finally we give expressions for the particle evaporation rate of a hot Fermi gas and compare them to the Weisskopf formula.

A Formalism for Calculating the Evaporation Rates of Rapidly Evaporating Interacting Particles

Abstract The transport equations governing the quasi-steady vapor density and temperature fields surrounding rapidly evaporating interacting spherical particles are reduced to the Laplace equation by using suitable variable transformations. Once reduced to this form, these equations can be solved by the method of images. This method is a fairly general one for solving the Laplace equation and can be applied to particle arrays consisting of an arbitrary number of arbitrarily arranged interacting particles which may differ in size and chemical composition. Interactions are shown to significantly affect particle evaporation rates even at large particle separations. For the arrays considered, however, particle temperatures are found to be unaffected by interactions.

The effects of nearest neighbor interactions on the evaporation rate of cloud particles

A modified images method is presented and used to determine the vapor density field in arrays of up to nine “quasi-stationary” evaporating particles. The evaporation rate of each particle in an array is calculated as a function of the particle separation. Correction factors to Maxwell's evaporation rate expression to account for nearest neighbor interactions are proposed. The “R 2-law” is assessed and found to be only approximately correct when applied to interacting particles.