Evaporation Rate Constant Research Articles

Insights into nanostructured silica particle formation from sodium silicate using spray drying: An experimental and theoretical study

Spray drying is a widely used operational procedure utilized in diverse sectors. The current knowledge of droplet mobility processesinside the drying chamber is not fully comprehensive due to the complex nature of droplet evaporation issues. The present study used a comprehensive methodology that integrated numerical models and actual testing to investigate the drying process of droplets in the context of spray drying. The precursor material utilized in this investigation was colloidal silica. The influence of external hydrodynamic variables, such as drying temperature (473, 673, and 873 K) and carrier gas flow rate (2, 4, 6 L/min), was thoroughly investigated in connection to the dynamics of droplets and the subsequent creation of silica particles. Distinct alterations in the size and shape of the silica particles were observed due to variations in drying temperature and carrier gas flow rate. The validity of this investigation was established by the generation of two discrete particle morphologies, namely spherical particles and doughnut-shaped particles. Spherical particles were produced under reduced carrier gas flow rates of 2 L/min and lower drying chamber temperatures of 473 K. In the quantitative evaluation, the computational fluid dynamics (CFD) computations properly established the evaporation rate constant (K) and the temperature difference (ΔT) inside the droplets. Furthermore, qualitative calculations provide insight into the complex dynamics of droplet motion inside the spray chamber. The results of this study have the potential to open up new possibilities for regulating nanoparticle creation via the use of the spray drying process.

The effect of oxygen fugacity on the evaporation of boron from aluminoborosilicate melt

Abstract. We present the results of B2O3 evaporation experiments from Ca- and Mg-bearing aluminoborosilicate melts. Our experiments were conducted at 1245 to 1249 ∘C and 1350 to 1361 ∘C for different run times (60–1020 min), and at oxygen fugacities (logfO2) relative to the fayalite–magnetite–quartz (FMQ) buffer of FMQ−6 to FMQ+1.5, and in air. Our results show that with increasing fO2, evaporation of B from the melt increases by a factor of 5 compared to reducing conditions. Using Gibbs free energy minimization calculations, we suggest two possible evaporation reactions for B2O3 which constrain its speciation in the gas phase to be either 3+ or 4+ (B2O3(g) and BO2(g)). The measured B2O3 contents of the B evaporated residual glasses were used to calculate evaporation rate constants (ki) for B2O3 in oxidizing conditions (air, ki=2.09×10-4 cm min−1 at 1350 ∘C) and reducing conditions (FMQ−4, ki=4.46×10-5 cm min−1 at 1350 ∘C). The absence of diffusion profiles in the experimental glasses suggests that the evaporation rates are slower than B2O3 diffusion rates and therefore the rate-limiting process. Overall, the rate of B evaporation in air is approximately a factor of 5 higher compared to reducing conditions at FMQ−4.

Theoretical Study of the Gas-Phase Hydrolysis of Formaldehyde to Produce Methanediol and Its Implication to New Particle Formation.

Aldehydes were speculated to be important precursor species in new particle formation (NPF). The direct involvement of formaldehyde (CH2O) in sulfuric acid and water nucleation is negligible; however, whether its atmospheric hydrolysate, methanediol (CH2(OH)2), which contains two hydroxyl groups, participates in NPF is not known. This work investigates both CH2O hydrolysis and NPF from sulfuric acid and CH2(OH)2 with quantum chemistry calculations and atmospheric cluster dynamics modeling. Kinetic calculation shows that reaction rates of the gas-phase hydrolysis of CH2O catalyzed by sulfuric acid are 11-15 orders of magnitude faster than those of the naked path at 253-298 K. Based on structures and the calculated formation Gibbs free energies, the interaction between sulfuric acid/its dimer/its trimer and CH2(OH)2 is thermodynamically favorable, and CH2(OH)2 forms hydrogen bonds with sulfuric acid/its dimer/its trimer via two hydroxyl groups to stabilize clusters. Our further cluster kinetic calculations suggested that the particle formation rates of the system are higher than those of the binary system of sulfuric acid and water at ambient low sulfuric acid concentrations and low relative humidity. In addition, the formation rate is found to present a negative temperature dependence because evaporation rate constants contribute significantly to it. However, cluster growth is essentially limited by the weak formation of the largest clusters, which implies that other stabilizing vapors are required for stable cluster formation and growth.

Evaporation of the tramp element antimony from molten steel under reduced pressure treatment

ABSTRACT The evaporation of antimony (Sb) in molten steel under reduced pressure is investigated with the gas phase pressure of 19–670 Pa, smelting temperature of 1823–1973 K and initial Sb content of 0.033–0.1 wt%. The findings indicate that Sb removal efficiency increases when smelting temperature rises or gas phase pressure decreases. However, the initial Sb content has little effect on the Sb removal. The Sb removal ratio can reach 94.1% within 40 min of treatment. The range of the apparent evaporation rate constant, k Sb, is 0.61 × 10−5 to 29.13 × 10−5 m s−1. The evaporation of Sb is limited by the gas phase mass transfer at 1903 K and 19–670 Pa. Additionally, the apparent activation energy of Sb evaporation, E Sb, is 128 kJ mol−1 at 226 Pa and 1823–1973 K, and the rate of the interfacial reaction severely restricts the evaporation of Sb.

Experiments and Numerical Simulation of N-decane/Ethanol Bi-Component Droplet Evaporation

The evaporation characteristics of n-decane-based bi-component or multi-component droplets have been veiled for application in advanced combustion. This paper proposes to experimentally investigate the evaporation of n-decane/ethanol bi-component droplets settled in the convective hot air, and numerically simulate the key parameters affecting the evaporation charactersitics. It was found that the evaporation behavior was interactively affected by the mass fraction of ethanol and the ambient temperature. For mono-component n-decane droplets, the evaporation process included the transient heating (non-isothermal) and steady evaporation (isothermal) stages. In the isothermal stage, the evaporation rate followed d2-law. The evaporation rate constant linearly increased as the ambient temperature enhanced (573~873 K). For n-decane/ethanol bi-component droplets, at low mass fractions (≤0.2), the isothermal evaporation processes were steady due to the good miscibility between n-decane and ethanol, like mono-component n-decane, whereas at high mass fractions (≥0.4), the evaporation process experienced ultrashort heating and fluctuating evaporation stages. During the fluctuating evaporation, the bubbles formed inside the bi-component droplets and expanded, resulting in the occurrence of the microspray (secondary atomization) and the microexplosion. The evaporation rate constant of bi-component droplets increased as the ambient temperature enhanced, and showed a "V-shaped" trend with the increase of the mass fraction, and the evaporation rate constant was the smallest at 0.4. The evaporation rate constants based on the numerical simulation by using the multiphase flow model and Lee model showed reasonable agreement with the experimental ones, suggesting a potential of application in practical engineering.

Investigation of supercritical transition and evaporation process of a hydrocarbon droplet under diesel engine-relevant conditions

Fuel spray supercritical transition in the diesel engine has been concerned widely, especially supercharged ones, however, whether the transition can significantly affect the fuel–air mixing under diesel engine-relevant conditions is an issue that still needs evaluating carefully. Thus, the transition and evaporation of individual n-heptane droplets in a nitrogen environment under various ambient conditions were investigated through a high-pressure evaporation model. The influence of ambient pressures on droplet lifetime (τlife) and evaporation rate constant (K) varies with ambient temperature. When Tr < 0.98, with increasing pressure, τlife first increases and then decreases, and K decreases. When 0.98 < Tr < 1.28, as pressure increases, τlife would increase even if K increases, because the increment of initial heat-up periods exceeds the decrease in quasi-steady evaporation periods. When Tr > 1.28, with increasing pressure, τlife decreases, and K increases. As the droplet heats up, the droplet interface is where the binary mixture would reach the critical mixing state if ambient conditions are high enough. Increasing ambient density decreases the minimum ambient temperature (Tm) required for the transition to occur. Above the Tm, transitions occur earlier with the increasing energy density of ambient gas. Increasing ambient density decreases the minimum ambient temperature required for diffusive fuel-gas mixing modes to dominate the mixing process. For droplets large enough to ignore gas–liquid interfacial thickness, the Tm required for the attainment of critical mixing states is almost independent of droplet diameter.

Purification of Si–Ti alloys via vacuum evaporation: A thermodynamic and kinetic study of Ca impurity removal

Vacuum refining is a green technology for separating Ca from Si–Ti alloys. This study investigated the kinetics of Ca evaporation from Si–Ti melts in vacuum refining, including the effect of holding time and melt temperature and composition. Although the Ca activity in Si–Ti melts is an essential thermodynamic property that affects the Ca evaporation kinetics, it has not been measured to date. Consequently, another objective of this study was to determine the Ca activity using a chemical equilibration technique and investigate its effect on Ca evaporation. The results show that the activity of Ca in the Si–Ti melt decreased gradually with increasing Ti content in the melt, which reduced the Ca removal ratio. The removal ratio of Ca increased with the temperature and holding time. The apparent evaporation rate of Ca was consistent with the first-order law, and the apparent evaporation rate constants of Ca in 15%Si–Ti melt at 1673 and 1773 K were 2.1 × 10−6 and 2.8 × 10−6 m/s, respectively. The apparent activation energy for Ca evaporation from Si–15 wt%Ti melt was determined to be 68.2 kJ/mol, and the rate-determining step to be Ca mass transfer at the melt boundary layer.

Experimental studies of evaporation of paraffin drops

Paraffin is a low-melting material that can be used as an environmentally friendly and high-energy fuel. It can also be considered as a promising material with phase transitions capable of accumulating heat. It is known that the period of induction of ignition of paraffin droplets consists of successive stages, the main ones of which are melting and evaporation. The paper examines the processes of evaporation of octadecane and docosane droplets in air at different temperatures. A method of studying the kinetics of evaporation of paraffin fuel droplets using digital photography of the research object and computer image processing has been developed.&#x0D; It is proved that the evaporation kinetics of the studied alkanes is described by the linear law of the change of the square of the diameter of the drop over time. Evaporation rate constants were found. There are no such data in the literature for these paraffins. The use of evaporation constants made it possible to find the time of complete evaporation of drops and to estimate the diffusion coefficient of paraffin vapor in air at different temperatures. As a result of the experiments, it was established that the rate of evaporation of octadecane is more than 4 times higher than the rate of evaporation of docosane. It was found that when the air temperature reaches the specified values, the rate of evaporation of the specified alkanes increases significantly.

Estimating evaporation rates and contaminant air concentrations due to small spills of non-ideal aqueous organic solvent mixtures in a controlled environment

Although small spills of non-ideal organic solvent mixtures are ubiquitous undesirable events in occupational settings, the potential risk of exposure associated with such scenarios remains insufficiently investigated. This study aimed to examine the impact of non-ideality on evaporation rates and contaminant air concentrations resulting from small spills of organic solvent mixtures. Evaporation rate constants alphas (α) were experimentally measured for five pure solvents using a gravimetric approach during solvent evaporation tests designed to simulate small spills of solvents. Two equations were used for estimating contaminants’ evaporation rates from aqueous mixtures assuming either ideal or non-ideal behavior based on the pure-chemical alpha values. A spill model also known as the well-mixed room model with exponentially decreasing emission rate was used to predict air concentrations during various spill scenarios based on the two sets of estimated evaporation rates. Model predictive performance was evaluated by comparing the estimates against real-time concentrations measured for the same scenarios. Evaluations for 12 binary non-ideal aqueous mixtures found that the estimated evaporation rates accounting for the correction by the activity coefficients of the solvents (median = 0.0318 min−1) were higher than the evaporation rates estimated without the correction factor (median = 0.00632 min−1). Model estimates using the corrected evaporation rates reasonably agreed with the measured values, with a median predicted peak concentrations-to-measured peak concentrations ratio of 0.92 (0.81 to 1.32) and a median difference between the predicted and the measured peak times of −5 min. By contrast, when the non-corrected evaporation rates were used, the median predicted peak concentrations-to-measured peak concentrations ratio was 0.31 (0.08 to 0.75) and the median difference between the predicted and the measured peak times was +33 min. Results from this study demonstrate the importance of considering the non-ideality effect for accurately estimating evaporation rates and contaminant air concentrations generated by solvent mixtures. Moreover, this study is a step further in improving knowledge of modeling exposures related to small spills of organic solvent mixtures.

Antimony recovery from antimony-rich slag by top blowing nitrogen into the molten slag bath

We study the evaporation of antimony trioxide from an antimony-rich slag by bubbling nitrogen gas through a submerged top lance at 1000–1300 °C. Experiments are carried out with 300 g slag at gas flow rates of 20–60 l/h with the lance height of 1.5 and 10 mm. Slag samples are taken and analyzed to identify changes in the antimony trioxide content during the evaporation. By fitting the antimony trioxide content evolution of the slag to the calculation according to the integrated rate law, the antimony trioxide evaporation under the present conditions is found to be a second-order reaction. The evaporation rate constants and Sb recovery are enhanced with increasing temperature and gas flow rate and decreasing lance height. The deposited antimony trioxide product is characterized to determine its impurity level and microstructure. The rate-limiting step of the antimony trioxide evaporation is discussed, and the residual slag is analyzed to understand the antimony trioxide evaporation limits.

Effects of nanoparticle concentration and peclet number on nanofluid droplet evaporation behavior

Effects of droplet parameters are reported using model simulations on the evaporation characteristics of droplets containing insoluble nanoparticles and related to the formation of shells of particles. The droplet parameters are initial droplet diameter (do), initial particle mass fraction (Yvo), and the Peclet number (Pe) based on the fluid evaporation rate constant (K) to particle diffusivity in the fluid (Dpl). Time-dependent phenomena of the instantaneous droplet size (d), d2 vs time (for example, following the d2-law or the deviation from it), instantaneous evaporation efficiency (i.e., effective area of evaporation due to particle inclusion on the droplet surface compared to a pure fluid droplet) are reported. Analytical expression of shell parameters at shell formation such as time to shell formation, shell size, and evaporation efficiency are derived using the initial droplet parameters and compared with the computational results. It is found that the evaporation behavior of droplets containing insoluble nanoparticles, including the deviation from d2-law, is independent of do but is strongly influenced by the product of Pe·Yvo.

Predicting first-order evaporation rate constant alpha (α) from small spills of organic solvents in a controlled environment

Exposures to vapors generated by small spills of organic solvents are common in the occupational hygiene practice. In these scenarios, contaminant mass release is exponentially decreasing, driven by an evaporation rate constant alpha (α). Knowing α is fundamental for adequately modeling peak concentrations and/or short-term exposures that occur and for achieving efficient occupational risk analysis and management. The purpose of this study was to measure alpha experimentally using a gravimetric approach in a controlled environment during solvent evaporation tests designed to simulate small spills of solvents. The effects of several factors on α were evaluated. Equations based on regression models derived from the experimental data were proposed for predicting α. Predictions were externally validated against experimental data. A total of 183 tests was performed. Data analyses found that alpha (α) values increased with vapor pressure, spill surface area-to-spill volume ratio, and air speed across the spill. Larger α were associated with petri dish containers compared to watch glasses. Three regression models were created for predicting α. They had four variables in common, namely vapor pressure, molecular weight, air speed above the liquid, and surface tension of the liquid. The fifth variable was either spill volume, spill surface area, or spill surface area-to-spill volume ratio. The R2 of the regression models were equal to 0.98. External validation showed mean relative errors of −32.9, −32.0, and −25.5%, respectively, with associated standard deviations of the relative errors of 17.7, 33.3, and 26.0%, respectively, and associated R2 of 0.92, 0.65, and 0.87, respectively. The proposed equations can be used for estimating α in exposure scenarios similar to those evaluated in this study. Moreover, these models constitute a step further in the improvement of knowledge on estimating evaporation rates for small spills of organic solvents.

Comparison of evaporation rate constants of a single fuel droplet entering subcritical and supercritical environments

This work compares the evaporation rate constants of a single fuel droplet under subcritical and supercritical environments by experimental visualizations (in millimeter scale) and molecular dynamics simulations (in nanometer scale). The objection is to understand the differences of fuel droplet evaporation rate constant variations under the subcritical and supercritical environments. Firstly, an experimental apparatus is designed to study the diesel droplet evaporation under subcritical and supercritical environments. The results show that the droplet evaporation obeys the D2 law in subcritical environments, while the D2 law is no longer strictly applicable in supercritical environments and the evaporation rate constant isn’t a constant but a variable that increases with the evaporation time. Secondly, the molecular dynamics simulations are applied to investigate the evaporation of diesel surrogate n-dodecane under subcritical and supercritical environments. The results show that the droplet temperature keeps at the equilibrium temperature in subcritical environments, but keeps increasing in supercritical environments. In supercritical environments, the reason for the evaporation rate constant keeps increasing is that the droplet density keeps decreasing with the increase of droplet temperature. The droplet surface tension will rapidly vanish in supercritical environments, which implies that the formation of fuel–air mixture is no longer dominated by the surface tension, but is the turbulent diffusion in diesel trans/supercritical injection conditions. It can deduce that the evaporation rate constants of a fuel droplet in micrometer scale (that is commonly generated in engines) will follow the same trend and within the same variation scale at the same ambient pressures and temperatures.

Effect of aluminum nanoparticles addition on the evaporation of a monodisperse ethanol droplet stream

Evaporation behavior of nanofluid droplets within a fuel cloud plays an important role in most applications. Reported here is an experimental study on the evaporation characteristics of a monodisperse droplet stream of pure ethanol containing aluminum nanoparticles with varying concentrations (up to 0.3 wt.%) at different environmental temperatures (296–743 K). The recently developed phase rainbow refractometry (PRR) is used to simultaneously measure droplet temperature, size, and size variation of nanoscale. The results show that droplet evaporation under the experimental conditions still follows the classical D2-law. Furthermore, the addition of aluminum nanoparticles gradually decreases the evaporation rate. And a 16.2% reduction in the evaporation rate constant is observed when n-Al concentration increases to 0.3 wt.%. The effect of interactions between droplets on evaporation is quantified by combining with isolated droplet evaporation constant predicted by the theoretical model. And the quantified interaction effect is compared with the proposed empirical correlation, indicating that the interaction effect on the evaporation is similar for the studied range of n-Al concentration.

Measuring evaporation rate constants of highly volatile compounds for use in predictive kinetic models

A kinetic model was previously developed in our laboratory to predict evaporation of compounds as a function of gas chromatographic retention index (IT). To define the initial model, evaporation rate constants were experimentally determined for compounds in the range IT = 800–1400 at temperatures from 5 to 35 °C. While the predictive accuracy was demonstrated, broader application of the model requires extension of the IT range to include more volatile compounds. However, such extension requires experimental determination of rate constants, which is challenging due to the explosive hazard and rapid evaporation of volatile compounds.In this work, rate constants of highly volatile compounds were experimentally determined and used to extend the kinetic model to predict evaporation. Prior to experimental evaporations, theoretical calculations were performed to optimize experimental parameters and to ensure that the vapor generated remained below the lower flammability limit for each compound. Compounds were then experimentally evaporated at three different temperatures (10, 20, and 30 °C) and analyzed by gas chromatography-mass spectrometry. The evaporation rate constants for each compound, corrected for condensation, were determined by regression to a first-order rate equation. These rate constants were combined with previously collected data to extend the kinetic model at each temperature. Comparison of predicted and experimentally determined chromatograms of an evaporated validation mixture indicated good model performance, with correlation coefficients ranging from 0.955 to 0.997 and mean absolute percent errors in predicting abundance ranging from 3 to 26%.

Application of a kinetic model to predict extracted ion profiles for the identification of evaporated ignitable liquids

A kinetic model was previously developed in our laboratory to predict evaporation rate constants of compounds in ignitable liquids as a function of gas chromatographic retention index. The model is applied to the chromatogram of an unevaporated liquid and used to predict chromatograms corresponding to any evaporation level of the liquid. In previous work, accuracy in predicting total ion chromatograms (TICs) for gasoline and petroleum distillates was demonstrated. In this work, the model is applied for the first time to predict extracted ion profiles (EIPs) corresponding to different compound classes.Fifteen ignitable liquids representing five classes (isoparaffinic, naphthenic-paraffinic, aromatic, petroleum distillate, and gasoline) defined in ASTM E1618 were experimentally evaporated to 50–90% (v/v). The kinetic model was applied to predict TICs and EIPs corresponding to the alkane, cycloalkane, aromatic, indane, and polynuclear aromatic compound classes for each evaporated liquid. Strong correlation between experimental and predicted TICs and EIPs indicated the predictive accuracy of the model. Reference collections were then generated containing predicted TICs and EIPs for each liquid corresponding to evaporation levels ranging from 10 to 90%. The reference collections were used to identify the ignitable liquid class in samples of increasing complexity, based on maximum correlation between the experimental and predicted chromatograms. In all cases, the correct liquid class was identified, even in the presence of substrate interferences. Overall, this work demonstrates the potential of a kinetic model to generate TICs and EIPs as a practical tool for identification of ignitable liquids in fire debris samples.

(Invited) Oxidation, Nitridation and Volatilisation of Noval Chromium-Based Alloys Above 1000°C

Cr-Si-based alloys are promising candidates for high temperature materials which can resist temperatures beyond the commonly used Ni-based superalloys (> 1150°C). Thereby, higher working temperatures can be used in high temperature applications, which increases the process efficiency. The main drawback of Cr-based alloys is their nitridation and embrittlement at high temperatures during exposure in air. Additionally, at temperatures above 1000°C, Cr-based alloys are prone to spallation of the protective oxide scale and to the formation of volatile oxides.The aim of this work was to develop heat-treatable Cr-Si-based alloys with Cr ≥ 89 at. % and to optimize their oxidation and nitridation resistance. Ge, Mo, and Pt were chosen as additional alloying elements and the microstructure as well as the oxidation resistance of the alloys were investigated.All investigated binary, ternary, quaternary, and quinary alloys developed a two-phase structure consisting of a chromium solid solution Crss matrix and A15 phase precipitates, which was proven using EPMA, SEM, and XRD.Depending on the alloy composition and on the heat treatment, the fraction of A15 precipitates and thereby the alloy's properties can be adjusted. Annealing at 1200°C led to a maximum A15 phase fraction of 37% with respect to the investigated alloy compositions. Increasing the A15 phase fraction goes along with an increase in hardness. Si, Ge, and Pt are A15 phase formers while Mo was found in comparable concentrations in both phases. The precipitates formed in Mo-containing alloys are smaller compared to the other systems, which is important in terms of maintaining the material's properties during high temperature exposure.Additionally, Mo and Pt were found to decrease the lattice misfit between A15 phase and Crss matrix. However, in contrast to Mo, Pt promotes coarsening of the A15 precipitates during annealing.Thermogravimetric analysis and isothermal oxidation tests at temperatures of 1050 -1350°C for up to 200 hours in synthetic air were conducted to investigate the alloy's oxidation and especially nitridation resistance. It was found that, in comparison to the binary system, both properties were improved by additional alloying with Ge, Mo, and Pt.During oxidation Cr-Si-alloys form SiO2 (interface metal/oxide) next to Cr2O3 (interface oxide/gas). This leads to a decrease in oxide growth and in nitridation attack. Alloying with Ge and Pt supports the formation of a continuous, nitridation-resistant A15 phase layer in the subsurface area which further reduces nitridation. Cr3PtN forms in the presence of Pt instead of detrimental Cr2N. Additionally, Ge decreases the effect of local scale failure of the protective oxide scale on the oxidation kinetics and improves scale adhesion.Alloying with Mo enhances SiO2 formation and reduces the evaporation rate constant, most likely due to a morphological change in the Cr2O3 scale. In the Cr-Si-Mo alloy nitridation is further hindered as a Mo-enriched Crss phase forms in the subsurface scale which is resistant against nitridation.To describe the oxidation in more detail, a kinetic model (kp-kv-P model) was developed. This model includes for the first time the impact of local oxide scale failure in oxidation kinetics. Thereby, the formation of local defects and scale adhesion depending on the alloy composition can be investigated using thermogravimetric measurements.The influence of the alloying elements on the Cr2O3 morphology was also investigated. Combining both, the assumption is made that the oxide scale is intrinsically protective against nitridation and Cr2N formation is solely caused by local scale failure. Therefore, the improvement of nitridation resistance by alloying is mainly attributed to a change in oxide morphology, induced growth stresses, and scale adhesion.In this work it was shown that appropriate alloying of Cr-Si further reduces the parabolic oxidation constant (kp) and the evaporation rate (kv) by one order of magnitude. Additionally, nitridation was reduced by 98%. In microstructural investigations and oxidation tests the effects of the different alloying elements Ge, Mo, and Pt were found to be combinable. All in all, the Cr-Si-Ge-Mo-Pt-system shows high potential for the future development of refractory-based high-temperature alloys.

Numerical analysis of heat and mass transfer in kiwifruit slices during combined radio frequency and vacuum drying

Kiwifruit slices subjected to combined radio frequency (RF) and vacuum drying undergo a complicated process, including electromagnetic heating, heat and mass transfer along with a phase change of evaporation and shrinkage under a low pressure. The aim of this paper was to obtain an in-depth understanding of this complicated drying process. A 3D multiphase porous model was established to describe heat and mass transfer within the kiwifruit slices in a 3 kW, 27.12 MHz RF-vacuum drying system using COMSOL Multiphysics software. The validation results showed that the established model could be applied to describe the RF-vacuum drying process of kiwifruit slices since the maximum relative errors of temperature, moisture content, and drying rate between simulation and experiment were less than 10%. The temperature distribution of kiwifruit slices both from simulation and experiment indicated that the sample temperature at corners and edges were higher than that at the center of the container, and the sample temperature at the center was the lowest for a single kiwifruit slice. But the distribution of moisture content was opposite to that of temperature. Evaporation rate constant was found to be sensitive to moisture content of treated samples compared with temperature. The influences of sample thickness, electrode gap, and vacuum pressure of the RF-vacuum drying system on temperature and moisture content-time histories were determined during the RF-vacuum drying process. The findings of this research may help to gain a comprehensive understanding of RF-vacuum drying and optimize treatment parameters for drying processes.

Experimental study on evaporation characteristics of single and multiple fuel droplets

In this work, evaporation experiments of multiple droplets are carried out in a stagnant hot atmospheric environment (573, 673 and 773 K) using high-speed backlit image technique. Three fuel droplets with nearly same initial diameter are suspended at intersections of two 0.1 mm quartz fibers. The normalized droplet spacing (s/d0) of three droplets is 2.25. The results show that the evaporation process of single, edge and central fuel droplet containing three stage: initial heating, unsteady evaporation and quasi-steady evaporation stage. Classical d2 law is still suitable for edge and central droplet at quasi-steady evaporation stage. The third stage of edge and central droplet accounts for more than 60% of droplet lifetime at low temperatures and about 50% at high temperatures. The evaporation rate constant of edge and central droplet increases and droplet lifetime decreases with increasing ambient temperature. The evaporation time of edge and central droplet at first and third stage is higher than single droplet, but lower than single droplet in the second stage. More importantly, the evaporation interactions between droplets is significant at low temperature. Compared with single droplet, the lifetime of central droplet is increased by 31.8%, 18.6% and 25.9%, respectively.

Controlling the Nanoscale Gaps on Silver Island Film for Efficient Surface-Enhanced Raman Spectroscopy.

We control the nanoscale gaps on silver island films by different processing methods and investigate the surface-enhanced Raman scattering (SERS) efficiency on the films. We propose a facile technique to control the film morphology by substrate bending while keeping the evaporation rate constant. The films developed by our new method are compared to the films developed by traditional methods at various evaporation rates. The SERS signals generated on the samples prepared by the new method have similar strengths as the traditional methods. Substrate bending allows us to reduce the gap sizes while using a higher evaporation rate, hence the film can be developed in a shorter time. This cost-effective and time-efficient method is suitable for the mass production of large-area SERS sensors with good sensitivity. Scanning electron microscope images are analyzed to quantify the gap densities and widths to elucidate the relationship between the film morphology and the SERS intensity. While the gap size appears to be the major factor influencing the enhancement, the shape of the nano-island also seems to influence the SERS efficiency.