Evaporation Coefficient Research Articles

Measurement of an evaporation coefficient in tissue sections as a correction factor for 10B determination.

Boron neutron capture therapy (BNCT) is a cancer treatment option that combines preferential uptake of a boron compound in tumors and irradiation with thermal neutrons. For treatment planning, the boron concentration in different tissues must be considered. Neutron autoradiography using nuclear track detectors (NTD) can be applied to study both the concentration and microdistribution of boron in tissue samples. Histological sections are obtained from frozen tissue by cryosectioning. When the samples reach room temperature, they undergo an evaporation process, which leads to an increase in the boron concentration. To take this effect into account, certain correction factors (evaporation coefficients, CEv) must be applied. With this aim, a protocol was established to register and analyze mass variation of tissue sections, measured with a semimicro scale. Values of ambient temperature, pressure, and humidity were simultaneously recorded. Reproducible results of evaporation curves and CEv values were obtained for different tissue samples, which allowed the systematization of the procedure. This study could contribute to a more precise determination of boron concentration in tissue samples through the neutron autoradiography technique, which is of great relevance to make dosimetric calculations in BNCT.

Influence of molecular anisotropy and quadrupolar moment on evaporation

The molecular interactions of numerous real fluids, like argon, nitrogen, or carbon dioxide, are adequately described by the two-center Lennard-Jones plus quadrupole potential. Applying this model class in molecular dynamics simulations, evaporation is investigated systematically. The influence of the molecular anisotropy and quadrupole as well as the boundary conditions, i.e., bulk liquid temperature and evaporation magnitude, is reported. A method for specifying the evaporation magnitude in terms of hydrodynamic velocity is further developed for that purpose. Analyses show that the largest molar flux and energy flux occur for spherical molecules and that anisotropy and quadrupole influence several quantities. Depending on the bulk liquid temperature, the quadrupole predominantly affects the interface temperature, while the anisotropy of the molecule significantly influences the interface temperature as well as both molar and energy fluxes. In addition, the preferred average orientation of the molecules in the interface region is investigated. The evaporation coefficient is determined, and thermodynamic states traversed during the evaporation process are discussed.

Wetting and evaporative performance analysis of wet channels in indirect evaporative cooler with hydrophilic nano-coating

The wetting condition is an important factor that affects the cooling performance of an indirect evaporative cooler (IEC). Applying hydrophilic coatings to the wet channels is an effective way to enlarge the wetting area and enhance evaporative cooling. However, long-term hydrophilic durability tests for the coated surface are rarely discussed in existing studies. Moreover, the evaluation of performance improvement of a hydrophilic-coated IEC is usually based on testing the prototype as a whole, without giving attention to the internal wetting behavior inside the core. Therefore, a TiO2/SiO2 nano-coated polypropylene (PP) IEC is manufactured for studying the wetting and evaporative performance by considering the hydrophilic durability and internal wetting behavior with a specially-designed visualized experiment, aiming at promoting its application and guiding the design. Firstly, the microstructure, thermal conductivity, drop-surface characteristics and hydrophilic durability of the coated samples were studied. Secondly, the internal water distribution was studied under the influence of spray time (5 s ∼ 180 s), water flow rate (2 L/min ∼ 6 L/min), air velocity (2 m/s ∼ 3.5 m/s) and nozzle height (50 mm ∼ 250 mm) by combining fluorescence tracer and digital image processing methods. Lastly, the evaporative performance of the nano-coated IEC was tested under two spray strategies. Results show the hydrophilicity of polished nano-coated PP is durable in a dark and water-scouring environment with a steady contact angle of 30.9°. The coated core shows super hydrophilicity by forming a thin water film and being fully wetted within 60 s. Compared to the continuous spray, the intermittent spray can provide a larger evaporative mass transfer coefficient by 23.5% and a higher coefficient of performance by 114%.

Experimental and analytical investigation of applying an asymmetric plate heat exchanger as an evaporator in a thermally driven adsorption appliance

This communication presents an experimental and analytical study on the evaporation mechanism in a closed-structured asymmetric plate heat exchanger (PHE) employed as a stagnant water evaporator for the application in an adsorption heat transformation appliance. To this aim, an experimental unit is constructed, which comprises two identical PHEs, one acting as an evaporator/condenser and the second, as an adsorber/desorber. Two endoscopes are mounted inside the investigated evaporator to visualize the evaporation mechanism when performing adsorption-evaporation processes under different boundary conditions. It turned out that the evaporation mechanism is a partially covered, thin film evaporation. A heat transfer analysis is performed to evaluate the heat transfer coefficient of the thin film evaporation (hf) inside the investigated evaporator, resulting in hf-values between 1330 and 160 [W∙m−2∙K−1] over the investigated adsorption-evaporation time. Correlating the obtained (hf) to the film thickness δ and the wetted area Awet results in δ-values between 0.34 and 0.78 [mm] and wetted to total area ratios Awet/Atotal of 0.78 to 0.16. Besides, an analytical model has been developed and introduced to correlate the overall evaporator heat transfer coefficient with the adsorption potential and the time rate of change of the water uptake.

Estimation of Evapotranspiration Based on a Modified Penman–Monteith–Leuning Model Using Surface and Root Zone Soil Moisture

Most of the current parameterization schemes for the Penman–Monteith–Leuning evapotranspiration (ET) model (PML) consider meteorological and energy factors and land use types, but the analysis of the effect of soil moisture (SM) changes on ET processes lacks sufficient attention. This paper proposes a parameterization scheme for the sensitive parameters of the PML model considering soil water content, i.e., coupling the land surface SM in the calculation of soil evaporation coefficient f and coupling the SM of the root zone layer in the calculation of maximum stomatal conductance gsx, respectively. The new parameterization scheme is validated at 13 flux sites worldwide and showed significant improvements in improving the correlation with measured values. Moreover, based on the analysis of the spatial distribution of soil evaporation and vegetation transpiration, and the correlation between SM and ET, the regional characteristics of the effect of SM on ET are further revealed. This study provides a new idea for conducting the fusion simulation of SM based on a PML model, which is useful for the subsequent development of the model.

Thin Film Evaporation Modeling of the Liquid Microlayer Region in a Dewetting Water Bubble

Understanding the mechanism of bubble growth is crucial to modeling boiling heat transfer and enabling the development of technological applications, such as energy systems and thermal management processes, which rely on boiling to achieve the high heat fluxes required for their operation. This paper presents analyses of the evaporation of “microlayers”, i.e., ultra-thin layers of liquid present beneath steam bubbles growing at the heated surface in the atmospheric pressure nucleate of boiling water. Evaporation of the microlayer is believed to be a major contributor to the phase change heat transfer, but its evolution, spatio-temporal stability, and impact on macroscale bubble dynamics are still poorly understood. Mass, momentum, and energy transfer in the microlayer are modeled with a lubrication theory approach that accounts for capillary and intermolecular forces and interfacial mass transfer. The model is embodied in a third-order nonlinear film evolution equation, which is solved numerically. Variable wall-temperature boundary conditions are applied at the solid–liquid interface to account for conjugate heat transfer due to evaporative heat loss at the liquid–vapor interface. Predictions obtained with the current approach compare favorably with experimental measurements of microlayer evaporation. By comparing film profiles at a sequence of times into the ebullition cycle of a single bubble, likely values of evaporative heat transfer coefficients were inferred and found to fall within the range of previously reported estimates. The result suggests that the coefficients may not be a constant, as previously assumed, but instead something that varies with time during the ebullition cycle.

Analysing Pan Evaporation to Understand Wastewater Treatment Plant Performance, A Case Study in A Manufacturing Industry

Water loss has become a problem with the balance of a water system, including one in the industry. General opinion has considered evaporation as one of the main justifications for explaining the water loss, especially in the area with a higher daily temperature. A study was conducted on a wastewater treatment system owned by a manufacturing industry consisting of semi-open-air sedimentation and aeration ponds, which suddenly experienced a significant deficit in its water balance. Problem-solving was performed by 8-Step Problem Solving approach and root causes were confirmed by estimating water evaporation. The actual water evaporation rate (E) was approached by pan evaporation (Epan) using the partial pressure of the water vapour and the pan evaporation coefficient (Kpan). The study revealed that evaporation (1.67±0.59mm.d-1 and 1.72±0.62mm.d-1, for sedimentation pond and aeration pond, respectively) was not the main cause of sudden significant water loss (R2 = .490, p-value .05) and confirmed another root cause. In parallel, a water balance model was constructed and fitted the actual condition (R = .987). A countermeasure was performed against the confirmed root cause followed by a monthly evaluation of water loss using the constructed model with a 3σ threshold value (UCL = 9.55%) which showed the elimination of the problem.

Evolution of droplet size of Jet-A surrogate fuel measured by High-Speed interferometric laser imaging for droplet sizing in high temperature region formed by Methane/Air flat flame in laminar counterflow field

The dynamic evaporation processes of fuel droplets in a steep temperature gradient formed by a spray flame were evaluated through high-speed interferometric laser imaging for droplet sizing (Hs-ILIDS). To examine the time history of the evaporation processes of droplets of kerosene and surrogate fuel, comprising of n-dodecane (C12H26), iso-octane (C8H18), and toluene (C7H8), evaporation coefficients were obtained for different fuel supply rates. The results indicate that the evaporation behavior of fuel droplets correlated with the d-square law, even for fuel droplets of several tens of micrometers in the spray combustion field. The evaporation coefficient of the surrogate fuel was larger than that of kerosene, regardless of the fuel supply rate.

The Development of Empirical Equations Estimation of Pan Evaporation Coefficient in the Fasa Region

The Development of Empirical Equations Estimation of Pan Evaporation Coefficient in the Fasa Region

Physical model and numerical simulation of high-temperature silicification of carbon composite material.

A mathematical model is developed to describe the process of high-temperature silicification of a carbon porous material. The cause of pores blockage is the condensation of gaseous silicon at the inner walls of tubules. Phenomenological temperature dependences for the coefficients of condensation and evaporation are proposed, which determine the intensity of the siliconizing process. It is assumed that the diffusion of the silicon vapour is the main mechanism of the material open pores filling. Numerical modelling is carried out by the finite difference method using an explicit scheme in the case of a constant value of the mobile concentration at the input and for different variants of boundary conditions on temperature. These temperature distributions on boundaries make it possible to describe siliconized regions and to delimit from them the sample parts that remain 'dry' duringthe silicification process. The dynamics of main physical quantities change, i.e. porosity of the composite material and concentration of the immobile component in the volume, are analysed. The results of numerical simulation for the time of full silicification and the weight gain of the product are in qualitative and quantitative agreement with all known experimental data. This article is part of the theme issue 'New trends in pattern formation and nonlinear dynamics of extended systems'.

Experimental investigation on energy and mass transport at steady-state evaporating interface in liquid methane storage tanks

In large-scale energy storage and transportation applications, a comprehensive understanding of the evaporation characteristics of liquid methane will benefit the extension of liquid methane storage duration and the improvement in energy storage efficiency. The complexities of achieving cryogenic steady-state evaporation and submillimeter visualization make experimental studies of liquid methane interfacial heat and mass transport difficult and insufficient. In this study, a cryogenic evaporation testing rig was constructed to obtain the interfacial temperature distributions of liquid methane with a spatial resolution of 50 µm. The evaporation coefficient was obtained directly from the measured temperature profile data near the liquid/vapor methane interface. The coupled effects of thermal conduction and Marangoni convection at the liquid-methane interface were revealed and quantitatively compared to those of liquid oxygen and water. A new high-precision dimensionless correlation for the evaporation coefficient was proposed for liquid methane, which explained the consistent dependence of the evaporation coefficient on the liquid-side temperature and wall interface height. The results help to enhance the simulation stability and accuracy and benefit the design of liquid methane or liquefied natural gas storage tanks.

Evapotranspiration dynamics of forage cultivated in the Southern Amazon with SimDualKC Model

Studies of the dynamics of water in the Soil-Plant-Atmosphere system have been constantly carried out, as they directly contribute to the increase in production and the rational use of water for the most diverse cultures. In this approach, the aim of the study was to assess the water demand in forage Urochloa decumbens, with application of the SIMDualKC model. The meteorological variables used in the paper were obtained by an automatic INMET station. And to estimate forage production, cuts were made at a height of 15 cm, then removing it, simulating cattle grazing. In each cut, the accumulation of degree-days was determined and the base and cut temperatures were 10°C and 30°C, respectively. For the dynamics of crop water demand, the SIMDualKc model was used, in a daily time interval to estimate the basal crop coefficient (Kcb), evaporation coefficient (Ke), crop evapotranspiration (ET), in addition to Kc and the water available in the soil in the forage growing cycle. Kcb and Ke had an inverse relation, which means that when Kcb increased, Ke showed a decreasing trend. The Kc values did not change until the 3rd cut, but in the 4th cut there were changes with a reduction in rainfall and water availability in the system. It is observed that the model showed good ability to estimate the amount of water available in the soil, in addition to the physiological attributes of the forage, being a great option in the study of water dynamics for the forage.

Experimental study of falling film evaporation of refrigerants, R32, R1234yf, R410A, R452B and R454B on horizontal tubes

This study investigates the heat transfer performance of the falling film evaporation and pool boiling of low-GWP refrigerants, R452B and R454B, for the replacement of R410A. Fluids including pure refrigerants R32, R1234yf, and near-azeotropic mixtures R410A, R452B, and R454B were tested on smooth tubes and 60FPI fin tubes at various conditions of saturation temperature (5, 15 °C), volume flow rate (150∼300 ml/min), and heat flux (10∼50 kW/m2). For the falling film evaporation on smooth tubes, the R32 yielded the highest heat transfer coefficient among all test fluids. R410A has higher heat transfer among the three mixtures with the help of the component of R32, with lower Marangoni number and consequent lower critical heat flux. In addition, the heat transfer coefficients of film evaporation of R452B and R454B is about 18.2% and 14.5% lower than that of pool boiling, respectively, at Ref=833–865. The heat transfer coefficient of fin tubes is 2.84–3.87 folds that of the smooth tubes for R452B. Based on 339 sets of falling film test data of near-azeotropic fluids, an empirical correlation was established. It predicts experimental data with mean absolute error (MAD%) of 8.28%.

Research on the cooling of corn grain in the mound

Two-stage drying is widely used in agriculture in Russia and abroad when grain is under-dried to condition humidity by several percent and left to dry by atmospheric air, thereby saving costs of heat. When using this method, there are questions about determining the minimum air velocity in the layer, the calculation of retention time before and during the ventilating process itself. For different thicknesses of the material layer, the retention time varies greatly - from 6 h to 24 h. This is due to the fact that the duration is usually set empirically by the increased or decreased moisture content during the process. Cooling duration is calculated from the recommended normative specific air supply to the bulk and its consumption per ton of cooled grain, which often leads to overestimated cooling duration. Therefore, the purpose of this work is to substantiate and calculate the permissible specific air supply, as well as the duration of tempering and cooling of grain. Minimum specific air supply during cooling of heated grain depends on the layer thickness, the coefficient of moisture evaporation and the logarithm of the difference between the relative humidity of moisture vapor on the surface of grain and air. The minimum duration of tempering of heated grain is caused by moistening its outer surface and depends on the coefficient of moisture diffusion, the radius of the grain, the thickness of the shell and the logarithm of the difference between the current and equilibrium moisture content. The duration of cooling of the embankment can be calculated according to the dependencies for drying grain at the actual values of air and embankment parameters. Economic studies have shown the admissibility of using the obtained dependencies for calculating the main parameters of the process without reducing the quality of grain.

A study on droplet group evaporation modeling based on interface resolved numerical simulations of two-phase flow

Large-scale numerical analyses of the evaporation of groups of 640 droplets and 2,964 droplets were conducted by the interface-resolved simulation method, and the effect of mutual droplet interference on the evaporation rate of droplets in groups was modeled. The interference effect reduces the evaporation rate of each droplet due to the increase in its surrounding fuel concentration caused by the evaporation of neighboring droplets. The evaporation rate interference coefficient, which is the ratio of the evaporation coefficient of each droplet in a group to that of a single droplet without interference, was modeled using three parameters: the distance between the droplets, the size of the droplet interfering with an object droplet, and the evaporation state of the interfering droplet. A new droplet evaporation rate parameter containing these model parameters was proposed, and it was confirmed that the proposed parameter properly models the evaporation rate interference coefficient calculated using the droplet group evaporation numerical analysis results. In addition, the accuracy of the model was improved by including a further parameter to express the positional relationship of a droplet within a group.

Analysis of the evaporation coefficients of water, heavy water, and methanol in a high vacuum environment

The evaporation coefficient of liquids is an important factor for many practical situations but has been poorly understood due to the complexity of the experiments and uncertainty in the measurements. Indeed, the lack of current knowledge about the nature of the evaporation coefficient prevents accurate confirmation of its value. We have determined the evaporation coefficients of three liquids, water, heavy water, and methanol, by fitting Knudsen's evaporation rate equation to past measurements of the temperature of droplet trains and liquid jets evaporating in a high-vacuumed environment. Analysis of available experimental data suggests the value of 1 for the evaporation coefficient (αe) of water, and 0.725 for that of heavy water. A small-jet methanol study predicts αe≈1; however, analysis of experimental data from a large-jet methanol study was an exception—indicating coefficients well over unity (1.5<αe<9). These exceed the suggested maximum theoretical value of unity. We found that considering αe=1 for water, and most of the test fluids, is a fair approximation in a high-vacuumed environment that presents minimal kinetic barriers to evaporation. These results confirm that water should not be considered an anomalous liquid with respect to evaporation. This study provides insight that will help improve the design and development of the many processes in which evaporation plays a key role.

Dynamic correction on condensation time relaxation coefficient of Lee model based on mass conservation mechanism

Lee model is commonly accepted for modelling the mass transfer processes subject to both evaporation or condensation. However, the mass loss or mass excess always exist due to the mismatch of condensation and evaporation coefficients. Rare study focused on the proper ratio of them in closed systems. In present study, a dynamic correction model (named mass-conservation model) against to the condensation time relaxation coefficient is proposed, which can adjust the condensation coefficient in each time step considering the mass conservation between the initial state and the current state. Three typical closed structures are introduced for model validation in which severe mass loss and temperature prediction deviation are recognized. Result shows that the mass variation calculated by present dynamic correction model is less than 0.5% while the maximum mass loss with other models could reach 48.31%. Meanwhile, the prediction error of the three Cases calculated with the present model is the smallest, with the maximum decrement of 29.73% compared with other models. It is also realized that the insufficient heat release and the mass non-conservation are the key factor resulting in the temperature prediction deviation, which is well solved by present correction model on condensation time relaxation coefficient.

In-situ estimation of water transfer parameters in a proton exchange membrane fuel cell

A reduced model to describe steady state transport of water through a PEM fuel cell is proposed. It includes three rigorously estimated parameters: the effective gas diffusion coefficient of water vapor in hydrogen, of water vapor in air and of water in sorbed phase in the membrane. First, it is proved that the parameters are well decorrelated. Then, uncertainty on estimates is calculated by simulating an experiment with random noise. Finally, the parameters are estimated as a function of the average humidity. It is observed that the gas phase diffusion coefficients do not depend on humidity. The effective diffusion coefficient in sorbed phase decreases slightly with increasing average relative humidity. It is shown that the effect of electrodes on water transport is negligible by comparing the water fluxes through an MEA and a membrane alone. A temperature gradient is imposed between the two plates and allows to obtain the condensation of water on one side of the membrane. A discontinuity of the water flow through the MEA is observed and results are interpreted by an increase of the effective diffusion coefficient in sorbed phase and/or by an increase of the water content of the membrane in contact with liquid water.

Vapor condensation induced by fast-moving liquid film in the presence of noncondensable gas molecules

In this study, vapor condensation phenomena at the surface of a fast-moving planar liquid film in a gas mixture (vapor and noncondensable (NC) gas), aimed to imitate a cavitation bubble collapse, were simulated to investigate the evaporation and condensation coefficients of vapor molecules based on the Enskog–Vlasov equation. These coefficients indicate the evaporation and condensation rates of vapor molecules, and they were incorporated into the kinetic boundary condition (KBC) for the vapor/gas–liquid interface. We employed the Enskog–Vlasov direct simulation Monte Carlo method to simulate the condensation phenomena. Based on the results, we confirmed that the obtained coefficients decreased with an increase in the liquid temperature and NC gas content at the condensing interface. Additionally, the coefficients had the same values as those in the equilibrium state, even for such a moving system. Notably, the nonequilibrium effect on condensation phenomenon discussed in the field of cavitation bubble dynamics, whereby vapor molecules fail to condense when the liquid velocity is significantly high, could not be confirmed in this simulation. Furthermore, we conducted an analysis based on the Boltzmann equation using the direct simulation Monte Carlo method to verify the obtained coefficients.

Saliva Droplet Evaporation Experiment and Simple Correlation of Evaporation-falling Curve under Different Temperatures and RH

In the present study, water and saliva droplet evaporation rates are experimentally evaluated in various environmental conditions: temperature and relative humidity. We found that the ratio of saliva residue size to saliva initial droplet size is 0.216. We also found that the saliva correction factor which is defined as a ratio of water evaporation coefficient to saliva evaporation coefficient was not affected by environmental conditions and was determined as 0.857. By considering the evaporation and gravitational settling effects, the saliva airborne lifetime, i.e., the evaporation-falling curve is numerically calculated and characterized. In addition to this numerical calculation results, we present the simple correlation equation for obtaining the evaporation-falling curve under different temperatures and relative humidity.