Non-isothermal Evaporation Research Articles

NON-ISOTHERMAL MODELING OF SOIL VAPOR EXTRACTION SYSTEM INCLUDING SOIL TEMPERATURE EFFECT

Soil vapor extraction (SVE) is a proven effective in-situ technology for the removal of volatileorganic compounds (VOCs) from the subsurface. SVE process is highly sensitive to temperature.Studying annual soil temperature variation with depth declares that there is a considerable temperaturevariation in the upper few meters that may affect the overall efficiency of SVE process.A numerical model was developed to aid in investigation of field-scale soil vapor extractionprocess. The model is three-dimensional, time dependent that simulates nonisothermal vapor flow andtransport of multicomponent mixtures in soil and keeps track of the distribution of each compound inthe other three immobile phases (NAPL, aqueous, and sorbed). Rate limited interphase mass transferwith linear driving force expressions were used to model volatilization of oil into gas phase. A localequilibrium partitioning was assumed between gas, water, and solid phase. The model equations werediscretized using a standard Galerkin finite element method and solved using set iterative solutionalgorithm.Simulation of hypothetical field-scale problems was done. The physical domain described a threedimensionalsystem with flow to a single extraction well. A hypothetical soil temperature variationwith depth was incorporated with the model. The result of these simulations showed that thistemperature variation has a considerable effect on system efficiency and may play a role in optimumsystem configuration.

Fluoride Evaporation of Low-Fluoride CaF2-CaO-Al2O3-MgO-TiO2-(Na2O-K2O) Slag for Electroslag Remelting

To elucidate the behavior of fluoride evaporation in an electroslag remelting process, the non-isothermal evaporation of the low-fluoride CaF2-CaO-Al2O3-MgO-TiO2-(Na2O-K2O) slag is studied using thermogravimetric analysis. The evaporation law of the melted slag is further verified using thermodynamic calculations. Fourier transformation infrared (FTIR) spectroscopy is used to evaluate the change in slag structure. It is discovered that the principal evaporating substances are CaF2, KF, and NaF, while the evaporation of MgF2, AlF3, and AlOF is less. KF evaporates absolutely in the early stage of the reaction, and CaF2 evaporates in a large proportion during the late reaction period. At 1500 °C, the order of vapor pressure is KF > CaF2. When K2O and Na2O are added to the residue sample at the same time, the evaporation ability of KF is stronger than that of CaF2 and NaF. As the K2O content increases from 0 to 8.3 wt%, evaporation increases from 0.76% to 1.21%. The evaporation rates of samples containing more K2O and those containing more Na2O are 1.48% and 1.32%, respectively. Under the same conditions, K2O has a greater effect on evaporation than Na2O. FTIR results show that the addition of K2O depolymerizes the network structure and that K2O can depolymerize the network structure better than Na2O.

ZeoPTES: Zeotropic Pumped Thermal Energy Storage with an Ammonia–Water Mixture as Working Fluid

The rapid rise of renewable energy production necessitates the development of large‐scale electricity storage systems. Pumped thermal energy storage (PTES), where electricity is stored in hot and cold storage units, has recently garnered a lot of interest. Previously proposed PTES systems rely on pure fluids as working fluids in Brayton‐ or Rankine‐based power cycles. Herein, ZeoPTES, a PTES system using a zeotropic mixture of ammonia and water as working fluid, is introduced. The mixture exhibits nonisothermal evaporation and condensation, which allows utilization of industrially available sensible storage units, like water or molten salt, to store the thermal energy from these phase changes. A simulation using the REFPROP 10.0 database is written to analyze the cycle. Eight parameters are identified as having an effect on system round‐trip efficiency and 30 000 simulations with random values for these parameters are run to identify the impact of all. Although efficient compressors and expanders are necessary to reach a high power‐to‐power round‐trip efficiency, the largest system‐wide losses arise from entropy generation and thermal stream mismatch in the cold and hot side heat exchangers. Using state‐of‐the‐art microchannel heat exchangers with extremely low pinch points allows for round‐trip efficiencies in excess of 70%.

On the Numerical Modeling of Water Flows in Porous Media under Near-Critical Conditions

The problem of the numerical modeling of nonisothermal water and vapor flows in a porous medium under near-critical thermodynamic conditions is considered. It is shown that the equations of single-phase and two-phase flows in porous media become degenerate at the critical point of water. Some disadvantages of flow modeling methods that are able to exclude degeneracy by selecting the pressure and enthalpy as independent model variables are discussed. A new method of modeling under near-critical thermodynamic conditions in the standard pressure–temperature variables is proposed. This method is based on an approxinate description of the fluid phase diagram with linear splines that eliminates the degeneracy of conservation laws at the critical point and enables the application of equations of state for predicting the fluid properties in pressure and temperature variables.

Multistage and passive cooling process driven by salinity difference.

Space cooling in buildings is anticipated to rise because of an increasing thermal comfort demand worldwide, and this calls for cost-effective and sustainable cooling technologies. We present a proof-of-concept multistage device, where a net cooling capacity and a temperature difference are demonstrated as long as two water solutions at disparate salinity are maintained. Each stage is made of two hydrophilic layers separated by a hydrophobic membrane. An imbalance in water activity in the two layers naturally causes a non-isothermal vapor flux across the membrane without requiring any mechanical ancillaries. One prototype of the device developed a specific cooling capacity of up to 170 W m-2 at a vanishing temperature difference, considering a 3.1 mol/kg calcium chloride solution. To provide perspective, if successfully up-scaled, this concept may help satisfy at least partially the cooling needs in hot, humid regions with naturally available salinity gradients.

The crystallization behavior of the aqueous solution of CaCl2 salt in a drop and a layer

Non-isothermal evaporation during crystallization of CaCl2 salt in a droplet and a thin layer on a hot wall has been investigated experimentally. The growth of salt crystal hydrates (CaCl2·2H2O) on the interface has been studied. It has been found that the change in the initial salt concentration leads to different crystallization rates. The crystallization rate on the droplet interface is many times lower than for a thin layer. The description of crystallization in the salt solution droplet should take into account the crystallization anisotropy, which is associated with the direction of crystallization. The crystallization rate along the contact line of the droplet is many times higher than in the direction of the droplet radius. For a long time of crystallization, the area of the crystal film (outside the drop) increases several times. Four characteristic modes of crystallization to a drop of salt solution have been distinguished. When modeling crystallization, it is necessary to take into account multiple changes in the growth rate of salt crystallohydrates over time, as well as the anisotropic nature of crystallization.

Droplet evaporation on a structured surface: The role of near wall vortexes in heat and mass transfer

Experimental studies on the evaporation of a drop located on a horizontal hot wall with cavities of different diameters of 0.5–2.5 mm were carried out. The wall temperature Tw was constant (74 °C and 83 °C). The evaporation behavior on a structured surface was compared with that on a smooth wall. Instantaneous velocity profiles have been obtained over a single cavity and in the vicinity of several cavities using the Micro Particle Image Velocity method (Micro PIV). It has been established that a hotter liquid is periodically ejected from the cavity, which increases convection inside the drop. The strongest intensification of mass transfer is specific for the largest cavities with a diameter of 2.5 mm. The behavior of the droplet evaporation on a smooth wall coincides with that on a structured surface with a cavity diameter of 0.5 mm. Until now, there have been no data that would link the convection in the drop with the vortexes in the cavity at non-isothermal evaporation and at high heat fluxes. The strongest influence of cavities is manifested in the initial period of evaporation, when a cold drop is placed on a hot wall. Over time, the evaporation rate on a structured wall approaches evaporation on a smooth (unstructured) surface. The article considers the influence of several key factors on the convection in a drop.

Non-isothermal evaporation and heat transfer of the salt solution layer on a structured wall in the presence of corrosion

The layer evaporation with and without corrosion on an iron micro-structured wall with ceramic coating is investigated experimentally. SiO2 coating is obtained. A structured wall is used to intensify heat transfer. Hydrophobic ceramic coating is used to reduce the aggressive effect of salts on the metal wall. Rapid wall corrosion at high temperatures is a significant problem for desalination technologies, high-temperature generators of adsorption heat pumps, and chemical technologies using salts. The evaporation rate of the salt solution (CaCl2/H2O and NaCl/H2O) on the structured heated wall with SiO2 is 25–30% higher than on the smooth surface. Often, to simplify the simulation of heat transfer, free convection in the layer is neglected due to the low layer height and high viscosity of the solution. The novelty of this work lies in the fact that it is free convection that plays a key role. The combined effect of microstructured roughness and anticorrosion coating on heat transfer and convection is considered for the first time. The role of convection depends on whether the salt belongs to the first or second group, characterized by a certain character of evaporation. To date, there are no models that take into account structured surface, corrosion and natural convection on non-isothermal evaporation and heat transfer in a thin solution layer.

The influence of the wall microtexture on functional properties and heat transfer

Experimental studies of various microstructures of the surface have been carried out; wettability and heat and mass transfer at non-isothermal evaporation of a drop of water located on a horizontal heated wall have been investigated. Textured surfaces of different topology are obtained by laser exposure. The behavior of heat exchange on a textured surface has been compared with rough (unstructured) wall. It is shown that the instantaneous velocity fields inside the droplet, measured near the structured surface, are inhomogeneous. Heat flux depends on several key factors: droplet diameter, droplet height, convection due to Marangoni forces and convection, associated with surface microstructures. Maximum convection enhancement is registered for the hydrophobic surface. The maximum value of the heat transfer coefficient corresponds to the hydrophobic surface. Heat transfer intensification is about 30% higher than for rough surfaces.

Free Solution Convection at Non-Isothermal Evaporation of Aqueous Salt Solution on a Micro-Structured Wall

ABSTRACTEvaporation and heat transfer of layers of aqueous salt solutions have been studied. The behavior of salt solutions is compared for a smooth and micro-structured wall with a rectangular profile. The evaporation rate of the salt solution on the structured wall is 20–30% higher than on the smooth one at high salt concentration. Previously, it was thought that the heat transfer for solutions can be calculated for thin layers and films without taking into account the natural convection in liquid. In this paper, the liquid free convection is shown to play a key role. A simple model linking the solutal and the thermal Marangoni numbers and the Peclet number with free convection of the liquid on a hot structured wall is considered. For correct simulation of the non-isothermal heat and mass transfer, it is necessary to take into account local characteristics of thermal and velocity fields inside a layer of the salt solution, as well as to determine the average characteristic scales of circulation into the liquid. To simplify the analysis it is possible to effectively consider four types of characteristic convective scales, the role of which depends on the thickness and diameter of the solution layer, as well as on the wall temperature. The strong influence of free convection in a thin layer of the solution is extremely important for accurate modeling of a wide range of modern technologies. Intensification of heat transfer and evaporation due to the use of a structured wall can be applied in heat exchangers, to improve efficiency in desalination of water, in energy technologies (e.g., in heat absorption pumps), as well as in chemical technologies.

The influence of characteristic scales of convection on non-isothermal evaporation of a thin liquid layer

Here, the effect of convection in liquid on non-isothermal evaporation of a horizontal thin layer on a hot wall is investigated. It is considered that the evaporation rate of salts always decreases with the growth of salt concentration. Depending on the nature of evaporation rate, the aqueous salt solutions can be classified into two different types: (1) the equilibrium partial pressure of water vapor ps varies slightly with time; (2) with an increase in salt mass concentration, ps decreases many times, which leads to a sharp drop in evaporation rate j. The criteria for attributing the salt to characteristic types are proposed, and relation between j and thermodynamic properties of salt solutions is determined. Different approaches to modeling are proposed for each group. For the first time, a simple calculation method linking the Peclet and Marangoni criteria with convection in a liquid and non-stationary heat exchange is proposed. The analysis shows that it is impossible to simulate the heat transfer without knowing the local characteristics of the velocity field in the liquid phase and without clearly distinguishing the characteristic convective scales of the velocity and temperature fields. So far, it has been believed that the surface Marangoni flow can be neglected due to the negative impact of surfactants. However, the studies of this paper show that a noticeable increase in free convection relates to the thermal and solutal Marangoni flows. A strong influence of the Marangoni flow on liquid convection at high heat fluxes is extremely important for reliable simulation of layer evaporation in a wide range of modern technologies.

Non-isothermal Evaporation of Salt Solutions on a Microstructured Surface

ABSTRACTHeat transfer of a droplet and layer during evaporation of aqueous solutions of salts has been studied. The behavior of salt solutions on a smooth and microstructured surface is compared here. Evaporation rate of aqueous salt solutions is greater for a microstructured surface than for a smooth wall. The behavior of heat transfer coefficient α can be described by two time regimes: quasi-constant values of α and significant increase in heat transfer at a multiple decrease in the liquid layer height. Measurements made with application of the particle image velocimetry showed that the structured surface increases liquid speed inside the sessile drop. The largest value of the heat transfer coefficient α on the structured surface corresponds to water for the final stage of evaporation. For salt solutions, the heat transfer coefficient is lower than that for water in the entire period of evaporation on the structured surface. The maximal excess (20–30%) of α of the structured wall above the smooth surface corresponds to the maximal height of the liquid layer at the beginning of evaporation. With increasing time, the excess is reduced. A drop of heat transfer intensification with a decrease in the layer height relates to suppression of free convection (a multiple decrease in the average velocity in the drop).

The Anomalously High Rate of Crystallization, Controlled by Crystal Forms under the Conditions of a Limited Liquid Volume

Non-isothermal evaporation and crystallization in a thin layer of an aqueous salt solution have been studied experimentally. The results of these studies are important for a wide range of modern technologies, associated with obtaining thin coatings and crystallization in thin layers and films. The crystallization patterns are shown to differ for different salts and change both over the crystalline hydrate surface and over time. The growth kinetics of crystal hydrates has been examined. A qualitative behavior of crystallization curves may both coincide and differ from the behavior of the curves, obtained using a statistical approach. It is shown that this difference is determined by a changing ratio of rates of evaporation to crystallization. By changing the evaporation velocity, it is possible to change the crystal forms and crystallization velocity. It is well-known that the crystallization rate increases as a square root of time. However, according to our results, the crystallization rate may both rise ...

Evaporation of layers of salt solutions

Nonisothermal evaporation of layers of water and aqueous salts solutions of H2O/LiBr, H2O/CaCl2 and H2O/LiCl was studied experimentally. The liquid layer was placed on a horizontal heated wall. The initial concentration of salt C0 was 10 %. The wall temperature Tw = 75 °C and ambient air pressure was 1 bar. It was shown that the heat flux q increases for water for the final evaporation stage and falls for salt solutions due to the increase in salt concentration C and due to a significant drop in the equilibrium partial pressure of water vapor.

Non-isothermal evaporation in a sessile droplet of water-salt solution

Experimental data on nonisothermal evaporation of sessile droplets of water-salt solutions (LiBr + H2O; CaCl2+H2O) were obtained. Evaporation of droplets of volatile liquids occurs with almost constant evaporation rate, and the problem is solved in a stationary approximation. High-temperature evaporation of water-salt solutions leads to significant difficulties at modeling the heat and mass transfer. In this case, the evaporation rate substantially decreases with time. With the growth of salt concentration in the solution from 11% to 60%, the partial pressure of water vapor at the interface falls by an order of magnitude. In this work, we have performed simulation, considering diffusion in solutions, a non-isothermal character, and the Stefan flow, and proposed a simple method for calculating the mass flow. The resulting technique can qualitatively and quantitatively predict the solution behavior with a significant change in the external boundary conditions in time.

Optimal multicomponent working fluid of organic Rankine cycle for exergy transfer from liquefied natural gas regasification

A hybrid optimization methodology is proposed for the working fluid selection of an organic Rankine cycle (ORC). First, a stochastic global solver is used to select the chemical species of the working fluid; then, a deterministic global solver is used to optimize the composition. The first step is a mixed integer nonlinear programing (MINLP) and the second a nonlinear program (NLP). The methodology is applied to the recovery of cryogenic energy during the evaporation of liquefied natural gas (LNG), which is a promising way to produce electricity with relatively high efficiency from the large amount of otherwise wasted heat. Seawater or low-grade heat sources can be used as heat source. Multicomponent working fluids have advantages compared to pure fluids because of the nonisothermal evaporation of LNG. Herein, a ternary mixture is considered. A mixture of CF4, CHF3, n-pentane is identified as an optimum ternary working fluid. It produces about 1.1 MJ/kmol LNG with a simple organic Rankine cycle using seawater as heat source. The optimization results are quantitatively compared with the literature in terms of power generation and are shown to have substantially higher electricity generation.

Syntheses, single-crystal structures, vibrational spectra and DSC/TG analyses of orthorhombic and trigonal Ag[N(CN)2

Abstract Colorless powders of orthorhombic Ag[N(CN)2] were synthesized by blending aqueous solutions Ag[NO3] with stoichiometric amounts of Na[N(CN)2] equally dissolved in water. Single crystals of both modifications of Ag[N(CN)2] could be obtained by recrystallizing the powder with aqueous ammonia. Non-isothermal evaporation of the solvent at room temperature within a few hours yielded the orthorhombic modification as the main product crystallizing in the space group Pnma (no. 62) with the unit-cell parameters a = 1612.45(12), b = 361.58(3) and c = 599.02(4) pm (Z = 4). Upon evaporating the solvent much more slowly, isothermally within a few days, mainly thin hexagonal platelets of the trigonal modification of Ag[N(CN)2] were obtained, crystallizing in the space group P3121 (no. 152) with the lattice constants a = 359.86(3) and c = 2285.91(17) pm (Z = 3). Both results corroborate earlier structure determinations, but show higher precision. The vibrational spectra confirm the presence of the dicyanamide anion [N(CN)2]−, but exhibit slight differences to literature data. Differential scanning calorimetry/thermogravimetry (DSC/TG) analyses were performed to further characterize the two modifications.

Nonstationary evolution of the size, composition, and temperature of microdroplets of nonideal two- and three-component aqueous solutions

Results of numerical solution have been presented for a set of equations describing the nonstationary and nonisothermal growth or evaporation of microdroplets consisting of ethanol and water, sulfuric acid and water, and sulfuric and nitric acids and water. Time dependences of droplet size, temperature, and composition have been determined at low concentrations of a condensable vapor, as compared with the concentration of a carrier gas in an ambient vapor–gas mixture. The calculations have been performed using different initial conditions and approximations for the dependences of saturation vapor pressures, activity coefficients, and partial heats of condensation of the components, as well as average volumes per molecule on droplet composition and temperature. By the examples of ethanol–water and sulfuric acid–water droplets, it has been shown that nonmonotonic variations in the droplet radius are possible. Regimes of nonmonotonic variations in the temperature of a droplet that precede the onset of its stationary growth or evaporation have been revealed for all systems under consideration.

Thermoeconomic optimization of a Kalina cycle for a central receiver concentrating solar power plant

Concentrating solar power plants use a number of reflecting mirrors to focus and convert the incident solar energy to heat, and a power cycle to convert this heat into electricity. This paper evaluates the use of a high temperature Kalina cycle for a central receiver concentrating solar power plant with direct vapour generation and without storage. The use of the ammonia-water mixture as the power cycle working fluid with non-isothermal evaporation and condensation presents the potential to improve the overall performance of the plant. This however comes at a price of requiring larger heat exchangers because of lower thermal pinch and heat transfer degradation for mixtures as compared with using a pure fluid in a conventional steam Rankine cycle, and the necessity to use a complex cycle arrangement. Most of the previous studies on the Kalina cycle focused solely on the thermodynamic aspects of the cycle, thereby comparing cycles which require different investment costs. In this study, the economic aspect and the part-load performance are also considered for a thorough evaluation of the Kalina cycle. A thermoeconomic optimization was performed by minimizing the levelized cost of electricity. The different Kalina cycle simulations resulted in the levelized costs of electricity between 212.2$MWh−1 and 218.9$MWh−1. For a plant of same rated capacity, the state-of-the-art steam Rankine cycle has a levelized cost of electricity of 181.0$MWh−1. Therefore, when considering both the thermodynamic and the economic perspectives, the results suggest that it is not beneficial to use the Kalina cycle for high temperature concentrating solar power plants.

On sharp-interface level-set method for heat and/or mass transfer induced Stefan problem

A detailed sharp interface level set method (SI-LSM) based CFD development methodology as well as its implementation and application to the various types of Stefan problem is not found and reported here. The methodology and code-validation are presented in-detail for heat and/or mass transfer induced Stefan flow, 2D Cartesian as well as axi-symmetric coordinate system and Dirichlet as well as jump boundary conditions at the interface. For the Dirichlet BC, a higher order extrapolation methodology is presented – for the computation of ghost fluid value across the interface. Whereas, for the jump BC, a generic finite volume method based discretized equation is proposed – consisting of additional source term and modification in diffusion coefficient for the control-volumes adjoining the interface. An excellent agreement between the present and published results are shown separately for Stefan flow problems induced by heat transfer (melting as well as solidification), mass transfer (isothermal evaporation) and heat as well as mass transfer (non-isothermal evaporation). Finally, a performance study is presented demonstrating a superior performance of the present sharp as compared to diffused interface level set method. The performance study is also presented for two different methods of computation of normal gradient across the interface, with regard to numerical oscillations in the results.The present work – involving localized fluid flow and heat as well as mass transfer in the two-phase domain – will be extremely useful to CFD developer on multi-phase flow.