Water Vapor Diffusion Coefficient Research Articles

Changes of Temperature and Moisture Distribution over Time by Thermo-Hydro-Chemical (T-H-C)-Coupled Analysis in Buffer Material Focusing on Montmorillonite Content

Bentonite is used as a buffer material in engineered barriers for the geological disposal of high-level radioactive waste. The buffer material will be made of bentonite, a natural clay, mixed with silica sand. The buffer material is affected by decay heat from high-level radioactive waste, infiltration of groundwater, and swelling of the buffer material. The analysis of these factors requires coupled analysis of heat transfer, moisture transfer, and groundwater chemistry. The purpose of this study is to develop a model to evaluate bentonite types and silica sand content in a unified manner for thermo-hydro-chemical (T-H-C)-coupled analysis in buffer materials. We focused on the content of the clay mineral montmorillonite, which is the main component of bentonite, and developed a model to derive the moisture diffusion coefficient of liquid water and water vapor based on Philip and de Vries, and Kozeny–Carman. The evolutions of the temperature and moisture distribution in the buffer material were analyzed, and the validity of each distribution was confirmed by comparison with the measured data obtained from an in situ experiment at 350 m in depth at the Horonobe Underground Research Center, Hokkaido, Japan.

Evaluating moisture transfer properties of wood by inverse analysis of moisture content profiles determined during drying by X-ray attenuation

An X-ray density analyzer was used to determine the evolution of the moisture content (MC) profiles of a wood board during drying. Tests were performed on quartersawn boards of five wood species (oak, beech, Scotch pine, birch, alder) of two thicknesses (20 and 40 mm) dried at low temperature. The water vapor diffusion coefficient and liquid permeability of each wood species were determined by inverse analysis of the profiles using a computational model of coupled heat and mass transfer. These parameters were also measured by classical methods for validation purposes. The inverse method gives values close to the measured ones when using the evolution of the moisture profile, but also quite remarkably when using only the evolution of moisture at the core of the board. On the contrary, the method is less accurate when using only the average kinetics as experimental information.

Impact of clod (aggregate) size on evaporation rate and soil moisture profile during the drying process

AbstractLarge clods (centimetres in size) can be formed by tillage in clayey paddy fields where upland crops are planted. These clods cause early water depletion near the soil surface, which decreases crop germination and emergence rates. Because of the difficulty in reducing clod size, desiccation damage to seeds can be avoided by adjusting the seeding depth based on the clod size‐dependent soil moisture profile. This study aimed to clarify the effect of clod size on (1) the evaporation rate (E) and soil moisture profile and (2) the mobility of soil water during the drying process. Evaporation experiments were conducted in an air‐conditioned greenhouse with natural light using cylindrical columns filled with artificially made clods 3 (L columns) and 1 cm (S columns) in diameter. We measured E, potential evaporation rate (PE), and total soil moisture content (wtot) throughout the experiment and the soil moisture profiles at the end of the experiment. The water diffusivity (Dw) and apparent vapour diffusion coefficient (dvap) were calculated as the mobility of soil water and water vapour, respectively. We found that wtot was lower in the L column than in the S column, although not at the onset of the experiment. At the end of the experiment, the soil moisture content was lower in the L column than in the S column throughout the soil layer. In contrast, E/PE was higher in the L column than in the S column throughout the experiment and even at the same wtot. Regarding mobility, Dw was slightly greater in the L column than in the S column in the soil moisture content range, where vapour movement could be greater than liquid water movement. In addition, the ratio of dvap to the diffusion coefficient of water vapour in soil was higher than unity in both columns and was 2.4–3.2 times higher in the L column than in the S column. In summary, larger clods caused a higher evaporation rate and lower soil moisture content, owing to the increased enhancement of water vapour movement probably induced by wind.

Thymol@Natural Zeolite Nanohybrids for Chitosan/Polyvinyl-Alcohol-Based Hydrogels Applied as Active Pads.

Currently, food saving, a circular economy, and zero environmental fingerprints are of major interest. Scientific efforts for enhanced food preservation using "green" methods have been intensified. Even though chemicals could achieve such targets effectively, the global trend against the "greenhouse effect" suggests the use of environmentally friendly biobased materials for this purpose. In this study, the promising biopolymer chitosan is incorporated with the promising biodegradable polymer polyvinyl alcohol to produce an improved biopolymeric matrix. This biodegradable biopolymer was further mixed homogeneously with 15% thymol/nano-zeolite nanohybrid material. The properties of the final developed film were improved compared to the relevant values of chitosan/polyvinyl alcohol film. The mechanical properties were enhanced significantly, i.e., there was a 34% increase in Young's modulus and a 4.5% increase in the ultimate tensile strength, while the antioxidant activity increased by 53.4%. The antibacterial activity increased by 134% for Escherichia coli, 87.5% for Staphylococcus aureus, 32% for Listeria monocytogenes, and 9% for Salmonella enterica. The water vapor diffusion coefficient and the oxygen permeability coefficient decreased to -51% and -74%, respectively, and thus, the water vapor and oxygen barrier increased significantly. The active pads were used in strawberries, and the antimicrobial activity evaluation against the mold of fungi was carried out. The visual evaluation shows that the active pads could extend the shelf life duration of strawberries.

The influence of water vapor on the structural response of asphalt pavement

Abstract A series of water damage phenomena of asphalt pavement during service show that water is very important. To comprehensively analyze the influence of water vapor factors on the response of pavement structure, this article first tests the water vapor diffusion coefficient of asphalt pavement materials at different temperatures establishes the water vapor concentration field model of the pavement structure, and analyzes the actual water vapor distribution of the pavement structure; then, the relationship between viscoelastic parameters of the mixture and water vapor concentration is established. Based on this, the finite element model of the pavement structure considering the water vapor factor is established, and the influence of the water vapor factor on the pavement structure response and fatigue life is quantified. It is found that water vapor has an important influence on the mechanical response. On the hottest days in summer, the attenuation of surface modulus caused by water vapor has the greatest impact on the upper layer of the pavement structure. Under the influence of water vapor, the position just below the wheel of the upper layer at the bottom of the horizontal tension strain increases by 108.26%, and the horizontal tension strain of the other layers increases by about 5%, increasing the risk of upper layer cracking. At the same time, water vapor reduces the stress that can be borne by the surface layer, while the rest of the stress is borne by the base. Compared with the fatigue life calculation method in the specification, the actual non-uniform water vapor concentration field reduces the fatigue life of the surface layer by 12.246%. This shows that the current structural calculation does not consider the water vapor factor, which makes the pavement structure more dangerous, and the influence of the water vapor factor should be fully considered.

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.

Microstructural changes of young cement paste due to moisture transfer at low air pressures

It has been long recognized that low air pressure (AP) affects the moisture transfer in concrete, and thus its microstructure and bulk property, but less is known about the thermodynamic mechanism. This research investigated the microstructure evolution of young white Portland cement paste during wetting or drying at different APs (i.e. 101 kPa, 60 kPa, 20 kPa), the experimental and numerical analyses reveal the moisture transfer changes at low APs, which matters for the microstructural features of hardened cement paste. It was found that low APs increase the water vapor diffusion and convective mass transfer coefficients, and the intrinsic permeability of cement paste sample, which results in higher water contents within cement paste during wetting at 98 % RH but more rapid water losses during drying at 43 % RH. Owing to the distinct moisture transfer behaviors, the C-S-H structures were linked with water contents within confined pores.

Water vapor permeation in amorphous/polycrystalline inorganic layers on polyethylene terephthalate substrates

The barrier performance of amorphous and/or polycrystalline inorganic oxide thin films deposited on flexible polyethylene terephthalate (PET) substrates is evaluated by a developed model that accounts for the diffusion of water vapor through the pinholes and grain boundaries, respectively, in amorphous and polycrystalline sublayers. Three-dimensional diffusion simulations are performed for modeling the influences of layer thickness, defect spacing and grain size on the barrier properties of single (multi) layer films. The water vapor transmission rate (WVTR) is calculated for single/multi-inorganic layers with different structures (amorphous and polycrystalline), numbers of inorganic layers and thicknesses. The simulation results show that the WVTR decreases exponentially with the increase of the inorganic layer thickness. According to the fitted results, the diffusion coefficient of water vapor in the defects (grain boundaries or pinholes) of inorganic materials is much smaller than that in PET, and the diffusion coefficient of water vapor in the grain boundaries of the polycrystalline inorganic layer (PIL) is close to that in the defects/pinholes of the amorphous inorganic layer (AIL). In comparison to PET/AIL, PET/PIL and PET/PIL/AIL films, the barrier performance of PET/AIL/PIL film is superior. As examples, PET/Al2O3/ZnO and PET/ZnO/Al2O3 films were prepared for verification of the developed model. The simulated WVTR values agree well with the experimental ones.

Experimental performance comparison of helium-gap diffusion distillation and air–gap diffusion distillation

Freshwater shortage threatens the development of human society. Seawater accounts for approximately 71% of the Earth’s surface area, which makes seawater-desalination techniques an important method for alleviating freshwater shortage. As a novel desalination technique, air–gap diffusion distillation (AGDD) has gained popularity. However, because of the existence of air gap, the performance of heat and mass transfers in AGDD is not satisfactory. To enhance the heat and mass transfers in AGDD, helium-gap diffusion distillation (HGDD) based on AGDD is proposed in this paper. The biggest difference of HGDD compared with AGDD is that the gap is filled with helium gas, which has higher thermal conductivity and water–vapor diffusion coefficient than air. In this study, experimental performance comparison of HGDD and AGDD was performed by investigating the influence of different parameters on the relevant performance indicators of these two distillers. The results showed that the temperature difference between the hot and cold streams in HGDD was always smaller than that in AGDD at an arbitrarily same location, which indicated that the heat exchange capability of HGDD is better than that of AGDD. Moreover, the increase in the inlet temperature of the hot stream and mass flow rate of the cold stream both enhanced the heat and mass transfers in the device. However, the increase in gap thickness exerted a negative effect on the performance of HGDD and AGDD. From the comparison, irrespective of how the parameters varied, the relevant performance of HGDD was always better than that of AGDD. This work provides a new method for improving the performance of AGDD without changing its original structure or adding new components, which provides a significant guide for practical application of AGDD.

Effect of ion chelator on water sorption and transport behavior of unsaturated mortars with high volume fly ash and blast furnace slag

This paper investigated the effect of ion chelator on water dependency of water transport and fixation in mortars with high volume of fly ash (FA) and blast furnace slag (BFS). Ion chelator (CA) as a new type of crystalline admixture could improve self-healing performance of cementitious materials by crack-healing and pore filling. The porosity of mortar was measured by vacuum saturation and pore size distribution of mortar was tested by low-field nuclear magnetic resonance (LF NMR). The water sorption of mortar was evaluated by water vapor adsorption desorption balance. The water transport of mortar with and without crack was tested by capillary water absorption. Based on the experimental results, the water distribution of capillary water absorption in mortar was calculated. Results showed that the pore structure of mortar was improved evidently by CA, and the pores (>1μm) and the total porosity in mortar containing 50 %BFS were significantly reduced. CA had no obvious effect on the water vapor diffusion coefficient of mortar, but has a significant impact on the liquid water diffusion coefficient, especially in pure cement and containing 50 % BFS mortar. The total capillary water absorption and capillary absorption coefficient of mortars were reduced by CA, especially in pure cement and containing 50 % BFS mortar. The addition of CA decreased the water penetration in mortar, the reduction rates of water penetration depth in experimental groups (with CA) at 360 min were 12.4 % (100 %OPC), 6.0 % (30 %FA), 21.3 % (50 %BFS) and 10.8 % (16 %FA 24 %BFS) compared to control group (no CA). After curing for 42d, the normalized water absorption coefficients S” of cracked M100CA and M50BFS50CA were 55.1 % and 72.0 %, respectively. Ion chelator significantly improved internal pore structure and crack healing ability of the mortar containing 50 %BFS, and then delayed the water diffusion.

Development of a differential photoacoustic system for the determination of the effective water diffusion and water vapor permeability coefficients in thin films

This paper focused on developing a methodology and metrology using a differential photoacoustic (PA) system to determine the effective water vapor diffusion coefficient (Deff) and the effective permeability coefficient (Π) in thin films as a piece of paper and standard polystyrene for a controlled relative humidity. The methodology proposes a new differential photoacoustic system, including the water reservoir, relative humidity, and temperature detectors. Two cells, reference/sample, were used to obtain the instrumental function to reduce the electronic and environmental noises. A method based on the study of ln[1−(S−S0)/ΔS]=t/τD and the behaviors of R2 as a function of the number of data was proposed to assess the region in which the photoacoustic signal should be processed to determine each effective coefficient. S is the amplitude of the PA signal, S0 is the initial amplitude value, ΔS is the change, t (time), and τD is the water vapor diffusion time. The effective water diffusion coefficient (Deff) for water and polystyrene was 1.90 × 10−11 m2/s and 3.09 × 10−11 m2/s, respectively. The permeability coefficient value for the piece of paper was 4.18 × 10−9 mol kg−1 cm−2 s−1 Pa−1, while for polystyrene, it was 6.80 × 10−9 mol kg−1 cm−2 s−1 Pa−1 for 70% of relative humidity. This methodology can be extended by changing the moisture content on the chamber to obtain the dependence of Deff as a function of relative humidity.

Microstructure and barrier properties of reactive compatibilized PLA/PA11 blends investigated by positron annihilation lifetime spectroscopy

Poly(lactic acid)/polyamide11 (PLA/PA11) blends were prepared by reactive blending with multifunctional epoxy oligomer as compatibilizer, and the compatibility of PLA and PA11 was improved. The compatibilization, toughness mechanisms, and rheological behavior of PLA/PA11 blends were studied. Positron annihilation lifetime spectroscopy (PALS) was used to study the free volume and interfacial compatibility of the compatibilized blends. Moreover, the influence of free volume and void volume on the barrier properties of the blend films was discussed. The glass-transition temperature and storage modulus of PLA in the compatibilized PLA/PA11 blends decreased, and the rheological behavior in the blends changed from liquid-like to solid-like at low-frequency. This finding showed that the interfacial compatibility between PLA and PA11 improved, and the material properties underwent a rigid-to-tough transition. PALS analysis further showed the improved interfacial compatibility of the multifunctional epoxy reactive compatibilizer reduced the free-volume pores and increased the number of free-volume pores. The entanglement degree of the molecular-chain segment in the compatibilized blend system increased, and the volume of free-volume pore decreased. The reactively compatibilized PLA/PA11 blend had excellent oxygen and water barrier properties, and its oxygen transmissibility and water-vapor diffusion coefficient greatly decreased. Therefore the reactive compatible PLA/PA11 blends had potential application and research value as packaging-barrier film materials.

Water vapor diffusion properties of Obernkirchener sandstone: Analysis of DVS data

In this paper, experimental characterization by dynamic vapor sorption (DVS) experiments on Obernkirchener sandstone samples is presented, giving access to the sorption isotherm of the porous building material. The latter may be classified as sigmoidal type II with a hysteresis in the high-humidity range. As the specimens were prepared with a defined geometry (circular plate, thickness of 7 mm), the recorded mass history in response to a step-wise increase or decrease of the sample-enclosing humidity was employed to backcalculate diffusivity of the porous material. Hereby, modeling focuses on vapor diffusion in the unsaturated pore space with water vapor concentration gradients as the driving force. We disregard capillary transport in the model, which, hence, is only valid in the low-humidity range, i.e. modeling in the scopes of a porous, hygroscopic material. The vapor diffusion coefficient of the porous material is estimated as 2600 mm 2 /h (at a temperature of 25 ° C ), what relates well to the bulk diffusion coefficient of water vapor in air taking into account tortuosity and porosity of the considered sandstone. As regards the influence of surface resistance to mass transfer, the samples may be characterized as borderline thick with a Biot number of approximately 50. • Transport model taking into account instantaneous adsorption at pore walls. • Relation of macroscopic vapor diffusion coefficient to molecular diffusion coefficient. • Discussion of influence of surface resistance to mass transfer (Biot number). • Comparison with results for other sandstone lithotypes in the open literature.

Water vapor transmission parameters of the Kežmarok sandstone

Dynamic complex heat and water transport in building constructions can be calculated using modern simulation software such as Wufi, Delphin, Math, Comsol Multiphysics and other. This software is suitable for the evaluation of the thermal and hygric processes in constructions of historical buildings. Because it includes many important factors such as dynamic boundary conditions, solar radiation, driven rain, ground water, diffusion and others. The heat and water transport parameters of historical materials are needed for simulation of coupled heat and water transport in building materials. The Kežmarok sandstone was chosen for analysis. One of the important material parameters for evaluation of complex heat and water transport in building constructions are water vapor transmission parameters. Vapor transport can be described as water vapor diffusion resistance factor, water vapor diffusion coefficient and diffusion equivalent air layer thickness. The dry cup and wet cup method were used to determine the water vapor transmission parameters of Kežmarok sandstone.

Experimental and Numerical Simulation of Water Adsorption and Diffusion in Coals with Inorganic Minerals

The study on the adsorption and micropore filling of water vapor in coal is significant for predicting coalbed methane content in coal seams. The primary purpose of this study is to explain the effects of coal pore structure and its surface chemistry on water vapor monolayer adsorption, micropore filling, and diffusion coefficient. First, X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) analyzed inorganic mineral components of two kinds of coal samples and pore fissures structures. Then, we divide pores and fissures according to the theory of fractal dimensions. Furthermore, we carried out the water vapor adsorption and desorption experiments on two kinds of coal; in particular, we set 14 points of relative pressure between 0 and 0.2. Guggenheim–Anderson–de Boer (GAB), Frenkel–Halsey–Hill (FHH), and Freundlich models were used to analyze the data of water vapor adsorption to obtain the boundary pressure points of the monolayer, multilayer adsorption, and capillary condensation. Finally, the parameters of the models were obtained by fitting the adsorption data of water vapor according to the combined GAB, Freundlich, DA, and bidisperse adsorption (BDA) models to analyze the interaction mechanism between coal and water. We explain why the strongly adsorbed water minerals, such as pyrite, illite, and nacrite coal, can improve water vapor’s adsorption and diffusion capacity in coal pore fissures.

Modeling and analysis of water vapor dynamics in high-temperature proton exchange membrane fuel cell coupling gas-crossover phenomena

A 3-D numerical model coupling gas-crossover phenomena for high-temperature proton exchange membrane fuel cell (HT-PEMFC) is developed to investigate the water vapor behavior. After model validation, sensitivity analysis of the water vapor diffusion coefficient is carried out, which does not further affect the water vapor behavior, when the order of magnitude of diffusion coefficient is higher than 10−5m2/s. It is also found that the water vapor transport flux decreases with increasing membrane thickness. However, the flux increases slightly with increasing the catalyst layer. In addition, Increasing the pressure and humidity on the anode side will cause water vapor to diffuse from the anode to the cathode, while increasing the current density or the pressure of cathode, the rate of water vapor transport from the cathode to the anode is enhanced. In the dead-end mode, the accumulation of water vapor at the anode outlet is the main cause for the reversible performance decline, which can be restored through reasonable purge strategies. This work contributes to improve the water management strategy of HT-PEMFC operating in dead-end mode.

Dynamic model for the simultaneous adsorption of water vapor and methane on shales

Water is almost ubiquitous in shale gas reservoirs. To analyze simultaneous water vapor and methane adsorption in shales, a dynamic model with a time-dependent boundary and diffusivity and a noninstantaneous adsorption mechanism was proposed. The analytical solution of the model was obtained via integral transformation method. The model was applied to match the dynamic data of simultaneous CH4–H2O adsorption. The results suggest that the strength for gas adsorption to shale is characterized by adsorption rate coefficient and desorption rate coefficient, where the adsorption strength of shale to water vapor is stronger than that to methane, and the conversion rate from water vapor to adsorbed water on adsorption sites is larger. The methane and water vapor diffusion coefficients first increase to a maximum and then decrease with time. The maximum methane diffusion coefficient ranges from 5.83 × 10−13–1.08 × 10−11 m2/s; the maximum water vapor diffusion coefficient ranges from 2.06 × 10−19–4.53 × 10−13 m2/s. The methane diffusivity and its variation rate are larger than those of water vapor. The methane and water vapor diffusion coefficients first increase with the density of free gases in pores and then decrease in the late period of the adsorption process. Water vapor reduces the adsorption strength of shale to methane and free methane-to-adsorbed methane conversion rate on adsorption sites. The methane diffusion coefficient in dry shale increases with time, and its growth rate reaches zero at adsorption equilibrium, which ranges from 6.60 × 10−14–4.82 × 10−11 m2/s; the methane diffusivity increases exponentially with the methane adsorption amount and free methane density in pores.

Water vapor distribution and particle condensation growth in turbulent pipe flow

Heterogeneous condensation is a potential particle size-increasing technology that is progressively used in dust removal. In this paper, the condensation growth of fine particles under turbulent pipe flow was investigated. A numerical model was established and verified in terms of gas flow field, relative humidity, and particle size. Results showed that the particles in the center were larger than those in the near-wall area at the outlet of pipe. The mechanism for such improvement was analyzed. The relative humidity in the pipe was nonuniformly distributed, and this was because the low diffusion coefficient of the central water vapor could not effectively supplement the water vapor consumption caused by wall condensation. Furthermore, to achieve superior particle growth, three different spoilers were used to change the gas flow field and water vapor in the pipe. The results indicated that when the spoiler was in the middle, the average particle size increased and the standard deviation decreased, demonstrating an improvement for particle growth. In this case, the changed effective diffusion coefficient of water vapor in the central region can make up for the wall condensation loss without excessive transmission. Consequently, better particle growth was achieved, which was more favorable to particle removal by subsequent equipment. These findings establish theoretical guidance for the treatment of fine particles in the industrial process.

Experimental Study on Frost Crystal Morphologies and Frosting Characteristics under Different Working Pressures

Frost usually grows on the cooling surface of heat transfer equipment, which often operates under different working pressures. In such conditions, one of the fundamental influencing factors of frosting, the diffusion coefficient of water vapor around the cooling surface, is different from that in the normal atmosphere pressure. In order to investigate the pressure effect on frosting, a test rig for frosting visualization is designed. The role of working pressure around the cooling surface on the frost crystal morphologies and frost layer characteristics is evaluated under different air parameters and temperatures of cooling surface. The vertical growth and lateral distribution of frost crystals are strongly related to working pressure around the cooling surface. More frost crystal morphologies and faster growth rate of frost layer are presented at a lower pressure. The frost thickness, frost accumulation and density reduce with higher pressure. The dimensionless correlation of frost thickness is proposed considering working pressure around the cooling surface, humid air parameters, and temperature of cooling surface.

Sorption isotherms and moisture transfer properties of seven Central Africa hardwood species

ABSTRACT The aim of this article is to characterize the wood-water relationship and certain moisture transfer properties of Central African species for which very little data exists in the literature, i.e. Iroko, Padouk, Bibolo (Dibetou), Moabi, Doussié, Bilinga and Grenadilla. Moisture sorption isotherms were measured using the Dynamic Vapor Sorption method at 35°C. The Hailwood-Horrobin model was used to describe the sorption isotherm and the model parameters are given for each species. A rough estimation of the fiber saturation point could be made and is between 18.5% for Doussié and 26.5% for Bibolo. The water vapour diffusion coefficient in the radial direction of the wood was determined by the cup method at 35°C. Its value ranges from 1.72E-11 for Grenadilla to 1.71E-10 m2/s for Bibolo. It has been shown that the model proposed by Siau (1984) allows a relatively reliable estimation of the value of this parameter from wood density. The gas permeability was also measured on the same samples and ranged from 3.3E-19 m2 for Moabi to 3.78E-16 m2 for Bibolo. From these parameters, the apparent tortuosity and equivalent pore diameter for the mass flow were estimated for each species.