Indirect assessment of hydraulic diffusivity and permeability for unsaturated cement-based material from sorptivity

The hydraulic diffusivity, water permeability and relative gas permeability for cement-based materials are indirectly evaluated from measured sorptivity and water vapor sorption isotherms (WVSIs). The dependence of sorptivity on initial saturation degree is first established to help calculate hydraulic diffusivity and other transport properties. An experimental program with a self-scaled preconditioning strategy is also carefully designed and conducted on three concretes to measure their sorptivity, WVSIs as well as permeability to various fluids. It's found that hydraulic diffusivity of ambiguous physical significance may be not a good durability indicator. The predicted water permeability is larger than measured value but at the same order of magnitude. This overestimation is attributed to the required drying preconditioning. The predicted relative water permeability agrees well with reported data. However, the predicted relative gas permeability agrees with the measured data from classical CEMBUREAU method better than that from tri-axial permeameter with higher inlet gas pressure.     

Three-dimensional CFD modelling of PEM fuel cells: An investigation into the effects of water flooding

In this work, a three-dimensional PEM fuel cell model has been developed and is used to investigate the effects of water flooding on cell performance parameters. The presence of liquid water in the cathode gas diffusion layer (GDL) limits the flow of reactants to the cathode catalyst layer, thereby reducing the overall reaction rate and curtailing the maximum power that can be derived from the cell. To characterize the effects of water flooding on gas diffusion, effective diffusivity models that account for the tortuosity and relative water saturation of the porous fuel cell electrodes have been derived from percolation theory and coupled with the CFD model within a single phase flow skeleton. The governing equations of the overall three-dimensional PEM fuel cell model, which are a representative of the coupled CFD and percolation theory based effective diffusivity models, are then solved using the finite volume method. Parametric studies have been conducted to characterize the effects of GDL permeability, inlet humidity and diffusivity of the reactants on the various cell performance parameters such as concentration of reactants/products and cell current densities. It is determined that the GDL permeability has little or no effect on the current densities due to the diffusion dominated nature of the gas flow. However, through the incorporation of percolation theory based effective diffusivity model; a marked reduction in the cell performance is observed which closely resembles published experimental observations. This is a reasonable approximation for effects of water flooding which has been inherently used for further parametric studies.     

General solution of hydraulic diffusivity from sorptivity test

A general approach to solving hydraulic diffusivity from sorptivity test is established and verified in this paper. The diffusion equation governing capillary water absorption is first converted into normalized ordinary differential and integral forms via Boltzmann transformation, which are then directly solved by the method of weighted residuals. By this method, the approximate solution of Boltzmann variable is determined for any distribution law of diffusivity. The relationship between sorptivity and diffusivity is further analytically established. It's found that initial diffusivity is proportional to square of the ratio of sorptivity to the water content difference between saturated and initial states. Ignoring the water vapor diffusion leads to the underestimation of derived water content profile and diffusivity. The Boltzmann variable and diffusivity calculated by the proposed method are verified by experimental data. Finally, the relationship between coefficients of general solution for diffusion equation and shape parameter for exponential diffusivity is also derived.     

Fundamental salt and water transport properties in directly copolymerized disulfonated poly(arylene ether sulfone) random copolymers

Water and sodium chloride solubility, diffusivity and permeability in disulfonated poly(arylene ether sulfone) (BPS) copolymers were measured for both acid and salt form samples at sulfonation levels from 20 to 40 mol percent. The hydrophilicity of these materials, based on water uptake, increased significantly as sulfonation level increased. The water permeability of BPS materials in both the salt and acid forms increases more than one order of magnitude as sulfonation level increases from 20% to 40%, while NaCl permeability increases by two orders of magnitude. The water and salt diffusivity and permeability were correlated with water uptake, consistent with expectations from free volume theory. In addition, a tradeoff was observed between water/salt solubility, diffusivity, and permeability selectivity and water solubility, diffusivity and permeability, respectively. This finding suggests a water/salt permeability/selectivity tradeoff, similar to that operative in gas separation polymers, in this family of polymers.     

PEMFC带沟槽气体扩散层内传输特性孔隙网络模拟

采用三维孔隙网络模型计算了不同沟槽参数下气体扩散层(GDL)的液态水突破压力、毛细压力分布、气体扩散率和液相相对渗透率随饱和度变化,并从孔隙尺度角度探究了沟槽的作用机制。研究结果表明:沟槽改变了GDL的毛细压力分布,提供了液态水直接传输路径并优化了GDL内氧气和液态水的分布,从而提高了氧气有效扩散率。沟槽位置对氧气传输有明显影响,对液相传输的影响取决于是否形成贯穿GDL的传输路径;沟槽加深,氧气和液态水传输性能增强,沟槽穿透GDL时传输性能达到最佳;沟槽变宽,液相传输性能增强,氧气传输性能在低饱和度范围内先增强后减弱。综合各因素,给出了氧气和液态水传输性能最优时的沟槽参数。     

Gas permeability and electrical conductivity of structural concretes: Impact of pore structure and pore saturation

This paper investigates the gas permeability and the electrical conductivity of structural concretes under different pore saturations. The gas permeability was measured by CemBureau device and the electrical conductivity by alternating current method. The pore structure was characterized by mercury intrusion porosimetry (MIP) and gravimetric methods. The Archie's law is used to interpret the tortuosity of the pore structure. The impact of pore saturation is evaluated through the Van Genuchten-Mualem (VGM) and Kozeny's models. The results show that (1) the global correlation between the gas permeability in dried state and the electrical conductivity in saturated sate is weak; (2) both VGM and Kozeny's models can describe the relative permeability/conductivity, but the VGM model gives more consistent exponents for permeability and conductivity; (3) as the pore gas and liquid phases are both percolated, the gas permeability is correlated to the electrical conductivity for arbitrary pore saturation.     

A One-Dimensional Model of Sedimentation Using Darcy's Law

Abstract

Darcy's law together with the mass conservation equation is used to predict concentration profiles in a one-dimensional sedimenting column. Analytic solutions are obtained for two special cases of the diffusivity and hydraulic conductivity. For derived physical parameters the theoretical predictions are compared against experimental results from three sedimenting columns, each of different height. The predictions compare favorably with observations, indicating that this macroscopic approach of Darcy's law should be further developed, both numerically and into more than one spatial dimension.     

Prediction of the hydraulic diffusivity from pore size distribution of concrete

A model is presented in this work through which variation of hydraulic diffusivity of concrete with relative water content can be obtained from pore size distribution as an input. The specific water capacity and hydraulic conductivity of concrete are expressed in terms of pore size characteristics, considering laminar flow due to capillary suction through tortuous elliptic tubes, oriented equally in three orthogonal directions. Hydraulic diffusivity being the ratio of hydraulic conductivity and specific water capacity is thus expressed in terms of pore size characteristics. The input pore size distributions have been determined experimentally for normal strength concrete mixes through mercury intrusion porosimetry. Using the model the variation of hydraulic diffusivity with relative water content is determined for three cases viz. 1) ideal continuous wetting, 2) ideal continuous drying and 3) random access of pores by water. These results are then compared with an experimentally obtained variation.     

A model to predict the solubility and permeability of gaseous penetrant in the glassy polymeric membrane at high pressure

In this work, the transport properties of gaseous penetrant through several dense glassy polymeric membranes are studied. The nonequilibrium lattice fluid (NELF) in conjunction with the modified Fick's law and dual mode sorption model was used to simulate the gas transport in glassy polymeric membranes. The approach is based on the sorption, diffusion, in which solubility is calculated based on the NELF model, and diffusion coefficient is obtained from the product thermodynamic coefficient and molecular mobility. The governing equation is solved by the finite element method using COMSOL multi-physics software. The developed model for gas permeability of glassy polymeric membrane can be applied in a wide range of pressure and temperature. The comparison of the calculated permeability and solubility of gasses with the experimental data represented the ability of the developed model. Increasing feed gas temperature increases the gas permeability, while this variation leads to lower gas solubility in the glassy polymeric membranes. The effect of feed temperature and pressure on permeability and solubility is investigated, and the experimental data from literature are described by the developed model. A good prediction of the experimental data can be observed over the considered condition.     

Effect of water on permeation,diffusion, and solution of gases in cellophane

The laminate method for studying the permeability and diffusivity of moistened cellophane to gases is described and the humidity dependence of the transport parameters for H₂, He, and Ne is presented. In the relative humidity region of about 0% to 60%, a small increase in the permeability was observed, which is caused by a comparatively small increase in the diffusivity owing to the plasticizing effect of sorbed water and a decrease in the solubility. On the other hand, an extremely large increase in the permeability observed in the relative humidity region above 60% is mainly based on the diffusion coefficient of gas enhanced by the swelling effect of sorbed water. The presence of a minimum in the solubility–relative humidity curves has been confirmed and is discussed.     

Water retention and gas relative permeability of two industrial concretes

This experimental study aims at identifying the water retention properties of two industrial concretes to be used for long term underground nuclear waste storage structures. Together with water retention, gas transfer properties are identified at varying water saturation level, i.e. relative gas permeability is assessed directly as a function of water saturation level S_w. The influence of the initial de-sorption path and of the subsequent re-saturation are analysed both in terms of water retention and gas transfer properties. Also, the influence of concrete microstructure upon water retention and relative gas permeability is assessed, using porosity measurements, analysis of the BET theory from water retention properties, and MIP. Finally, a single relative gas permeability curve is proposed for each concrete, based on Van Genuchten–Mualem's statistical model, to be used for continuous modelling approaches of concrete structures, both during drying and imbibition.     

Water and methane in shale rocks: Flow pattern effects on fluid transport and pore structure

Using molecular dynamics simulations, the two‐phase flow of water and methane through slit‐shaped nanopores carved from muscovite is studied. The simulations are designed to investigate the effect of flow patterns on fluids transport and on pore structure. The results indicate that the Darcy's law, which describes a linear relation between flow rate and pressure drop, can be violated when the flow pattern is altered. This can happen when the driving force, that is, the pressure drop, increases above a pore‐size dependent threshold. Because the system considered here contains two phases, when the fluid structure changes, the movement of methane with respect to that of water changes, leading to the violation of the Darcy's law. Our results illustrate the importance of the capillary force, due to the formation of water bridges across the model pores, not only on the fluid flow, but also on the pore structure, in particular its width. When the water bridges are broken, perhaps because of fast fluid flow, the capillary force vanishes leading to significant pore expansion. Because muscovite is a model for illite, a clay often found in shale rocks, these results advance our understanding regarding the mechanism of water and gas transport in tight shale gas formations. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2993–2999, 2015     

Pore formation,permeability, and permselectivity in porogen derived membranes

Porogen derived membranes, formed on decomposition of porogen molecules homogeneously dispersed in premembranes, were divided into two groups according to permeability and permeation mechanism. The more open membranes were formed when premembranes were cast on nonwoven polyester cloth support, or when decomposition products were extracted by solvent. The molecular cut off found for these membranes was at about 1000 Daltons and permeability was mainly determined by permeants' diffusivity and concentration. Permeability through the denser membrane was mainly determined by the permeants' partial vapor pressure, leading to high permselectivities. These denser membranes allowed relatively high osmotic water permeation while practically blocking permeation of solutes, such as salt, in the opposite direction. These membranes were thus also suitable for concentration of aqueous solutions by water removal into concentrated electrolyte solutions. © John Wiley & Sons, Inc.     

Liquid permeability of packed bed with binary mixture of particles

We investigate the effect of binary sized packing on the permeability of water flow through a column packed with binary mixture of spherical particles. The size ratios λ of large to small particles are chosen to be 1.4 and 2.5. Particle packing density for binary mixture of the particles is larger than that for equal sized particles and shows the maximum around the particle blending ratio at which large particles are densely packed and all the small particles fill the void among the large particles. This behavior is observed in both experimental results and theoretical estimation. The variation pattern of packing density with the blending ratio does not agree with that of permeability. The permeability increases with relative fraction of large particles at the maximum packing. Experimental results for the permeability are compared with three theoretical models. Variation pattern of the permeability with the blending ratio from these theoretical models agrees with that from the experiment. Theses theoretical models are in good agreement each other.     

Sorption in zeolites—II. Tracer diffusion

The objective of the paper is to investigate the relationship between tracer diffusivity and intrinsic diffusivity for sorption in zeolites. It is argued that this relationship cannot be derived from irreversible thermodynamics. Dependence of tracer diffusivity on sorbate concentration for Langmuirian sorbed phase is investigated using irreversible thermodynamics. The analysis indicates that the reported relationship between tracer diffusivity and intrinsic diffusivity derived from molecular models are consistent with irreversible thermodynamics.     

Sorption and permeation properties of water and ethanol vapors in poly[bis(trifluoroethoxy)phosphazene] (PTFEP) membranes

The permeability and solubility for water and ethanol in PTFEP membranes were determined experimentally, and the data were analyzed by the solution-diffusion model. The permeability for water and ethanol ranged from several hundreds to several thousands Barrers, and they increased exponentially with the vapor activity and increased with temperature. At the same temperature and vapor activity, the permeability ratio between water and ethanol ranged from 5.7 to 2.3, and it decreased as the vapor activity increased. The sorption isotherms for water and ethanol were fitted by the Henry’s law relationship. The solubility decreased as the temperature increased so that the heat of sorption for both water and ethanol was negative. The solubility for water was more than twice the solubility for ethanol. The solubility seems to be inversely proportional to the molecular size of the penetrants in such a system. The solubility ratio between water and ethanol is smaller than their molecular volume ratio possibly due to the slightly stronger nonpolar interaction and the higher degree of plasticization in the ethanol-polymer system. The diffusivity for water and ethanol ranged from 10⁻⁸ to 10⁻⁷ cm²/s, and the values for water were larger than those for ethanol at the same temperature and vapor activity. The diffusivity for water and ethanol also increased exponentially with the vapor activity. The diffusivities for water and ethanol increased with temperature and their activation energies of diffusion were very similar possibly due to the same energy characteristic of polymer main chain movement.     

Determination of permeability and diffusivity of oxygen in polymers by polarographic method with inert gas

A modified polarographic method with inert gas for determination of the oxygen permeability in polymers immersed in liquids is described. Owing to the stream of an inert gas towards polymer from the cathode side, lateral oxygen diffusion (edge effects) is minimized. Unlike the standard Fatt method, the method with inert gas is suitable also for thick samples and, therefore, for high-permeable materials. The method was tested for prediction of oxygen permeability in poly(1-vinyl 2-pyrrolidone) (PVP) and poly(2-hydroxyethyl methacrylate) (PHEMA). As an electrolyte, solution of potassium chloride was used. The effect of additional resistances and small lateral diffusion was taken into account. Unexpectedly, oxygen permeability in both polymers was greater for 0.1 M KCl than for 0.5 M KCl. The experimental setup was also used for diffusivity estimation in thick samples of PHEMA and PVP. Here, the oxygen flux response at one sample surface to the stepwise change in oxygen concentration at the other surface is measured and evaluated. The effect of the additional boundary layer on the oxygen transport is taken into account. A simple procedure for the diffusivity determination from the characteristic time of response as a function of the sample thickness is given. Solubility of oxygen in polymer is calculated from the obtained permeability and diffusivity.     

The hydrodynamics of trickling flow in packed beds operating at high pressures. The relative permeability concept

One of the largest experimental databases of measured pressure drops and liquid holdups in high-pressure trickle-bed reactors is presented in order to evaluate the currently existing models for the prediction of hydrodynamic parameters of cocurrent two-phase flow through packed beds. Our findings support the conclusions of Carbonell (Oil & Gas Science and Technology—Revue de l'IFP 55 (2000) 417) based on theoretical analysis of existing models that only the relative permeability model and the fluid–fluid interaction model are based on solid hydrodynamic principles, which are able to predict the hydrodynamic parameters within the experimental error. Special emphasis has been given to the relative permeability model to demonstrate its practicalness in describing the complex phenomena existing within the two-phase flow through porous media.     

Effect of uniaxial compressive loading on gas permeability and chloride diffusion coefficient of concrete and their relationship

Knowledge of the transport properties of damaged concrete in marine environments is essential for predicting its durability. The objective of this study was to fill this gap by correlating the change in permeability and chloride diffusivity with an increasing uniaxial load on ordinary concrete (OC) and high performance concrete (HPC). Concrete cylinders were induced microcracks by mechanical uniaxial compression between 60% and 90% of the ultimate strength to get diffuse damage. The damage variable of specimens was evaluated by elastic stiffness degradation and ultrasound pulse velocity. After unloading intrinsic gas permeability was measured using a constant head permeameter, the chloride migration coefficient was evaluated by migration test in steady state conditions, with the same concrete specimen. The damage variable of specimens showed relationship with gas permeability and chloride diffusion of concrete in this experiment. A correlation was obtained between intrinsic permeability coefficient and chloride diffusion coefficient depending on the damage variable, specific for each concrete type (OC and HPC).     

A Two‐dimensional Network Study on Oxygen Transport in Porous Gas Diffusion Layer

Oxygen transport in the porous gas diffusion layer (GDL), which is generally characterised by the oxygen effective diffusivity, is of great importance for the performance of proton exchange membrane fuel cells (PEMFCs). The determination of the oxygen effective diffusivity is challenging due to the complex structure of the porous GDL samples. In the present study, a two‐dimensional network consisting of arms and nodes is adopted to illustrate how oxygen effective diffusivity is affected by the GDL structure under the condition with/without water invasion. Water permeation in the network is simulated using the invasion percolation algorithm and oxygen transport in the arms is described by Fick's law. The simulation results reveal that oxygen effective diffusivity under dry condition decreases with increase in the network heterogeneity. With water permeation, the oxygen effective diffusivity goes to zero even though water saturation is rather less than unity. The critical water saturation, above which the oxygen effective diffusivity becomes zero, is found to decrease with increasing heterogeneity. To enhance oxygen transport, four different modified networks are introduced in the present study. It is found that the network with large arms in oxygen transport direction has the best oxygen and water transport properties.