Isothermal Moisture Transport in Partially Saturated Porous Media

The prediction of a hydrodynamic model for the isothermal transport of liquid in partially saturated porous media is compared with experimentally obtained values of water transport in clay. The transport obeys the diffusion equation, The diffusion coefficient is described as a function of porosity, permeability and pore size distribution. The comparison indicates that the model needs some refinement.     

A Possible Revision of the Results of a Model for Moisture Transport in Partially Saturated Porous Media

ABSTRACT

It is shown that the discrepancy between the prediction of the model of van der Zanden et al. for the diffuslion coefficient of liquid in porous materials and experiments can be explained with inaccuracies in the measurement of the pore size distribution or small cracks in the drying porous material.     

Isothermal Vapour and Liquid Transport Inside Clay During Drying

A model is presented to describe the moisture transport inside a partially saturated porous material. The transport is caused by vapour diffusion and liquid diffusion. The evaporation inside the porous material is described with a mass transfer coefficient and a specific evaporating surface. Predictions of the model for moisture profiles are compared to experimentally obtained profiles found in the literature. The model needs further extension in the form of incorporating sorption isotherms.     

Isothermal Vapour and Liquid Transport Inside Clay During Drying

A model is presented to describe the moisture transport inside a partially saturated porous material. The transport is caused by vapour diffusion and liquid diffusion. The evaporation inside the porous material is described with a mass transfer coefficient and a specific evaporating surface. Predictions of the model for moisture profiles are compared to experimentally oblained profiles found in the literature. The model needs further extension in the form of incorporating sorption isotherms.     

Isothermal Vapour and Liquid Transport Inside Clay During Drying

ABSTRACT

A model is presented to describe the moisture transport inside a partially saturated porous material. The transport is caused by vapour diffusion and liquid diffusion. The evaporation inside the porous material is described with a mass transfer coefficient and a specific evaporating surface. Predictions of the model for moisture profiles are compared to experimentally oblained profiles found in the literature. The model needs further extension in the form of incorporating sorption isotherms.     

Isothermal Vapour and Liquid Transport Inside Clay During Drying

ABSTRACT

A model is presented to describe the moisture transport inside a partially saturated porous material. The transport is caused by vapour diffusion and liquid diffusion. The evaporation inside the porous material is described with a mass transfer coefficient and a specific evaporating surface. Predictions of the model for moisture profiles are compared to experimentally obtained profiles found in the literature. The model needs further extension in the form of incorporating sorption isotherms.     

Simulation of Solute Diffusion through Porous Media

Abstract

Simulation of solute diffusion through porous media (membrane) was carried out by a random walk procedure. The porous media used were three kinds of two-dimensional square networks of channels with almost the same average pore diameters and porosities, but with different pore size distributions. In the simulation the partition equilibrium of solute between the bulk feed phase and the membrane phase was established, and the apparent permeability in the steady state was evaluated. First, only the steric interaction between the solute and the pore wall was considered. In spite of the similar average pore diameters and porosities of the networks, the apparent permeability and selectivity were dependent on the kinds of networks. The network with more small pores showed the smaller permeability and the higher selectivity. When the diameter of solute which can be actually transported is fairly smaller than the average pore diameter, the network with broad pore distribution and with more small pores is found to be useful for obtaining higher selectivity. Next, the electrostatic and dispersion (van der Waals) interactions between the solute and the pore wall were introduced in this simulation. By such long-range interactions the selectivity increased while the apparent permeability decreased. This result showed that the introduction of the repulsive electrostatic interaction between membrane and solute is one useful method for enhancing the selectivity. The effects of the kinds of networks on transport properties in the presence of long-range interactions were similar to those in the absence of such interactions.     

Procedure for setting up the drying regime that is consistent with the nature and properties of the clay raw material

Over the past three decades, traditional ceramic facilities, such as chamber and tunnel dryers, are improved. Better thermotechnical equipment, operational strategies, and reliable scale-up methodologies have lead to higher quality of the dried clay roofing tiles. Although there has been a progress, up to this study, there is no universally or even widely applicable criterion, which could be used to precisely define the change of drying air parameters (humidity, temperature, and velocity) during the drying process. The objective of this study was to specify the variable air parameters that should be used during the drying process to approach as much as possible to the theoretically defined optimal drying process.     

多孔介质中的分数扩散方程

多孔介质复杂的几何结构可看作是随机分形。分形空间中的扩散与欧几里德空间中的扩散不同,为反常扩散,扩散方程不能用普通微分方程准确描述,需用分数微分方程。从微观和宏观相结合的角度讨论了三个比较有影响的分形介质中的扩散方程。     

Moisture Profiles Determination During Convective Drying Using X‐Ray Microtomography

The knowledge of internal moisture profiles which develop during drying is essential for model validation purposes, but they are difficult to determine experimentally. This paper shows that X‐ray microtomography, together with image analysis, provides an accurate, non destructive and easy to use technique to determine moisture profiles. Results reported concern the drying of wastewater sludges whose management is becoming a real environmental challenge. An analysis of the development of moisture gradients at the sample external wall shows an influence of drying operating conditions. Finally, mass diffusion coefficients are estimated from the knowledge of both the moisture gradients and the drying flux.     

Physicochemical Factors Influencing Colloidal Particle Transport in Porous Media

Abstract

The mobilization/immobilization of colloidal-sized particles which have high surface areas per unit mass is an important process occurring in groundwater flow systems. Association of contaminants with mobile colloidal particles may enhance the transport of adsorbed pollutants, or deposition of colloidal particles in porous media may decrease permeability and reduce contaminant transport. The general objective of this work was to elucidate physical and chemical factors affecting colloidal particle (Brownian and non-Brownian) transport in porous media under typical groundwater flow velocities. The most critical chemical factor influencing Brownian particle (0.1 and 1.0 μm) transport in a packed column was found to be pH. The next most critical factor was electrolyte concentration (calcium ion and sodium ion concentration). Gravitational force was found to be an important factor for non-Brownian particle (10 μm) transport. The non-Brownian particle transport was observed to be independent of solution chemistry. Increases in superficial velocity (from 0.9 to 2.7 m/day) resulted in different types of behavior for Brownian and non-Brownian particle transport under different conditions. The Brownian particle throughputs at a superficial velocity of 0.9 m/day were mainly controlled by the surface interaction forces, that is, hydrodynamic action was not important. The difference in Brownian diffusivity between 0.1 and 1.0 μm particles caused opposite results in particle throughputs in all experimental columns regardless of solution chemistries. Particles of 0.1 μm produced the maximum transport in the column filled with the smallest glass beads, while 1.0 μm particles produced the maximum transport in the column packed with the largest glass beads.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

Modeling of Thermo-Hydro-Viscoelastic Behavior of a Partially Saturated Ceramic Material During Drying

A mathematical model to simulate the drying of a partially saturated ceramic material has been developed. The investigated problem involves coupling equations considering heat, mass, and mechanical aspects. Transport in unsaturated porous medium includes two mean mechanisms: convection for liquid and gases, and molecular diffusion in the gas mixture.

Viscoelastic rheology is applied by using the generalized Maxwell model in which the relaxation modulus depends on time and moisture content. The numerical resolution is performed to foresee the variation of moisture content, shrinkage, liquid pressure, gas pressure, temperature, and mechanical stresses during drying. The model was validated partially for porcelain material in classical convective drying. Simulation results showed the importance of using a viscoelastic model with two dependent variables in evaluating the developed mechanical stresses. The Von Mises criterion is used as a crack initiation criterion, which allows localizing the area of risk of the material's fracture.     

Mass Transport Through PDMS/Clay Nanocomposite Membranes

Poly(dimethylsiloxane)/clay nanocomposite membranes have been synthesized and mass transport properties through those nanocomposite membranes have been investigated. The effect of mechanical deformation on the transport properties of the PDMS (nanocomposite) membranes has also been studied. With the introduction of clay particles into the polymer matrix, mass transport is reduced, likely due to the longer diffusion path, which slows the diffusion process. The effect of membrane extension on diffusion is more complicated. Under small deformation, the permeation flux decreases, but under high deformation, it shows an enhanced diffusion. As the clay particle concentration increased, the effect of external deformation is reduced, and an enhanced diffusion is observed.     

多孔介质内部传热传质规律的研究进展

多孔介质中的热质交换理论及其实验研究是一个涉及面广、研究难度大而又颇具工程应用价值的课题。本文对多孔介质内部传热传质规律的研究从理论和实验两方面作了简要的综述及展望,并对一些描述热质传递过程的数学模型及方程作了介绍。     

Investigation on Moisture Transfer Mechanism in Porous Media During Rapid Drying Process

Abstract

In this work, moisture transfer mechanism in wet porous media during rapid drying process is investigated experimentally and analytically. By use of scanning electron microscopic device, the rapid drying processes for potato, carrot, and radish species were observed and recorded. The microscopic drying experiments show that during high intense and rapid drying process, the mechanism of moisture migration in materials is mainly considered as a displacement flow driven by pressure gradient along a capillary passage. A simplified displacement flow model during rapid drying process is proposed and the time needed for moisture transfer in porous media is calculated. To examine this drying mechanism, one-dimensional displacement flow test device is built up and a set of experiments under different pressure gradients and temperatures are conducted. Glass beads of 0.8 mm in diameter are used as the porous material. The experimental results show that when pressure gradient is getting greater at constant temperature, the moisture removal time is getting smaller. On the other hand, under the same pressure gradient, when liquid temperature increases, the time for moisture transfer from the internal to the external surface decreases. The calculated moisture removal times are well agreed with the experimental data.     

Investigation on Moisture Transfer Mechanism in Porous Media During Rapid Drying Process

In this work, moisture transfer mechanism in wet porous media during rapid drying process is investigated experimentally and analytically. By use of scanning electron microscopic device, the rapid drying processes for potato, carrot, and radish species were observed and recorded. The microscopic drying experiments show that during high intense and rapid drying process, the mechanism of moisture migration in materials is mainly considered as a displacement flow driven by pressure gradient along a capillary passage. A simplified displacement flow model during rapid drying process is proposed and the time needed for moisture transfer in porous media is calculated. To examine this drying mechanism, one-dimensional displacement flow test device is built up and a set of experiments under different pressure gradients and temperatures are conducted. Glass beads of 0.8 mm in diameter are used as the porous material. The experimental results show that when pressure gradient is getting greater at constant temperature, the moisture removal time is getting smaller. On the other hand, under the same pressure gradient, when liquid temperature increases, the time for moisture transfer from the internal to the external surface decreases. The calculated moisture removal times are well agreed with the experimental data.     

非水溶性超分散纳米催化剂在多孔介质中的运移机理研究

随着原位改质热采稠油工艺的不断发展,水溶性超分散纳米催化剂作为丸粒状催化剂的替代者,具有良好的应用前景。为评价注入的纳米催化剂在地层中的效用,需要研究其在地层内的运移机理。以单球型聚集器作为地层(即多孔介质)的假想模型,定义并计算聚集效率η以定量分析各因素对粒子运移的影响,从五个方面对影响纳米粒子在聚集器内的因素进行了综述分析。研究发现,捕获作用、水动力作用对粒子运移的影响较大。此外,注入纳米粒子浓度较高时,变形作用的影响也应予以关注。