Determination of the effects of the pore size distribution and pore connectivity distribution on the pore tortuosity and diffusive transport in model porous networks

An efficient computation method to study flow and transport process of small molecules in porous media using a dual site-bond lattice model, DBSM, is described. The microscopic properties of the porous network take into account the influence of local heterogeneities during the simulations. The numerical experiments demonstrated the combined effect of pore size distribution and connectivity distribution on the mass transport properties and the structural tortuosity. The results indicate that the pore size distribution and percolation phenomena related with pore shielding effects, influence significantly the tortuosity and the effective diffusivity of the porous network. Also, the simulations raise the important role of the connectivity distribution among the various pores in the gas diffusive properties of the poorly connected networks.     

FLOW THROUGH A HIGHLY POROUS ANISOTROPIC MULTIFILAMENT KNIT

Experimental results for isothermal Newtonian flow through a high porosity anisotropic porous medium are provided. The porous medium is formed by a knitted stainless steel wire mesh rolled up to form a cylindrical plug. This kind of porous medium is generally used as a mist eliminator in the chemical and petroleum industries. Experimental results are compared with pressure drop predictions obtained with a theoretical pore-scale method. The model allows for anisotropy and is applied predictively over both the Darcy and non-Darcy regimes, requiring only that the porosity, dimensions of the wire and characteristic lengths of the plug be known. The predictions compare reasonably well with the experimental results.     

APPLICATION OF THE PRECONDITIONED CONJUGATE GRADIENTS METHOD IN THE SIMULATION OF RELATIVE PERMEABILITY PROPERTIES OF POROUS MEDIA

A model that is after the determination of the relative permeability characteristics of porous media is presented. It is part of a general model that deals with the simulation of capillary phenomena and immiscible fluid flow behaviour in porous media. The relative permeability characteristics in a water wet porous medium are simulated with the use of three-dimensional (3-D) network models of pore structure with pore body and pore throat size distributions. The major assumption involved is that a cubic network of pore bodies connected by pore throats with pore body and pore throat size distribution respectively, is a realistic representation of the pore structure of a porous medium. The physical laws that apply in real media are simulated in the network analysis, and the computed results are compared with experimental findings. A new algorithm was developed that leads to the solution of a large set of linear equations, with a sparse and positive definite coefficient matrix. Results obtained with the application of the Preconditioned Conjugate Gradients method and numerical aspects of the simulation are presented and discussed. Comparison of this method with other available numerical methods is also made. It is concluded that the Preconditioned Conjugate Gradients method is advantageous for large networks with regards to time of solution, convergence, and accuracy. The validity of the algorithm is tested against other methods in the literature.     

ONE- AND TWO-PHASE FLOW IN NETWORK MODELS OF POROUS MEDIA

We discuss the low Reynolds number flow of one or two immiscible Newtonian fluids in network models of microscopically random porous media. For the case of a single fluid, we reduce the flow problem to an analog random electrical resistor problem and use an 'effective medium theory' to express the permeability of such networks in terms of the pore space geometry. For the flow of two fluids we use the Washburn approximation to incorporate capillary pressure differences, and show that this problem may also be formulated as a random electrical network. In this case, the capillary menisci correspond to moving batteries, and we follow the motion of the fluid-fluid interface (the ensemble of analog batteries) by a time-step procedure. We study the time evolution of the interface and the dynamics of blobs of one fluid contained in the other, as a function of the network geometry.’     

ONE- AND TWO-PHASE FLOW IN NETWORK MODELS OF POROUS MEDIA

We discuss the low Reynolds number flow of one or two immiscible Newtonian fluids in network models of microscopically random porous media. For the case of a single fluid, we reduce the flow problem to an analog random electrical resistor problem and use an 'effective medium theory' to express the permeability of such networks in terms of the pore space geometry. For the flow of two fluids we use the Washburn approximation to incorporate capillary pressure differences, and show that this problem may also be formulated as a random electrical network. In this case, the capillary menisci correspond to moving batteries, and we follow the motion of the fluid-fluid interface (the ensemble of analog batteries) by a time-step procedure. We study the time evolution of the interface and the dynamics of blobs of one fluid contained in the other, as a function of the network geometry.'     

An experimental study on fluid flow characteristics of superposed porous and fluid layers

In the present study, fluid flow characteristics of a porous layer overlaid by a fluid layer were investigated through experiments. The experimental results were analyzed in comparison with theoretical results of a porous medium bounded by impermeable walls. With spheres, the slip coefficient was found to be 0.0107 for Poiseuille flow over a porous layer. As the permeability decreased, the experimental results approached the values calculated by Darcy’s law and Forchheimer’s equation. In addition, the effects of the presence of a fluid layer over a porous medium were examined in terms of the friction factor. The present experimental data placed in the range of the Darcy to the non-Darcy region are shown to be in reasonable agreement with the proposed correlation.     

气体通过颗粒层突破压力和达西渗透率的关系

对空气通过窄筛分球型颗粒层和窄筛分沙粒在水中自然沉积层孔隙率和突破压力进行实验,发现随着多孔颗粒层内的流通通道的微细化,即使在常压下,气流也会出现速度滑移的稀薄性效应,并且颗粒越小速度滑移的影响越显著,使得实验测量结果不再与平均粒径(或平均孔隙直径)的平方成正比,而受制于Kn_p数的大小.给出了考虑这种速度滑移影响的数据综合方法.     

Characterisation of porous solids by simplified gas relative permeability measurements

A novel design for the application of a simplified experimental procedure, capable of providing satisfactory relative permeability data for porous media at low relative pressures, is presented. The technique avoids the two-component mixing complications as well as the problems arising from the sample macroscopic inhomogeneity. The latter has been reduced with the aid of a variable pressure compaction procedure. Thus, it becomes easier to utilise the powerful relative permeability technique for the characterisation of the structure of porous solids. Emphasis is given to the combination of the relative permeability data obtained with an analytical approach based on Effective Medium Theory for the calculation of pore connectivity.     

吸胶膜对复合材料构件固化过程中树脂流动行为的影响

热固性树脂基复合材料构件成型过程中的树脂流动行为决定了其最终成型后的纤维体积分布情况,严重影响构件的最终成型质量。而吸胶膜作为预浸料中的多余树脂流出构件后的吸收载体,其材料特性对树脂的流动过程起主导作用。以吸胶膜为关注重点,基于AS4/3501-6复合材料体系,建立了厚截面复合材料构件在热压罐中固化压实的仿真模型,模型包括了固化反应、热传导和紧密压实三个模块。模型与实验中构件的压实情况对比,验证模型的可行性。通过模型计算结果研究了吸胶膜的孔隙率、渗透率及厚度等关键参数在压力、温度等工艺条件的协同作用下对树脂流动行为的影响规律。结果表明:吸胶膜的孔隙率和厚度作为吸收树脂载体的空间度量,对构件的最终压实厚度起主要作用;吸胶膜的渗透率通过树脂流动速度影响固化压实过程,渗透率过小时,构件的压实厚度明显减小。     

Multiscale modeling of ion transport in porous electrodes

Ion transport through nanoporous materials is of fundamental importance for the design and development of filtration membranes, electrocatalysts, and electrochemical devices. Recent experiments have shown that ion transport across porous materials is substantially different from that in individual pores. Here, we report a new theoretical framework for ion transport in porous materials by combining molecular dynamics (MD) simulations at nanopore levels with the effective medium approximation to include pore network properties. The ion transport is enhanced with the combination of strong confinement and dominating surface properties at the nanoscale. We find that the overlap of electric double layers and ion–water interaction have significant effects on the ionic distribution, flux, and conductance of electrolytes. We further evaluate the gap between individual nanopores and complex pore networks, focusing on pore size distribution and pore connectivity. This article highlights unique mechanisms of ion transport in porous materials important for practical applications.     

Scale-up of pore-level relative permeability from micro- to macro-scale

Scaling up relative permeability curves of wetting and nonwetting phase of drainage and imbibition processes from pore scale to macro scale is a challenge. A new method for scaling up relative permeability from micro- to macro-scale is proposed based on electrical analogy of multiphase fluid flow at pore scale. The method is validated against four synthetic porous media generated using homogeneous and heterogeneous grain size distributions, each of which were cut into eight sub-segments. Single-phase and two-phase flow properties were calculated for the main blocks and the subsequent sub-segments using random network modelling technique. Then, the subsegments were randomly distributed in space to reconstruct the main blocks and the proposed scale-up method was employed to calculate the relative permeability curves of the reconstructed blocks. Results were compared to the ones obtained directly from the network model of the original blocks and show good agreement between the calculated and scaled-up relative permeability curves of primary drainage and secondary imbibition. Furthermore, the model was tested on real media. Eight network models were extracted from pore size distribution of core samples obtained from the Green River basin located in the Mesaverde Formation. Flow properties obtained from the network models were validated against experimental data and good agreement was observed. These network models show a higher level of heterogeneity at micro-scale. Then, the scale-up methods were employed in order to reconstruct the macro-scale sample and predict its properties. Scale-up methods successfully predict the single-phase and two-phase flow properties of the sample.     

Modeling drying in thin porous media after coupling pore-level drying dynamics with external flow field

This article uses a novel, fully coupled method to solve the isothermal slow drying of porous media in laminar flow. The film effect is included and a novel logistic equation is used to relate the pore network variables with the external field variables. The model is used to simulate the drying of several thin porous media with different aspect ratios in a flow. One, two, or all sides of the pore network are opened to the flow. The studies show that the higher exposed area vs. total volume ratio leads to faster drying while the orientation of the porous media is immaterial.     

Optimization of flow in additively manufactured porous columns with graded permeability

Chemical engineering systems often involve a functional porous medium, such as in catalyzed reactive flows, fluid purifiers, and chromatographic separations. Ideally, the flow rates throughout the porous medium are uniform, and all portions of the medium contribute efficiently to its function. The permeability is a property of a porous medium that depends on pore geometry and relates flow rate to pressure drop. Additive manufacturing techniques raise the possibilities that permeability can be arbitrarily specified in three dimensions, and that a broader range of permeabilities can be achieved than by traditional manufacturing methods. Using numerical optimization methods, we show that designs with spatially varying permeability can achieve greater flow uniformity than designs with uniform permeability. We consider geometries involving hemispherical regions that distribute flow, as in many glass chromatography columns. By several measures, significant improvements in flow uniformity can be obtained by modifying permeability only near the inlet and outlet.     

A model for resin flow during composite processing: Part 1—general mathematical development

A generalized three-dimensional model for resin flow during composite processing has been developed. The model is based on a theory of consolidation and flow through a porous medium, which considers that the total force acting on a porous medium is countered by the sum of the opposing forces, including the force due to the spring-like effect of the fiber network and the hydrostatic force due to the pressure of the liquid within the porous medium. The flow in the laminate is described in terms of Darcy's Law for flow in a porous medium, which requires a knowledge of the fiber network permeability and the viscosity of the flowing fluid. Unlike previous resin flow models, this model properly considers the flows in different directions to be coupled and provides a unified approach in arriving at the solution. Comparison of numerical solutions with the closed form analytical solution shows good agreement. Resin pressure profiles show that the pressure gradients in the vertical and horizontal directions are not linear, unlike the assumption of linearity made in several previous resin flow models. The effects on the resin pressure of both linear and nonlinear stress-strain behavior of the porous fiber network were considered. The nonlinear behavior simulates a rapidly stiffening spring and the resin pressure decreases much more rapidly after a given initial period compared to the linear stress-strain behavior.     

Porosity and particle size effects on the gas flow characteristics of porous metals

Several models have been proposed in the past which relate the gas flow characteristics of a porous material to the pore microstructure. Such approaches require metallographic measurements to predict the permeability or inertial flow coefficients. The present study approaches the problem through the dominant adjustable extrinsic processing variables. Specifically, porosity and mean particle size are used to predict the gas flow coefficients for porous 316L stainless steel. Although such an approach proves successful, it is established that traditional processing variables (compaction pressure, sintering temperature and sintering time) have a significant influence on pore shape and therefore on gas flow. Such factors must be included in any comprehensive future predictive models of gas flow through porous metals.     

Analysis of compaction and life‐time prediction of porous polymer membranes: influence of morphology,diffusion and creep behaviour

In membrane applications, large values of permeability and selectivity are generally desired during the whole period of application. The permeability of porous polymer membranes often is reduced by the effect of compaction. Compaction of polymer membranes is a time‐dependent process which is strongly determined by the viscoelastic properties of the polymer and its plasticisation caused by the feed medium (e.g. a liquid medium or a process gas in the case of porous support structures). In this study, the time‐dependent compaction of porous polymer membranes under pressure is modelled. The influence of viscoelastic and diffusion properties of the polymer material on the permeability of the membrane is analysed for different types of membrane morphologies. The life‐time of a porous polymer membrane is associated with the time at which the glass transition is achieved in a creep experiment. Equations are derived in order to estimate the maximum life‐time of polymer membranes based on compaction. The analysis reveals that the diffusion coefficient, the average retardation time in creep, the magnitude of creep compliance and the time–temperature–pressure shift factor strongly influence compaction of microporous membranes. Generally, a larger tortuosity at constant porosity yields a lower life‐time of the membrane. Buckling of cell struts is the dominant failure mechanism in porous membranes with a very high porosity and allows an estimation of life‐time. © 2016 Society of Chemical Industry     

Permeability measurement of fresh cement paste

Fresh cement paste permeability is a key parameter to understand the hydro-mechanical behavior of cement-based materials, i.e., rhelogical properties and static stability. However, its permeability measurement is not easy to assess. The porous medium is not rigid and tends to change due to hydration kinetics. Two measurement methods, with 70 mm and 20 mm initial height specimens respectively, are presented and compared in this paper. The first uses a basic cell of soil permeability measurement and consists of simultaneous consolidation and percolation tests. The second uses a displacement-controlled oedometer cell equipped with pore water pressure transducers, and consists in inducing consolidation to a given void ratio first and, consecutively, in accurately measuring the permeability. A good correlation of results is observed. A comparison with theoretical models confirms that, from one fitted parameter relative to particle characteristics, a relationship between permeability and void ratio can be established.     

Study on diffusion and flow of benzene, n-hexane and CCl4 in activated carbon by a differential permeation method

In this paper the diffusion and flow of carbon tetrachloride, benzene and n-hexane through a commercial activated carbon is studied by a differential permeation method. The range of pressure is covered from very low pressure to a pressure range where significant capillary condensation occurs. Helium as a non-adsorbing gas is used to determine the characteristics of the porous medium. For adsorbing gases and vapors, the motion of adsorbed molecules in small pores gives rise to a sharp increase in permeability at very low pressures. The interplay between a decreasing behavior in permeability due to the saturation of small pores with adsorbed molecules and an increasing behavior due to viscous flow in larger pores with pressure could lead to a minimum in the plot of total permeability versus pressure. This phenomenon is observed for n-hexane at 30°C. At relative pressure of 0.1-0.8 where the gaseous viscous flow dominates, the permeability is a linear function of pressure. Since activated carbon has a wide pore size distribution, the mobility mechanism of these adsorbed molecules is different from pore to pore. In very small pores where adsorbate molecules fill the pore the permeability decreases with an increase in pressure, while in intermediate pores the permeability of such transport increases with pressure due to the increasing build-up of layers of adsorbed molecules. For even larger pores, the transport is mostly due to diffusion and flow of free molecules, which gives rise to linear permeability with respect to pressure.     

Influence of the boundary conditions on capillary flow dynamics and liquid distribution in a porous medium

After depositing a wetting liquid onto a porous medium surface, and under the influence of the capillary pressure, the liquid is imbibed into the porous medium creating a wetted imprint. The flow within the porous medium does not cease once all the liquid is imbibed but continues as a secondary capillary flow, where the liquid flows from large pores into small pores along the liquid interface. The flow is solved using the capillary network model, and the influence of the boundary condition on the liquid distribution within the porous medium is investigated. The pores at the porous medium boundaries can be defined as open or closed pores, where an open pore is checked for the potential threshold condition for flow to take place. In contrast, the closed pore is defined as a static entity, in which the potential condition for flow to take place is never satisfied. By defining the pores at distinct porous medium boundaries as open or closed, one is able to obtain a very different liquid distribution within the porous medium. The liquid saturation profiles along the principal flow direction, ranging from constant to steadily decreasing, to the profile with a local maximum, are found numerically. It is shown that these saturation profiles are also related to the geometrical dimension that is perpendicular to the flow principal direction, and changing the boundary type from open to closed allows the liquid distribution within the porous medium to be controlled. In addition to the liquid distribution, the influence of the boundary conditions on capillary pressure and relative permeability is investigated, where both parameters are not influenced by variation of the boundary condition types. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

鄂尔多斯华庆地区长6储层特征分析

运用岩心观察、扫描电镜、铸体薄片、测井、压汞等资料,对华庆地区长6储层特征进行研究。结果表明,研究区储层砂岩类型以岩屑长石砂岩和长石岩屑砂岩为主;储层岩石成分成熟度低、结构成熟度中等、成岩程度较高,岩屑成分、填隙物成分都很复杂;孔隙类型以粒间孔和长石溶孔为主;储层为典型的低孔、低渗、超低渗储层,储层非均质性强;压实和胶结是造成储层孔隙度大幅度降低的主要成岩作用,但绿泥石膜胶结一定程度上改善了储层物性;沿解理或压实作用所造成的颗粒破裂缝发生的强烈溶蚀对次生孔隙贡献最大,对储层物性起建设性作用。