多孔介质/纯流体耦合区域内可压缩气体的流动

采用统一形式的修正N-S方程描述多孔介质/纯流体耦合区域的流动,提出了方程中用于处理多孔介质内流动的源项确定方法,并利用成熟的CFD技术对圆管内具有前后台阶的耦合区域内可压缩气体的流动进行了数值模拟,得到了与实验吻合的计算结果.     

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     

Flow of viscoelastic surfactants through porous media

We compare the flow behavior of viscoelastic surfactant (VES) solutions and Newtonian fluids through two different model porous media having similar permeability: (a) a 3D random packed bed and (b) a microchannel with a periodically spaced pillars. The former provides much larger flow resistance at the same apparent shear rate compared to the latter. The flow profile in the 3D packed bed cannot be observed since it is a closed system. However, visualization of the flow profile in the microchannel shows strong spatial and temporal flow instabilities in VES fluids appear above a critical shear rate. The onset of such elastic instabilities correlates to the flow rate where increased flow resistance is observed. The elastic instabilities are attributed to the formation of transient shear induced structures. The experiments provide a detailed insight into the complex interplay between the pore scale geometry and rheology of VES in the creeping flow regime. © 2017 American Institute of Chemical Engineers AIChE J, 64: 773–781, 2018     

Anisotropic convective heat transfer in microlattice materials

Fluid dynamics and heat transfer of flow through periodic open‐cellular microlattice structures are characterized for varying superficial flow orientations and flow rates to investigate heat transfer and pressure loss anisotropy. For given Reynolds number, friction factor is lowest when flow is aligned with the largest straight‐through passages in the microlattice. A maximum friction factor, over twice the optimally aligned friction factor, exists for flow orientations between π/8 and π/4 rad off the optimal alignment, with little variation in friction factor for π/8 and π/4 rad. Heat transfer is maximized at π/4 rad off axis from the largest straight‐through passages; however, less angular variation occurs in Nusselt number than in friction factor. Empirical correlations involving superellipses yield analytical equations describing Nusselt number dependence on flow angle and Reynolds number. This work enables selection of optimal flow orientations and optimal cellular architecture in convective heat transfer implementations of microlattice materials for lightweight and multifunctional applications. © 2012 American Institute of Chemical Engineers AIChE J, 59: 622–629, 2013     

多孔介质中流体流动的格子气自动机模拟

介绍了 13-Bit正六边形多速格子气自动机模型的特点 ,讨论了多孔介质流体流动的格子气自动机模型的渗透率等参量算方法 ,应用该模型对计算机产生的多孔介质几何构型和焦炭多孔介质中的流体流动进行了模拟 ,其结果既可以给出多孔介质中的流动细节 ,也可统计得到表征多孔介质宏观流动特征的物理参量 .初步研究表明 :格子气自动机模型可用于模拟复杂边界条件下多孔介质的流体流动     

Increase of flow-through pores in rationally designed organosilica-PVDF nanocomposite membrane

Organosilica-polyvinylidenefluoride nanocomposite membrane has shown excellent performance in emerging technology of membrane distillation process for treatment of highly saline water stream. In present work, a systematic study using capillary flow porometry was carried out to evaluate the constricted part of the flow-through pores, which is active pores of the nanocomposite membranes. Mean flow pore size and distributions of the membrane pores were found to be influenced due to the phenomenon of micro-gelation by air exposure prior to the immersion in the coagulation bath of the preparation method, polymer concentration, polymer chain length and nature of the solvent in the casting dope solution. The best membrane in terms of the largest mean flow diameter of 0.12 μm with narrow distribution of flow-through pores were observed in the membrane with optimum organosilica content of 1.4 wt%.     

Electrochemical detection of large channels in porous rocks

An electrochemical method for studying channels in porous rocks and similar insulators, has been developed. In principle, the method consists of displacing a nonconducting liquid from the pores by an inflow of an ionically conducting solution. The conductance is monitored continually as the rate of flow through the rock is gradually increased. The rate of conductance increase is interpreted in terms of a simple model, based on Poiseuille flow through a capillary network, to provide information about the sizes and numbers of large pores in the rock. Three rock samples have been analysed.     

Numerical calculation with experimental validation of pressure drop in a fixed-bed reactor filled with the porous elements

To study flow dynamics in a fixed-bed reactor, experiment and numerical simulation are used. In this work, the flow dynamics in a fixed-bed reactor filled with porous particles was simulated. For verification, experimental data were used. Porous particles were prepared from grains with sizes from 0.2 to 2.0 mm. Porous particles made by sintering grains in a muffle furnace. The study of pressure drop was performed in the velocity range up to 3 m/s. Numerical calculation was performed for a realistic computational domain obtained by RBD-algorithm (rigid body dynamics). It was found that if the pore size is less than 0.5 mm, then the flow through the porous medium of particles is minimal; if the pore size is larger than 0.5 mm, then there is a flow through the porous elements of the fixed-bed reactor. To take into account the porosity of the particles, γ correlation coefficient was proposed for the Ergun equation.     

The flow and displacement in porous media of fluids with yield stress

We study the mobilization and subsequent flow in a porous medium of a fluid with a yield stress, modeled as a Bingham plastic. We use single-capillary expressions for the mobilization and flow in a pore-throat, and a pore-network model that accounts for distributed yield-stress thresholds. First, we extend the statistical physics method of invasion percolation with memory, which models lattice problems with thresholds, to incorporate dynamic effects due to the viscous friction following the onset of mobilization. Macroscopic relations between the applied pressure gradient and the flow rate for single-phase flow are proposed as a function of the pore-network microstructure and the configuration of the flowing phase. Then, the algorithm is applied to model the displacement of a Bingham plastic by a Newtonian fluid in a porous medium. The results find application to a number of industrial processes including the recovery of oil from oil reservoirs and the flow of foam in porous media.     

纤维增强材料渗透性能的测量研究

渗透性是纤维预成型坯的特征常数，其值越大，说明树脂流过纤维集合体的阻力就越小，它是充模流动研究的主要内容。本文以平流法为基础，从理经和实验上对RTM充模中的面内渗透率张量作了研究。用三种有代表性的纤维材料即随机纤维毡、多层机织物和整体纺织物进行实验，所测各渗透率与计算结果吻合较好。并对影响渗透率的因素作了分析。     

Relationship of threshold diameter and Darcean permeability in unconsolidated porous structures

Experiment was conducted on the threshold pressure for atmospheric air through unconsolidated narrow size distributed mini sphere and sand particles at low flow rates. The threshold diameter calculated from measured threshold pressure showed that it does not follow the traditional similarity theory. This is consistent with our experiment on accurate permeability measurement, and can be explained as a result of gas slip flow within such micro pore structure. Our current work tend to find the method to predict the permeability-threshold pressure relationship for unconsolidated porous structures.     

泡沫碳化硅波纹规整填料内的液体流动特性

通过采用配备微距镜头的高速摄像仪记录单波纹片上的液体流动行为,发现了泡沫碳化硅波纹规整填料内同时存在液膜流动和准壁流两种液体流动模式。同时测定了组合波纹片之间的液体传递量,并将泡沫碳化硅波纹规整填料与金属丝网波纹填料和金属板波纹规整填料的测试结果进行比较,结果表明泡沫碳化硅波纹规整填料的单波纹片上的液体扩散程度较大,组合波纹片之间的液体传递量高于金属丝网波纹填料和金属板波纹规整填料。研究结果揭示了泡沫碳化硅波纹规整填料具有高效传质特性的根本原因在于其独特的三维空间网孔结构增强了液体在单波纹片上的横向扩散能力和在组合波纹片之间的径向传递能力,为新型高效规整填料的机理研究和进一步开发开拓了思路。     

The effect of gas diffusion layer compression on gas bypass and water slug motion in parallel gas flow channels

Water slugs form in the gas flow channels of polymer electrolyte membrane fuel cells (PEMFCs) which hinder reactant transport to the catalyst layer. We report a study correlating video images of slug formation and motion with pressure/flow measurements in parallel gas flow channels. Slugs move when the differential gas pressure exceeds the force to advance the contact lines of the slug with the channel walls. Water slugs can divert the gas flow through the gas diffusion layer (GDL) beneath the ribs to adjacent channels. The flow diversion can cause slugs to stop moving. Slug size and motion has been correlated with in situ GDL permeabilities as functions of GDL compression. Compression reduces the GDL permeability under the ribs much more than the GDL permeability under the channel. A model is presented to describe the spatio‐temporal location of slugs in a PEMFC flow field. © 2014 American Institute of Chemical Engineers AIChE J, 61: 355–367, 2015     

Convection in a Krogh cylinder: Putting back fluid flow in the extravascular tissue

Models for drug delivery are based on the use of stirred tanks to represent organs that contain no mass transfer resistances. In the original Krogh cylinder model, a mass transfer resistance shows up but there is no convection in the tissue where convection should matter. In the present study, a two-dimensional flow field is used to show that when a liquid enters the capillary, some leave through the walls into the tissue at the arterial end and then doubles back into the capillary at the venous end. Some flow does not return which is taken to be the flow to the lymphatic system. We can get the measured transcapillary pressure drop of about 2,666 Pa if in addition the compliance of the tube wall is taken into account. Very realistic flow fields have been shown for a model liver and a tumor.     

Freeze-cast honeycomb structures via gravity-enhanced convection

The effect of gravity on directional solidification was investigated in solution-based freeze casting. A preceramic siloxane-based polymer was freeze-cast with a cyclohexene solvent from two different directions: that against the direction of the gravitational force and that in concert with the gravitational force. Because the density of preceramic polymer is higher than the solvent, the segregated polymer creates a denser solution ahead of the freezing front than the underlying solution when the freezing direction is the same as the gravity direction. This results in convective flow in the liquid phase. This convective flow influences constitutional supercooling, which changes not only the pore size of freeze-cast structure but also the pore morphology from dendritic to cellular pores.     

NETmix®, a new type of static mixer: Modeling,simulation, macromixing,and micromixing characterization

NETmix® is a new technology for static mixing based on a network of chambers connected by channels. The NETmix® model is the basis of a flow simulator coupled with chemical reaction used to characterize macro and micromixing in structured porous media. The chambers are modeled as perfectly mixing zones and the channels as plug flow perfect segregation zones. A segregation parameter is introduced as the ratio between the channels volume and the whole network volume. Different kinetics and reactants injection schemes can be implemented. Results show that the number of rows in the flow direction and the segregation parameter control both macro and micromixing, but the degree of micromixing is also controlled by the reactants injection scheme. The NETmix® model enables the systematic study of micromixing and macromixing for different network structures and reaction schemes, enabling the design of network structures to ensure the desired yield and selectivity. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Flow-induced degradation of hydrolyzed polyacrylamide in porous media

Summary In this work we present an experimental study of flow-induced degradation of hydrolyzed polyacrylamide in aqueous solutions flowing through porous media. The degradation is analyzed by passing the solution repeatedly through the medium at a constant flow velocity and the degraded solution is then characterized by porous media and opposed jets flows. When the polyacrylamide is dissolved in deionized water, it exhibits a gradual extension thickening in the flow through porous media and opposed jets. In this case, the polymer degrades as it passes through the porous medium even at relatively low flow rates. When the polyacrylamide is dissolved in an NaCl solution, it exhibits critical extension thickening in porous media flows, and it only degrades at Reynolds numbers that are higher than the onset of the extension thickening behavior. Chain degradation is therefore only encountered when extension thickening is produced. The results also show that the extent of degradation decreases as the pore size decreases.     

Fluid dynamics of flow through microscale lattice structures formed from self‐propagating photopolymer waveguides

The fluid dynamics of flow through microscale lattice structures is characterized for different unit cell sizes, flow angles, and flow rates. The structures consist of an octahedral‐type periodic unit cell, which is formed from an interconnected pattern of self‐propagating photopolymer waveguides. The periodic unit cell of each sample has a node‐to‐node spacing between 800 and 2400 μm and a truss member diameter between 148 and 277 μm. Water is directed through the microscale lattice structures, and the resulting pressure drop is investigated for two different flow angles and superficial flow rates between 0.5 and 4.8 L/min. Finite element analysis is used to determine pressure drop in the laminar flow regime. The results are used to develop a correlation describing friction factor as a function of flow direction, geometric characteristics, and Reynolds number. This work enables control of the fluid dynamics in microarchitected multifunctional truss materials through design and superficial flow angle. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Penetration of Shear Flow Into an Array of Rods Aligned With the Flow

Shear flow over a square array of widely‐spaced rods aligned with the flow is investigated using singularity methods to solve Stokes equation. The flow field is determined for various arrays occupying a fraction of a Couette channel, for solid volume fractions from 0.001 to 0.1. Flow penetration into an array is quantified by the slip velocity at the array edge. This velocity is much greater than when the flow is across the array but still less than the value predicted by the use of Brinkman's equation.