Water diffusion in biomedical systems as related to magnetic resonance imaging

Water diffusion within the brain is studied numerically for various clinical conditions. The numerical procedure used in this work is based on the Galerkin weighted residual method of finite-element formulation. A wide range of pertinent parameters such as Lewis number, cell volume, and the buoyancy ratio are considered in the present study. Comparisons with previously published work show excellent agreement. The results show that the diffusion coefficient, cell volume, and the buoyancy ratio play significant roles on the characterization of the mass and heat transfer mechanisms within the cell. Concentration maps are developed for various clinical conditions. Pertinent results for the streamlines, isotherms and the mass and heat transfer rates in terms of the average Sherwood and Nusselt numbers are presented and discussed for different parametric values. Experimental tests are also conducted to produce an 8 Tesla image which is compared with our numerical simulation. The present study provides essential maps for brain disorders classified based on several pertinent clinical attributes.     

微流燃料电池空气阴极物质传输孔隙尺度模拟

采用格子Boltzmann方法研究了微流燃料电池空气阴极多孔扩散层内多组分物质传输特性。随机重构了扩散层,获得渗透率及有效扩散系数。建立了耦合边界电化学反应的二维模型,研究了过电位、孔隙率对氧气、水蒸气浓度分布及局部反应速率的影响。结果表明,常用的Bruggeman经验关联式会高估氧气有效扩散系数;扩散层孔隙结构对物质传输有重要影响,孔隙率减小使得传质阻力增大,导致局部氧气浓度降低,局部反应速率降低,而水蒸气浓度增大,当孔隙率从0.83降至0.7,催化界面平均氧气浓度从8.472降至8.466 mol·m^-3。     

Effect of adsorbent heat and mass transfer resistances on performace of an open-cycle adiabatic desiccant cooling system

A mathematical model describing the heat and mass transfer processes that occur during sorption of water vapor in a dehumidifier consisting of parallel channels lined on either side width desiccant felt, is presented. The model includes both gas-side and solid-side resistances for thermal and mass transport. The moisture transport in the desiccant is expressed by considering both gas phase diffusion and surface diffusion. The heat and mass transfer in the desiccant matrix occurs simultaneously and is coupled by an equilibrium sorption relation which is applied locally within the desiccant. Solutions are obtained by using an explicit finite-difference scheme.The effect of heat and mass transfer Biot numbers and other system parameters on the performance of an adiabatic open-cycle cooling system are investigated.     

Porous structure of membranes of an acrylonitrile copolymer. Porosity, 1H-NMR permeability

Nanoporous polymer membranes (porosity ) used for dialysis are studied from NMR relaxation times of water confined in the pore space. Fast interpore water diffusion is observed. Two structural parameters are evidenced: i) a reduced NMR relaxation time, , which reflects the width of the pore-size distribution; ii) the average polymer-grain size of the solid matrix deduced from NMR experiments performed on membranes partially filled by water. A relation is found between the ratio , where k is the permeability to water and the porosity. This relation is in qualitative agreement with numerical simulations reported in the literature on low-porosity systems and with experimental results obtained for sedimentary rocks and for fused glass model systems. It supports the idea that is the relevant structural parameter to describe convective transport in a wide class of porous systems. Received 8 July 1999     

多孔介质中盐指现象的数值模拟

运用基于杂交网格的高精度数值方法研究了多孔介质中的盐指现象.该算法将基于边界拟合坐标下的高精度有限差分法和高精度的泊松方程快速求解器有效地结合在一起,从而达到提高整体的计算精度、计算效率和稳定性的目的.通过比较不同孔隙率的多孔介质对盐指对流的传热传质效应的影响,发现在标准孔隙率较低的多孔介质中,盐度扩散的速度明显比热扩散的速度快,盐指很快触及上下壁面,使得上下层的盐度梯度迅速减小,这是与非多孔介质具有明显差异之处. 关键词： 多孔介质 双扩散对流 盐指     

Natural convection of water-copper nanoliquids confined in low-porosity cylindrical annuli

Natural convection in cylindrical porous annuli saturated by a nanoliquid whose inner and outer vertical radial walls are respectively subjected to uniform heat and mass influxes and out fluxes is studied analytically using the modified Buongiorno-Darcy model (MBDM) and the Oseen-linearization technique. Nanoliquid-saturated porous medium made up of water as base liquid, copper nanoparticles of five different shapes, viz., spheres, bricks, cylinders, platelets and blades, and glass balls porous material is considered as working medium for investigation. The thermophysical properties of nanoliquid -saturated porous medium is modeled using phenomenological laws and mixture theory. The effect of various parameters and individual effects of five different shapes of copper nanoparticles on velocity, temperature and heat transport are found. From the study, it is clear that the addition of a dilute concentration of nanoparticles increases the effective thermal conductivity of the system and thereby increases the velocity and the heat transport, and decreases the temperature. In other words, the heat transport is more in the case of heat and mass driven convection compared to purely heat-driven convection. Among the five different shapes of nanoparticles, blade-shaped nanoparticles facilitate the transport of maximum temperature compared to all other shapes. Maximum heat transport is achieved in a shallow cylindrical annulus compared to square and tall circular annuli. The increase of the inner solid cylinder’s radius is to decrease heat transport. The results of the KVL single-phase model are obtained from the present study by setting to zero the value of the nanoparticles’ concentration Rayleigh number. Also, neglecting the curvature effect in the present problem, we obtain the results of the rectangular enclosure problem.     

Non-stationary heat conduction of a porous medium

The thermal diffusion process is examined for a porous sample with idealized arrangement and form of the pores, when its surface is illuminated by a modulated light. A formula for the frequency dependence of the average surface temperature is derived. It is shown that it depends on the porosity, the form of the pores, and the ratio between a characteristic pore size and the thermal wavelength. In the limiting cases of low frequency of modulation and low porosity the results agree well with those quoted in the literature. The frequency dependence of the surface temperature of a microporous rubber sample, glass filtering crucibles, and leather samples have been measured by a PA cell and compared with the analytical results. The influence of various processes on the heat diffusion in porous media is discussed.     

Method of Measuring the Diffusion Resistance of the Structures of Porous Electrode Materials Based on a Proton-Conducting Ionomer and Carbon Nanomaterials

A method has been developed for measuring the oxygen mass-transport resistance of porous electrodes based on micro- and nanostructures of a Nafion proton-conducting ionomer, Pt/C, and carbon nanowires with the ultrahigh porosity. The method suggests measuring the limit oxygen electroreduction current density at the controlled oxygen flow through an electrochemical system and calculating the oxygen mass transport resistance using the ratio based on the Fick’s law. The method has been used to study the mass-transport loss of a membrane-electrode assembly electrode and electrode material based on a rotating disk electrode and can be applied in the development of new electrodes.     

Complimentary aspects of diffusion imaging and fMRI: II. Elucidating contributions to the fMRI signal with diffusion sensitization

Tissue water molecules reside in different biophysical compartments. For example, water molecules in the vasculature reside for variable periods of time within arteries, arterioles, capillaries, venuoles and veins, and may be within blood cells or blood plasma. Water molecules outside of the vasculature, in the extravascular space, reside, for a time, either within cells or within the interstitial space between cells. Within these different compartments, different types of microscopic motion that water molecules may experience have been identified and discussed. These range from Brownian diffusion to more coherent flow over the time scales relevant to functional magnetic resonance imaging (fMRI) experiments, on the order of several 10s of milliseconds. How these different types of motion are reflected in magnetic resonance imaging (MRI) methods developed for "diffusion" imaging studies has been an ongoing and active area of research. Here we briefly review the ideas that have developed regarding these motions within the context of modern "diffusion" imaging techniques and, in particular, how they have been accessed in attempts to further our understanding of the various contributions to the fMRI signal changes sought in studies of human brain activation.     

颗粒烧结多孔芯蒸发/沸腾换热的实验研究

以水为工质,在热管工况（真空减压条件）下对具有不同颗粒种类（电解粉和水雾粉）、颗粒直径和多孔芯厚度的铜粉颗粒烧结多孔芯进行了蒸发/沸腾换热实验研究。结果表明：随着热流密度的上升,换热系数先上升后下降;在孔隙率一定的情况下,存在最优多孔芯厚度使得蒸发/沸腾换热性能最佳;当多孔芯厚度一定时,在热流密度不是很大时存在着最优孔...     

直接甲醇燃料电池两相非等温模型

本文建立了直接甲醇燃料电池的两相、非等温模型.采用多孔介质中的经典多相流动模型来计算电池内与电化学反应相耦合的传质、传热问题;模型中考虑了水的汽化凝结过程和甲醇窜流对电池性能的影响.计算结果表明电池内温度分布不均匀,温度最高点出现在阴极催化层;阳极甲醇浓度分布不均匀是造成阳极催化层内局部反应速率不均匀分布的主要原因,而阴极催化层局部反应速率主要依赖于阴极过电势的分布;大的流场板开口比条件下电池整体均匀性较好,性能得到提高.     

Regimes of the filtration combustion of structured heterogeneous systems

An analysis of the characteristics of the combustion front in a multilayer porous system with radiative heat transfer and filtration mass transfer of gaseous reactants into the exothermic conversion zone is presented. At moderate pressures, the mass of the gas in the porous layer is smaller than that required by stoichiometry, and, therefore, filtration transport without diffusion from the ambient medium occurs. It was taken into account that the bulk heat release in the porous media can be limited by both the kinetics of the exothermic chemical reaction and the filtration transport of a gaseous reactant from the ambient medium. The effect of filtration on the characteristics of relay-race combustion was examined. The characteristics of the front and the dynamics of the conversion of the elements of the discrete system were determined. The characteristics of the relay-race filtration combustion front under conditions of heat losses into the ambient medium were examined, and the possibility of existence of two steady regimes, with a low- and a high-temperature relay-race combustion front, was demonstrated. At heat losses above a critical level, relay-race combustion extinguishes. A numerical analysis of relay-race combustion regimes under nonadiabatic conditions showed that the low-temperature front is absolutely unstable and made it possible to study the dynamics of the onset of high-temperature relay-race filtration combustion.     

Homogeneous averaging of the stochastic diffusion equation in chaotic porous media

The diffusion of solution components in the pore space of a chaotic porous medium is treated. The problem of homogeneous averaging of the diffusion boundary problem is developed. In the end a system of homogeneous equations is obtained on whose basis one can treat particular transport processes. Russian University of Friendship for the People. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 52–58, September, 1996.     

地下坑道周围山体内氡运移力学解析

基于连续介质力学原理,建立氡在地下坑道周围山体内运移的-维径向对称模型,分析在浓度扩散和气体渗流条件下,坑道周围山体内氡浓度分布,并给出氡浓度的解析表达式.用有效运移距离的概念,定量描述对坑道内氡浓度有影响的山体范围,给出有效运移距离和有效孔隙率、比渗流速度之间的关系式,同时给出坑道壁面上氡析出率的表达式.     

基于图象处理和傅里叶变换的三维多孔介质重构方法

本文基于傅里叶变换的截断高斯场方法的理论,通过从实际多孔介质二维切片的二元化图像中提取孔隙率和自相关函数来构造整个三维多孔结构.模拟结果显示通过傅里叶变换技术得到的随机过程确实满足高斯分布,且构造出的三维多孔结构的统计特性基本与二维切片类似,说明了该技术能够构造出符合要求的三维多孔介质,从而为进一步从孔隙尺度研究其内部传热传质过程奠定了基础.     

Tracer dispersion in a percolation network with spatial correlations

We analyze the transport properties of a neutral tracer in a carrier fluid flowing through percolationlike porous media with spatial correlations. We model convection in the mass transport process using the velocity field obtained by the numerical solution of the Navier-Stokes and continuity equations in the pore space. We find that the resulting statistical properties of the tracer show a transition from a subdiffusion regime at low Peclet number to an enhanced diffusion regime at high Peclet number.     

微细板翅与烧结多孔结构中对流换热实验研究

本文对水和空气流过4个微细板翅结构和1个烧结多孔结构中的对流换热进行了实验研究,并对其流动与对流换热性能进行了分析和比较。结果表明:在本文实验参数范围内,与空槽道相比,这4种微细板翅结构分别使水的对流换热系数增加10—24倍,分别使空气的对流换热增强了16~40倍;与相同孔隙率的烧结多孔结构相比,微细板翅结构中的流动阻力相近,而对流换热系数却增大。存在最优的微细板翅结构,其换热性能大大强于烧结多孔结构,而流动阻力更小。     

Water signal attenuation in diffusion-weighted 1H NMR experiments during cerebral ischemia: influence of intracellular restrictions,extracellular tortuosity,and exchange

The “concept of restricted intracellular water diffusion at permeable boundaries,” which was recently used to model diffusion-weighted ¹H NMR experiments on glioma cells, was applied to measurements on the rat brain in vivo. Combined with the “concept of extracellular tortuosity,” various physiological states of the brain were simulated. Hereby, a variable intracellular volume fraction, intracellular exchange time, and extracellular tortuosity factor were considered for young, adult, and ischemic rat brains. The model simulated the cytotoxic shift of extracellular water, changes in membrane permeability and tissue morphology, and was able to explain the diffusion time dependence as well as the non-monoexponentiality of the diffusion attenuation curves. Preliminary diffusion time dependent experiments on the healthy rat brain (¹H NMR imaging) agreed well with the theoretical concept. Hereby, the intracellular water signal was separated from extracellular signal contributions by large diffusion weighting. It showed the characteristic of restricted diffusion as well as a signal decay due to the exchange of intracellular water across the plasma membrane. A map of the mean intracellular exchange time for water in living animal brain was determined, and the upper limit in rat brain was evaluated to 15 ms. The presented methods can be applied to correlate local differences in a map of exchange times with tissue morphology and to detect pathological deviations of the exchange time, e.g., during ischemia.     

Lattice Boltzmann simulation of double diffusive natural convection in heterogeneously porous media of a fluid with temperature-dependent viscosity

Double diffusive natural convection inside a porous cavity with non-uniform porosity has been numerically studied. The cavity was filled with two parallel porous layers with different porosity. Considering the effects of temperature-dependent viscosity, simulations have been done via the Lattice Boltzmann method (LBM) at representative elementary volume (REV) scale. In this study, the effect of porosity, buoyancy ratio, the viscosity-variation number and thermal Rayleigh number on heat and mass transfer rates was investigated. The streamlines, isotherms, isoconcentrations, average Nusselt number and average Sherwood number curves of different parameters were discussed in detail. It was observed that the governing parameters has significant impact on the fluid flow, temperature and concentration distributions. In addition, the average Nusselt and Sherwood numbers are increase with an reduce in the viscosity-variation number. Further, as the absolute value of buoyancy ratio and thermal Rayleigh number increases, the effect of porosity and viscosity changes on the heat and mass transfer enhancement was augmented.