在多孔介质内气体扩散的分形表征

将多孔介质内孔抽象为分形曲线,并以该曲线的维数作为多孔介质的结构参数—谱维数d,藉此建立了气体在介质内的扩散通量计算式,并给出了测量分形结构码尺的计算方程和多孔介质谱维数的测定方法.由双组分气体在Ni/r- Al_2O_3催化剂上的扩散实验数据,计算出该催化剂的谱维数d=1.10.     

多孔氧化铝中气体的有效扩散系数

在两种氧化铝中 ,分别采用C₂H₄ 、Ar和CO₂ 进行了两组分和三组分的Wicke -Kallenbach定态扩散实验 .由尘气模型和Stefan -Maxwell方程建立起的扩散通量方程 ,对实验结果加以处理 ,得到了两组分扩散的有效扩散系数及三组分扩散的虚拟两组分有效扩散系数 .同时 ,针对过渡区扩散推出了等温下两组分扩散的有效扩散系数为常数应满足的基本条件 ,所得结论和实验结果相吻合 .而对三组分扩散中的虚拟两组分有效扩散系数不能按照常数处理 .此外 ,无论是两组分或三组分扩散 ,对每一种氧化铝而言 ,有着相近的曲折因子 ,从而证实了曲折因子是与多孔介质结构密切相关的参数     

分形多孔材料的一种改进化气体扩散分形模型

基于分形理论与技术,提出了气体在由一簇弯弯曲曲、横截面积大小不等的椭圆形毛细管组成的多孔材料中的气体扩散率分形模型。研究结果表明:归一化气体扩散率是最大孔隙面积、最小与最大孔隙面积之比、多孔材料总横截面积、形状因子及分形维数等多孔材料微结构参数的函数;模型能清楚地揭示影响气体扩散率的物理机制。文中气体扩散率分形模型与已有的实验数据进行对比,结果显示它们之间吻合较好;提出的改进化气体扩散分形模型更具有一般性。     

基于分形理论的油页岩有效扩散系数研究

以直径为1.6—2 mm柳树河油页岩为原料,采用扫描电镜法与氮吸附法得出油页岩吸附-脱附等温线,孔径分布曲线等;分析了油页岩内部孔隙结构,计算出孔道弯曲分形维数与孔隙面积分形维数,并研究油页岩内部的有效扩散系数。结果表明:油页岩吸附-脱附曲线属于Ⅲ型等温线,是一种典型的二端都开放的管状毛细孔型结构的多孔物质;其含有较发达孔隙,孔径分布为0.4—40 nm;油页岩孔道弯曲分形维数越小,孔道越平坦,孔隙面积分形维数为2.464;油页岩有效扩散系数与其内部结构和操作温度有关,温度越高,粒径越小,有效扩散系数越大。     

气体吸附法研究活性炭表面分形维数

在77K下对几种活性炭进行氮气吸附，得到了孔结构参数，并利用等温吸附数据分析了活性炭的分维。结果表明：在多层吸附早期阶段和高度覆盖期所得的分形维数，变化趋势基本一致；分形维数与活性炭的BET比表面积和总孔容都没有直接的关系，而与特征吸附能和极微孔相对含量较为一致。     

蚓孔的分形模型

蚓孔具有分形性[1],本文通过分形理论中的L-系统生成法和DLA法对蚓孔进行计算机模拟[2],其分形图形和蚓孔的形态极其相似,这也暗示了可能反映出蚓孔生长的一些因素。并对蚓孔的分形模型建立了数学模型并求解[3],从而得出基质酸化中蚓孔的穿刺距离[4]。     

铜基催化剂孔隙率对气体有效扩散系数的影响

研究了18种具有不同孔结构参数的粒状铜基甲醇合成催化剂孔隙率对有效扩散系数的影响。结果发现，催化剂微孔孔隙率增大，曲节因子亦增大，扩散阻力增加。将孔隙率与催化剂曲节因子关联，得到了半经验方程，用它可估算催化剂的曲节因子。     

分形多孔介质的孔隙特性对气体扩散的影响

采用随机行走方法建立了分形多孔介质生成模型,生成的颗粒在形貌上与真实多孔颗粒接近,且能够反映其固有分形特征。在此模型基础上,根据经典分子动理论建立扩散控制方程,对气体在多孔介质中的扩散进行数值模拟。分析了比表面积、平均孔径、孔隙率等孔隙特性参数对扩散的影响,获得了分形多孔介质中气体扩散系数与平均孔径的函数关系。结果表明,扩散系数随平均孔径的增大以幂函数形式增大,相应的指数表征扩散系数对平均孔径的敏感度,其值随孔隙率的增大呈线性减小。     

分形多孔介质中气体的非稳态扩散

通过引入平均修正系数修正Fick第二扩散定律得到了描述分形多孔介质气体非稳态扩散的理论方程。基于“塞状流”扩散实验法建立了分形多孔介质非稳态扩散实验系统,对3个孔隙结构不同的分形多孔介质样品进行了非稳态扩散实验,通过实验对理论方程进行验证和修正。结果表明,Fick第二扩散定律不适用于分形多孔介质中气体非稳态扩散,分形多孔介质中气体非稳态扩散存在一定规律,且多孔介质孔隙结构不同其扩散规律不同。     

Determination of oxygen effective diffusivity in porous gas diffusion layer using a three-dimensional pore network model

In proton exchange membrane fuel cell (PEMFC) models, oxygen effective diffusivity is the most important parameter to characterize the oxygen transport in the gas diffusion layer (GDL). However, its determination is a challenge due to its complex dependency on GDL structure. In the present study, a three-dimensional network consisting of spherical pores and cylindrical throats is developed and used to investigate the effects of GDL structural parameters on oxygen effective diffusivity under the condition with/without water invasion process. Oxygen transport in the throat is described by Fick's law and water invasion process in the network is simulated using the invasion percolation with trapping algorithm. The simulation results reveal that oxygen effective diffusivity is slightly affected by network size but increases with decreasing the network heterogeneity and with increasing the pore connectivity. Impacts of network anisotropy on oxygen transport are also investigated in this paper. The anisotropic network is constructed by constricting the throats in the through-plane direction with a constriction factor. It is found that water invasion has a more severe negative influence on oxygen transport in an anisotropic network. Finally, two new correlations are introduced to determine the oxygen effective diffusivity for the Toray carbon paper GDLs.     

Anisotropic, effective diffusivity of porous gas diffusion layer materials for PEFC

A comparative, experimental diffusivity study of gas diffusion layer (GDL) materials for polymer electrolyte fuel cells (PEFC) is presented for the first time. The GDL plays an important role for electrochemical losses due to gas transport limitations at high current densities. Characterization and optimization of these layers is therefore essential to improve power density. A recently developed method which allows for fast diffusimetry is applied and data compared to the literature values. Measurements are made as a function of direction and compression and the effect of different binder structures and hydrophobic treatments on effective diffusivities are discussed. A better understanding of the results is gained by including novel GDL cross-section images and a meaningful unit cell model for the interpretation of the data. The diffusivity data is valuable for GDL manufacturers and future PEFC models. The study reveals that a binder-fiber ratio larger than 50% has a negative impact on the effective diffusion properties. The hydrophobic treatment which is necessary to improve the water management can impede diffusion and thus reduce the power density. Furthermore binder has an isotropic effect while compression pronounces the in-plane orientation of the fibers.     

The determination of effective diffusivity by gas chromatography. Time domain solutions

Time domain solutions applicable to the GC pulse dispersion experiment are presented in the form of dimensionless numerical plots. It is shown that when the particles are nonporous, or diffusion in both the macro- and micropore systems is rapid, a Gaussian peak results. In this case the peak broadening is due to axial dispersion alone. When either the macro- or micropore resistance becomes significant, the peak assumes a skewed shape. The experiment should be conducted under these circumstances if possible.Selection of the proper particle size to employ in the experiment is important. Large particles should be employed in measurements of macropore diffusion. Small particles enhance the microparticle contribution to peak broadening and measurements of micropore diffusion in amorphous materials should be conducted over crushed samples. With some materials, namely zeolite systems in which strong molecule-pore entrance interactions exist, the tailing may be impossible to determine over crushed particles. This situation can sometimes be improved by using full-size particles.     

A diffusivity model for gas diffusion through fractal porous media

A fractal model for gas diffusivity in porous media is derived by using fractal theory and by considering rarefied gas effect in micro-/nano-channels/capillaries. The proposed gas diffusivity model is expressed as a function of micro-structural parameters (the fractal dimensions for pore area and for tortuosity of tortuous capillaries, porosity and pore sizes) of porous media. The effects of parameters such as porosity, microstructures of porous media and fractal dimensions on gas diffusivity are analyzed. The effective diffusivities predicted by the present fractal model are compared with the available experimental data, and a fair agreement between them is found when porosity is less than about 0.70.     

Measurement of effective diffusivity in catalyst-coated monoliths

A review is provided about techniques that have been used to evaluate the effective diffusivity of gases in catalyst/washcoat layers, as used in catalytic monoliths.

The importance of making such measurements is described, in order to ensure that the choice of model for effective diffusivity can be verified, and if necessary an appropriate value of tortuosity can be back-calculated. Based on methods described in the literature, it is concluded that, where possible experiments should be performed on actual monolith structures, rather than those that have been reformed. The chromatographic technique is applied to a catalytic monolith and preliminary results of unpublished work are presented. A method of using a cut section from a catalytic monolith in a modified form of ‘Wicke–Kallenbach diffusion cell’ is also described.

Examples from the patent literature are provided showing, how interest in layered catalyst systems has started to grow, illustrating how diffusion in porous layers can be exploited to develop ‘designer catalyst systems’.     

Effect of catalyst pore structure on effective diffusivity

The effective diffusivity of the catalyst was experimentally measured in both octanedecane and polystyrene-chloroform systems for comparison with effects identified in the coal liquefaction model work. The diffusivity data were in good agreement with the theoretical solution and their reproducibility was satisfactory. In both experimental systems, the effective diffusivity was strongly dependent upon the ratio of diffusing species to catalyst pore size. When the steric exclusionhydrodynamic drag theory was applied, the tortuosity significantly varied with the ratio of diffusing molecule to catalyst pore size. Using the diffusivity data of unimodal catalysts, an empirical equation relating the size ratio to the restrictive factor was derived to evaluate the diffusivity data of bimodal catalysts.     

Determination of appropriate effective diffusivity for different food materials

Effective diffusivity is the most important key parameter needed in the analysis, design, and optimization of heat and mass transfer during food drying process. In general, two types of effective diffusivities are used to develop the mathematical modeling of food drying, namely, moisture-dependent effective diffusivity (MDED) and temperature-dependent effective diffusivity (TDED). However, no study has extensively investigated which effective diffusivity is more accurate in predicting drying kinetics. The main goal of this study is to determine the appropriate effective diffusivity for predicting the drying kinetics. Drying models were developed for different fruits and vegetables based on moisture-dependent and temperature-dependent effective diffusivities. COMSOL Multiphysics, a finite element-based engineering simulation software is used to solve the coupled heat and mass transfer equations. 3D moisture profiles were developed to investigate the spatial moisture distribution during drying. Extensive experimental investigation on five types of fruits and vegetables was conducted and results were compared with the simulated results. The experiments were repeated thrice, and the average of the moisture content at each value was used for constructing the drying curves. Close agreement between experimental and simulated results validates the models developed. It was observed that the moisture profile and temperature profile in case of MDED were more closely fitted with the experimental results. For all fruits and vegetables, the moisture ratio with MDED was significantly lower than moisture ratio with TDED. This finding confirms that the MDED is more accurate for predicting kinetics in food drying. Moreover, the moisture ratio of apple was lowest whereas pear showed the highest moisture ratio. On the other hand, carrot showed a considerably lower moisture ratio compared to potato.     

Modeling of the link between microstructure and effective diffusivity of cement pastes using a simplified composite model

The knowledge of the relationship between porosity and transport properties of concrete is a point of major importance to run properly the models coupling chemistry, transport and mechanics in order to simulate the engineered barrier degradations in the context of the nuclear waste deep repository. The present work proposes a simplified composite model aiming at linking microstructure and effective diffusivity of cement pastes. The proposed analytical method allows the estimation of the evolution of effective diffusivity of such materials submitted to porosity opening or plugging, at the scale of the Representative Elementary Volume (REV). The method is then applied to Ordinary Portland Cement (OPC) pastes. The porosity-diffusion evolutions determined from the composite model for various OPC pastes are implemented into simplified chemo-transport simulations aiming at describing the leaching of cementitious materials. Using these evolutions, OPC paste leaching simulations are in good agreement with the available experimental data, indicating a good reliability of the simplified composite model.     

节能型变换催化剂曲折因子和气体有效扩散

设计自制改进的Wicke-Kallenbach扩散池,按稳态法实验测定了常压313K下N2-CO,N2-CO2,CO-CO2二组分气体和CO+CO2-N2,CO+N2-CO2,N2+CO2-CO三组分气体在还原态LB型节能高温变换催化剂内的逆流扩散通量,实验所得二组分和三组分气体扩散通量之比符合Graham气体扩散规律,表明扩散池设计合理,测定过程稳定、数据可靠。同时说明气体在还原态LB型节能高温变换催化剂内的扩散是过渡区扩散。根据平行交联孔模型,计算出以平均孔半径为基准的曲折因子和以孔分布为基准的曲折因子以及各气体的有效扩散系数。