Local Porosity Distributions Research Articles

A method for the determination of local packing factor distribution of a packed pebble bed by the improved line-based averaging method

The packing factor is an important parameter for describing the internal structural features in pebble beds, which have a significant influence on the heat and mass transfer behavior and the thermo-mechanical properties of the pebble bed. A comprehensive understanding of the packing structure is essential for the design and optimization of the pebble bed and can promote the application of the pebble bed. In this work, an improved line-based averaging method was proposed to calculate the local packing factor or local porosity distribution and validated by comparing the results with those obtained from experimental and numerical studies of cylindrical packed pebble bed. Furthermore, the local packing factor distributions in the angular-radial plane of the cylindrical pebble bed were revealed for the first time. In addition, the line-based averaging method has been applied to reveal the local packing factor distributions in the annular pebble beds, U-shaped pebble beds and hexagonal pebble beds. The main feature of this method is the ability to calculate and plot contour maps of local packing factor or porosity distributions for columnar pebble beds of arbitrary shapes, especially the local packing factor distributions in the cross-sectional plane and the angular-radial plane.

Laser–Ultrasonic Study of Local Porosity Distribution in Carbon Fiber Reinforced Plastic Stringer Panels

Laser–Ultrasonic Study of Local Porosity Distribution in Carbon Fiber Reinforced Plastic Stringer Panels

Joint Distributions of Local Pore Space Properties Quantitatively Explain Simulated Air Flow Variations in Paper

The gas flow through sheet-like porous materials such as paper can show marked lateral variations due to a heterogeneous, locally varying microstructure. Hence, reliable predictions of such lateral flux variations require an appropriate consideration of local variations in the microstructure. The flow through such sheet-like materials is commonly described with Darcy’s law in which permeances are formulated in terms of microstructure properties, such as porosities, tortuosities, or hydraulic radii. This work proposes an extension of existing permeance models that directly considers the variation and the cross-dependence between local microstructure properties. The extended model is applied to local air fluxes through a paper sheet to exemplarily reveal the joint impact of local porosities and local tortuosities on the air flux. The key extension is to consider a joint distribution of porosity and tortuosity. The latter is constructed from the univariate property distributions using a copula approach and yields local tortuosities including their variation for any encountered local porosity. These values jointly enter any permeance model that qualitatively captures the dependence of the air flux on the porosity. To assess the merit of the model, variations in the air flux and in the pore space properties are independently determined from the same measured microstructure of paper. Air flux variations are provided by computational fluid dynamics simulations on multiple, nonoverlapping segments taken from the microstructure. A statistical analysis of the entire microstructure provides the distribution of local porosity, tortuosity, and thicknesses. Our model quantitatively explains that porosity-dependent variations in the tortuosity, in particular the ones associated with high-volume pathways, decisively determine air flux variations.

Investigation on the microstructure of a 3D-printed mortar through a novel leaching-subsidiary tomography

The microstructure of a 3D-printed mortar was investigated in present study. To enhance the density contrast between sand and cementitious slurry, a novel leaching strategy was used as an auxiliary method, based on which the spatial dispersion of sands in 3D-printed mortar was rendered. Meanwhile, to alleviate the problem of CT’s limited resolution, X-ray attenuation method (XRAM) was introduced in this study to investigate the spatial distribution of local porosity in 3D-printed mortar. Besides, focusing on single filaments, the upper part of the filaments presented lower sand rate and higher porosity than the lower part, and the difference between the filaments located near the top of 3D-printed mortar was more significant. Finally, based on the sliding method, the interlayer width, average porosity and average sand ratio of 3D-printed mortar were estimated as 640 21.9% and 43.1%, respectively. The research results would deepen the understanding of 3D-printed concrete.

Impact of tissue porosity and asymmetry on the oxygen uptake of the human placenta: A numerical study

IntroductionThis study proposes a computational fluid dynamics model of a human placenta's independent exchange unit (placentome) to assess the effect that the inner villi distribution and decidual veins (DVs) location and number, have on the oxygen uptake. MethodsThe internal placentome porosity distribution was altered in symmetric morphology, while asymmetry was introduced by varying the location and number of DVs. The DV asymmetry was introduced by either displacing them circumferentially, thereby changing the angle between them, or by adding DVs in the inlet cross-section. The results were analyzed by the changes in the normalized oxygen mass fraction and the oxygen uptake. ResultsOxygenated blood was shown to be delivered deeper into the placentome when the area of non-homogeneous porosity was larger. The largest oxygen uptake was achieved in the asymmetric model with the smallest angle distance between the DVs, where a 10% decrease relative to the farthest case was obtained. Placing DVs adjacent to the spiral artery opening enhanced the drainage of oxygenated blood. DiscussionThis study demonstrates the importance of the local porosity distribution for the proper perfusion of the intervillous space and proposes a novel approach to improve our understanding of the role of the DVs in placental oxygen uptake.

Combined effects of microstructural characteristics on anisotropic transport properties of gas diffusion layers for PEMFCs

Gas diffusion layer (GDL) plays a key role in proton exchange membrane fuel cells, which provides multi-functions for gas transport, thermal-electrical conduction and mechanical support. Coupling manipulation of different microstructural characteristics could potentially improve transport properties of GDLs. This work proposes an approach to reconstruct heterogenous GDLs and conduct pore-scale modeling to evaluate the anisotropic transport properties. The models are reconstructed using X-ray computed tomography, stochastically reconstruction methods and morphological processing techniques, which consider different fiber diameter, GDL thickness and local porosity distribution type. Combined effects of microstructure characteristics on tortuosity, diffusivity, thermal-electrical conductivity and anisotropic ratios are investigated comprehensively. The results show that the diffusivity with fiber diameter of 7 μm is approximately 7% lower, and the conductivity is 8% higher than that of 9 μm. The anisotropic ratios of diffusivity, thermal conductivity and electrical conductivity range from 1.25 to 1.65, 5 to 20, and 20 to 55, respectively. Local porosity distribution of uniform-fluctuated type, fiber diameter of 7 μm and GDL thickness of 126 μm are suggested to balance diffusivity and thermal-electrical conductivity simultaneously. The methods and results can guide microstructure design of other porous electrodes with higher performance.

CFD Investigation of helium gas flow in sphere packed (Pebble bed) in a rectangular canister using OpenFOAM

The lead-lithium cooled ceramic breeder uses spherical lithium titanate pebbles, which act as the breeder for generating tritium as the fuel for nuclear fusion reaction and helium as the purging gas. In this work, the flow-field of helium gas and temperature distribution inside a rectangular canister is investigated numerically, considering the pebble bed as the porous medium. In the numerical model, the local thermal non-equilibrium (LTNE) is assumed to exist between the pebbles and the purging gas, and a local porosity distribution is considered. Considering the radiation effect, a heterogeneous and LTNE based solver developed in OpenFOAM software is validated with the published results. Further, the effects of different heat generation rates, mass flow rates, and packing arrangements (random and uniform arrangements, i.e. BCC and FCC) on the flow-field and temperature profile are studied. The BCC and FCC packing arrangements have better purging behaviour than random packing. Further, the effect of inlet and outlet configuration is evaluated to find the optimal locations for uniform distributions of purging gas in the canister. With a higher value of area-weighted average velocity, the diagonally opposite inlet-outlet is the best choice for removing tritium from the packed bed.

Effects of fixed wall and pebble size ratio on packing properties and contact force distribution in binary-sized pebble mixed beds at the maximum packing efficiency state

The binary-sized pebble mixed beds at the maximum packing efficiency state were numerical simulated by the DEM simulation to investigate the effect of fixed wall and pebble size ratios. The evolution of the distribution of packing fraction and porosity, radial distribution function and contact force were given and analyzed with emphasis on the effects of the fixed wall and pebble size ratio. The results showed that the fixed wall will result in a reduction of average packing fraction and an obvious wall effect in local porosity distribution. With the increase of the pebble size ratio, the volume fraction of the wall affected regions gradually decrease. The variation of the local packing fraction of the binary-sized pebble bed is mainly determined by the partial packing fraction of large pebbles and the partial packing fraction of small pebbles, respectively, in the regions close to the fixed wall. Furthermore, the fixed wall has little effect on the radial distribution function and contact force. However, the pebble size ratios have great influence on the radial distribution function and the contact force in binary-sized pebble bed at the maximum packing efficiency state. With the increase of the pebble size ratio, the radial distribution functions of the whole pebble beds are consistent with that of small pebbles, and approach to that of the mono-sized pebble bed. In addition, with the increase of the pebble size ratio, a higher contact force can be obtained in pebble beds.

X-ray computed tomography-based porosity analysis: Algorithms and application for porous woody biomass

The properties of lignocellulosic biomass, such as bulk physical, thermal, and mechanical properties, as well as the mobility of enzymes or catalysts, are largely affected by porosity, pore structures, and pore size distribution. While X-ray computed tomography (CT) has been introduced to effectively produce 3D volumetric images of material microstructure, a quantitative porosity analysis from the 3D images has never been done. This work introduces a first-of-its-kind X-ray CT-based quantitative porosity analysis method and applies the developed toolkit to characterize the internal porosity distribution of loblolly pine. A sample loblolly pine chip was scanned by a nano-resolution X-ray CT system and digitally reconstructed after a sequence of image processing operations. A comprehensive porosity analysis was then performed to quantify the envelope porosity, the spatial distribution of local porosity, and the directional porosity. Solutions to the various challenges in image processing, porosity calculation, and large data handling are provided.

Statistical variation of characteristics of random packed beds of Raschig rings: The influence of the sample size

Many studies on random packed beds have been dedicated to local porosity (void fraction) or orientation distribution of particles. However, despite the random nature of the considered packings, very little attention has been devoted to examination whether the number of particles used in experiments/simulations is sufficiently large to get reproducible results. The reproducibility of the key packing parameters depends on the size of the population of particles (sample size) and ordering effects induced by the confining walls. This work investigates quantitatively the influence of the sample size on the statistical variation of the packed bed characteristics. Packed beds of Raschig rings were generated with a sequential algorithm and three column diameters were considered. It has been found that in the case of the orientation distribution the results depend strongly on the sample size, especially for slender columns, while the porosity profiles are well-reproducible characteristics even for relatively small packings of rings. Some complementary results for full cylinders are also included.

Evaluation of Tensile Adhesion Strength in Simulated Dust Cakes on Fabric Filters

AbstractBased on the results of a 3D simulation of cake formation on characteristic filter models for fabric filters with different surface structure, the tensile adhesion strengths in simulated dust cakes are evaluated on the basis of the resulting inhomogeneous local porosity distribution of the particle packing by calculating fracture points at a balance between adhesion and detaching strengths. Both the evaluation method itself and the evaluation results for a computation example are presented and discussed. A significant proportion of fracture points are found in deeper levels in the filter medium, and the distribution of tensile adhesion strengths there differs strongly from the distributions in top levels of the filter medium. The fracture points in the top layer of the filter medium, where a special surface texture is generated, can contribute a considerable part of the tensile adhesion strengths over the whole filter area.

Application of Laser-Ultrasonic Technique of Acoustic Impedance Measurement with Signals Detection by Backward-Mode Scheme for Porosity Content Evaluation in CFRP Laminates

The aim of this work is the development and experimental implementation of the laser-ultrasonic method for CFRPs porosity assessment with one-side access to an investigated object. The scheme with the laser thermooptical generation and backward-mode piezoelectric detection of longitudinal acoustic waves is used. The proposed method is based on the measurement of the acoustic impedance of a composite specimen by using the value of the integral of the ultrasonic pulse reflected from an immersion layer-specimen interface. The existence of the relationship between the porosity content, ultrasound velocity, and material’s density allows one to assess the porosity of CFRPs by the measured acoustic impedance. The proposed method does not require plane parallelism of the input and output surfaces of the studied objects, as well as measuring the thickness of the object. This allows studying CFRP structures with complex shapes. This method may be of use for various types of composite materials, regardless of acoustical waves propagating features related to a periodical material’s structure. CFRP specimens with three reinforcement schemes and different average volume porosity contents were investigated. It is shown that the local porosity distribution in the studied CFRPs is nonuniform along the fiber stacking plane. The porosity content obtained by the laser-ultrasonic method was checked using the X-ray computer tomography. The porosity values, averaged from the laser-ultrasonic measurement results, coincide with the X-ray tomography data within the error limits. The method can be useful for quality control of obtained composite structures with the aim of modernization of technology processes and selecting optimal production conditions and also for the detection of changes in composite structures during their operation or during fatigue testing processes.

Quantitative Evaluation of Porosity in Unidirectional CFRPs Using Laser Ultrasonic Method

A method for estimating the porosity of CFRPs based on measuring their acoustic impedances is proposed and experimentally implemented. The acoustic impedance of the test sample is derived from the magnitude of the antiderivative of an ultrasonic pulse reflected from the immersion-liquid–sample interface. The studied samples are unidirectional CFRPs with various volume contents of the matrix and filler. It has been established that the distribution of local porosity in the samples is uneven along the carbon fiber stacking plane. The value of porosity averaged over the results of optical–acoustic measurements with allowance for measurement errors is consistent with the X-ray tomography data. The method for estimating the value of porosity presented in this paper does not involve measuring the volume and mass of the object under study and can be used in diagnostics of complex shaped composite structures.

LASER-ULTRASONIC METHOD OF ACOUSTIC IMPEDANCE MEASUREMENT FOR QUANTITATIVE POROSITY ESTIMATION OF CROSS-PLY CARBON FIBER REINFORCED PLASTIC MATERIALS

A method of measuring the acoustic impedance of carbon fiber plastics based on the laser optoacoustic effect is proposed and experimentally realized. Measurement of the acoustic impedance of the studied composite is made by the value of the antiderivative of ultrasonic pulse reflected from the interface between the immersion liquid and the sample. A method for determining the porosity of a material by the measured value of the acoustic impedance, based on the dependence of the material density and the velocity of propagation of longitudinal acoustic waves in it on its porosity, is presented. Porous samples of crossply reinforced carbon plastics with three types of carbon fiber lay-up schemes were studied. It was found that the studied carbon fiber plastics have a non-uniform distribution of local porosity in the plane of carbon fabric stacking. It is also shown that the variation of the local porosity in the sample depends on the fiber laying scheme. It is shown that the porosity value obtained by X-ray computed tomography coincides with the results of laser-ultrasonic measurements. The advantage of the proposed method is the possibility of rapid diagnosis of porosity with one-way access to the object under study without measuring its dimensions and mass, which can be used for composite structures of complex shape.

Super-resolution of real-world rock microcomputed tomography images using cycle-consistent generative adversarial networks.

Digital rock imaging plays an important role in studying the microstructure and macroscopic properties of rocks, where microcomputed tomography (MCT) is widely used. Due to the inherent limitations of MCT, a balance should be made between the field of view (FOV) and resolution of rock MCT images-a large FOV at low resolution (LR) or a small FOV at high resolution (HR). However, large FOV and HR are both expected for reliable analysis results in practice. Super-resolution (SR) is an effective solution to break through the mutual restriction between the FOV and resolution of rock MCT images, for it can reconstruct an HR image from a LR observation. Most of the existing SR methods cannot produce satisfactory HR results on real-world rock MCT images. One of the main reasons for this is that paired images are usually needed to learn the relationship between LR and HR rock images. However, it is challenging to collect such a dataset in a real scenario. Meanwhile, the simulated datasets may be unable to accurately reflect the model in actual applications. To address these problems, we propose a cycle-consistent generative adversarial network (CycleGAN)-based SR approach for real-world rock MCT images, namely, SRCycleGAN. In the off-line training phase, a set of unpaired rock MCT images is used to train the proposed SRCycleGAN, which can model the mapping between rock MCT images at different resolutions. In the on-line testing phase, the resolution of the LR input is enhanced via the learned mapping by SRCycleGAN. Experimental results show that the proposed SRCycleGAN can greatly improve the quality of simulated and real-world rock MCT images. The HR images reconstructed by SRCycleGAN show good agreement with the targets in terms of both the visual quality and the statistical parameters, including the porosity, the local porosity distribution, the two-point correlation function, the lineal-path function, the two-point cluster function, the chord-length distribution function, and the pore size distribution. Large FOV and HR rock MCT images can be obtained with the help of SRCycleGAN. Hence, this work makes it possible to generate HR rock MCT images that exceed the limitations of imaging systems on FOV and resolution.

Laser Optoacoustic Method for Quantitative Porosity Assessment of Carbon Fiber Reinforced Plastic Composites Based on Acoustic Impedance Measurement

Abstract—A method for measuring the acoustic impedance for the assessment of the porosity of carbon fiber reinforced plastic composites using laser thermo-optical excitation of longitudinal acoustic waves is proposed and experimentally realized. The acoustic impedance of the test sample is measured from the magnitude of the antiderivative ultrasonic pulse reflected from the immersion liquid–sample interface. A method for calculating the porosity of carbon fiber plastic composites based on the measured acoustic impedance is presented. Composite samples with three different carbon fiber layers stacking schemes were investigated. It is shown that in the studied composites, the local porosity distribution is nonuniform along the fiber stacking plane. The porosity value, averaged from optoacoustic measurement results, actually coincides with X-ray tomography data within the error limits. The presented method for determining porosity does not require determining the volume and mass of the studied object and can be used for investigation of composite structures with complex shapes.

Morphology on Reaction Mechanism Dependency for Twin Polymerization.

An in-depth knowledge of the structure formation process and the resulting dependency of the morphology on the reaction mechanism is a key requirement in order to design application-oriented materials. For twin polymerization, the basic idea of the reaction process is established, and important structural properties of the final nanoporous hybrid materials are known. However, the effects of changing the reaction mechanism parameters on the final morphology is still an open issue. In this work, the dependence of the morphology on the reaction mechanism is investigated based on a previously introduced lattice-based Monte Carlo method, the reactive bond fluctuation model. We analyze the effects of the model parameters, such as movability, attraction, or reaction probabilities on structural properties, like the specific surface area, the radial distribution function, the local porosity distribution, or the total fraction of percolating elements. From these examinations, we can identify key factors to adapt structural properties to fulfill desired requirements for possible applications. Hereby, we point out which implications theses parameter changes have on the underlying chemical structure.

Optimization of local porosity in the electrode as an advanced channel for all-vanadium redox flow battery

Experimental and numerical studies have been carried out to improve the flow distribution by optimizing the local porosity of the electrodes of the all-vanadium redox flow batteries (VFBs) to increase the energy efficiency at high current density. We control the local porosity by inserting extra layer of electrode at inlet and outlet, and the flow field of electrolyte is analyzed numerically. First, the flow field of electrolyte is analyzed numerically to understand distribution of electrolyte in the electrode. Then, the charge and discharge curve is analyzed to understand the effect of local porosity of electrode on the energy efficiency. At 50 mA/cm2, the energy efficiency is the highest when using electrode with uniform porosity. At 150 mA/cm2, however, the energy efficiency of the cell using the electrode with low porosity at inlet is similar to that using the uniform electrode which is 66.6%. Lastly, we suggest an empirical equation for optimal local porosity distribution of the electrode according to current density. Using the empirical equation, we can increase the energy efficiency of the cell to 67.7% at 150 mA/cm2. This study shows the possibility of increase of the energy efficiency of VFBs by controlling local porosity of the electrode.

Local non-vuggy modeling and relations among porosity, permeability and preferential flow for vuggy carbonates

It is challenging to predict the permeability of a vuggy carbonate based on a bulk porosity indicator such as the effective porosity obtained by helium porosimetry. The primary reason is that heterogeneity in the local porosity distribution within the rock may cause nonuniform fluid flow or preferential flow due to the large amount of hydraulically ineffective pore spaces. Here we present the results of a new X-ray computed tomography (CT) based numerical fluid-flow analysis for various types of vuggy carbonate samples, together with experimental visualization of the flow paths within some samples. To numerically analyze the preferential flow, the samples are modeled as aggregates of submillimeter-scale non-vuggy carbonates with different porosities for which the widely-used Lucia bulk porosity-permeability relations of non-vuggy carbonate rock are applied (local non-vuggy modeling). The local porosity distribution is estimated using X-ray computed tomography of the samples. The local permeability distribution is then estimated based on the local porosity distribution, and a single-phase Darcy flow simulation is performed. This provides the bulk porosity and permeability and the preferential flow within the samples, which are consistent with experimentally measured values and visualized flow paths. Both lognormal and normal distributions of local porosity are identified. Preferential flow is associated with a lognormal distribution, and the geometric standard deviation is related to the degree of preferential flow. We show that, in the case of a normal local porosity distribution, the bulk permeability can be predicted based on a bulk porosity indicator such as the effective porosity measured using helium porosimetry. Additionally, we show that in the case of a lognormal local porosity distribution, the permeability is a function of the geometric mean and the geometric standard deviation of the distribution. This new method will contribute to better evaluation and modeling of vuggy carbonate reservoirs/aquifers for hydrocarbon production, carbon storage, and water resource conservation.

Multiscale local porosity theory, weak limits, and dielectric response in composite and porous media

A mathematical scaling approach to macroscopic heterogeneity of composite and porous media is introduced. It is based on weak limits of uniformly bounded measurable functions. The limiting local porosity distributions that were introduced in the work [Adv. Chem. Phys. XCII, 299–424 (1996)] are found to be related to Young measures of a weakly convergent sequence of local volume fractions. The Young measures determine frequency dependent complex dielectric functions of multiscale media within a generalized self-consistent effective medium approximation. The approach separates scales by scale factor functions of regular variation. It renders upscaled results independent of the shape of averaging windows upon reaching the scaling limit.