Structure Of Porous Media Research Articles

Numerical study on flow-boiling heat transfer in square channels partially centrally filled with gradient porous media

Boiling heat transfer is widely used in industry and daily life and can be enhanced by porous media structure. Previously HPM have been shown to significantly improve heat transfer performance, but how the porosity gradient affects boiling heat transfer remains unknown. In this paper, the subcooled flow boiling in vertical square channels partially filled with gradient porous media was numerically investigated by using a newly developed solver named GPMPhaseChangeFoam embedded in the open-source platform OpenFOAM. The results indicate that in positive GPM inserted channels, when z ≤ 0.16 m Nuz increases with increasing Hp*, and when z ≥ 0.16 m Nuz increases with increasing Hp* from 0.2 to 0.8 but deceases with Hp* rises from 0.8 to 1.0, and the effect of Hp* on Nuz is more pronounced in the entrance region over the exit region. In negative GPM inserted channels, Nuz increases with increasing Hp* throughout the channel length in positive GPM inserted channels, and the effect of Hp* on Nuz is less pronounced in the entrance region than the exit region. Flow resistance increases with Hp*, but changes little with Δε, and Nufd increases with Hp* in both positive and negative GPM inserted channels. However, Nufd decreases with an increase in ∆ε from 0 to 0.3 in positive GPM inserted channels, from -0.3 to 0 in negative GPM inserted channels, and ∆ε = -0.3 at Hp* = 1.0 is 13.4 % higher than that of ∆ε = 0.3. Furthermore, the negative GPM inserted channel behaves higher flow boiling heat transfer performance than the HPM and positive GPM.

Impact of diatomit on the transport behavior of unmodified and carboxyl-modified nanoplastics in saturated porous media

Due to the extensive use of plastic products and unreasonable disposal, nanoplastics contamination has become one of the important environmental problems that mankind must face. The composition and structure of porous media can determine the complexity and diversity of the transport behavior of nanoplastics. In this study, the influence of diatomite (DIA) on the nanoplastics transport in porous media is investigated by column experiments combined with XDLVO interaction energy and transport model. Results suggest that the recovery rates of unmodified polystyrene nanoparticles (PSNPs) and carboxyl-modified polystyrene nanoparticles (PSNPs-COOH) in the porous media containing DIA decreases compared with that in the pure quartz sand (QS), and the BTCs showed a “blocking” pattern. The presence of DIA inhibits the transport of both PSNPs and PSNPs-COOH, but the inhibition is not significant. This may be because the presence of DIA provides more favorable deposition sites for PSNPs and PSNPs-COOH to some extent. However, since DIA itself carries a certain negative charge, this can only play a role in compressing the double electric layer for PSNPs and PSNPs-COOH with the same negative charge, and cannot destabilize them. The migration capacity of PSNPs and PSNPs-COOH is strongest in the DIA-QS porous media at pH = 7, and is weak at pH = 9 and pH = 5. The inhibition of migration at pH = 9 can be attributed to the dissolution of the DIA surface under alkaline conditions and the formation of pore and defect structures, which provide more deposition sites for PSNPs and PSNPs-COOH. The presence of humic acid (HA) leads to an increase in the mobility of PSNPs and PSNPs-COOH, and the mobility is enhanced with HA concentration. The mobility of PSNPs and PSNPs-COOH in DIA-QS decreases with ionic valence and ionic strength, and PSNPs-COOH is more significantly inhibited compared to PSNPs.

A new fractal pore-throat chain model for non-Darcy flow through porous media

Non-Darcy flow through porous media is of great significance in hydraulics, oil and gas engineering, biomedical science, chemical and civil engineering etc. However, it is difficult to fully grasp the nature of fluid flow through porous media from macroscopic scale alone. Based on the statistically fractal scaling laws of pore structures, a new fractal pore-throat chain model (FPTCM) for non-Darcy flow through the isotropic porous media is developed. The analytical expressions for the Darcy and non-Darcy permeability as well as non-Darcy coefficient are derived accordingly. In order to explore the local flow field of high-speed non-Darcy flow through porous media, the finite element method is also carried out on an equivalent pore-throat unit. The predicted permeability by FPTCM shows better agreement with present numerical results and available experimental data, compared with commonly used semi-empirical formulas including Kozeny-Carman and Ergun equations. It has been found that both Darcy and non-Darcy permeability as well as non-Darcy coefficient strongly relate to the pore structures of porous media. The non-Darcy permeability is positively correlated to porosity, pore fractal dimension and Darcy permeability, while it is negatively related to tortuosity fractal dimension and pore size range. The non-Darcy coefficient shows opposite correlation with these parameters. The present work can provide theoretical basis for oil and gas development, nuclear waste treatment, carbon dioxide geological sequestration etc.

Periodic Stratification of Colloids in a Liquid Phase Produced by a Precipitation Reaction and Gel Swelling.

Pattern formation is a frequent phenomenon occurring in animate and inanimate systems. The interplay between the mass transport of the chemical species and the underlying chemical reaction networks generates most patterns in chemical systems. Periodic precipitation is an emblematic example of reaction-diffusion patterns, in which the process generates a spatial periodic structure in porous media. Here, we use the dormant reagent method to produce colloidal particles of Prussian blue (PB) and PB analogues at the liquid-gel interface. The generated particles produced a stable periodic stratification pattern in time in the liquid phase placed on top of the solid hydrogel. The phenomenon is governed by periodic swelling of the gel driven by the osmotic stress and stability of the formed particles. To illustrate the phenomenon, we developed an extended reaction-diffusion model, which incorporated the gel swelling and sedimentation effect of the formed colloids and could qualitatively reproduce the pattern formation in the liquid phase.

Regulation strategy and analysis of the ultra-low temperature heat and mass transfer capacity of 3He-4He mixture in the mixing chamber of dilution refrigerator

As an important method for achieving millikelvin temperatures, dilution refrigerators (DRs) have been widely utilized in the fields of low-temperature superconductivity, low-temperature measurement, and space exploration. The mixing chamber (MC) uses a 3He-4He mixture as the refrigerant and serves as the cold end of the DR. However, the current research lacks a detailed study of the effect of the structure of porous media on the MC cooling capacity. In this study, the heat and mass transfer model in the MC is established using the finite element method. The model is based on the improved two-fluid model to evaluate its cooling capacity and propose regulation strategies. The reliability of the model is verified through experiments. When the load power stabilizes at 10 μW, the steady-state temperature is 59.6 mK, and the Kapitza thermal resistance significantly affects the cooling process of the MC. Three strategies for regulating Kapitza thermal resistance are provided. First, the optimal porosity of the sintering cylinder for the porous medium is ε = 0.5. Second, the extreme point of the sintering cylinder is at a height of h = 1.75 cm, and the cooling capacity is at its minimum at this location. Third, the steady-state temperature initially increases and then decreases with an increase in the cylinder radius. This study is of great significance as it reveals the influence of a porous medium heat exchanger on the heat and mass transfer process of mixtures at ultra-low temperatures, guiding the design of DR.

Effects of microscopic pore-throat structure on gas–liquid relative permeability: Porous media construction and pore-scale simulation

The porous media structure of the oil/gas reservoir changes greatly after long-term development, which subsequently influences the macroscopic relative permeability. To clarify the effects of microscopic pore-throat structure on macroscopic relative permeability, we first proposed a method to generate two-dimensional porous media images with adjustable structure parameters. The method is based on Delaunay triangulation and is similar to the pore-network generation process, which can provide binary images for direct numerical simulation of flow through porous media. Then, we established the single component multiphase Shan–Chen lattice Boltzmann method coupling the real gas equation of state. Finally, we discussed the effect of pore radius, coordination number, pore-throat ratio, pore shape, and wettability on the gas–liquid relative permeability curve using the lattice Boltzmann simulation. This study provides an effective method to generate porous media and explain the mechanism of relative permeability change at the pore scale.

Lattice Boltzmann simulation of dissolution patterns in porous media: Single porosity versus dual porosity media

Understanding the influence of porous media structure, particularly dual porosity, on solvent transport and pore geometry evolution during chemical reactions is a complex and critical area of study. This research leverages the lattice Boltzmann method to investigate how the presence of aggregates in a medium affects solvent transport and pore space development, focusing on distinct dissolution regimes: face and wormhole dissolution. The study addresses the challenge of managing variable pore sizes in dual porosity media by developing specialized GPU algorithms, which efficiently handle fine grids and complex pore spaces. The findings reveal that dual porosity significantly enhances dissolution rates in both the face and wormhole dissolution regimes. Intriguingly, while the pattern of face dissolution remains largely unchanged, dual porosity markedly alters the pattern of wormhole dissolution. In dual-porosity media, the wormholes tend to be narrower and more elongated compared to the wider wormholes observed in single-porosity media. This variation is attributed to the reaction area dynamics, where the reduced reactive surface area along the main wormhole path in dual-porosity media results in less solvent engagement in the reaction processes. Moreover, the research provides insights into the microscale interactions in porous media, emphasizing how variations in microscale porosity can have substantial impacts on the overall dissolution dynamics. The study results are not only significant for understanding the fundamental aspects of chemical dissolution in porous media but also have practical implications in fields such as geo-energy and groundwater remediation. These findings help optimizing reaction processes in complex and heterogeneous porous systems, highlighting the need for detailed consideration of microstructural characteristics in modeling and industrial applications.

Pore scale numerical simulation of flow and heat transfer non-equilibrium in randomly packed bed

The complex pore structure of porous media in packed beds leads to randomness and non-uniformity of flow and heat transfer. In this paper, the pore-scale method is used to numerically study the flow and heat transfer in 3D randomly packed bed. The average value and root-mean-square (RMS) of variables are used to evaluate the non-equilibrium characteristics of temperature and velocity. The results demonstrate that the packed bed consistently exhibits non-equilibrium phenomena due to the influence of porous media structure. The peak of thermal non-equilibrium occurs at the location with the largest temperature difference between gas and solid phases, while the peak of flow non-equilibrium occurs after reaching maximum temperature. With increasing inlet velocity, there is an expansion in both local velocity and temperature disturbance range, leading to a wider non-equilibrium region. However, the trend of non-equilibrium between flow and heat transfer is different.

A novel micro-scale structure reconstruction approach for porous media and characterization analysis: An application in ceramics-based diesel particulate filter

Diesel particulate filter (DPF) made from cordierite with a porous filter media structure has become an essential component in diesel engines to meet the increasingly the strict emission standards. Hence, a more accurate and universal DPF porous filter media model is necessary to analyze the complex mechanism and optimize its performance. In this work, a new optimization method was proposed for the initial DPF model constructed by the quartet structure generation set (QSGS), and the fractal and spectral dimension were used to quantify the actual effects of the optimized DPF model. Results showed that the method could effectively repair dead zones and eliminate burr boundaries, thus decreasing the complexity of the pore structure and increasing the effective flow area of the DPF model. The increase in porosity decreased the fractal dimension and increased the spectral dimension in porous media. Additionally, simulation results showed that the proposed method was more effective on burr boundaries. This allows the optimization method to be adopted more effectively in porous media modeling and provides improved guidance for optimization. Not only DPF, the proposed method offers a new idea for the structure optimization of porous filter media.

Packing Soft Polygons in a Minimum Height Rectangular Target Domain

The paper studies packing polygons of variable shapes, regarding the stretching coefficient, in a rectangular target domain of minimum height. Packing objects of a variable shape have a wide spectrum of applications, e.g, in biology, materials science, mechanics, land allocation, and logistics. Interest in these problems is also due to the modeling of the structures of porous media under pressure, e.g., for creating test models of artificial digital cores. Elements of porous media can be deformed under the influence of an external force, but the mass of each particle remains unchanged. This corresponds to conservation of area for the two-dimensional case. Polygonal objects must be completely contained within the target domain (containment constraint) and do not overlap (non-overlapping constraint), provided they have free translations, continuous rotations, stretch transformations, and conserve their area. The phi-function technique is used for an analytical description of the placement constraints for variable shape polygons. Quasi-phi-functions for describing non-overlapping constraints and phi-functions for describing containment constraints are defined. The packing problem is presented in the form of a nonlinear programming model. A solution strategy is proposed, which consists of the following stages: generation of feasible starting points; search for local minima of the problem of packing soft polygons for each starting point using the decomposition algorithm; choosing the best local minimum found at the previous stage. To search for smart starting arrangements, an optimization algorithm for packing original polygons using their homothetic transformations is applied. Decomposition of the problem of packing polygons of variable shapes is based on an iterative procedure that allows reducing a large-scale problem to a sequence of smaller nonlinear programming problems (linear to the number of objects). Numerical examples are provided for oriented rectangles and non-oriented regular polygons. Keywords: packing, polygons, stretch transformation, rectangular container, quasi-phi-functions, optimization, decomposition algorithm.

Numerical simulation of diesel particulate filter flow characteristics optimization: From the perspective of pore structure parameters and inlet velocity

The diesel particulate filter (DPF) with porous media structure has become an essential component for diesel engines to satisfy increasingly strict emission standards. This study constructed a porous media structure model of DPF, and optimized its boundary to conduct a simulation on the flow characteristic with different influencing parameters using the lattice Boltzmann method (LBM). Results showed that the increase of the average flow velocity of the DPF model reduced the pressure drop, indicating that the flow probability of model improved. In addition, the porosity application range was expanded. The visualization and quantization of the velocity and pressure distribution with the DPF revealed that an increase of the wall thickness resulted in a higher pressure drop of the DPF, but a lower flow velocity. Further, the fractal dimension of the porous media exhibited no direct relationship with the DPF pressure and velocity performance; however, the outlet velocity and pressure drop of the model were optimized within the different porosity. Moreover, both the increase in the spectral dimension and model optimization improved the DPF permeability. The impact of increasing the inlet velocity on the pressure drop was particularly significant as it accelerated the rate of pressure drop, illustrating that a smoother porous media boundary was conducive to improve the flow performance of DPF, which facilitated a better scheme for the design of DPF porous media.

PORE STRUCTURE AND PERMEABILITY EVOLUTION OF POROUS MEDIA UNDER IN SITU STRESS AND PORE PRESSURE: DISCRETE ELEMENT METHOD SIMULATION ON DIGITAL CORE

In stress-sensitive oil and gas reservoirs, formation rock deformation occurs under in situ stress and pore pressure, affecting the rock's porosity and permeability. Pore deformation is the fundamental mechanism. However, the literature on numerical simulation of rock porosity and permeability at the pore scale is rare. In this paper a numerical simulation framework of pore scale is proposed based on the discrete element method. The pore geometry and permeability evolution of the core are quantitatively analyzed by the digital core method. Firstly, the coupled fluid-discrete element method (CFM-DEM) is used to simulate the samples' deformation under different stress and pore pressures. We then reconstruct the digital core using Avizo. Finally, the pore geometric topological structures are analyzed, and the permeability changes are calculated. The results show that stress can reduce porosity, modify pore shape, and lead to poor porosity connectivity and permeability, while pore pressure can weaken such trends.

Investigation of the effect of alumina porous media on the polyethylene waste pyrolysis with continuous feed

This study investigates the pyrolysis of polyethylene waste in a continuous-feed mode to provide novel insights into recycling waste plastics. Alumina (Al2O3) porous media has been introduced into the pyrolysis reactor to study its effect on continuous-feeding plastic waste pyrolysis. The pyrolysis product yields, oil fractions, and gas components have been determined under varying porous media structures, i.e., thickness and pore size, and operating parameters, i.e., temperature and gas-carrying velocity. To the best of our knowledge, it is the first attempt to investigate the influence of porous media pore size on the thermal decomposition of waste polyolefin in continuous feeding mode. The results reveal that higher thicknesses and smaller pore sizes of porous media can enhance the properties of pyrolysis oil with lower heavy fractions (>C20). The aromatization of light olefins is triggered when the temperature exceeds 520 °C, resulting in a rapid increase in light fraction (<C12) in pyrolysis oil. Moreover, the gas-carrying velocity determines the pyrolysis product distribution by regulating the volatiles' residence time.

Experimental and Numerical Simulations of Pore Structures and Seepage Characteristics of Deep Sandstones

Previously conducted studies have established that deep underground rock masses have complex pore structures and face complex geological conditions. Therefore, the seepage problem of such rock masses seriously affects engineering safety. To better explore the seepage law of deep rock masses and ensure engineering safety, indoor experimental methods such as casting thin sections, scanning electron microscopy, and mercury intrusion testing were utilized in this study. The microscopic pore shape, size, distribution, and other structural characteristics of sandstone in coal bearing strata were analyzed. The tortuosity calculation formula was obtained by the theoretical derivation method. And a numerical model was established for seepage numerical simulation research through microscopic digital image methods. The seepage law of surrounding rocks in the Tangkou Coal Mine roadway under different conditions is discussed. The research results indicate that the complexity of the pore structure in porous media leads to an uneven distribution of flow velocity and pressure within the medium. Meanwhile, with the change of physical properties, the fluid flow characteristics also undergo significant changes. The research results can effectively guide micropore water blocking, reduce the impact of groundwater on the environment, ensure the environment and safety of the project, and provide guidance for other geological projects.

Multiscale evolution observation of the pore structures of porous media under an electrokinetic treatment

Direct current (DC) stimulation is a promising electrokinetic method for enhancing oil recovery in porous media, especially in tight sandstones, due to the more remarkable effect of electroosmosis in materials with narrower pore throats. However, an interesting reversal of enhanced oil recovery (EOR) occurs in which oil recovery begins to decrease with DC voltages; this phenomenon hinders the application of this electrokinetic technology in petroleum industry. To date, the reason for this phenomenon remains unclear. Thus, in this study, we applied nuclear magnetic resonance (NMR) and in-situ scanning electron microscope (SEM) methods to investigate the underlying mechanisms. We classified and inspected the evolution of pore structures in tight sandstones at multiple scales ranging from the nanoscale to micron scale as the strength of DC voltage increases. The effects of both low DC and high DC voltages were analyzed in detail. Results show that the size and number of nanopores increased as the DC voltage increases. A low DC voltage can transform submicropores to micropores, and a high DC voltage can fasten this transformation. Furthermore, a low DC voltage increased the number and size of micropores, but a high voltage decreased the number and size of micropores. This study provides insights into changes in the pore structures of reservoir rocks under the treatment of the DC voltage, helping develop DC voltage-related technologies to recover hydrocarbons from porous media.

Gas Dispersion Coefficient Test System and Dimensionless Inversion Method for Porous Media.

Due to the complex porous media structure of the longwall gob area, it has been difficult to determine the gas dispersion coefficient of oxygen when studying spontaneous coal combustion in the gob area. In this work, we first designed an experimental device for testing the gas diffusion coefficient of porous media. Then, the distribution law of gas concentration in porous media under different particle size conditions was obtained by experiments. Subsequently, we established a dimensionless mathematical model of gas dispersion in porous media and developed a corresponding numerical simulator based on the finite volume method (FVM). The influence of the dimensionless gas dispersion coefficient on the gas concentration distribution was analyzed, and then a dimensionless inversion method of the gas dispersion coefficient was summarized and put forward. Finally, we obtained the values of the gas dispersion coefficient in the experimental device under different particle size conditions by inversion and discussed its effect on the gas dispersion behavior in porous media. The results show that (1) the distribution of gas concentration obtained from the experimental test and numerical simulation is consistent, which verifies the reliability of our work; (2) the dimensionless gas concentration is the highest near the injection point and gradually decreases along the depth and both sides of the test container; (3) with the increase of the dimensionless gas dispersion coefficient, the distance required for uniform gas mixing in the test container is gradually shortened and the gas dispersion coverage is wider; and (4) the larger pore space facilitates the dispersion behavior of the gas, and the gas dispersion coefficient shows a parabolic trend with the increase of porous medium particle size.

Fractal analysis of rock pore structure: A comparative study of the Matlab-based image method and the mercury intrusion method

As a naturally porous medium, rocks have a very strong heterogeneity in their internal pore structure, which has an important impact on the mechanical and chemical properties of rocks, etc. Therefore, it is very important to quantitatively characterise the heterogeneity of the pore structure of rocks. Fractal dimension has long been recognised as an effective means of characterising the heterogeneity of the pore structure of porous media and is widely used in oil and gas exploitation, construction materials, mining, and water engineering. There are different methods to calculate the fractal dimension. To verify the consistency between different methods, this paper compares the results of fractal analysis using the mercury intrusion method and MATLAB image fractal analysis using three rock samples with large differences in porosity and calculates the fractal dimension in three different ways on the basis of the mercury intrusion method. The results demonstrate that the fractal dimension of the mercury intrusion method and the box counting dimension of the image analysis obtained by the three methods, although slightly different in numerical value, are consistent in their numerical relationship, i.e., they all conform to the rule that the stronger the non-homogeneity of the pore throat, the larger the fractal dimension. The results of this paper show that fractal dimension is indeed an effective means of characterising rock homogeneity.

Theoretical analysis of shape factor of gas hydrate sediments under stress dependence

The Kozeny-Carman (KC) equation is used extensively to characterize the effective permeability of gas hydrate sediments. However, most existing modeling approaches ignore the changes in the effective stress, hydrate saturation, and pore structures of porous media, which significantly affect the extraction and exploration of gas hydrates. In addition, theoretical models that can accurately describe shape factors during the imbibition process and predict the effective permeability of gas hydrate sediments are rarely reported. This paper proposes a theoretical stress-dependent shape factor model to study the physical relations between the shape factor and relevant parameters (e.g., effective stress, hydrate saturation, effective porosity, and pore fractal dimension). Then, the derived model is extended to study the shape factor during the imbibition process. The two main novel features of the proposed model are: (1) it takes the effective stress, hydrate saturation, and pore structures into account; and (2) it can be used to study the evolution of shape factors during the imbibition process in gas hydrate sediments. The predictions from the developed model provide a good agreement with experimental results, which validates the model. In addition, based on the derived model, the effects of relevant parameters on shape factors are studied theoretically. From a practical standpoint, the model can not only estimate the effective permeability of gas hydrate sediments under stress dependence but also provide insight into the evolution of shape factor, pore structures (e.g., capillary total number, pore fractal dimension, tortuosity fractal dimension, and effective porosity) and hydrate pore habits during the extraction of hydrates.

Numerical Study of the Flow Through Porous Structures Built from Gray–Scott Patterns

Numerical solutions of the reaction–diffusion system of equations in three dimensions provide a systematic procedure to generate a variety of porous media structures of a chosen porosity. A way to characterize such geometries is by their Minkowski functional densities and their average pore diameter. We generated multiple porous geometries of the granular and foam-like types. Hydraulic flow through these series of geometries is simulated to study the dependence of the permeability k and the Forchheimer parameter β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta$$\\end{document} on the chosen geometrical identifiers. Both k and β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta$$\\end{document} exhibit two distinct behaviors aligned with the two distinct generated types of geometries when scaled with the Euler characteristic. A good correlation for dimensionless k as a function of the Minkowski functional densities and the average pore diameter was found for both types of geometries with an accuracy of 15% and 18%, respectively, while no good correlation was found for β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta$$\\end{document}, implying that more geometrical information is needed beyond the proposed one. From the correlations, it was found that, together with the porosity, the mean curvature is also a good parameter to characterize the permeability.

Visualization of local two-phase flow structure in a packed bed of spheres

A two-phase flow structure in porous media is essential to clarify the debris cooling characteristics during a severe accident in nuclear power plants. In previous experimental studies, a packed bed of spheres has been used instead of the porous debris bed. Many models to predict the flow and heat transfer characteristics in the packed bed have been proposed based on the experimental data in the sphere-packed bed. However, those models have not considered the local flow structure, which should be studied to enhance the prediction accuracy. In this study, the local flow structure of the air-water two-phase flow in the unit model of the sphere-packed bed was measured by neutron computed tomography. The distribution of the gas phase was reconstructed by acquired images. From these results, the breakup and coalescence of the bubbles in the packed bed were evaluated.