Gas Transport In Porous Media Research Articles

Combining wall interactions, fluid momentum balances and the Maxwell-Stefan equations for gas transport in porous media: An alternative approach

This paper presents a modelling framework for incorporating wall interactions into the Maxwell-Stefan equations for ideal gases in porous media. In contrast to already existing models in literature, such as the Dusty Gas, Binary Friction and Modified Binary Friction models, the framework makes use of N−1 (with N being the number of gas species) independent mass balances for the gas species in combination with a total fluid momentum balance. This approach has the advantage that already existing numerical schemes for fluid momentum balances, such as Darcy’s law or the Brinkman equations, can be used with only minor modifications. It also allows for separate initialisation of the velocity and pressure fields, something that is often beneficial when solving convection–diffusion equations numerically. The derived model is validated against both stationary and time-dependent experimental data from literature for binary Ar-He systems in porous media, using Knudsen diffusivities to define the wall friction terms.

Multiphase flow gas transport in a deep geological repository

Gas transport in porous media is of interest in many industrial applications, such as the oil and gas industry, geological storage, and deep geological repositories for radioactive waste. In a deep geological repository, gas will be generated due to the corrosion of metallic components and the degradation of organic materials. This leads to a build-up of gas pressure, which may activate gas transport through the host rock as well as the excavation-damaged zone around backfilled galleries.
 In order to understand different transport mechanisms involved, numerical simulations were performed, and the results were compared with laboratory data. In the framework of the EURAD/GAS project, a gas pressure-dependent permeability model was implemented into the finite element code OpenGeoSys-6 (OGS-6) [1]. The permeability alteration in this model is a function of gas pressure. The laboratory experiments showed that the rate of permeability change are different at low and high gas pressures. Therefore, the permeability model employed a threshold pressure ( to categorize this behaviour.
 and are empirical parameters. Moreover, two other permeability models were employed to study the hydro-mechanical behaviour of the host rock and permeability changes. In the strain-dependent permeability model, the permeability change was related to the elasto-plastic behaviour of the host rock, and in the embedded fracture model, it was related to the opening and closure of fractures [3] [4]. Thus, volumetric elastic strain and equivalent plastic strain are employed to be the controlling variables.
 The initial permeability of the intact rock samples were determined by applying a constant pressure at the upstream and downstream of the samples (i.e. constant pressure gradient). The imperical parameters were determined by matching experimental and numerical results.
 Two types of gas injection tests carried out by the Institute for Rock Mechanics (IFG GmbH, See Figure 1) were used to investigate the gas transport through Opalinus Clay and to examine the permeability models [2]. The first experiment demonstrates an advective-diffusiion gas transport through the sample and an elastic deformation. The second experiment highlights the formation of a tensile fracture (plastic deformation and preferred flow path). In both experiments the advective transport is the dominant transport mechanism. The strain-dependent permeability model was successfully applied to reproduce the hydro-mechanical behaviour of the host rock in both elastic deformation test and tensile fracture test (see Figures 2 and 3). The hydro-mechanical response of a saturated single phase flow model was compared with the behaviour of a saturated two-phase flow model. Both single phase and two-phase flow models were able to describe the hydraulic as well as mechanical behaviour of the experiments performed. Therefore, one can conclude that in these experiments the water phase (wet-phase) was immobile.
 A gas injection test under triaxial conditions was performed by École Polytechnique Fédérale de Lausanne (EPFL) in saturated Opalinus Clay (Fig. 4). The numerical simulations reproduced the hydro-mechanical behaviour of the sample during the gas injection test. A two-phase flow model was applied to simulate the experiment. The relative permeabilities and capillary pressure functions followed Mualem approach and van Genuchten formulation, respectively. The outflow volume and mechanical response of the sample were measured. The experimental results were in good agreement with the numerical ones. The results of the modelling illustrated the penetration of gas into the sample and hence, the displacement of water (see Figures 5 and 6).

Review of Shale Gas Transport Prediction: Basic Theory, Numerical Simulation, Application of AI Methods, and Perspectives

The gas transport mechanism in shale reservoirs is extremely complex and is a typical multiscale and multiphysics coupled transport process, considering the complex shale rock structure, wide distribution of micropores and nanopores in shale gas reservoirs, diverse gas occurrence forms, and large pore size spans. An accurate understanding of the shale gas transport process and mechanism is important for effective exploration of shale gas reservoirs. In this work, a review of the recent progress in the prediction of shale gas transport in porous media is presented. The basic theory of gas transport in nanopores is discussed. The gas transport in organic and inorganic matter and the gas adsorption effect are covered. Then, gas transport simulations with conventional multiscale numerical methods, including molecular dynamics and lattice Boltzmann simulations, are reviewed, and the multiscale modeling methods are discussed. Furthermore, the application of artificial intelligence (AI) methods in shale gas transport research is discussed. The focus is on the characterization of the shale porous geometry, including porosity, tortuosity, pore size distribution, and reconstruction of the shale porous medium. The application of AI-based methods such as neural networks and machine learning for the prediction of porous flow properties is discussed. This study intends to provide a comprehensive review of shale gas transport characteristics and to enable the accessibility of AI tools in the research of shale gas.

Numerical study of the impact of geometrical parameters on the rarefied gas transport in porous media

To understand the transport mechanism of rarefied gas in the shale rock, the geometry of the porous media is generated based on the Voronoi tessellation, and the gas flows in continuum and rarefied flow regimes are simulated by using the Multiple-Relaxation-Time Lattice Boltzmann Method and Linearized Discrete Velocity Method, respectively. Based on explicit topological relations, porous media with diverse geometries are constructed to investigate the effect of pore-to-throat radius ratio, dead pore number and low-tortuosity throat on the rarefied flow in porous media. The relationships of the mass flow rate, apparent permeability and tortuosity versus the Knudsen number are obtained for varying pore-to-throat radius ratios. Results show that the intrinsic and apparent permeabilities of the porous media decrease as the blockage rate increases, while the reduction in intrinsic permeability is more significant than that in apparent permeability. Moreover, the phenomenon of Knudsen minimum reappears after the addition of low-tortuosity throats to porous media, in which the contribution to mass flow rate increases significantly in rarefied regimes relative to continuum regime, and it is supported by the observation that the tortuosity decreases with increasing Knudsen number. The increase in flow rate with increasing rarefaction effect in the low tortuous regions not only reveals the critical impact of fractures created by hydraulic fracturing on exploitation but also can identify the potential causes for the rapid production decay of shale gas reservoirs.

GAS PERMEABILITY IN POROUS MEDIA WITH ROUGH SURFACES BY FRACTAL-MONTE CARLO SIMULATIONS

Gas permeability is an important parameter for gas transport in microporous and nanoporous media. A probability model of gas permeability of fractal porous media with rough surfaces is proposed and numerically simulated by the Monte Carlo technique. This model consists of two gas flow mechanisms: the Poiseuille flow and the Knudsen flow, and can be expressed by structural parameters, such as the pore fractal dimension, the tortuosity fractal dimension, the relative roughness and porosity. The validity of the proposed model is investigated through the available experimental data, and a good agreement is obtained. The predicted results indicate that gas permeability of microporous and nanoporous media with rough surfaces decreases with the increase of the relative roughness and the tortuosity fractal dimension, and increases with the increase of porosity and the pore fractal dimension. Our gas permeability model could reveal the physical mechanisms of gas transport in porous media with rough surfaces.

EFFECTIVE GAS DIFFUSION COEFFICIENT OF FRACTAL POROUS MEDIA WITH ROUGH SURFACES BY MONTE CARLO SIMULATIONS

Gas diffusion coefficient is an important parameter in characterizing gas transport in porous media. Based on the fractal characteristics of microscopic capillaries, the effect of capillary surface roughness on gas diffusion is considered by the Monte Carlo technique. We proposed a probability model of the effective gas diffusion coefficient of the porous media with rough surfaces. This model is expressed as a function of the geometrical parameters of porous media, such as the porosity, the pore fractal dimension, the tortuosity fractal dimension, and the relative roughness. The simulated effective gas diffusion coefficient is compared with the available experimental data and a good agreement between them is obtained. The effect of the structural parameters of porous media on gas diffusion is analyzed in detail.

On multicomponent gas diffusion and coupling concepts for porous media and free flow: a benchmark study

Multicomponent gas transport in porous media and at the interface between porous media and free flow occurs in a wide range of technical and environmental systems. Modeling tools are required for the quantitative description of such coupled environmental compartments. We present a benchmark study to compare different diffusion models as well as different coupling concepts employed for the description of multicomponent gas flow and transport at the porous medium/free-flow interface. The benchmark problems allowed us to compare two diffusion models (Fickian and Maxwell-Stefan formulations) for predicting the transport behavior of multicomponent mixtures in porous media and in presence of a free flow. The examples highlighted the importance of interaction between different diffusing components by applying the Maxwell-Stefan formulation, as well as the impact of the free flow velocity on gas migration at the interface under diffusion- and advection-dominated conditions. The problems were solved using two simulation tools, incorporating different coupling concepts to describe the physics of flow and transport processes. In COMSOL Multiphysics, we implemented a single-domain Brinkman approach, whereas in the DuMuX simulator we used a two-domain approach with the Navier-Stokes equation for the free flow and Darcy’s law for the porous medium. Also, the formulation of the Maxwell-Stefan equation for multicomponent transport was different between the two codes. Despite the different modeling concepts, mathematical descriptions and numerical schemes implemented in the two simulators, their outcomes demonstrate excellent agreement for all benchmark problems, as well as the capability to reproduce the results of previous modeling and experimental studies.

Stochastic Estimation of the Slip Factor in Apparent Permeability Model of Gas Transport in Porous Media

In this paper, we introduce an estimation of the random Klinkenberg slip coefficient in the apparent permeability model using a chaos decomposition technique. The apparent permeability expression (Klinkenberg model) is used to describe natural gas transport in low-permeability media. In this process, the Klinkenberg factor is considered as a random parameter that depends on two random variables. The mean and variance (or standard deviation) of the two random variables can be estimated from the empirical data available in the literature. Therefore, the variation in the pressure is related directly to the random variation in the Klinkenberg factor. The polynomial chaos expansion is used to decompose the governing equation into a set of coupled deterministic equations that are solved and then used to compute the mean and variance of the solution. The algorithm of how to solve the deterministic coupled system is also presented. For verification, the model and its solution have been compared with the analytical solution of the basic steady-state version of the model. The comparison shows a very good agreement. The effects of a number of important parameters have been presented in graphs and discussed. It was found that the stochastic model works very well with small values of the liquid equivalent permeability, which meets the characteristics of low-permeability reservoirs. Also, the stochastic model works very well with small values of gas viscosity. On the other hand, the porosity seems to be not engaged well with the low-permeability model. The sensitivity of selection of random parameters is also investigated as well as the transient effect.

Upscaled Model for Multicomponent Gas Transport in Porous Media Incorporating Slip Effect

A two-scale model for multicomponent gas transport in porous media is developed. At the pore-scale, Stefan–Maxwell formulation is used to describe the multi-gas transport together with the mass and momentum conservation equations, whereas on the solid/fluid interface, a slip velocity according to the Kramers–Kistemaker condition is taken into account. The pore-scale equations are then upscaled using a formal homogenization procedure. The macroscopic model shows that the total average velocity is modified by a slip velocity which depends mostly on the diffusive flux of the light gas in the mixture. Application to hydrogen transport in electrochemical devices such as fuel cell, electrochemical hydrogen purifier/compressor, in which considerable contrast of molar mass between the gases occurs, is carried out. As a result, the gas slip effect can modify considerably the gas transport behavior within the porous medium of the devices. This is an important result because the gas transport mechanisms play a crucial role in their efficiency.

Relating wind-induced gas transport in porous media to wind speed and medium characteristics

Impact of wind speed characteristics and porous medium properties on wind-induced gas dispersion in near surface porous media, was investigated using experimental data for four porous materials and 11 wind conditions. The data were used to evaluate two recent empirical models (involving 2 and 3 empirical parameters, respectively) for predicting wind-induced gas dispersion in porous media as a function of distance to the wind-exposed surface. Regression analyses were used to identify relationships between the empirical model parameters, wind speed characteristics (wind speed, wind turbulence index, wind speed power spectrum) and porous medium properties (particle diameter, porosity, hydraulic conductivity, tortuosity). While only one parameter in the three-parameter model correlated well with wind speed power spectrum characteristics and porous medium properties, both parameters in the two-parameter model correlated well with r2 ranging up to 0.99 where r2 is the regression coefficient. The strongest relationships were observed between model parameters and vertical wind speed, minimum particle diameter, particle surface roughness, saturated hydraulic conductivity and molecular gas diffusion coefficient with r2 values generally on the order of 0.85–0.95. Overall, the results indicated that wind-induced gas dispersion porous media can be predicted based on wind speed characteristics and porous medium properties.

Drying of porous media by concurrent drainage and evaporation: A pore network modeling study

Drainage and evaporation can occur simultaneously during the drying of porous media, but the interactions between these processes and their effects on drying are rarely studied. In this work, we develop a pore network model that considers drainage, evaporation, and rarefied multi-component gas transport in porous media with nanoscale pores. Using this model, we investigate the drying of a liquid solvent-saturated porous medium enabled by the flow of purge gas through it. Simulations show that drying progresses in three stages, and the solvent removal by drainage effects (evaporation effects) becomes weaker (stronger) as drying progresses through these stages. Interestingly, drainage can contribute considerably to solvent removal even after evaporation effects become very strong, especially when the applied pressure difference across the porous medium is low. We show that these phenomena are the results of the coupling between the drainage and evaporation effects and this coupling depends on the operating conditions and the stage of drying.

Molecular Dynamics Study of Oxygen Diffusivity in Catalyst Layer

Proton exchange membrane fuel cell (PEMFC) is one of the promising systems for electricity generation. To achieve the high power density operation, the oxygen gas diffusion phenomenon in the porous media of PEMFCs is a key factor. Thus, gas transport in the porous media, i.e. gas diffusion layer (GDL), microporous layer (MPL), and catalyst layer (CL), has been investigated by experiments and simulations. Gas transport in such porous media is governed by both the molecular and Knudsen diffusion. In particular, the Knudsen diffusion becomes a predominant diffusion mechanism in CL because the characteristic pore size of CL is comparable to the mean free path of oxygen molecules, where gas molecules collide with solid surfaces more frequently than other gas molecules. In this case, the behavior of oxygen molecules on ionomer surfaces should be taken into account for the accurate analysis of gas transport in CL. Therefore, we have performed molecular dynamics (MD) simulations to investigate the scattering behavior of oxygen molecules on ionomer surfaces. Our previous study revealed that the scattering processes and energy accommodation strongly depend on the surface composition of ionomer film. While oxygen molecules that collide with Nafion chains tend to be reflected directly from the surface, they tend to be adsorbed on water molecules long enough and desorbed with better accommodation. The conventional scattering model does not fully describe such scattering behavior. The diffusion of adsorbed molecules on solid surfaces is called “surface diffusion”. This result suggests that both the Knudsen and surface diffusion coexist as the diffusion path in CL. Although some experiments show the contribution of the surface diffusion on the overall diffusion in nanopores, the impact of the surface diffusion on the overall gas transport in CL remains to be unclear. The elucidation of diffusion mechanism incorporating the surface diffusion is of great importance to analyze accurately the oxygen gas transport in CL. In this study, MD simulations have been performed to investigate the oxygen diffusivity in nanopore of CL. In order to quantitatively evaluate the contribution of the diffusion in gas phase and surface diffusion to the overall oxygen diffusivity, a simulation system of a slit nanopore composed of an ionomer thin film was constructed. The diffusion mechanism was classified into the gaseous diffusion and surface diffusion in the pore. The diffusion coefficients of oxygen molecules in each region were compared at different water contents and pore sizes. The overall oxygen diffusivity increases remarkably with an increase in the pore size. In contrast, at the same pore size, the oxygen diffusivity in the surface region decreases significantly as the water content increases. This result suggests that the oxygen diffusivity in the nanopore is affected by the adsorption and surface diffusion on the water layer. The elucidation of diffusion mechanisms in nanopore of CL allows us to construct a more accurate model of oxygen behavior on an ionomer surface compared to the conventional Knudsen diffusion and provides a quantitative estimation of the oxygen transport resistance.

Multi-Physics HPC Simulations for PEM Fuel Cell with the Open-Source Code TRUST

Numerical simulations for PEMFC (Proton Exchange Membrane Fuel Cell) for understanding and co-optimization of designs (bipolar plate with MEA (Membrane Electrode Assembly)) play a key role to achieve the objective of cost reduction for PEMFC. To improve the overall simulation of PEMFC used for the design validation step of the bipolar plates, very detailed physical mechanisms are included in the PEMFC models in order to describe the main electrochemical and transport mechanisms (local 2D models) [1]. Present limitation of these approaches is that they are difficult to integrate in higher scale simulations, in terms of geometry. Pseudo 3D models allow to integrate the real geometry of the bipolar plate, but with reduction in term of meshing and physics [2]. To go further, the objectives are: i/ to perform reference simulations with no compromise (in term of geometry and physics); ii/ to promote multi-physics model with open-source code, to the international community. TRUST platform is used. The platform TRUST-FC is built on the CEA/DEN thermohydraulic TRUST framework. This C++ framework is an open-source software package of Computational Fluid Dynamics (CFD) which supports massively parallel computations with a distributed memory model (MPI) [3]. TRUST-FC contains physical models for gas, liquid, electron, ion transport and heat transfer. A model of anisotropic heat conduction is developed in TRUST-FC for taking into account the in-plane and through-plane anisotropic conductivity according to the real deformation of the gas diffusion layer (GDL) by the bipolar plate in the mechanical assembly. The multi-scale modelling framework for solving PEMFC specific physics are also developed: electrochemical reactions, multi-components gas transport in porous media for GDL and catalyst layer, coupling of free flow with porous media flow [2]. The used physical models are validated on the CEA-LITEN multi-physics and multi-scale simulation platform MUSES built on Comsol Multiphysic [1-2] and the experimental measurements [4]. TRUST-FC advantages are the robust numerical methods and the massive parallelism that allows to simulate coupled multi-physics phenomena on large scale domains. TRUST-FC can currently simulate a real CAD design containing tens millions of elements and numerous state variables in a few hours on cluster. SALOME is used as the meshing and visualization tool [5]. A full simulation on a design of bipolar plate with the MEA is presented (Fig.: temperature profile on the bipolar plate, with the coupling of flow cooling and heat production of the reactions) and discussed and compared to pseudo 3D simulations on Comsol Multiphysics. [1] Randrianarizafy B., Schott P., Chandesris M., Gerard M. and Bultel Y. Design optimization of rib/channel patterns in a PEMFC through performance heterogeneities modelling. Int. J. Hydrogen Energy, 43(18):8907 8926, (2018) [2] Nandjou F.,Poirot-Crouvezier J.-P.,Chandesris M. and Bultel Y.A pseudo-3D model to investigate heat and water transport in large area fPEMg fuel cells - Part 1: Model development and validation. Int. J. Hydrogen Energy, 41(34):15545-15561, (2016) [3] https://sourceforge.net/projects/trust-platform/[4] Robin C., Gerard M., d'Arbigny J., Schott P., Jabbour L. and Bultel Y. Development and experimental validation of a PEM fuel cell 2D-model to study heterogeneities effects along large-area cell surface. Int. J. Hydrogen Energy, 40(32):10211-10230, (2015) [5] https://www.salome-platform.org/ Figure 1

Saturation-dependent gas transport in sand packs: Experiments and theoretical applications

Understanding gas transport in porous media and its mechanism has broad applications in various research areas, such as carbon sequestration in deep saline aquifers and gas explorations in reservoir rocks. Gas transport is mainly controlled by pore space geometrical and morphological characteristics. In this study, we apply a physically-based model developed using concepts from percolation theory (PT) and the effective-medium approximation (EMA) to better understand diffusion and permeability of gas in packings of angular and rounded sand grains as well as glass beads. Two average sizes of grain i.e., 0.3 and 0.5 mm were used to pack sands in a column of 6 cm height and 4.9 cm diameter so that the total porosity of all packs was near 0.4. Water content, gas-filled porosity (also known as gas content), gas diffusion, and gas permeability were measured at different capillary pressures. The X-ray computed tomography method and the 3DMA-Rock software package were applied to determine the average pore coordination number z. Results showed that both saturation-dependent diffusion and permeability of gas showed almost linear behavior at higher gas-filled porosities, while deviated substantially from linear scaling at lower gas saturations. Comparing the theory with the diffusion and permeability experiments showed that the determined value of z ranged between 2.8 and 5.3, not greatly different from X-ray computed tomography results. The obtained results clearly indicate that the effect of the pore-throat size distribution on gas diffusion and permeability was minimal in these sand and glass bead packs.

Scaling law for slip flow of gases in nanoporous media from nanofluidics, rocks, and pore-scale simulations

In unconventional reservoirs, as the effective pore size becomes close to the mean free path of gas molecules, gas transport in porous media begins to deviate from Darcy’s law. The objective of this study is to explore the similarities of gas flows in nanochannels and core samples as well as those simulated by direct simulation BGK (DSBGK), a particle-based method that solves the Bhatnagar-Gross-Krook (BGK) equation.Due to difficulties in fabrication and experimentation, previous study on gas flow experiments in nanochannels is very limited. In this work, steady-state gas flow was measured in reactive-ion etched nanochannels with a controlled channel size on a sillicon wafer. A core-based permeability measurement apparatus was used to perform steady-state gas flow measurements on carbonate and shale samples. Klinkenberg permeability was obtained under varying pore pressures but constant temperature and effective stress. Methane was used in nanofluidic and rock experiments, making them directly comparable. Results from both experiments were then compared to gas flow simulations by DSBGK method carried out on several independently constructed geometry models. DSBGK uses hundreds of millions of simulated molecules to approximate gas flow inside the pore space. The intermolecular collisions are handled by directly integrating the BGK equation along each molecules trajectory, rather than through a sampling scheme like that in the direct simulation Monte-Carlo (DSMC) method. Consequently, the stochastic noise is significantly reduced, and simulation of nano-scale gas flows in complex geometries becomes computationally affordable.The slippage factors obtained from these independent studies varied across three orders of magnitude, yet they all appear to collapse on a single scaling relation where the slippage factor in the slip flow regime is inversely proportional to the square root of intrinsic permeability over porosity. Our correlation could also fit the data in the literature, which were often obtained using nitrogen, after correcting for temperature and gas properties. This study contributes to rock characterization, well testing analysis as well as the understanding of rarefied gas transport in porous media.

A high-order hybridizable discontinuous Galerkin method with fast convergence to steady-state solutions of the gas kinetic equation

The mass flow rate of Poiseuille flow of rarefied gas through long ducts of two-dimensional cross-sections with arbitrary shape is critical in the pore-network modeling of gas transport in porous media. Here, for the first time, the high-order hybridizable discontinuous Galerkin (HDG) method is used to find the steady-state solution of the linearized Bhatnagar–Gross–Krook equation on two-dimensional triangular meshes. The velocity distribution function and its traces are approximated in piecewise polynomial spaces (of degree up to 4) on the triangular meshes and mesh skeletons, respectively. By employing a numerical flux that is derived from the first-order upwind scheme and imposing its continuity weakly on the mesh skeletons, global systems for unknown traces are obtained with fewer coupled degrees of freedom when compared to the original discontinuous Galerkin formulation. To achieve fast convergence to the steady-state solution, a diffusion-like equation for flow velocity, which is asymptotic-preserving into the fluid dynamic limit, is solved by the HDG simultaneously on the same meshes. The proposed HDG-synthetic iterative scheme is proved to be accurate and efficient. Specifically, for flows in the near-continuum regime, numerical simulations have shown that, to achieve the same level of accuracy, our scheme could be faster than the conventional iterative scheme by two orders of magnitude, also it is faster than the synthetic iterative scheme based on the finite difference discretization in the spatial space by one order of magnitude. In addition, the implicit HDG method is more efficient than an explicit discontinuous Galerkin gas kinetic solver, as well as the implicit discontinuous Galerkin scheme when the degree of approximating polynomial is larger than 2. The HDG-synthetic iterative scheme is ready to be extended to simulate rarefied gas mixtures and the Boltzmann collision operator.

Analytical solution of the apparent-permeability gas-transport equation in porous media

The current paper is concerned with the power-series solution of the apparent permeability (Klinkenberg) model of gas transport in porous media. The one-dimensional radial partial differential equation has been transformed into an ordinary differential equation using a universal transform. The transformed equation has been solved using the infinite power-series technique. Results are obtained for various values of the given physical parameters and represented in graphs. We have presented results for both Dirichlet and Neumann boundary conditions at the well to express, respectively, the pressure and the flow rate at the well.

Effect of wind turbulence on gas transport in porous media: experimental method and preliminary results

We demonstrate a novel experimental arrangement for measuring wind turbulence‐induced gas transport in dry porous media under controlled conditions. This equipment was applied to assess the effect of wind turbulence on gas transport (quantified as a dispersion coefficient) as a function of distance to the surface of the porous medium exposed to wind. Two different strategies for the measurement of wind‐induced gas transport were compared. Experiments were carried out with O2 and CO2 as tracer gases with average vertical wind speeds of 0.02–1.06 m s−1. Oxygen breakthrough curves as a function of distance to the wind‐exposed surface of the porous medium were analysed numerically with a finite‐difference‐based model to assess gas transport. We showed that wind turbulence‐induced gas transport is an important transport mechanism that can be 20–70 times larger than molecular diffusion‐induced transport. Wind conditions and properties of the porous medium had strong controlling effects on this relationship. Importantly, we show that even though wind‐induced gas transport is greatest near to the wind‐exposed surface, it can have marked effects on the variation in gas concentration at much greater depths.Highlights We explored the effect of atmospheric wind turbulence on gas transport in porous media. We measured the depth relation of wind‐induced dispersion in porous media for real wind conditions. Wind‐induced gas dispersion coefficients were 20–70 times larger than molecular diffusion. Wind turbulence can potentially have a considerable effect on gas transport in porous media.

A study on pressure-driven gas transport in porous media: from nanoscale to microscale

Gas flow in porous media can be seen in various engineering devices such as catalytic converters and fuel cells. It is important to understand transport phenomena in porous media for improvement of the performance of such devices. Porous media with pores as small as the mean free path of gas molecules are used in such devices as proton exchange membrane fuel cells. It is difficult to measure molecular transport through such small pores in the experimental approach. In addition, even when using theoretical or numerical approaches, gas flow through nanoscale pores must be treated by the Boltzmann equation rather than the Navier–Stokes equations because it cannot be considered as a continuum. Thus, conventional analyses based on the continuum hypothesis are inadequate and the transport phenomena in porous media with nanoscale pores are not yet clearly understood. In this study, we represented porous media by randomly arranged solid spherical particles and simulated pressure-driven gas flow through the porous media by using the direct simulation Monte Carlo (DSMC) method based on the Boltzmann equation. DSMC simulations were performed for different porosities and different sizes of solid particles of porous media. It was confirmed that Darcy’s law holds even in the case of porous media with micro-/nanoscale pores. Using the obtained results, we constructed expressions to estimate the pressure-driven gas transport in porous media with micro-/nanoscale pores and porosity ranging from 0.3 to 0.5. The flow velocities estimated by using the constructed expressions agreed well with those obtained in the DSMC simulations.

Computational Study of Pressure-Driven Gas Transport in Nanostructured Carbons: An Alternative Approach.

We demonstrated a computationally efficient method in nonequilibrium molecular dynamics (NEMD) simulations to study pressure-driven gas transport in porous media. The reflecting particle method (RPM)14 was used to establish a steady-state gas flow along the transport channel, and the gas density in the feed chamber was properly adjusted to allow a constant pressure drop under various conditions by using a perturbation-relaxation loop developed here. This method was validated for methane flow through carbon nanotubes over a wide range of temperatures, giving results comparable to those of the commonly used dual control volume grand canonical molecular dynamics (DCV-GCMD) method but at least 20 times more efficient, even though the transport condition tested is favorable for the latter. This made it possible to perform systematic studies on the effects of temperature, pressure, and channel size on the transport behaviors. Our study shows that adsorption density varies significantly with temperature, which dramatically influences the transport mechanisms, especially in small channels at low temperatures and under high pressures. This newly developed NEMD method can be readily extended to study gas transport through channels with more complex surface morphology.