Pore-scale Numerical Simulation Research Articles

Pore-scale simulation of two-phase flow in biporous media

Enhancing both permeability and capillary pumping in porous structures has emerged as a key focus for researchers, leading to the development of biporous media. While experimental studies on these structures have been conducted recently, there is a lack of numerical simulations due to difficulties in describing the geometry. To address this gap, the present study explores pore-scale numerical simulation of two-phase capillary flow in biporous media. A new simplified biporous structure is proposed, featuring a staggered arrangement of clusters, with each cluster composed of closely packed solid particles. For comparison, a monoporous media case is contrasted and represented using a conventional staggered arrangement of solid particles. Both passive and active capillary flow modes are considered in the present study. The numerical results align well with previous experimental findings on biporous media, indicating that the proposed biporous geometry effectively models two-phase flow in complex structures at a reasonable computational cost. The results show that capillary effects in biporous media are up to two times more effective than in monoporous structures. Simultaneously, permeability is enhanced by a factor of four in biporous media under similar circumstances, with most of the mass flow rate (more than 95%) passing through the larger pores between clusters. This combined impact of increased capillary action and higher permeability leads to enhanced wicking performance in biporous structures. The outcomes can help to understand two-phase flow physics in the biporous structure and develop reliable models for the simulation of biporous media on a macroscopic scale. Numerical modeling and comprehension of capillary structures play a crucial role in designing optimized geometries to enhance their performance.

Pore-Scale Numerical Simulation of Acid-Rock Reaction Processes in Carbonate Reservoirs.

In the process of acidizing carbonate reservoirs, dissolution is employed for reservoir modification to enhance recovery rates. This study establishes a numerical model at the pore scale for acid-rock reaction flow based on a microscopic continuum medium model, integrating phase-field theory and component transport models. Subsequently, the results of the Darcy-Brinkman-Stokes model are compared to those of the arbitrary Lagrange-Euler method to validate the accuracy of the model. Finally, the flow behavior of the acid solution at the pore scale and the complex dissolution mechanisms in carbonate reservoirs are analyzed. The research indicates that the microscopic pore-scale dissolution in carbonate reservoirs mainly manifests as five dissolution modes: uniform dissolution, compact dissolution, conical wormholes, dominant wormhole, and ramified wormholes. Different distributions of microfractures will alter the flow state of the acid solution and the rock-acid reaction process within the pores. Once the wormhole breakthrough occurs, there is an increased probability of acid flow through the wormhole to the outlet, leading to a decrease in the effectiveness of the acidizing carbonate reservoirs. A proper understanding of pore-scale acid-rock reaction laws is of great significance for the development of carbonate oil and gas reservoirs.

Melting of PCM-graphite foam composites with contact thermal resistance: Pore-scale simulation

Latent heat energy storage systems have emerged as a solution to intermittency and instability in renewable energy sources, the phase transition of phase change materials (PCMs) within these systems is hindered by their poor thermal conductivity. Graphite foam, renowned for its superior thermal properties, stands as an ideal choice to improve the melting efficiency of PCMs. In the current study, the melting behavior of PCM-graphite foam composites is simulated by the pore-scale numerical simulation (PNS) by employing cubic idealized cells with spherical pores and cylindrical channels. The computed results demonstrate that the porosity and contact thermal resistance of graphite foam are key factors affecting the melting efficiency of PCMs. The full melting time (FMT) of PCMs increases with porosity, achieving a 53.1 % improvement as porosity decreases from 0.90 to 0.75. In addition, the absence of natural convection markedly diminishes the maximum melting rate of PCM, reducing it by 33.5 % to 50.3 % for porosity from 0.75 to 0.90. Notably, it needs to be emphasized that the melting rate of PCM is significantly reduced when the contact thermal resistance surpasses 1.0 × 10–4 m2·K·W−1. Compared to the idealized condition without contact thermal resistance, the dimensionless FMTs for porosities of 0.75, 0.80, 0.85, and 0.90 extend by 27.3 %, 27.1 %, 22.2 % and 23.9 % for a contact thermal resistance of 5.0 × 10–4 m2·K·W−1, respectively.

Multiphase flow simulation in hybrid porous structure

Recent research has primarily focused on creating biporous and hybrid porous structures with multiple pore sizes and length scales to optimize capillary pressure and permeability. Despite numerous experimental investigations on biporous and hybrid media, there is a noticeable absence of numerical simulations that explore the multiphase flow within these media. Therefore, the present study aims to conduct a pore-scale numerical simulation of two-phase flow in a biporous structure. The biporous structure is proposed by arranging clusters of solid particles in a staggered regular pattern, with each cluster consisting of closely packed particles. The dimensions and characteristics of the simulated structure are based on previous experimental literature on biporous and hybrid media. A monoporous structure simulation is also included for comparison with biporous results. ANSYS Fluent is utilized to carry out the numerical simulations of capillary pumping flow. The simulation results indicate that the permeability and average capillary pressure of the biporous media are four times and over one and a half times higher, respectively, compared to those of the monoporous media. The presence of smaller pathways within each cluster of a biporous and hybrid porous media enhances the capillary effect in comparison to conventional monoporous structures. Additionally, the larger pores between the clusters contribute to a higher permeability of the hybrid porous structure. As a result, the combined effect of increased capillary action and higher permeability leads to improved performance of the hybrid porous structure. Overall, the proposed simplified biporous geometry accurately models fluid flow in real, complex biporous and hybrid structures.

Pore-scale investigation of transpiration cooling based on triply periodic minimal surface

Triply periodic minimal surface (TPMS) can control the structural characteristic parameters through implicit functions, which possesses great significance for the customization of porous matrix topologies used in transpiration cooling. An innovative geometric reconstruction and meshing method is developed to reconstruct different TPMS porous matrix, a pore-scale numerical simulation of transpiration cooling coupled with mainstream is performed, and a hot wind tunnel experiment is developed to validate the numerical method. The main conclusions are as follows: The flow resistance loss of coolant within porous matrix is greatly influenced by the shape and size of pore throat, and due to the interaction between mainstream and coolant, higher flow resistance loss doesn't mean a higher coolant injection pressure. The influence of pore morphology on cooling efficiency is more obvious at higher coolant dosage, and considering the coolant injection pressure and cooling efficiency at the same time, the W(B) structure shows the best comprehensive performance. The generation of counter-rotating vortex pairs (CRVPs) in coolant film might lead to the increase in skin-friction coefficient on the structure surface, and the complex outflow angle of coolant can inhibit the generation of CRVPs, so for the case needs to reduce the skin-friction resistance, W(A) structure is recommended.

Pore-scale study of the effects of grain size on the capillary-associated interfacial area during primary drainage

This study investigates the capillary-associated interfacial area during primary drainage in unconsolidated porous media for different mean grain diameters, employing pore-scale numerical simulation with the phase-field method. Five, two-dimensional porous media are generated with the same grain/pore size uniformity, but different mean grain diameters, ranging between 0.24 mm and 0.94 mm. Three cases are considered, representing stable displacement, crossover zone and viscous-fingering regimes. The findings reveal that the capillary-associated interfacial production rate increases with decreasing grain size. Specifically, the capillary-associated interfacial production rate is higher for the viscous-fingering regime compared to the stable displacement. The maximum capillary-associated interfacial area obtained for different grain sizes agrees with the correlation established in our previous study using the kinetic interface-sensitive tracer. Furthermore, the capillary-associated interfacial area is divided into two parts: one related to the disconnected water clusters and the other related to the connected water. The disconnected capillary-associated interfacial area correlates with the average volume of the clusters, following an exponential function. Furthermore, the average volume of the clusters depends on both, the grain size and displacement regime. Moreover, it is found that the displacement regime has a great impact on the connected capillary-associated interfacial area. For stable displacement, the connected capillary-associated interfacial area is larger for larger mean grain diameter. In contrast, for fingering, the connected capillary-associated interfacial area is smaller for increasing mean grain diameter.

Melting characteristics of phase change material inside graded aluminum foam based on tetrakaidecahedron model: Pore-scale simulations

Metallic ligaments in open-cell metal foams have effectively improved the heat conduction of phase change materials (PCMs). However, when the gradient direction of porosity is perpendicular to the direction of heat conduction, the mechanism by which graded foam (GF) influences the phase transition of PCM remains elusive. To address this issue, this study employs the pore-scale numerical simulation and develops a numerical model to study the impact of GF on the charging of PCM-foam composites. The analysis identifies the local convection and heat conduction as the most crucial factors influencing heat transfer during phase transition. To effectively examine the role of metallic ligaments in the phase transition, simplified tetrakaidecahedron cells are adopted. The results demonstrate that the GF delays the charging of PCM. Specifically, compared to the homogeneous foam, the full melting time (FMT) of PCM in the GF increases by over 29%. Additionally, the integral-mean temperature response rate (ITRR) of PCM in the GF decreases by 20.2% to 22.1%. Within the GF, the PCM requires more time to melt completely in the tetrakaidecahedron cells with high porosity. Moreover, local convection plays a pivotal role, considerably reducing the FMTs of PCMs by over 85% and increasing the ITRRs of these structures by 588.1% to 599.0%.

Three-dimensional pore-scale study of methane hydrate dissociation mechanisms based on micro-CT images

<p>Methane hydrate is a promising source of alternative energy. An in-depth understanding of the hydrate dissociation mechanism is crucial for the efficient extraction. In the present work, a comprehensive set of pore-scale numerical studies of hydrate dissociation mechanisms is presented. Pore-scale lattice Boltzmann (LB) models are proposed to simulate the multiphysics process during methane hydrate dissociation. The numerical simulations employ the actual hydrate sediment pore structure obtained by the micro-CT imaging. Experimental results of xenon hydrate dissociation are compared with the numerical simulations, indicating that the observed hydrate pore habits evolution is accurately captured by the proposed LB models. Furthermore, simulations of methane hydrate dissociation under different sediment water saturations, fluid flow rates and thermal conditions are conducted. Heat and mass transfer limitations both have significant effects on the methane hydrate dissociation rate. The bubble movement can further influence the dissociation process. Dissociation patterns can be divided into three categories, uniform, non-uniform and wormholing. The fluid flow impacts hydrate dissociation rates differently in three-dimensional real structures compared to two-dimensional idealized ones, influenced by variations in hydrate pore habits and flow properties. Finally, upscaling investigations are conducted to provide the permeability and kinetic models for the representative elementary volume (REV)-scale production forecast. Due to the difference in the hydrate pore habits and dissociation mechanisms, the three-dimensional upscaling results contrast with prior findings from two-dimensional studies. The present work provides a paradigm for pore-scale numerical simulation studies on the hydrate dissociation, which can offer theoretical guidance on efficient hydrate extraction.</p>

Pore-scale numerical simulation of spontaneous imbibition in porous media containing fractures

Pore-scale numerical simulation of spontaneous imbibition in porous media containing fractures

Effect of natural convection on charging of phase change materials in graded metal foam: Pore-scale simulation

Open-cell metal foams (MFs) possess exceptional thermal conductivity and are often utilized to enhance the phase transition efficiency of phase change materials (PCMs) that have insufficient thermal conductivity. The movement of energy within MFs during phase transition is influenced by natural convection and heat conduction. In this study, pore-scale numerical simulation (PNS) was employed to study the effect of natural convection on the phase transition of graded MFs (positive gradient foam-PGF, homogenous foam-HF, and negative gradient foam-NGF). Simplified tetrakaidecahedron cells were used for PNS to capture the primary geometric features of MFs. Results have shown that natural convection significantly influences the phase transition process of PCMs. Compared to pure thermal conductivity, the full melting times (FMTs) of PCMs in PGF, HF, and NGF were reduced by 85.2% - 88.3% when natural convection was considered. Furthermore, the integrated average temperature response rates of PCMs in PGF, HF, and NGF, considering natural convection, were improved by 378.4%, 301.9%, and 342.5%, respectively. The presence of natural convection resulted in a gradient in the phase interface distribution and temperature field. The PNS method proved useful in illustrating the influence of metallic ligaments on phase interface and temperature distributions.

Pore-scale numerical simulation of supercritical CO2-brine two-phase flow based on VOF method

CO2 capture and storage technology is favorable for the reduction of CO2 emissions. In recent years, a great number of research achievements have been obtained on CO2 geological storage from nano scale to oil/gas reservoir scale, but most studies only focus on the flow behaviors in single-dimension porous media. Besides, the physical experiment method is influenced by many uncertain factors and consumes a lot of time and cost. In order to deeply understand the flow behaviors in the process of CO2 geological storage in microscopic view and increase the volume of CO2 geological storage, this paper established 2D and 3D models by using VOF (Volume of Fluid) method which can track the dynamic change of two-phase interface, to numerically simulate supercritical CO2-brine two-phase flow. Then, the distribution characteristics of CO2 clusters and the variation laws of CO2 saturation under different wettability, capillary number and viscosity ratio conditions were compared, and the intrinsic mechanisms of CO2 storage at pore scale were revealed. And the following research results were obtained. First, with the increase of rock wettability to CO2, the sweep range of CO2 enlarged, and the disconnection frequency of CO2 clusters deceased, and thus the volume of CO2 storage increased. Second, with the increase of capillary number, the displacement mode transformed from capillary fingering to stable displacement, and thus the volume of CO2 storage increased. Third, as the viscosity of injected supercritical CO2 gradually approached that of brine, the flow resistance between two-phase fluids decreased, promoting the "lubricating effect". As a result, the flow capacity of CO2 phase was improved, and thus the volume of CO2 storage was increased. Fourth, the influence degrees of wettability, capillary number and viscosity ratio on CO2 saturation were different in multi-dimensional porous media models. In conclusion, the CO2-brine two-phase flow simulation based on VOF method revealed the flow mechanisms in the process of CO2 geological storage at pore scale, which is of guiding significance to the development of CCUS technology and provides theoretical guidance and technical support for the study of CO2 geological storage in a larger scale.

Insights into the Microscopic Oil-Water Flow Characteristics and Displacement Mechanisms during Waterflooding in Sandstone Reservoir Rock Based on Micro-CT Technology: A Pore-Scale Numerical Simulation Study.

The low oil recovery rate observed in current oil fields is largely attributed to the presence of remaining oil trapped in the pores of porous media during waterflooding. To improve the recovery rate, it is imperative to gain an understanding of the oil-water flow characteristics and displacement mechanisms during waterflooding, as well as to elucidate the underlying mobilization mechanisms of residual oil at the pore scale. In this paper, we explore these issues in depth by numerically investigating the influence of factors such as water injection velocities, oil-water viscosity ratios, and wettability conditions on pore-scale oil-water flow characteristics and oil recovery rate. To this end, we employ a direct numerical simulation (DNS) method in conjunction with the volume of fluid (VOF) method to study the microscopic displacement mechanisms of waterflooding in a reconstructed two-dimensional digital rock core based on micro-CT technology. In addition, the particle tracing method is adopted to identify the flow path and dominant areas during waterflooding in order to mobilize the residual oil within the pores. The findings indicate that the oil-water flow characteristics in porous media are determined by the interplay between capillary and viscous forces. Furthermore, the oil recovery rate is 10.6% and 24.7% lower under strong water-wet and oil-wet conditions than that (32.36%) under intermediate wettability conditions, and the final oil recovery rate is higher under water-wet conditions than under oil-wet conditions. The seepage path and the dominant areas are directly linked to the capillarity formed during waterflooding. The findings of this study are significant in terms of enhancing the recovery rate of residual oil and provide a novel perspective for understanding the waterflooding process.

Upscaling methane hydrate dissociation kinetic model during depressurisation

In the present work, a pore-scale numerical simulation of methane hydrate dissociation by depressurisation is conducted to analyze the effect of heat and mass transfer on the dissociation rate for scaling up the kinetic model at the representative element volume (REV) scale. The mass transport limitation shows that the hydrate dissociation preferred to occur near the gas phase. The effective reaction surface area is introduced to measure the exposed hydrate surface to the gas phase during gas and water migration and is modelled as a function of local hydrate and water saturation and hydrate pore habits. Heat transport limitation is computed with the one-temperature model due to the local thermal equilibrium. Compared to the pore-scale simulation, the proposed REV-scale kinetic model predicts dissociation rates with a relative error of less than 10%, which is expected to increase the precision of the hydrate recovery forecast.

Pore-scale numerical simulation and LTNE analysis for fully-developed forced convective heat transfer in packed beds of mono-sized rough spheres covering near-wall and core regions

This work presents a novel method to acquire the effective convection heat transfer coefficients on the interfaces of wall-fluid contacts and sphere-fluid in both the near-wall region and core region of sphere-packed beds. Pore-scale numerical simulations are first conducted for different porosities pertaining to two regular body-centered cubic and face-centered cubic packings, packing lattice angles, inflow velocities, and ratios of the thermal conductivities of the solid and fluid. A finite-thickness metal reactor wall is included in the pore-scale simulation to make the heating boundary conditions more realistic. The outer surface of the metal wall is subjected to a uniform heat flux. The heat then passes through the metal wall and dispenses into the solid and liquid phases of the packed bed through the wall-sphere and the wall-fluid interfaces at the inner wall according to the physical mechanisms. Within the reactor, the heat transfer mechanism is separately considered in the near-wall region and the core region due to the intrinsic differences in the flow pattern. For each packing, different inflow angles are examined and exhibit different flow patterns and heat transfer performances. After the whole domain is hydrodynamically and thermally fully-developed, the local effective convection heat transfer coefficients can then be calculated. These coefficients reflect the realistic local convection either on the reactor wall or on the sphere-fluid interface. These convection heat transfer coefficients, along with the effective solid thermal conductivity through the contacting spheres obtained in our previous work, are implanted into a modified two-equation LTNE model. This modified LTNE model yields linear longitudinal fully-developed temperature distributions within the reactor having discrepancies in the temperature slopes less than ±10% from those by the pore-scale numerical simulations. In contrast, the conventional model even fails to reach linear distributions of the solid- and liquid-phase temperatures, which are expected for fully-developed packed-bed flows.

Effects of hollow skeleton on melting process in copper foam

The compositing of porous medium and phase change material is an effective way to improve the heat transfer performance of solid-liquid phase change energy storage system. In this paper, we reconstruct the three-dimensional numerical structure of the copper foam by using the micro computed tomography, and then conduct the pore-scale numerical simulation of the melting process in a cubic cavity filled with the phase change material comprised of the copper foam via the lattice Boltzmann method. The effects of the hollow skeleton on the melting process are discussed in detail under different Rayleigh numbers and ratios of thermal conductivity of the copper foam to that of the phase change material. The results show that the hollow skeleton copper foam possesses a lower average Nusselt number along the left wall at the early stage of the melting process, a slower melting rate, and a higher energy storage efficiency than the solid skeleton copper foam. Comparing with the skeleton region of the copper foam, the heat transfer rate entering the cubic cavity through the hollow region of the skeleton is almost negligible. Because of the competition between heat conduction and natural convection, the heat transfer enhancement efficiency of copper foam first increases, then decreases, and then increases again with the increase of the Fourier number. When the Rayleigh number decreases, the energy storage efficiency increases, and the natural convection also weakens. Meanwhile, the fluctuation of the heat transfer enhancement efficiency decreases as the Fourier number increases, and the gap of the heat transfer enhancement efficiency between the hollow skeleton copper foam and the solid skeleton copper foam becomes smaller. When the ratio of the thermal conductivity of the copper foam skeleton to that of the phase change material increases, the energy storage efficiency is relatively high at the early stage of the melting process but becomes relatively low when the melting process is completed. With a larger thermal conductivity ratio, the heat transfer rate entering the cubic cavity through the skeleton region of the copper foam becomes dominant, which reduces the effect of the hollow skeleton on heat transfer, and thus the gap of the heat transfer enhancement efficiency between the hollow skeleton copper foam and the solid skeleton copper foam becomes relatively small.

Investigation on permeability anisotropy in unconsolidated hydrate-bearing sediments based on pore-scale numerical simulation: Effect of mineral particle shape and pore-filling

Resulting from the preferential alignment of sediment particles, permeability anisotropy of unconsolidated strata is an essential parameter for natural gas hydrate (NGH) exploitation. Fundamentally, the effect of mineral particle shape and pore-filling NGH on the permeability anisotropy are considered in this study. A simulation with combining methods of particle flow code and computational fluid dynamics is conducted to construct a series of numerical samples and calculate the anisotropic permeabilities. Different particles shapes and hydrate saturations are preinstalled in these samples. It is firstly proved that the elongated particles have little effect on permeability anisotropy. Meanwhile, the preferential alignment of plate-shaped particles is the main reason for permeability anisotropy. In principle, the difference between intermediate orthogonal axes and short orthogonal axes of plate-shaped particles makes the tortuosity different in directions, thus inducing the permeability anisotropy. The anisotropy ratio decreases with the increase of hydrate saturation, which is caused by the nonlinear decline of main channel flow resulting from the thrombus-like hydrate particles. The results provide an insight into the formation mechanism of permeability anisotropy and display potential application value in NGH exploitation, groundwater environment protection and geological disaster prevention.

Influence of solder condition on effective thermal conductivity of two-directional random fibres: Pore-scale simulation

It is indicated that the solder joint of the metal fibrous materials is a critical factor impacting the heat conduction. To reveal the mechanism by which solder joint sizes, solder joint skips, solder flux materials, and filling media affect the thermal conductivity of fibres, pore-scale numerical simulation is employed to study the thermal transport in two-directional (2-D) random fibres. Satisfactory agreement with existing data validates the numerical model. The dimensionless effective thermal conductivity (ETC) of the porous fibres increases with the solder joint sizes. As the solder joint size (i.e., solder joint ratio) increases by 3.06%, the in-plane (ke-in) and out-of-plane (ke-out) dimensionless ETC increase by 9.0% and 437.2%, respectively. However, the solder joint skips will weaken the thermal conductivity of the fibres. For the same fibre, the ETC of the fibre increases as the thermal conductivity of solders increases. Further, when the dissimilarity in thermal conductivity between the filling medium and the fibre is reduced, the fibre is less affected by the solder joint skips. Finally, it should be supplemented that the in-plane and out-of-plane ETC (ke-in and ke-out) of the fibre without any solder joint are reduced by an average of 14.3% and 98.8%, respectively.

Pore-scale numerical simulation of the heat transfer and fluid flow characteristics in metal foam under high Reynolds numbers based on tetrakaidecahedron model

In this paper, the pressure drops and interfacial heat transfer coefficient in metal foam with different pore density which was constructed by tetrakaidecahedron models were numerically studied under high velocity by Computational Fluid Dynamics (CFD). The velocity fields, temperature fields and pressure fields in the computational domain of metal foam were investigated. The equation of pore density and hydraulic diameter of metal foam was obtained. Influence of flow rate on the whole porous zone was also studied and obtaining the fitting formula to predict pressure loss in porous metal which was applied to the pore density range from 10PPI(Pore Per Inch)to 40PPI at high flow rate. Comparing with the previous literatures, the maximum error of pressure drop value was 12.23%. Meanwhile, the relationship between Reynolds number and interfacial heat transfer coefficient was studied, the results shown that the values of heat transfer coefficient increase with the rising of Reynolds number, but the lifting rate decreases gradually. And according to the data points, the numerical formula of interfacial heat transfer coefficient in porous metal was obtained which had a great correctness through contrasting with other data. As a result, the fitting formula could accurately predict the convection heat transfer coefficient of the metal foam interface and provide relevant data for the Thermal Non-equilibrium Model.

Pore-scale electrical numerical simulation and new saturation model of fractured tight sandstone

Pore-scale electrical numerical simulation and new saturation model of fractured tight sandstone

Three-dimensional pore-scale simulation of non-equilibrium in a two-layer porous burner

A pore-scale numerical simulation for combustion of methane-air mixtures in a two-layer porous media burner made of staggered arrangement of connected particles is preformed. Combustion is modeled by a one-step reaction mechanism, and solid surface radiation heat transfer is computed by discrete ordinates (DO) model. The bridge approach is used to deal with the point contact between adjacent particles. The present study is intended to investigate the non-equilibrium characteristics of gas velocity and temperature under two operating parameters such as the inlet velocity and equivalence ratio. A comparison of the numerical values and experimental values was carried out to prove the effectiveness of numerical model, and a good agreement is shown. The results showed considerable non-equilibrium of velocity and temperature along the space coordinate due to the inhomogeneous porous structure. The extent of flow non-equilibrium increases as inlet velocity and equivalence ratio are increased. The extent of gas temperature non-equilibrium decreases slightly with the inlet velocity, but the equivalence ratio effect on thermal non-equilibrium can be neglected.