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Liquid nitrogen (LN2) is of great potential in stimulating reservoirs. Thanks to the low viscosity and ultralow temperature, which makes it easy to penetrate rock pores and intensified rock failure. However, the flow and heat transfer characteristics of LN2 in rock pores are still not well understood, which limits the development of LN2 fracturing technology. To address this issue, a flow and heat transfer model was established based on Lattice Boltzmann Method (LBM). A connectivity sandstone pore model was presented, combining CT scanning and the improved connectivity algorithm proposed in this paper. A series of simulations were conducted to investigate how the existence of pores and the fluid flow affect heat transfer. Moreover, a pore-scale Lattice Boltzmann Method (LBM) simulation optimization method was proposed to improve the calculation efficiency. The results indicate that the existence of pores and fluid flow will inhibit and assist the heat transfer, respectively. Besides, the effect of pore itself on heat transfer is greater than that of pore flow. Meanwhile, the flow and heat transfer of LN2 and two other common fracturing fluids were compared. Under the same condition, the velocity of LN2 and its rate of cooling rock are the greatest, followed by supercritical carbon dioxide (SC-CO2) and water. As a kind of fracturing fluid, LN2 is better than the other two fluids in terms of flow and heat transfer characteristics. This study provides an in-depth understanding of the mechanism of heat transfer between rocks and LN2. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrate phase transition and gas–water multi-phase flow were examined under pore-scale. • A LBM model of clayey silt hydrate-bearing sediment pore structure was constructed. • A quantitative description of the relationship between relative permeability and hydrate saturation was included. Unlike conventional oil and gas sources, natural gas hydrate (NGH) is stored in solid form within the pores of loose submarine sediments. Production by any method poses a complex problem involving phase transition and multi-phase flow of gas and water. The evolution of hydrate dissociation from the solid phase to gas and liquid phases in a hydrate reservoir is a dynamic process incorporating pore structure and multi-phase seepage. The solid–gas-liquid phase dissociation of NGH affects reservoir seepage properties by altering the micropore structure of the hydrate-bearing sediments. This study, based on the lattice Boltzmann method of modeling fluids flow, clearly demonstrates the micropore-scale changes in pressure and heat transfer that take place during hydrate dissociation. Pore-scale heterogeneity is found to influence the complex development of hydrates in a porous clayey silt medium during dissociation. The effect of hydrate, gas and water saturation on the evolution of relative permeability is identified, and the relationship between relative permeability and hydrate saturation during hydrate dissociation is quantified. These findings provide theoretical support for the implementation of marine natural gas hydrate production testing and future commercial production in China. [ABSTRACT FROM AUTHOR]

In this work, a hybrid method coupling a pseudo-potential lattice Boltzmann model (LBM) and a pore network model (PNM) to simulate drying in porous media is proposed. Based on the watershed method, the porous medium is firstly decomposed into pore regions. According to the liquid–vapor phase distribution at a given time, the pore regions are further divided into four pore types, namely two-phase pores where a liquid–vapor interface exists, buffer pores next to the two-phase pores, single-liquid and single-vapor phase pores. The pseudo-potential LBM is used in the two-phase and buffer pores to simulate liquid drying and track the movement of the interfaces, while the single-phase PNM simulations are conducted in the buffer and single-phase pores to simulate vapor or liquid flow. LBM and PNM are coupled in the buffer pores through exchange of boundary information. The hybrid method is applied to simulate liquid drying in a porous medium. The whole-domain LBM simulation is considered as the reference solution to validate the hybrid method. Liquid saturation variation during the drying process and detailed phase and pressure distributions obtained by the two methods match quite well, demonstrating the accuracy of the hybrid method. For the specific case studied, the hybrid method saves more than 60% computational time compared to the whole-domain LBM simulation. In addition, the speedup of the hybrid method becomes more significant for a larger computational domain. In summary, the hybrid method developed in this work combines the accuracy of LBM and the efficiency of PNM to simulate drying in porous media at pore scale and can lead to significant reduction of computation time, thus allowing the pore-scale consideration of drying in larger porous systems. [ABSTRACT FROM AUTHOR]

The development of high-resolution in situ imaging has allowed contact angles to be measured directly inside porous materials. We evaluate the use of concepts in integral geometry to determine contact angle. Specifically, we test the hypothesis that it is possible to determine an average contact angle from measurements of the Gaussian curvature of the fluid/fluid meniscus using the Gauss-Bonnet theorem. We show that it is not possible to unambiguously determine an average contact angle from the Gauss-Bonnet theorem. We instead present an approximate relationship: 2 π n (1 - cos θ) = 4 π - ∫ κ G 12 dS 12 , where n is the number of closed loops of the three-phase contact line where phases 1 and 2 contact the surface, θ is the average contact angle, while κ G 12 is the Gaussian curvature of the fluid meniscus which is integrated over its surface S 12 . We then use the results of pore-scale lattice Boltzmann simulations to assess the accuracy of this approach to determine a representative contact angle for two-phase flow in porous media. We show that in simple cases with a flat solid surface, the approximate expression works well. When applied to simulations on pore space images, the equation provides a robust estimate of contact angle, accurate to within 3 ° , when averaged over many fluid clusters, although individual values can have significant errors because of the approximations used in the calculation. [ABSTRACT FROM AUTHOR]

The change of wettability toward more water-wet by the injection of low salinity water can improve oil recovery from porous rocks, which is known as low salinity water flooding. To simulate this process at the pore-scale, we propose that the alteration in surface wettability mediated by thin water films which are below the resolution of simulation grid blocks has to be considered, as observed in experiments. This is modeled by a wettability alteration model based on rate-limited adsorption of ions onto the rock surface. The wettability alteration model is developed and incorporated into a lattice Boltzmann simulator which solves both the Navier-Stokes equation for oil/water two-phase flow and the advection-diffusion equation for ion transport. The model is validated against two experiments in the literature, then applied to 3D micro-CT images of a rock. Our model correctly simulated the experimental observations caused by the slow wettability alteration driven by the development of water films. In the simulations on the 3D rock pore structure, a distinct difference in the mixing of high and low salinity water is observed between secondary and tertiary low salinity flooding, resulting in different oil recoveries. [ABSTRACT FROM AUTHOR]

In the last decade, the study of fluid flow in porous media has developed considerably due to the combination of X-ray Micro Computed Tomography (micro-CT) and advances in computational methods for solving complex fluid flow equations directly or indirectly on reconstructed three-dimensional pore space images. In this study, we calculate porosity and single phase permeability using micro-CT imaging and Lattice Boltzmann (LB) simulations for 8 different porous media: beadpacks (with bead sizes 50 µm and 350 µm), sandpacks (LV60 and HST95), sandstones (Berea, Clashach and Doddington) and a carbonate (Ketton). Combining the observed porosity and calculated single phase permeability, we shed new light on the existence and size of the Representative Element of Volume (REV) capturing the different scales of heterogeneity from the pore-scale imaging. Our study applies the concept of the ‘Convex Hull’ to calculate the REV by considering the two main macroscopic petrophysical parameters, porosity and single phase permeability, simultaneously. The shape of the hull can be used to identify strong correlation between the parameters or greatly differing convergence rates. To further enhance computational efficiency we note that the area of the convex hull (for well-chosen parameters such as the log of the permeability and the porosity) decays exponentially with sub-sample size so that only a few small simulations are needed to determine the system size needed to calculate the parameters to high accuracy (small convex hull area). Finally we propose using a characteristic length such as the pore size to choose an efficient absolute voxel size for the numerical rock. [ABSTRACT FROM AUTHOR]

We study the effects of coal wettability on gas bubble/water two-phase flow behaviour in micro-cleats of the coal seam gas reservoir using a free energy based two-phase lattice Boltzmann model. The model is validated by comparison with analytical results and published results, and good agreement is achieved in general. Then we use this model to simulate bubble-water flow in both smooth capillary and a capillary with a narrow throat to systematically study the influences of contact angle, capillary pressure and bubble size on the flow behaviour. The simulation results indicate that both the bubble size and contact angle have significant impacts on the flow capacity of bubble and water, especially in a channel with a narrow throat. A decrease in water flow rate is observed when larger bubbles occur, and the water flow rate increases when the gas wettability becomes stronger. The bubble flow dynamics significantly influence the drainage of water and the further gas production. [ABSTRACT FROM AUTHOR]

• A lattice Boltzmann model for modeling the gas hydrate dissociation is proposed. • The gas-water flow during methane hydrate dissociation is analyzed. • Fluid flow promotes hydrate dissociation. • The phenomenon of bubbles formed by liquid curling flows is observed. A multi-phase multi-component lattice Boltzmann model for modeling the gas hydrate dissociation process is proposed, which takes into account the dissociation kinetics, gas-water migration, and heat-mass transfer processes. This model is validated by simulating the xenon hydrate dissociation process. Subsequently, the dissociation mechanism of methane hydrate in microporous media, the gas-water migration, and the effect of fluid flow on the dissociation are investigated by using the present model. The simulation results indicate that methane hydrate dissociation facing the inlet is faster than that away from the direction of inlet flow. Meanwhile, fluid flow promotes methane hydrate dissociation. In addition, the rapid dissociation process within the pore space may lead to a large amount of water production accumulation. Another noteworthy phenomenon is the observation of bubbles formed by liquid curling flows. Liquid curling is one of the reasons for bubble formation near the gas-liquid interface. [ABSTRACT FROM AUTHOR]

Effects of heterogeneity of mineral distribution and reaction rate on the rock dissolution process are investigated using a pore-scale reactive transport model based on the lattice Boltzmann method. Coupled fluid flow, species transport, chemical reaction and solid structure alternation due to dissolution are simulated. Effects of mineral distributions and chemical heterogeneity on the dissolution behaviors and evolutions of hydrologic properties are studied under different reactive transport conditions. Simulation results show that the coupling between advection, diffusion and reaction as well as the mineralogical heterogeneity leads to complex reactive transport behaviors and complicated temporal evolutions of hydrologic properties including porosity, permeability and reactive surface. Diverse relationships between surface area and volume are predicted, which cannot be described by simple models such as the spherical-grain model. Porosity-permeability relationships also differ under different mineral distributions and reactive transport conditions. Simulation results indicate that it is extremely challenging to propose general relationships for hydrologic properties for dissolution of rocks with mineralogical heterogeneity, due to the complicated interactions between reactive transport and mineralogical heterogeneity. [ABSTRACT FROM AUTHOR]

A fundamental understanding of flow in porous media at the pore-scale is necessary to be able to upscale average displacement processes from core to reservoir scale. The study of fluid flow in porous media at the pore-scale consists of two key procedures: Imaging - reconstruction of three-dimensional (3D) pore space images; and modelling such as with single and two-phase flow simulations with Lattice-Boltzmann (LB) or Pore-Network (PN) Modelling. Here we analyse pore-scale results to predict petrophysical properties such as porosity, single-phase permeability and multi-phase properties at different length scales. The fundamental issue is to understand the image resolution dependency of transport properties, in order to up-scale the flow physics from pore to core scale. In this work, we use a high resolution micro-computed tomography (micro-CT) scanner to image and reconstruct three dimensional pore-scale images of five sandstones (Bentheimer, Berea, Clashach, Doddington and Stainton) and five complex carbonates (Ketton, Estaillades, Middle Eastern sample 3, Middle Eastern sample 5 and Indiana Limestone 1) at four different voxel resolutions (4.4 µm, 6.2 µm, 8.3 µm and 10.2 µm), scanning the same physical field of view. Implementing three phase segmentation (macro-pore phase, intermediate phase and grain phase) on pore-scale images helps to understand the importance of connected macro-porosity in the fluid flow for the samples studied. We then compute the petrophysical properties for all the samples using PN and LB simulations in order to study the influence of voxel resolution on petrophysical properties. We then introduce a numerical coarsening scheme which is used to coarsen a high voxel resolution image (4.4 µm) to lower resolutions (6.2 µm, 8.3 µm and 10.2 µm) and study the impact of coarsening data on macroscopic and multi-phase properties. Numerical coarsening of high resolution data is found to be superior to using a lower resolution scan because it avoids the problem of partial volume effects and reduces the scaling effect by preserving the pore-space properties influencing the transport properties. This is evidently compared in this study by predicting several pore network properties such as number of pores and throats, average pore and throat radius and coordination number for both scan based analysis and numerical coarsened data. [ABSTRACT FROM AUTHOR]

Recent progress in the understanding of soil microbial processes at micrometric scales has created a need for models that accurately predict the microscale distribution of water, and the location of air–water interfaces in pores. Various models have been developed and used for these purposes, but how well they fare against real data has yet largely to be determined. In this context, for the first time, this article compares the prediction of two of these models to experimental data obtained on soil material. The distribution of water and air in soil samples constituted of repacked aggregates, equilibrated at three matric potentials ( − 0.5 kPa, − 1 kPa and − 2 kPa), was measured via synchrotron X-ray computed tomography at a resolution of 4.6 µ m. Water distribution was simulated by a two-phase lattice Boltzmann model (LBM) and a morphological model (MOSAIC). Results indicate that, when one lifts the assumption, motivated by capillary theory, that a pore can drain only if a connecting pore is already full of air, MOSAIC gives an acceptable approximation of the observed air–water interfaces. However, discretization of pores as geometrical primitives causes interfaces predicted by MOSAIC to have nonphysical shapes. By contrast, LBM is able to predict remarkably well the location of air–water interfaces. Nevertheless, given the huge difference in computing time (minutes versus tens of hours) required to run these two models, it is recommended that further research be carried out on the development of both, in parallel. [ABSTRACT FROM AUTHOR]

A coupled numerical model is employed to investigate the emission process of volatile organic compounds (VOCs) in the building material and chamber. In this model, for the first time the Langmuir-isotherm sink model is introduced by taking into account of the adsorption/desorption process between the absorbed VOCs on the material surfaces and the gas phase VOCs in pores. The porous material adopted in the model is reconstructed with a random generation-growth algorithm for 3D microstructures fibers. This model is numerically implemented by a multiscale strategy with the lattice Boltzmann method (LBM) being used to simulate pore-scale diffusion and finite volume method (FVM) for macroscopic transport. Information transferring from the macro scalar obtained from FVM zone to the distribution functions in LBM zone is executed with a reconstruction operator. The model is validated with the well-mixed diffusion model, and then applied to predict the VOCs emission process. A comprehensively investigation of the effects of sorption parameters, including adsorption/desorption rate constants, partition coefficient, Schmidt number and Reynolds number on the VOCs adsorption/desorption process in building material and emission process in chamber is conducted. The simulation results reveal that the equilibrium VOCs concentration in the building material pores could be numerically estimated with our proposed model; the equilibrium concentration of VOCs in a closed chamber depends on the initial emitted VOCs concentration and partition coefficient; Reynolds number can promote the emission process in a ventilated chamber. [ABSTRACT FROM AUTHOR]

Multicomponent multiphase reactive transport processes with dissolution–precipitation are widely encountered in energy and environment systems. A pore-scale two-phase multi-mixture model based on the lattice Boltzmann method (LBM) is developed for such complex transport processes, where each phase is considered as a mixture of miscible components in it. The liquid–gas fluid flow with large density ratio is simulated using the multicomponent multiphase pseudo-potential LB model; the transport of certain solute in the corresponding solvent is solved using the mass transport LB model; and the dynamic evolutions of the liquid–solid interface due to dissolution–precipitation are captured by an interface tracking scheme. The model developed can predict coupled multiple physicochemical processes including multiphase flow, multicomponent mass transport, homogeneous reactions in the bulk fluid and heterogeneous dissolution–precipitation reactions at the fluid–solid interface, and dynamic evolution of the solid matrix geometries at the pore-scale. The model is then applied to a physicochemical system encountered in shale gas/oil industry involving multiphase flow, multicomponent reactive transport and dissolution–precipitation, with several reactions whose rates can be several orders of magnitude different at a given temperature. The pore-scale phenomena and complex interaction between different sub-processes are investigated and discussed in detail. [ABSTRACT FROM AUTHOR]

A versatile lattice Boltzmann (LB) based pore-scale multicomponent reactive transport approach is presented in this paper. This approach is intended to capture mineral phase and pore structure evolution resulting from geochemical interactions applicable, for example to model microstructural evolution of hardened cement paste during chemical degradation. In the proposed approach heterogeneous reactions are conceptualized as pseudo-homogenous (volumetric) reactions by introducing an additional source term in the fluid node located at the interface adjacent to a solid node, and not as flux boundaries as used in previously proposed approaches. This allows a complete decoupling of transport and reaction computations, thus different reaction systems can be introduced within the LB framework through coupling with external geochemical codes. A systematic framework for coupling an external geochemical code with the LB including pore geometry evolution is presented, with the generic geochemical code PHREEQC as an example. The developed approach is validated with a set of benchmarks. A first example demonstrates the ability of the developed approach to capture the influence of pH on average portlandite dissolution rate and surface evolution. This example is further extended to illustrate the influence of reactive surface area and spatial arrangement of mineral grains on average dissolution rate. It was demonstrated that both location of mineral grains and surface area play a crucial role in determining average dissolution rate and pore structure evolution. [ABSTRACT FROM AUTHOR]

The gaseous fluid flow and heat transfer with the consideration of rarefaction effects are important at micro/nano-scales. The formulation of accurate predictive models is beneficial to optimize the heat transfer and friction across the porous domain, in micro- and nano-devices. In this work, the lattice Boltzmann method (LBM) with multiple-relaxation-time (MRT), which offers a natural and robust way to simulate the slip gaseous fluid flow and heat transfer, was applied to the confined porous medium at a pore-scale level. An efficient boundary scheme was utilized at the walls of porous media to consider the velocity slip and temperature jump effect. A two-dimensional porous domain composed of different types of micro obstacles (circle, ellipse and diamond) was established for the pore-scale simulations. Nusselt number (Nu) and friction factor (C f) were obtained and analyzed across the obstacles with varying Knudsen number (0.0 ≤ Kn ≤ 0.1), Reynolds number (1 ≤ Re ≤ 10) and porosity (0.4 ≤ ε ≤ 0.8). With increasing Re number, Nu number shows an increasing trend for all obstacles, however, for increasing Kn number, Nu number decreases accordingly. Whereas, the friction factor C f shows a decreasing trend for both increasing Kn and Re numbers. Based on the obtained results, the new predictive correlations for Nusselt number and friction factor were proposed, which will be valuable to understand the slip gaseous flow characteristic in confined porous space. • Slip gaseous fluid flow in confined porous medium is simulated with LBM. • Flow and heat transfer in slip regime are analyzed at pore scale. • Correlations for Nu number and friction factor of slip flow are proposed. [ABSTRACT FROM AUTHOR]

The asymmetric ceramic microfiltration (MF) membrane is widely used in water treatment processes, yet the dependence of its intrinsic permeability on the microstructure parameters of different layers are not thoroughly understood. In this study, a numerical model combining the Discrete Element Method (DEM) with Lattice Boltzmann Method (LBM) is developed to simulate the fluid flow through the detailed pore structure of an asymmetric ceramic MF membrane. The model is validated by comparing the simulated pure water permeability to experimental data with an average deviation of 8% (Max∼25%). The simulation results show that the pore size D 50 and porosity ε , of the top and intermediate layers are dominant microstructure parameters that determine the membrane pure water permeability. A new asymmetric ceramic MF membrane intrinsic permeability K m as a function of D 50 , ε and layer thickness h of both top and intermediate layers is derived. The verification of this K m correlation is conducted by comparison with experimental data and Carman-Kozeny (C–K), Hagen-Poiseuille (H–P) correlations, indicating the proposed K m correlation can accurately predict the membrane permeability. It provides a useful tool for a fundamental understanding of the effect of membrane microstructures on intrinsic permeability. [Display omitted] • A LBM-DEM model simulates the fluid flow in an asymmetric ceramic MF membrane. • Effect of membrane layers' microstructures on permeability is investigated. • A new membrane intrinsic permeability Km correlation is derived. • Verifcation of the new correlation is conducted with experiment results. [ABSTRACT FROM AUTHOR]

Abstract: A general numerical scheme based on the lattice Boltzmann method (LBM) is established to investigate coupled multiple physicochemical thermal processes at the pore-scale, in which several sets of distribution functions are introduced to simulate fluid flow, mass transport, heat transfer and chemical reaction. Interactions among these processes are also considered. The scheme is then employed to study the reactive transport in a posted micro reactor. Specially, ammonia (NH3) decomposition, which can generate hydrogen (H2) for fuel of proton exchange membrane fuel cells (PEMFCs), is considered where the endothermic decomposition reaction takes place at the surface of posts covered with catalysts. Simulation results show that pore-scale phenomena are well captured and the coupled processes are clearly predicted. Effects of several operating and geometrical conditions including NH3 flow rate, operating temperature, post size, post insert position, post orientation, post arrangement and post orientation on the coupled physicochemical thermal processes are assessed in terms of NH3 conversion, temperature uniformity, H2 flow rate and subsequent current density generated in PEMFC. [Copyright &y& Elsevier]

Abstract: Lattice Boltzmann method (LBM) is employed to investigate pore-scale flow and mass transport in a carbon paper gas diffusion layer (GDL) of interdigitated PEMFC. The carbon paper GDL is reconstructed using the stochastic method, and its macroscopic transport properties are numerically predicted. The predicted anisotropic permeabilities and effective diffusivity of the reconstructed GDL agree well with existing measurements. Then, effects of the porous structures of the carbon paper GDL are explored in terms of fluid flow, species transport and electrochemical reaction. The GDL porous structures greatly affect flow and mass transport, creating distinct specie concentration distribution and local current density distribution. Besides, simulations are performed to explore liquid water behaviors in the reconstructed GDL. The simulation results present a detailed description of the pore-scale liquid water behaviors. Further, simulations are performed to investigate the effects of land width and GDL contact angle on liquid water removal time and residual saturation. Narrower land reduces liquid water removal time and residual saturation. Higher contact angle increases the removal time and reduces the residual saturation. [Copyright &y& Elsevier]

Abstract: Pore-scale simulations of flow and transport through a filter were used to predict clean-bed filtration of nanoparticles from a 3D image of the filter. X-ray micro-computed tomography was used to obtain the geometry and topology of the filter consisting of flattened half-spherical collectors. A Lattice-Boltzmann method was used to model fluid flow and particle transport in the volumetric image of the filter. Nanoparticles are considered that are so small that diffusion, as compared to the processes of gravitational settlement and interception, dominates particle deposition. A correlation equation is determined for the average collector efficiency for diffusion through regression analysis performed for a set of numerical breakthrough experiments for a range of suspended particle sizes and Darcy velocities. This correlation quantifies the collector size by the surface average equivalent sphere diameter. The newly derived correlation agrees well with experimental data. [Copyright &y& Elsevier]

Abstract: Pore-scale simulations of two phase flows in a packed-sphere bed and in a carbon paper gas diffusion layer (GDL) are carried out using the free energy multiphase lattice Boltzmann method (LBM). The simulations are performed based on the detailed microstructure of the porous media under periodic boundary conditions such that the average phase saturations in the porous medium remain constant. A comparison of the simulated and measured relative permeabilities for the packed sphere bed as a function of non-wetting phase saturation is performed, and effects of the wettability and the anisotropic characteristics of relative permeabilities of the GDL are investigated. [Copyright &y& Elsevier]

In this work, the hydraulic tortuosity (T h) specific for the binary-sized elliptical particle packings is studied using the Lattice Boltzmann method (LBM) and discrete element method (DEM). The model is validated by comparing with experimental and simulation data with an average deviation of 6% (Max 10%). Then the model is used to develop a reliable T h correlation to describe the fluid streamlines in elliptical particle packings. Firstly, a generalized T h correlation format is proposed, containing porosity (ε) and two empirical coefficients (a, b). Then the effects of ε and size ratio (r) on the T h are studied, and by curve-fitting the simulation results, the empirical coefficients (a, b) are formulated as a function of r. Meanwhile, the generalized T h correlation format is applicable for predicting the T h in porous media with different microstructures. The T h correlations provide a theoretical basis for macroscopic modelling of fluid flow through porous media of elliptical particles. [Display omitted] • An LBM-DEM method studies fluid flow through binary elliptical particle packings. • A generalized tortuosity format, containing porosity and coefficients, is developed. • Particle packings' porosity and size ratio affect its Hydraulic tortuosity (T h). • Correlations are proposed to estimate T h for binary elliptical particle packings. [ABSTRACT FROM AUTHOR]

Fracture propagation in rocks is a complex thermal-hydro-mechanical (THM) phenomenon present in geotechnics, such as enhanced geothermal systems, CO 2 sequestration, and shale gas exploitation. Numerical simulation is an essential way to study its mechanism. In this study, a pore-scale model combined with the lattice Boltzmann method (LBM) and the discrete element method (DEM) is developed, in which the comprehensive THM coupling phenomena are considered, consisting of convective heat transport, conjugate heat transfer, hydrodynamic force, and thermal strain. In addition, to couple the temperature and anisotropic thermal expansion of rock in LBM-DEM, a novel method based on the deformation of sphero-triangle particle is presented, where the deformation is calculated using the principle of minimum potential energy. The main features of the proposed THM model are that it can accurately calculate the fluid flow in fractures and the temperature in both fractures and DEM particles, and it takes full consideration of multiple anisotropic properties in rock. After validations, the THM fracturing mechanism is preliminarily investigated using the proposed model. [ABSTRACT FROM AUTHOR]

High-efficiency thermal protection is becoming a critical design criterion for supersonic aircraft. Currently, there is significant research on accurately and efficiently designing heat protection systems, particularly at the high temperature stagnation point region of supersonic aircraft. In this work, a three-dimensional high thermal conductivity carbon/carbon composite thermal protection system with a directional heat transfer function is proposed to realize a high-efficiency thermal protection design in the stagnation point region. A comprehensive method that combines the lattice Boltzmann method (LBM) and finite volume method (FVM) was designed to investigate the heat transfer process in the proposed structure at the pore scale. The FVM was applied to calculate the heat radiative information, which was required to solve the energy equation with the LBM. A failure temperature is defined to quantitatively describe how well the heat transfer occurs in the designed direction in the proposed structure. The effects of the temperature of the stagnation point, thermal conductivity of the carbon fibers, carbon fiber diameters, and position of the carbon fibers on the heat transfer in the proposed structure were studied in detail. The results show that the proposed comprehensive method can predict the heat transfer in the proposed structure accurately. A competitive relationship exists between the directional heat conduction and the radiation heat transfer. The radiation heat transfer became more dominating as the temperature increased. The failure temperature for the directional heat transfer in the proposed structure increased with the thermal conductivity of the carbon fibers and the carbon fiber diameters, while it decreased as the distance between the position of the carbon fibers and the nose of the blunt structure increased. The above phenomena will be helpful for designing high-performance thermal protection systems with carbon/carbon composites. [ABSTRACT FROM AUTHOR]

• Up to 150 × speedup to steady state LBM by preconditioning with PNM and Laplace Solvers. • Velocity field and permeability error of PNM/Laplace Solvers corrected within 2,000 LBM timesteps. • Most effective on tight porous media; useful for reactive transport, fines migration, etc. Permeability characterises flow in porous rocks/media for upscaling, while steady-state flow fields allow analysis of reactive transport, fines migration, and tight unconventional rocks. Fast calculation of permeability and flow fields obtained from Pore Network Models (PNM) and Laplace Semi-Analytical Solvers (SAS) deviate from computationally demanding simulation of Navier Stokes Equations (NSE) due to flow and geometry simplifications. Coupling PNM/SAS with direct simulation via Lattice Boltzmann Method (LBM) provides 5-- 150 × speed-up without accuracy loss over 100 samples from 0.7mD to 3.5D. Permeability errors in PNM/SAS show 10-20% (up to 50-70%) error. PNM shows higher variance from geometric simplifications compared to SAS which only makes flow-based assumptions. PNM/SAS errors are eliminated by coupling with LBM at a fraction of LBM-only compute cost. Steady-state conditions with PNM/SAS-LBM are reached in < 2 , 000 timesteps, compared to LBM-only which can require > 10 5 timesteps in tight domains such as cemented sandstone, carbonate, coal, and shale. [ABSTRACT FROM AUTHOR]

• Melting of composite PCMs with void cavities is characterized by lattice Boltzmann model. • Void cavities can suppress thermal energy performance of composite PCMs. • Melting process is hindered with increasing volume fraction of void cavity. • Difference between near-wall and randomly distributed void cavities is identified. Composite Phase Change Materials (PCMs) incorporated with metal foams are promising candidates for thermal management for space exploration. However, void cavities are generated as a result of the volume change of PCM during the melting process, introducing a resistance to heat transfer due to the low thermal conductivity of the void. In this work, the effects of void cavity distribution on the conduction-dominated melting of composite PCMs under microgravity conditions are studied by a two-dimensional pore-scale lattice Boltzmann method, in which a microstructural description of the metal foam is experimentally characterized with the help of X-ray micro-Computed Tomography. Two typical distribution patterns of void cavities are analsed and computed performance is compared (1) a near-wall void cavity, and (2) randomly distributed void cavities. The evolutions of temperature distributions and melting interfaces are compared, and the average liquid fraction and energy stored per width are deduced to describe the energy storage performance. Moreover, the influence of the volume fraction of void cavities is investigated by comparing temperature distributions and energy storage performances of composite PCMs with four different volume fractions of void cavities (0%, 3.7%, 7.6%, 15.2%). After introducing void cavities, the energy stored per width is reduced by 5.7%, 12.3% and 20.2% for randomly distributed void cavities when volume fraction of void cavities is 3.7%, 7.6%, and 15.2%, respectively, and reduced by 42.2%, 64.1% and 79.7% for the near-wall void cavity, respectively. This work initiates the study of the effects of void cavity distribution on composite PCMs, which will stimulate work on structural optimization of thermal management systems. [ABSTRACT FROM AUTHOR]

In order to promote the applicability of redox flow batteries (RFBs), the deep eutectic solvents (DESs) have been proposed as a better choice for non-aqueous electrolyte of RFBs. The reactive transfer process within the RFB plays a key role in determining the cell performance. In this work, cyclic voltammetry experiment is conducted to obtain the reactive transfer property parameters of iron and vanadium ions in ethaline DES electrolyte. Then, a lattice Boltzmann method (LBM) model is utilized to reveal the reactive transfer mechanism of vanadium-iron RFB with DES electrolyte at the pore-scale. The numerical results present the influences of porous electrode morphology and DES electrolyte transfer properties on the performance of vanadium-iron RFB during galvanostatic discharging. This work suggests that the porous electrodes with the lower porosity and smaller fibre diameter lead to a more sufficient depletion time of reactant in DES electrolyte under the galvanostatic discharging and a given pumping power condition. Meanwhile, the larger fibre diameter yields the more widespread pore size distribution, resulting in the more apparent non-uniformity of local current density into the porous electrode. In addition, the results show the similar state of charge(SOC) in the cathode and anode under the same flow condition. By contrast, the absolute value of overpotential in the anode is significantly higher than that in the cathode. Furthermore, the reactive transfer process of this DES electrolyte-based vanadium-iron RFB is compared to all vanadium RFB with aqueous electrolyte under the galvanostatic discharging. [ABSTRACT FROM AUTHOR]

• A series of 3D heterogeneous and anisotropic shale structures are reconstructed. • 3D pore-scale lattice Boltzmann method is used to mimic gas diffusion process in shales. • Comprehensive sensitivity analysis is performed to disclose gas diffusion properties in shales. • An empirical formula is proposed to estimate the effective diffusivity of isotropic shales. Understanding methane gas mass diffusion in shale is crucial for estimating gas production. However, shale is a typical ultra-tight porous medium that is characterized by its spatially heterogeneous and anisotropic features. The heterogeneous feature is derived from coexisting components including organic matter (OM), inorganic matter (IM), interparticle pores, microfractures, and bound water; the anisotropic feature is induced by the IM particles appearing in the form of flake-like structure. In this work, a three-dimensional (3D) pore-scale lattice Boltzmann model, which considers above heterogeneous and anisotropic features, is presented to study the methane gas diffusion in the shale. The elementary building block model and quartet structure generation set method are integrated to reconstruct 3D shale structures. Reconstructed shale structures are then characterized in terms of porosity, pore size distribution, and pore connectivity. The results show that the shale diffusivity increases with temperature, porosity, and OM volume fraction, while logarithmically decreasing with the increased pressure. Considering the anisotropic feature, the effective diffusivity parallel to the bedding plane is approximately 1.2–4.2 times higher than that perpendicular to the bedding plane. The microfracture orientation and aperture, compared to the microfracture shape and distribution pattern, play a more important role in determining the effective diffusivity. In contrast, the presence of bound water hampers gas diffusion. In addition, a general formula is proposed to conveniently estimate the diffusivity of isotropic shale, where the influences of temperature, pressure, porosity, OM volume fraction, and water saturation are fully considered. [ABSTRACT FROM AUTHOR]

A precise heat transfer simulation of a three-dimensional impinging jet porous heat sink is presented and is analyzed from thermodynamics vantage point under local thermal non-equilibrium condition. To increase the computational efficiency of the analysis, pore-scale modeling based on lattice Boltzmann method (LBM) is used inside the porous media (at a meso-scale), whilst finite volume method (FVM) is employed around it (at a macro-scale). The effects of the Reynolds number, porous layer thickness, solid/fluid thermal conductivity ratio, and porosity on the critical heat transfer and entropy generation parameters are investigated. Additionally, the relations between viscous entropy generation and pressure drop and thermal entropy generation with thermodynamic non-equilibrium are presented. The results indicated that among all parameters, the effects of the porous layer thickness on the fluid permeability are more substantial than other investigated parameters. Also, it is found that increasing the Reynolds number or porous layer thickness increases the total pressure drop, average viscous entropy generation number, and Nusselt number. For each porous layer thickness and Reynolds number, the minimum thermal conductivity ratio (that porous layer had no significant effect on heat transfer) is obtained 200. Moreover, it is determined that increasing the porous layer thickness or reducing the solid/fluid thermal conductivity ratio reduces the thermal entropy generation number, leading to a move toward the local thermal equilibrium condition. Additionally, in contrast to the fluid-phase thermal entropy generation number, the total entropy generation number in most porous layer cases was greater than the entropy generated by the surface without porous media. • Heat transfer analysis of a 3-D impinging jet heat sink with embedded porous material was carried out. • Entropy generation (EG) analysis of the problem under LTNE condition was performed, using hybrid LBM-FVM. • The relations between VEG and pressure drop and TEG with thermodynamic non-equilibrium are presented. • Total entropy generation number was greater than the entropy generated by the surface without porous media. [ABSTRACT FROM AUTHOR]

• An improved rough-walled discrete fracture network (RW-DFN) model is applied to reconstruct rough-walled fracture. • Initial fluid distribution pattern affects the two-phase flow behaviours. • The influences of surface roughness on relative permeability are investigated and discussed. Two-phase flow in fractured geo-material plays a significant role in various subsurface processes. Despite some success in studying two-phase flow in ideal smooth-walled fractures, the more realistic issues regarding two-phase flow in rough-walled fractures are still not well understood. In this study, an improved approach based on free energy lattice Boltzmann model is proposed to analyze the two-phase immiscible flow in the rough-walled fracture and to further estimate the relative permeability curves. The proposed model is validated by comparison with analytical results of individual fracture, and good agreement is achieved in general. Then it is used to investigate the effects of wall roughness and the initial phase distribution patterns on the co-current flow of two immiscible fluids. The simulation results demonstrate that roughness impacts phase distribution, flow patterns and relative permeability curves in a single rough-walled fracture. Rough surface increases two-phase interference and flow resistance, and an unsteady flow state appears for the non-wetting phase (NWP) when the wetting phase (WP) saturation is small. In addition, the NWP flow can either be lubricated or be resisted by the WP depending on the flow patterns and the WP saturation. [ABSTRACT FROM AUTHOR]