Pore-scale Lattice Boltzmann Research Articles

Model PEM water electrolyzer cell for studies of periodically alternating drainage/imbibition cycles

Investigating the counter-current two-phase flow within the anodic porous transport layer (PTL) of polymer electrolyte membrane water electrolyzers (PEMWEs) is a complex yet intriguing challenge. Until now, the gas-liquid invasion processes inside PTLs are only to a limited extent accessible under operation conditions, usually using sophisticated and expensive optical approaches, i.e., neutron imaging. We propose a model-supported experimental method with a fully operating microfluidic PEMWE, that allows to examine pore-scale oxygen-water distributions at the anode with high spatial and temporal resolution. The microfluidic cell is made of transparent Poly-Methyl-Methacrylate (PMMA), and the PTL is simplified by a quasi-2D pore network (PN) with a uniform pore-throat structure in the first preliminary study. The proposed setup is a significant advancement over previous studies, where gas was only injected at a constant flow rate from a single point. Test cases with current densities of 0.1, 1, and 2 A/cm2 and water flow rates of 1, 3, 5, and 10 ml/min were realized in the novel setup. We found periodically alternating invasion of oxygen (drainage) and water (imbibition), which were analyzed based on image sequences as well as voltage measurements. The experimental data is additionally supported by pore-scale Lattice Boltzmann (LB) and PN simulations. The preliminary results with the simplified PN structure are used to study the dominating transport mechanisms, revealing that drainage and imbibition occur simultaneously and are affected by evaporation and wetting liquid films formed in sharp pore corners. These phenomena are also expected to occur in more complex PTL structures. The preliminary results can, therefore, be regarded as an important basis for PTL studies, which are structurally more complex.

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>

The Application of REV-LBM Double Mesh Local Refinement Algorithm in Porous Media Flow Simulation

REV-scale LBM (representative elementary volume scale lattice Boltzmann method) provides a possible way for the unified microscopic simulation of transscale seepage combined with pore-scale LBM. However, shown by numerical results, when simulated fluid flows in tight porous media with discrete microfractures or dispersed dissolution pore, the nonphysical oscillation phenomenon will occur in the region of relatively greater velocity gradient, due to the insufficient computational node caused by computational capability constraints. And for that reason, the increase in simulation scale under certain computational capability will be restricted. In order to solve this problem, this paper extends the application of the original LBM local refinement algorithm into REV-LBM and corrects the original REV-LBM velocity processing method, such that REV-scale LBM has a more reasonable local refinement algorithm. Through conventional computation example, this paper proves that the local refinement algorithm can maintain well computational accuracy while at least double the time efficiency and could save more than 30% of CPU time in the practical application. This algorithm has a great potential of application in simulating flows in larger size porous media with complex micro-nano structures and can provide a new idea to transscale simulation in porous media.

Pore-scale modeling of mass transport in the air-breathing cathode of membraneless microfluidic fuel cells

The air-breathing cathode of membraneless microfluidic fuel cells (MMFCs) is in direct contact with flowing aqueous electrolyte rather than a solid polymer membrane, suggesting the mass transport characteristics can be different from the air-cathode in conventional membrane-based fuel cells. To investigate the mass transfer and reaction, pore-scale Lattice Boltzmann (LB) models were developed for the air-breathing cathode, including the gas diffusion layer (GDL), the catalyst layer and the electrolyte microchannel. A modified multi-relaxation-time LB model was developed to handle the very large diffusivity ratio (104) between gaseous and dissolved oxygen. The GDLs were numerically reconstructed to mimic the real microstructure. The effect of GDL property (porosity, isotropy) and operation conditions (overpotential, electrolyte flow rate) were investigated and discussed. The results indicated that the GDL microstructure played an important role for the oxygen transport and reaction in the air-breathing cathode. Oxygen was found to penetrate the electrolyte microchannel and could form a thin layer of dissolved oxygen covering the catalyst layer, which in-turn affected oxygen transfer in the GDL. The electrolyte flow can also affect local oxygen transport but only showed minimum impacts.

Study of Fingering Dynamics of Two Immiscible Fluids in a Homogeneous Porous Medium with Considering Wettability Effects Using a Pore-Scale Multicomponent Lattice Boltzmann Model

In the present study, a pore-scale multicomponent lattice Boltzmann method (LBM) is employed for the investigation of the immiscible-phase fluid displacement in a homogeneous porous medium. The viscous fingering and the stable displacement regimes of the invading fluid in the medium are quantified which is beneficial for predicting flow patterns in pore-scale structures, where an experimental study is extremely difficult. Herein, the Shan-Chen (S-C) model is incorporated with an appropriate collision model for computing the interparticle interaction between the immiscible fluids and the interfacial dynamics. Firstly, the computational technique is validated by a comparison of the present results obtained for different benchmark flow problems with those reported in the literature. Then, the penetration of an invading fluid into the porous medium is studied at different flow conditions. The effect of the capillary number (Ca), dynamic viscosity ratio (M), and the surface wettability defined by the contact angle (θ) are investigated on the flow regimes and characteristics. The obtained results show that for M<1, the viscous fingering regime appears by driving the invading fluid through the pore structures due to the viscous force and capillary force. However, by increasing the dynamic viscosity ratio and the capillary number, the invading fluid penetrates even in smaller pores and the stable displacement regime occurs. By the increment of the capillary number, the pressure difference between the two sides of the porous medium increases, so that the pressure drop Δp along with the domain at θ=40∘ is more than that of computed for θ=80∘. The present study shows that the value of wetting fluid saturation Sw at θ=40∘ is larger than its value computed with θ=80∘ that is due to the more tendency of the hydrophilic medium to absorb the wetting fluid at θ=40∘. Also, it is found that the magnitude of Sw computed for both the contact angles is decreased by the increment of the viscosity ratio from Log(M)=−1 to 1. The present study demonstrates that the S-C LBM is an efficient and accurate computational method to quantitatively estimate the flow characteristics and interfacial dynamics through the porous medium.

Predicting the CO2 Footprint in Saline Aquifers: A Numerical-analytical Hybrid Model

In this study, we present a hybrid framework in which we combine pore-scale lattice Boltzmann (LB) simulations with a macroscopic analytical model for predicting CO2 plume migration and its residual trapping in saline aquifers. These integrated methods combine the flexibility of a pore-scale simulation method, allowing for implementing the micro-scale properties such as wettability, with the efficiency of an analytical method, for a quick estimation of the CO2 plume extension and its residual trapping during the injection and post-injection stages. The LB model is adopted to perform two-phase flow simulations of CO2/brine in the microstructure of a rock sample of Tuscaloosa sandstone taken from the Cranfield site in Mississippi. Employing the pore-scale LB simulations, we estimate pore-scale properties such as connate brine saturation, CO2 residual saturation, and CO2 end-point relative permeability. Subsequently, these parameters have been used in the analytical model for predicting the macro-scale behavior of CO2 plume during the injection and post-injection periods. To gain a better insight into the effect of wettability on the footprint of CO2 plume and its residual trapping, we run pore-sale simulations in different samples under various wetting conditions and calculate the pore-scale properties as a function of wettability. Thus, incorporating the pore-scale simulation results into the analytical model helps us evaluate the effect of pore-scale properties such as wettability on storage efficiency. We believe that this approach provides an appropriate tool for the estimation of storage efficiency in CO2 sequestration projects.

Comprehensive study of the interactions between the critical dimensionless numbers associated with multiphase flow in 3D porous media

Multiphase flow in porous media is of great interest in many engineering applications, such as geologic carbon sequestration, enhanced oil recovery, and groundwater contamination and remediation. In order to advance the fundamental understanding of multiphase flow in complex three-dimensional (3D) porous media, the interactions between the critical dimensionless numbers, including the contact angle, viscosity ratio, and capillary (Ca) number, were investigated using X-ray micro-computed tomography (micro-CT) scanning and lattice Boltzmann (LB) modeling. In this study, the 3D pore structure information was extracted from micro-CT images and then used as interior boundary conditions of flow modeling in a pore-scale LB simulator to simulate multiphase flow within the pore space. A Berea sandstone sample was scanned and then two-phase flow LB simulations were performed based on the micro-CT images. The LB-simulated water/CO2 distributions agreed well with the micro-CT scanned images. Simulation results showed that a decreasing contact angle causes a decrease in wetting-fluid relative permeability and an increase in non-wetting fluid relative permeability. A rising Ca number increases both wetting and non-wetting fluid relative permeabilities. An increasing viscosity ratio (the ratio of non-wetting fluid viscosity to wetting fluid viscosity) facilitates the increase of non-wetting fluid relative permeability and mitigates the reduction of wetting fluid relative permeability, when the contact angle decreases continuously. The primary novel finding of this study is that the viscosity ratio affects the rate of change of the relative permeability curves for both phases when the contact angle changes continuously. To the best of our knowledge, it is the first time that comprehensive interactions between these dimensionless numbers are demonstrated in a sandstone sample based on real 3D structures. We also investigated the role of the changes of flow direction and sample location on relative permeability curves. Simulation results showed that the change in non-wetting fluid relative permeability was larger when the flow direction was switched from vertical to horizontal, which indicated that there was stronger anisotropy in larger pore networks that were primarily occupied by the non-wetting fluid.

Characterization of immiscible phase displacement in heterogeneous pore structures: Parallel multicomponent lattice Boltzmann simulation and experimental validation using three-dimensional printing technology

Understanding and characterization of immiscible-fluid displacement mechanisms in pore structures are required for enhanced hydrocarbon resource recovery in unconventional tight reservoirs. The displacement process of immiscible fluids in a porous structure on the pore scale has been widely simulated and characterized via lattice Boltzmann (LB) methods. However, owing to the heterogeneity and tortuosity of rock pore structures, it is difficult to numerically represent the real irregular geometry and accurately describe the varied immiscible interfaces in experiments using LB simulation. A low mesh resolution is usually applied, which causes simulation inaccuracy compared to realistic displacement behaviors. In this study, we introduce a multicomponent LB model with high-resolution meshes to investigate immiscible oil-water displacement in heterogeneous pore structures. The pore-scale LB model is established using information extracted from a realistic reservoir rock. A parallel computing algorithm is integrated into the model to accelerate the LB simulation. To verify the LB simulation, an oil-water displacement experiment is conducted using a transparent pore model replicated using three-dimensional printing (3DP) technology and information on the real porous rock. Owing to the sensitivity of the interfacial condition at the inlet, the inlet buffer was designed to relieve injecting perturbations in experiments, and an artificial velocity profile was applied in the simulation to approximate this relieved interface. Comparison of the LB simulation and experiment data indicates that the suggested parallel multicomponent LB method can simulate the oil-water displacement process in heterogeneous pore structures well. This study provides a method for characterizing the immiscible fluid displacement mechanism in a porous structure and forecasting the preferential flow path in a heterogeneous structure with a certain water wettability.

Pore-scale lattice Boltzmann simulation of flow and mass transfer in bioreactor with an immobilized granule for biohydrogen production

The photo bioreaction combined with flow and mass transfer is simulated with pore-scale lattice Boltzmann (LB) method, which is the scenario of a bioreactor filled with a porous granule immobilized photosynthetic bacteria cells for hydrogen production. The quartet structure generation set (QSGS) is used to generate porous structure of the immobilized granule. The effects of porosity of the immobilized granule on flow and concentration fields as well as the hydrogen production performance are investigated. Higher porosity facilitates the substrate solution smoothly flowing through the porous granule with increasing velocity, and thus results in higher product concentration inside the immobilized granule. Additionally, the substrate consumption efficiency increases, while hydrogen yield slightly decreases with increasing porosity, and they tend to stable for the porosity larger than 0.5. Furthermore, the LB numerical results have a good agreement with the experimental results. It is demonstrated that the pore-scale LB simulation method coupling with QSGS is available to simulate the photo hydrogen production in the bioreactor with porous immobilized granules.

Lattice Boltzmann modeling of carbon deposition in porous anode of a solid oxide fuel cell with internal reforming

A pore scale Lattice Boltzmann (LB) model is developed for multi-component mass transport and carbon deposition in the porous anode of a solid oxide fuel cell (SOFC) fed with methane. In this model, the reforming and electrochemical reactions are discretely coupled via local molar fluxes of reactive gas species at catalytically active sites and triple phase boundaries (TPBs). By considering the heterogeneity of anode microstructure, the present pore scale LB model is capable of quantitatively predicting the local distributions of both the position and the amount of deposited carbon in the irregular porous anode. Using this model, the characteristics of carbon deposition in the anode are investigated. It is found that the heterogeneity of anode microstructure has significant influence on mass transport and carbon deposition. Increasing the pre-reforming extent of methane and decreasing the operating temperature are beneficial for the suppression of carbon deposition, whereas they will cause a relatively inferior cell performance. The LB model and results presented in this study are beneficial to mechanism investigation of carbon deposition. Furthermore, they are also helpful for studying the correlations between anode microstructure and carbon deposition and then bridging the manufacture of SOFC electrodes and their performance, which is useful to propose effective suggestions for avoiding performance degradation of SOFC induced by carbon deposition.

Pore scale investigation of gaseous mixture flow in porous anode of solid oxide fuel cell

A two-fluid finite-difference LB (Lattice Boltzmann) model is developed for multi-component flow. Then, a pore scale LB model is further developed for multi-component flow in porous media with heterogeneous microstructure. Thus, the present LB model is independent on any structure statistical parameters of the porous media, which is contrary to the numerical methods assuming the porous media is homogeneous. Compared with existing LB models for multi-component flow in the SOFC (solid oxide fuel cell) anode, the present LB model is capable of simulating mixture flow with larger ratios of molecular weights, thus it is qualified to meet the fuel flexibility of the SOFC. By using this model, the mass transfer of gaseous mixture flow in a SOFC anode is studied and the local molar fraction distributions of gaseous mixture are obtained within the irregular porous anode directly. The influences of anode microstructure, dimensionless reaction flux and fuel composition on anode performance are investigated. The quantitative relationship between the porous microstructure and anode performance is obtained.