Search

Porous structures of agglomerates in cathode catalyst layers (CLs) of proton exchange membrane fuel cells are reconstructed, in which all the four phases are resolved including Platinum, carbon, ionomer and pore. A pore-scale reactive transport model based on the lattice Boltzmann method is developed, in which oxygen dissolution reaction at pore-ionomer interface, oxygen diffusion inside ionomer, and electrochemical reaction at ionomer-Pt interface are considered. Emphasis is put on structural parameters, especially Pt/C mass ratio, on the reactive transport process and the volumetric reaction rate (or current density). Pore-scale results show that while under high Pt loading oxygen is depleted quite close to the surface of the spherical agglomerate, it has to penetrate deep into the porous agglomerate before it is completely consumed under low Pt loading which is not captured by classical agglomerate model based on homogeneous mixture assumption. Pore-scale results also found that effects of transport inside the agglomerate decreases as reaction rate, porosity or ionomer thickness increases. Finally, local transport resistance inside the agglomerate is evaluated, and it increases as the agglomerate size increases or the dissolution reaction rate decreases. • 3D porous agglomerates are reconstructed and pore-scale model is developed. • Pt loading sensitive volumetric reaction rate is predicted. • Effects of agglomerate size, porosity and ionomer thickness are studied. • Local transport resistance is evaluated. [ABSTRACT FROM AUTHOR]

For applications of reactive transport in porous media, optimal porous structures should possess both high surface area for reactive sites loading and low mass transport resistance. Hierarchical porous media with a combination of pores at different scales are designed for this purpose. Using the lattice Boltzmann method, pore-scale numerical studies are conducted to investigate diffusion-reaction processes in 2D hierarchical porous media generated by self-developed reconstruction scheme. Complex interactions between porous structures and reactive transport are revealed under different conditions. Simulation results show that adding macropores can greatly enhance the mass transport, but at the same time reduce the reactive surface, leading to complex change trend of the total reaction rate. Effects of gradient distribution of macropores within the porous medium are also investigated. It is found that a front-loose, back-tight (FLBT) hierarchical structure is desirable for enhancing mass transport, increasing total reaction rate, and improving catalyst utilization. On the whole, from the viewpoint of reducing cost and improving material performance, hierarchical porous structures, especially gradient structures with the size of macropores gradually decreasing along the transport direction, are desirable for catalyst application. [ABSTRACT FROM AUTHOR]

Solubility trapping is crucial for permanent CO 2 sequestration in deep saline aquifers. For the first time, a pore-scale numerical method is developed to investigate coupled scCO 2 -water two-phase flow, multicomponent (CO 2 (aq), H + , HCO 3 − , CO 3 2− and OH − ) mass transport, heterogeneous interfacial dissolution reaction, and homogeneous dissociation reactions. Pore-scale details of evolutions of multiphase distributions and concentration fields are presented and discussed. Time evolutions of several variables including averaged CO 2 (aq) concentration, scCO 2 saturation, and pH value are analyzed. Specific interfacial length, an important variable which cannot be determined but is required by continuum models, is investigated in detail. Mass transport coefficient or efficient dissolution rate is also evaluated. The pore-scale results show strong non-equilibrium characteristics during solubility trapping due to non-uniform distributions of multiphase as well as slow mass transport process. Complicated coupling mechanisms between multiphase flow, mass transport and chemical reactions are also revealed. Finally, effects of wettability are also studied. The pore-scale studies provide deep understanding of non-linear non-equilibrium multiple physicochemical processes during CO 2 solubility trapping processes, and also allow to quantitatively predict some important empirical relationships, such as saturation-interfacial surface area, for continuum models. [ABSTRACT FROM AUTHOR]

Self-cleaning porous membrane can serve as condensing surface for its capacity of transporting condensate to the back through the pores thus keeping the surface clean during the whole condensing period. However, the three-dimensional (3D) condensate behaviors including the initial nucleation, growth and permeation in the porous structures have seldom been clarified comprehensively. In this work, the 3D pseudopotential LB model with a phase change model are employed, and the condensation process inside the porous membrane with various porosity, wettability, additional external force and section geometric configurations are investigated. It's found that with the increase of porosity, surface hydrophobicity and additional external force, more rapid penetration of condensate in the porous media and correspondingly faster removal rate of liquid can be obtained, and the subsequent vapor condensation is enhanced. But the nucleation time gets longer with high porosity and surface hydrophobicity. The additional external force has little effect on the nucleation time. Larger porosity and lower flow resistance inside the straight channel lead to higher condensation rate at the initial stage of condensation, while the porous structure with distorted pores with the same porosity contributes to improve the total condensation amount. This work helps to select the porous membranes with different porosity, wettability or pore scale geometric characteristic and external field for different condensation conditions in specific applications. [ABSTRACT FROM AUTHOR]

Representative elementary volume (REV)-scale structure of shale matrix is reconstructed based on elementary building block (EBB) model using a stochastic reconstruction method called Quartet Structure Generation Set. In the EBB model, various constituents with different pore morphologies in shale matrix including organic matter and inorganic minerals are considered as different EBBs, and in each EBB, specific structural parameters and transport properties are locally defined. A generalized lattice Boltzmann model for fluid flow through tight porous media with slippage is employed to simulate fluid flow through the reconstructed REV-scale structures. A four-EBB shale matrix including clay, calcite, pyrite and organic matter is studied and its permeability is predicted. Effects of organic content, grain size, interparticle pores on the permeability of the REV-scale matrix are investigated. It is found that smaller grain size and interparticle pores can increase the permeability. The influences of complex physical processes such as slippage and adsorption on the REV-scale permeability are also explored. Slippage effect increases as the pore size decreases. Adsorption has two opposite effects on the permeability, and which one dominates depend on pressure. The present study can help understand gas transport in shale matrix and improve reservoir scale studies. [ABSTRACT FROM AUTHOR]

Fundamental understanding of coupled physicochemical processes is crucial for improving the heat storage/release performance of thermochemical heat storage systems. In this study, for the first time a coupled lattice Boltzmann model is developed to simulate the particle-scale physicochemical processes during Ca(OH) 2 dehydration, including fluid flow, heat transfer, vapor mass transport and chemical reaction. The dehydration processes of a single Ca(OH) 2 particle, a single particle with coated ceramic shell and packed particles are studied, and thorough parametric studies are performed. The results show that the dehydration reaction rate of a single particle is mainly determined by the temperature and Ca(OH) 2 concentration. Introducing more micro-pores or meso-pores into the particle is favorable to achieve quicker heat storage, but at the cost of lower energy density. Increasing the Reynolds number from 0.3 to 3 or increasing the inlet temperature by 50 K can shorten the reaction time t c by at least 33.8%. Dedicate design of the shell coated on the particle can enhance the dehydration process with t c decreased by 3.5%. The underlying heterogenous structures greatly affect the reaction rate of packed particles, and local cracks should be prevented to achieve fast and stable heat storage response. • Reactive transport process of Ca(OH) 2 dehydration at particle scale is simulated. • Temperature and solid concentration determine the dehydration of single particle. • Coating ceramic shell would increase the dehydration reaction rate of particle. • Structural heterogeneity strongly affects the dehydration of packed particles. • Effects of structural and operational parameters on the dehydration are studied. [ABSTRACT FROM AUTHOR]

A numerical approach is developed to simulate the formation of pseudomorphs with presence of chemical fluids at the mesoscopic scale. This approach consists of the lattice Boltzmann method (LBM) for transport of chemical species in the pore space, a chemical reaction model including basic kinetics of the coupled dissolution and precipitation reactions, and a mesoscopic model for nucleation and crystal growth. Our study confirms the mechanism of the solution chemistry-driven interface-coupled dissolution-precipitation for the formation of pseudomorphs and identifies several sources for the generation of porosity in the pseudomorphs. We demonstrate that epitaxial precipitation is not necessary and random crystal growth may be more favorable for pseudomorphs. We show that the difference of precipitation barrier on the surface of the primary and secondary minerals should not be too large. Otherwise only the rim of the primary phase is roughly preserved. [ABSTRACT FROM AUTHOR]

Three immiscible fluids systems are often encountered in energy and environmental disciplines, especially in the oil exploration field. It is essential to enhance oil recovery (EOR) by identifying the three-phase fluid flow mechanism of oil-gas-water system in complex geological structures. In the present study, a lattice Boltzmann (LB) color gradient model for three immiscible fluid flow in three-dimensional porous media is employed and validated using five benchmarks (i.e., the multiphase spinodal decomposition; the multiphase Young-Laplace test; the liquid lens; contact angles and 3D channel layered three-phase fluid flow). The effects of wettability, viscosity ratio (M) and capillary number (Ca) on the relative permeability curves are analyzed in detail. When the wetting condition is enhanced, the results indicate that the relative permeability of non-wetting phase raises while that for the wetting phase exhibits a decline. When M is not equal 1, the relative permeability of high viscosity phase is significantly increased, while the other two phases are less affected. Meanwhile, the maximal value of relative permeability does not exceed unity because of the low porosity in the porous medium. For the influence of Ca , simulation results indicate the relative permeability shows weak sensitivity to Ca. • A lattice Boltzmann color gradient model for three-phase fluid flow is proposed. • Pore-scale three-phase fluid flow in porous media is studied. • Three-phase relative permeability is predicted. • Effects of wettability, viscosity ratio and capillary number are explored. [ABSTRACT FROM AUTHOR]

• Nucleation position rises from bottom to top with the increase of surface hydrophobicity. • For C3 the condensation mode transits from dropwise to film condensation. • Cases with hybrid surface have shorter nucleation time than homogeneous surfaces. • Hydrophilic spots on hydrophobic pillars can shorten nucleation time and increase heat flux. Condensation is one of the most widely investigated interface phenomenon in science and engineering. However the three-dimensional (3D) condensate behaviors including the initial nucleation, growth, coalescence, penetration, spreading or expansion on textured surfaces have seldom been clarified comprehensively. In this work, the 3D pseudopotential LB model with a phase change model is employed, and the condensation process on textured structures with homogeneous and inhomogeneous wettability is investigated. It is found that the nucleation position rises to the top of the structure and the nucleation time gets longer with the increase of surface hydrophobicity when condensation occurs on homogeneous textured structure, and the dropwise condensation mode transits into the film condensation. The condensation on structures with inhomogeneous surface wettability at the top, side and bottom of the pillars and structures with hybrid top (namely hydrophilic spots in the center of the top of the hydrophobic pillars) is then investigated. It is concluded that hybrid top of the textured structures can enhance condensation heat transfer, shorten the nucleation time, and at the same time maintain the condensate in the Cassie state for a long time before the condensation mode transition, leading to the best condensation performance. [ABSTRACT FROM AUTHOR]