Two-phase Fluid Research Articles

Integration of CFD simulated abrasive waterjet flow dynamics with the material removal model for kerf geometry prediction in overlapped erosion on Ti-6Al-4V alloy

Complex surfaces in difficult-to-machine materials can be milled by maneuvering the abrasive waterjet (AWJ) according to the local part geometry by suitably overlapping the kerf cross-sectional profiles (CPs). The CP of a kerf milled in a single pass (i.e., trench) and overlapped pass (i.e., cavity) erosion by the AWJ are the building blocks that need to be known apriori for developing a jet path strategy to achieve a complex surface. This demands an efficient model for the kerf CP prediction. The present work proposes a novel framework for overlapped kerf CP prediction, wherein the jet characteristics of the AWJ exiting the focusing tube are evaluated from the two-phase flow computational fluid dynamics simulation coupled with the material removal model developed based on energy conservation principles. Towards incorporating the non-linearities observed in the kerf formation from the overlapped experimental trials into the model for CP prediction enhancement, the following are considered: (i) a hypothesis: ‘secondary erosion by the jet on the previously eroded kerf increases material removal’, (ii) the varying jet-material property of the ductile material (Ti-6Al-4V alloy), and (iii) local particle impact angles. Furthermore, the variation in the above mentioned non-linearities is incorporated into the model under the change in the AWJ conditions. From the results, it is observed that the proposed models predicted (i) the trench CP with a mean absolute error (MAE) of 27 μm, and the maximum erosion depth (hmax) with an error < 2%, and (ii) cavity at different overlaps (7% – 82%) and jet traverse rate (3000 mm/min – 5000 mm/min) with a MAE of 47 μm, and the hmax with an error < 6%. The predicted geometry of the CP correlates well with the experimental ones, with an average R2 of 0.98 and 0.95 in single- and overlapped- pass erosions.

Curvature and shape relaxation in surface-viscous domains.

The mechanics of curved, heterogeneous, surfactant-laden surfaces and interfaces are important to a variety of engineering and biological applications. To date, most models of rheologically complex interfaces have focused on homogeneous systems of planar or fixed curvature. In this study, we investigate a simple, dynamical model of a two-phase surface fluid on a curved interface: a condensed, surface-viscous domain embedded within a surface-inviscid, spherical interface of time-varying radius of curvature. Our aim is to understand how changes in surface curvature generate two-dimensional Stokes flows inside the domain, thereby resisting curvature deformation and distorting the domain shape. We model the surface stress within the domain using the classical Boussinesq-Scriven constitutive equation, simplified for a near-spherical cap undergoing a small-amplitude curvature deformation. We then analyze the frequency-dependent dynamics of the surface stress and curvature within the domain when the pressure difference across the surface is sinusoidally oscillated. We find that the curvature relaxes diffusively, and thus define a Peclet number (Pe) relating the rate of diffusion to the oscillation frequency. At small enough Pe, the surface deforms quasi-statically, whereas at high Pe, the curvature varies sharply within a thin boundary layer adjacent to the domain border. Consequently, the curvature of the domain appears discontinuous from the rest of the surface under rapid oscillation. We then examine the linear stability of the domain shape to small, non-axisymmetric perturbations when the surface is steadily compressed (i.e., the pressure difference across it is increased). While the line tension at the domain border tends to maintain circular symmetry, surface-viscous stresses generated by surface compression tend to destabilize the perimeter. A shape instability arises above a critical surface capillary number (Ca) relating surface-viscous stresses to line tension. Moreover, we show that the mechanism of instability is distinct from that of the famous Saffman-Taylor fingering instability. Various extensions of our model are discussed, including materials with finite dilatational surface viscosity, linear and nonlinear (visco)elasticity, and large-amplitude deformations.

Dynamic analysis of the large strain deformation of flexible pipes conveying two-phase fluids. Part II: nonlinear vibration analysis

This work presents the nonlinear analysis of the dynamics of large strain deformation of subsea flowlines and jumpers conveying two-phase fluid. Precisely, flexible pipes operating in the stated condition undergo large strain deformations. Thus, the known nonlinear deterministic model of the system is solved using method of discretized perturbation. Precisely, this study obtained the nonlinear natural frequency of simply supported flexible pipes modelled using the large strain deformation theory. Results show that both hardening and softening nonlinear behaviors are indicated for pipe undergoing large strain deformation compared with deformations modelled with small strain theory. Thus, operational insights and parameters for sustainable management of large strain deformed subsea flow lines and jumpers conveying two phase flow are made available.

Assessing heat transfer and nanofluid laminar flow in the curved micro-mixers by adopting two-phase model

Micromixers are devices used for mixing fluids at the microscale level and are used for a variety of applications, including chemical reactions, mass transport, and heat transfer processes. In heat transfer processes, micromixers can enhance the heat transfer rate by creating a more homogeneous fluid mixture. In this research, the flow and heat transfer of a nanofluid are simulated in four different geometry of micromixers. The objective is to investigate which one has a better performance compared to a smooth microchannel with regard to the performance evaluation criterion (PEC), which proposes an evaluation with regard to the trade-off between heat transfer and pressure drop. For this purpose, the Ag/water nanofluid is simulated in micromixers of different geometries at Re numbers ranging from 200 to 800 and solid volume fractions (SVF) of 0, 2, and 4% using a finite volume method. The mixture model is adopted to simulate the nanofluid as a two-phase fluid, which is subjected to a laminar flow regime. The micromixers’ walls are exposed to constant heat flux. The effect of increasing the Re number and the SVF is assessed and compared in all geometries, and the results indicate that an increase in both these figures increases the PEC considerably. Moreover, the PEC for each case is assessed and compared, and the best and worst geometry in terms of heat transfer as well as pressure drop are presented. Specifically, in the best case with the highest PEC at the Re = 800 and SVF = 4%, the Nu number is increased by 2.3 fold while its pressure drop is increased by 1.42 fold compared to a smooth microchannel.

Flow structure transition and identification of two-phase fluid flow through rough rock fractures

Flow structure transition and identification of two-phase fluid flow through rough rock fractures

ANALYSIS OF THE Z WELL PRODUCTION TEST USING THE HORIZONTAL LIP PRESSURE METHOD AT PT. PERTAMINA GEOTHERMAL ENERGY ULUBELU AREA

The Ulubelu area of PT. Pertamina Geothermal Energy is a geothermal field that produces two-phase hot fluids with water domination and has a generation capacity of 2 x 110 MW. The production well fluids are conveyed to a separator and scrubber before being directed to the power generation unit (PLTP). Prior to production, a well production test is necessary to determine the well's condition, reservoir characteristics, production capacity, and enthalpy contained in the well fluids. This activity is carried out after drilling and completion testing have been conducted. The method used in testing the Z well is the horizontal lip pressure method developed by Russell James. The purpose of this production test is to determine the maximum and optimum production by adjusting the throttle valve opening in various ways so that the supplied fluid amount matches the PLTP conditions. The production test yields data in the form of parameters to be processed, such as dryness fraction, fluid mass flow rate, enthalpy, and generation. From this data, a deliverability curve can be generated in the form of a graph that shows the well's production capacity at different wellhead pressures.

Computational study of the thermal performance of water/Fe3O4 nanofluid in an oscillating heat pipe: A molecular dynamics approach

Recently, oscillating heat pipes (OHPs) filled with nanofluid (NF) as the operating fluid has drawn researchers’ attention because of their improved thermal conductivity, and heat/mass transfer (HT/MT) characteristics. An OHP is an HT device based on a two-phase fluid flow that transfers heat between heat sources and heat sinks which is applicable in industries in terms of its highly effective thermal conductivity. According to previous research, in previous experimental and computational studies, the effect of adding metal oxide NPs into the operating fluid of an OHP was not studied. Therefore, adding Fe3O4 NPs to the operating fluid of the water flowing into an OHP with nano dimensions will be the research work ahead that can increase the efficiency of designed structures. The maximum density, velocity, temperature, and heat flux after 20 ns are examined to determine the effects of NP size and an external magnetic field (EMF). The numerical findings show that heat flux increased from 1561 to 1602 W/m2 when the NPs' size grew from 5 to 10. Therefore, the HT/MT of Fe3O4-H2O simulated NF showed enhanced thermal behavior as NP's radius increases. Furthermore, the results show that the presence of an EMF enhanced the thermal behavior of NF in the OHP. The heat flux increased from 1563 to 1586 W/m2 when the magnetic field magnitude increased from 1 to 5 T.

Optimal displacement of immiscible two-phase fluids in a pore doublet

Multiphase fluid flow in a pore doublet is a fundamental problem and is important for understanding the transport mechanisms of multiphase flows in porous media. During the displacement of immiscible two-phase fluids in a pore doublet, the transport process is influenced not only by the capillary and viscous forces, but also the channel geometry. In this paper, a mathematical model is presented of the two-phase fluid displacement in a pore doublet considering the effects of capillary force, viscous force, and the geometric structure. This leads to a new and more general analytical solution for the pore doublet system, and it is found that the displacement process is dominated by the capillary number, viscosity, and radius ratios. Also, the optimal displacement is defined, which refers to the wetting fluids in the two daughter channels breaking through the branches simultaneously (i.e., both having the same breakthrough time). Also, the critical capillary number corresponding to the optimal displacement is obtained, which is related to the radius ratio of the two daughter channels and the viscosity ratio of the two immiscible fluids. Finally, analytical results are presented for the displacement in the pore doublet, which can be used to explain and understand the preferential flows in porous media, such as for improving oil recovery from porous media; these are usually observed in oil recovery, groundwater pollution, and the geological sequestration of carbon dioxide.

Impact of wettability on immiscible displacement in water saturated thin porous media

The characterization of immiscible displacement processes at the pore scale is crucial in order to understand macroscopic behaviors of fluids for efficient use of multiphase transport in various applications. In this study, the impact of porous material wetting properties on gas invasion behavior at various gas injection rates was investigated for thin hydrophilic porous media. An experimentally validated two-phase computational fluid dynamics model was employed to simulate the dynamic fluid–fluid displacement process of oxygen gas injection into liquid water saturated thin porous media. A phase diagram was developed through a parametric characterization of the thin porous media in terms of the material hydrophobicity and gas flow rates. In addition to calculating the saturation of the invading gas, gas pressure variations were calculated and used to identify the locations of phase diagram boundaries. Non-wetting phase streamlines resolved at the microscale were visualized and presented as a novel indicator for identifying displacement regimes and phase diagram boundaries. It was observed that the crossover from the capillary fingering regime to the stable displacement regime occurred between contact angles of 60° and 80°. By increasing the gas injection rate, due to viscous instabilities, flow patterns transitioned from the capillary fingering and stable displacement regimes to viscous fingering regime.

Computational Analysis of Erosion Wear Rate by Multiphase Flow in Optimum Bend Ratio of Pipeline

Abstract: Pipe bends are an integral part of transportation system as they help by providing flexibility in the path. In this paper, flow of mono-dispersed particles with carrier flow is simulated using Computational Fluid Dynamics by implementing Eulerian two-phase model in FLUENT. This paper is a forward stepping stone as it depicts the effect of pipeline bend on flow characteristic such as erosion rate, concentration profile, wall shear stress, etc. after numerous researches on pressure drop and velocity profiles. In this paper, a two-phase fluid of silica sand, having mean diameter of 450 µm (density = 2650 kg/m3 ), and water are mixed at the ratio of 8.82% solid concentration by weight and flown through a pipeline with diameter 53.0 mm and the optimum pipe bend ratio of 5.60 for minimum pressure loss.

Prediction of two-phase flow properties for digital sandstones using 3D convolutional neural networks

Multiphase fluid flow within porous media is of great importance in a wide range of environmental and industrial fields, such as CO2 sequestration, geothermal systems, fuel cells, enhanced oil recovery, and groundwater remediation. Pore-scale modeling is a promising tool to estimate macroscopic multiphase flow properties from micro-scale images of porous materials; however, it is a complex, time-consuming procedure and would be highly resource-intensive in the case of direct numerical simulation. We present a framework that consists of (1) extraction of sub-samples from rock images, (2) computation of relative permeability and capillary pressure from two-phase pore network modeling with respect to their contact angle and interfacial tensions, (3) training a convolutional neural network (CNN), and (4) validation with unseen datasets and parameters. 500 sub-samples of grayscale and binary types are extracted from images of 12 different sandstones. Relative permeability, capillary pressure, and residual saturation of binary sub-samples are computed from two-phase fluid flow simulation in their representative pore network models The proposed CNN model is trained with grayscale images, eliminating the need for image processing. The results demonstrate a good agreement between CNN predictions and simulation results, and the computational time was reduced by several orders of magnitude.

Gas composition measurements in randomly distributed and fast moving gas bubbles in two-phase fluids.

Gas composition in randomly distributed and fast-moving bubbles was optically measured aided by laser-induced breakdown spectroscopy (LIBS). Laser pulses were focused at a point in a stream of bubbles to induce plasmas for the LIBS measurements. The distance between the laser focal point and liquid-gas interface, or 'depth,' plays a major role in determining the plasma emission spectrum in two-phase fluids. However, the 'depth' effect has not been investigated in previous studies. Therefore, we evaluated the 'depth' effect in a calibration experiment near a still and flat liquid-gas interface using proper orthogonal decomposition, and a support vector regression model was trained to exclude the influence of the interfacing liquid and extract gas composition information from the spectra. The gaseous molecular oxygen mole fraction in the bubbles was accurately measured under realistic two-phase fluid conditions.

Simplified scaling relations for the depth-dependence and IFT reduction of fluid imbibition in gas-saturated reservoir rocks

Imbibition of water into fractured unconventional reservoirs is a significant factor contributing to the large volumes of permanently lost water by hydraulic fracturing. The lost water partitions at the fracture-matrix interface thereby restricting gas flow into the fractures. Thus, minimizing the large water footprint during the fracturing process is very attractive for water conservation and gas production. A key step towards achieving this goal is quantifying uncertainties needed to reliably predict water imbibition and to evaluate the success of efforts aimed at minimizing it. In this contribution, we present new scaling relations for water imbibition in gas-saturated rocks that account for the depth-dependence of fluid properties and chemical alteration of IFT through the use of a surfactant. We then propose a new method for quantifying uncertainties relating to rock properties, two-phase fluid properties, and the driving force for imbibition. We applied the proposed approach to analyze glass-bead pack and shale experiments involving water imbibition into air-filled pores, and validated our model predictions against published values from X-ray CT images and transient permeability measurements. Our work provides a methodology for quantifying the uncertainties pertaining to water imbibition, thereby enabling optimization of post-fracturing interventions aimed at minimizing water loss in deep unconventional shale gas reservoirs.

Pulsating pressurization of two-phase fluid in a pipe filled with water and a little gas

Although two-phase flows containing gas and water have received extensive attention, the pulsating pressurization effect of a two-phase fluid in a pipe is unclear and the influence of the gas-phase content has not been revealed. This paper discusses the pulsating pressurization of such a two-phase fluid. First, the two-phase Navier–Stokes equations are derived and an algorithm is developed based on MacCormack's method. The reliability of the algorithm is examined and validated using Poiseuille's theory and existing experimental two-phase flow data. Finally, the influence of several key factors is discussed, including the gas-phase fraction and pipe slenderness. Our results show that a significant pulsating supercharging phenomenon occurs when the gas-phase fraction is less than 10−3. When the gas-phase fraction is greater than this critical value, the pulsating supercharging effect decreases significantly with the increasing gas-phase fraction. The equivalent elastic modulus of the two-phase fluid rapidly decreases as the gas-phase fraction increases, and the pressure disturbance is absorbed by the gas bubbles, causing an apparent weakening of the pulsating supercharging effect. Thus, decreasing the gas-phase content can enhance the pulsating supercharging effect. The pipe slenderness has a very limited influence on the pulsating pressurization process, and the maximum reduction is only 7.3% for slenderness ratios of up to 2000. Moreover, we derive and propose a new mathematical expression for the inlet boundary that is applicable to gas–liquid two-phase flows. To our knowledge, this paper extends the pulsating pressurization range from the single-phase to two-phase fluid for the first time and reports different physical phenomena and regularity. The present research clarifies the pulsating pressurization phenomenon in two-phase flows, providing a valuable reference for pulsating pressurization design.

The Peña do Seo W-Sn deposit, NW Iberia: Petrology, fluid inclusions and O-H-S isotopes

The Peña do Seo W-Sn ore deposit in NW Iberia consists mainly of quartz veins hosted in schists. Vein mineralogy comprises wolframite, cassiterite and minor molybdenite. Peraluminous S-type granites and minor metaluminous granitoid breccias crop out in the same location. Whole rock geochemistry, mineral characterization, fluid inclusions and stable isotope geochemistry have been combined to infer hydrothermal mineralization conditions. A pre-ore stage (0) involving the alteration of host rocks, three stages of mineralization (I, II and III), and a supergene stage (IV) have been identified. Stage I (oxide-halide-sulfide stage) consisted of a cassiterite-pyrite-fluorite mineralization in granitoid breccias. Stage II (main oxide stage) involved wolframite-rich selvages and wolframite-cassiterite-(molybdenite) quartz veins. Stage III (main sulfide stage) consisted of a sulfide mineralization in quartz veins comprising pyrite and minor arsenopyrite and chalcopyrite. Three types of fluid inclusions were found in vein quartz: (1) aqueous two-phase inclusions, with homogenization temperatures (Th) between 445 °C and 280 °C and moderate salinities (9-14 wt.% NaCl eq.), (2) aqueous-carbonic three-phase fluid inclusions, with Th from 340 °C to 260 °C and low salinities (2-7 wt.% NaCl eq.), and (3) aqueous two-phase fluid inclusions, with Th from 270°C to 155 °C and low salinities (0-6 wt.% NaCl eq.). δ18O values in quartz from mineralized veins range from +11.2‰ to +13.4‰, and between +15.0‰ and +15.4% in quartz from mineralized granitoid breccias. δ34S values in sulfides (pyrite, arsenopyrite and chalcopyrite) range between +13.0 ‰ and +37.1 ‰, thus suggesting a marine source of sulfate and possible equilibration with host-rock sulfides. δD values in muscovite and chlorite from quartz veins range between -105.7 ‰ and -71.5 ‰ and between -69.4 ‰ and -67.1 ‰, respectively, indicating a transition from magmatic to magmatic-metamorphic conditions. An aqueous (H2O–NaCl) magmatic-hydrothermal fluid led to the W-Sn mineralization, as deduced from the studied fluid inclusions and isotopic signatures. High W and Sn contents in the peraluminous granites indicate that the W-Sn mineralization in Peña do Seo could partially be related to the granites and granitoid breccias. Whereas the Sn (± W) likely derived from hydrothermal fluids exsolving from the crystallizing magmas, host quartz schists would have supplied other elements required for wolframite deposition such as Fe and Mn. Sulfide mineralization in stage III was probably driven by fluid dilution.

Numerical simulation of water shut-off performance for heterogeneous layered oil reservoirs

This paper proposes a hydrodynamic model of two-phase three-component fluid filtration in a layered heterogeneous reservoir that takes into account cross-flows between layers and allows prediction of the process of oil displacement by water under conditions of water shut-off near injection and production wells. The numerical calculations show that, in the absence of cross-flow between layers, the oil recovery factor for the entire reservoir is higher with water shut-off near the injection well than with water shut-off near the production well, and this effect grows as the water cut of the high permeability layer decreases. In the event of cross-flow across layers, the oil recovery factor in the entire reservoir is higher with water shut-off near the production well than with water shut-off near the injection well, and this impact grows when water cut in the high-permeability layer decreases. In the absence of crossflow, the gel washout time during water shut-off in the region of both injection and production wells is substantially longer than in the presence of crossflow. Keywords: hydrodynamic model; water shut-off; oil reservoir; gel; well.

Relative permeability of two-phase fluid flow through rough fractures: The roles of fracture roughness and confining pressure

The relative permeability of two-phase fluid flow through rough fractures was investigated by conducting the triaxial two-phase fluid flow experiment on sandstone fractures. The relative permeability in rough rock fractures is greater than that predicted by the Corey model and the viscous coupling model due to interphase resistance. Therefore, the flow resistance between two-phase fluids is lower than that of the Corey model and viscous coupling model. The resistance between two-phase fluids increases with the increase of fracture roughness and confining pressure, causing the decrease of relative permeability of two-phase fluid flow. It is also found that the influence of fracture roughness on relative permeability of water phase is more obvious than that of gas phase, while the influence of confining pressure on relative permeability of gas phase is more obvious than that of water phase. The relative role of fracture roughness and confining pressure in two-phase fluid flow decreases while the relative permeability approaching the equal-permeability point. Based on the influence of fracture roughness and confining pressure, a relative permeability model for rough rock fractures under the effect of confining pressure is established. This model describes the two-phase fluid flow behavior in rough rock fracture under different confining pressure well. Further analysis on relative permeability in terms of fractional flow of water was conducted. It shows that the confining pressure and fracture surface roughness play a hindering role in the two-phase fluid flow. This hindering effect is mainly on nitrogen at the low fractional flow of water and mainly on water at the high fractional flow of water.

Geology, geochemistry, fluid inclusion data, stable isotope characteristics, and ore genesis of the Barout Aghaji gold deposit, NW Zanjan, Iran

The Barout Aghaji gold deposit is located ∼90 km northwest of Zanjan, within the Takab-Takht-e-Soleyman subzone of the Sanandaj-Sirjan metamorphosed-deformed zone. Ore-bearing quartz veins are hosted by Neoproterozoic amphibolite and Eocene to Oligocene granitic gneisses. Oligo-Miocene Upper Red Formation unconformably overlies the amphibolite and granitic gneisses. Field observations and petrographic studies show that two deformation stages occurred in this area. The first deformation stage was ductile, producing mylonitic and proto-mylonitic microstructures, but the second one was brittle, represented by sheeted quartz veins and veinlets. In the first stage, barren milky quartz veins occurred containing minor sulfide minerals, but dark to light gray ore-bearing quartz veins and veinlets are formed in the latter stage. The mineralized veins appear as massive microcrystalline quartz cut by sheeted quartz veins with comb, druse, and crustiform textures. The gold-bearing quartz veins contain as much as 3% sulfide minerals. Pyrite is the main sulfide mineral and is associated with minor chalcopyrite. Sulfides are commonly altered to hematite, goethite, and rarely malachite. Hydrothermal alteration around the quartz veins consists of silicification, pyritization, and sericitization. The whole-rock geochemistry of the collected samples from the granitic gneisses and quartz veins shows that Au is enriched in the quartz veins (average of 114 ppb) relative to host rocks (average of 22.5 ppb). Au shows strong positive correlations with As, Ba, Mo, Pb, Sc, Tl, Ag, and negative correlations with Cu, Bi, Se, and Te in the granitic gneisses. It also shows strong positive correlations with S, Hg, Th, Co, Bi, Pb, and Ag and negative correlations with P, V, Te, W, Sc, Zn in quartz veins. Four types of primary fluid inclusions were identified, including type I, two-phase aqueous-rich fluid inclusions (liquid > vapor; LV); type II, two-phase vapor-rich fluid inclusions (gas > liquid; VL); type III, three-phase fluid inclusions containing CO2 with clathrate formation (L1L2V); and type IV three-phase fluid inclusions (aqueous, vapor, and solid; LVS). The homogenization temperatures of fluid inclusions in auriferous quartz veins range from 199 −446 with a mode of 270–300 °C. Salinities range from 0.8 to 49.02 wt% NaCl Equiv. with two distinct populations at 0.8–8.5 and 31.1–49.02 wt% NaCl Equiv. The large variations in the homogenization temperatures and salinities can be attributed to the cooling and isothermal mixing of fluids. The δ34S values for four pyrites separated from auriferous quartz veins range from +2.9 to +7.1‰, with an average of 4.5‰. δ34S values of fluids in equilibrium with pyrite were calculated from +3.5 to +7.3‰, with an average of 5.4‰, indicating a metamorphic source for the sulfur using temperatures estimated from the fluid inclusion study. The Field observations, vein textures, mineralogy, ore geochemistry, fluid inclusion studies, and sulfur isotope data indicate that gold mineralization in the Barout Aghaji area has many similarities to orogenic and intrusion-related gold deposits, such that low salinity fluids derived from metamorphic rocks are mixed with high salinity fluid inclusions possibly derived from granitic gneisses during syn to post tectonic magmatism.

Directional Dependency of Relative Permeability in Vugular Porous Medium: Experiment and Numerical Simulation

Carbonate reservoirs are a highly heterogeneous type of reservoir characterized by the presence of a large amount of vugs and pores. During two-phase displacement, the two-phase flow regime in the vugs might be gravity segregated. The distribution pattern of two-phase fluid in the vugs would accelerate the water flow in downward and horizontal directions, meanwhile decelerating in an upward direction, resulting in a different oil recovery ratio. This gives rise to the question of whether the relative permeability should be modeled as a directional dependent in a vugular porous medium since it is usually treated as an isotropic quantity. In this study, via both experiment and numerical simulation, we demonstrate that the relative permeability of vugular porous medium is dependent on the angle between the flow direction and the horizontal plane and should be considered for oil recovery estimation for carbonate reservoirs. Using the transmissibility-weighted upscaling method and a single-vug model, the relative permeability curves for different flow directions are obtained by numerical simulation. A directional relative permeability model for a vugular porous medium is also proposed.

Effects of Cathode GDL Gradient Porosity Distribution along the Flow Channel Direction on Gas–Liquid Transport and Performance of PEMFC

The gas diffusion layer (GDL) is an important component of proton exchange membrane fuel cells (PEMFCs), and its porosity distribution has considerable effects on the transport properties and durability of PEMFCs. A 3-D two-phase flow computation fluid dynamics model was developed in this study, to numerically investigate the effects of three different porosity distributions in a cathode GDL: gradient-increasing (Case 1), gradient-decreasing (Case 3), and uniform constant (Case 2), on the gas-liquid transport and performance of PEMFCs; the novelty lies in the porosity gradient being along the channel direction, and the physical properties of the GDL related to porosity were modified accordingly. The results showed that at a high current density (2400 mA·cm-2), the GDL of Case 1 had a gas velocity of up to 0.5 cm·s-1 along the channel direction. The liquid water in the membrane electrode assembly could be easily removed because of the larger gas velocity and capillary pressure, resulting in a higher oxygen concentration in the GDL and the catalyst layer. Therefore, the cell performance increased. The voltage in Case 1 increased by 8% and 71% compared to Cases 2 and 3, respectively. In addition, this could ameliorate the distribution uniformity of the dissolved water and the current density in the membrane along the channel direction, which was beneficial for the durability of the PEMFC. The distribution of the GDL porosity at lower current densities had a less significant effect on the cell performance. The findings of this study may provide significant guidance for the design and optimization of the GDL in PEMFCs.