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The effective thermal conductivity of unsaturated fibrous media is simulated by Fractal-Monte Carlo simulations in this work. Based on the probability model and thermal-electrical analogy method, the proposed model of the effective thermal conductivity can be expressed as an explicit function of fractal dimensions, D t , D f , intrinsic thermal conductivities, k s , k w , k g , the straight capillary length of a unit cell L 0 , liquid saturation S , pore size λ i as well as porosity ϕ. Each parameter of the proposed model has physical meaning and the function has no empirical constant. The results calculated by Fractal-Monte Carlo simulations have been compared with the experimental data available, and an excellent agreement has been shown for an appropriate range of porosity except for the condition at a lower liquid saturation. It is found that, when k s / k g > 1 and k w / k g > 1 , the dimensionless effective thermal conductivity of unsaturated fibrous media decreases with increment in fractal tortuosity dimension and porosity and decrement in liquid saturation. On the other hand, when k s / k g < 1 and k w / k g < 1 , the correlations are opposite. The present Fractal-Monte Carlo technique may have its own advantages in predicting other transports in unsaturated fibrous media. [ABSTRACT FROM AUTHOR]

The unrecovered hydraulic fracturing fluid will invade the matrix and induce water blockage, creating formation damage and hindering the oil or gas production rate. First, the synergistic effect of cationic Gemini surfactant (MQAS) and nonionic fluorosurfactant (N‐2821) mixtures on reducing the surface tension and wettability alteration was investigated in this paper. The critical micelle concentration (CMC) of the surfactant mixture is one or two orders of magnitude lower than that of N‐2821 and MQAS, indicating that the MQAS/N‐2821 mixtures exhibit an apparent synergistic effect in reducing surface tension. Moreover, the maximal contact angle of MQAS/N‐2821 mixtures reached 83.55° at αN‐2821 = 0.5, and the total surfactant concentration of 1 × 10−4 mol/L due to the adsorption of surfactant. The adsorption mechanism of surfactants on the surface of quartz sand was then examined. The adsorption kinetics is consistent with the pseudo‐second‐order model at different surfactant concentrations, while the Freundlich model is suitable for describing the adsorption behavior of surfactants on the sandstone surface. This finding indicates that surfactant adsorption is multilayered. The MQAS/N‐2821 surfactant mixtures have excellent surfactant activity due to the relationship of the capillary pressure to the surface tension, pore radius, and contact angle; thus, the addition of surfactant mixtures can reduce the liquid saturation effectively. Furthermore, the sequential imbibition experiments indicate that MQAS/N‐2821 mixtures alter the wettability of the core plug, which results from the adsorption of surfactants. Compared with brine water, the MQAS/N‐2821 mixtures decreased the liquid saturation and increased the permeability recovery ratios of the core plug. [ABSTRACT FROM AUTHOR]

As a porous medium with rich pore structure, activated carbon (AC) was once considered the best carrier for hydrate storage and transportation. However, the harsh conditions of the hydrate reaction in the wet carbon environment have always limited the sufficient and rapid formation of hydrate. Therefore, the influences of particle size (4–8, 8–16, 20–40 and 100 mesh) and liquid phase saturation (fully/partially saturated) in the sodium dodecyl sulfate system on hydrate reaction were investigated. The results showed that small particle in the fully saturated liquid phase system led to the increase in hydrate generation rate, with the highest hydrate reaction rate of 3.16 mmol/min in the 100 mesh AC layer, which was 1.7–2.9 times higher than other AC layers. The gas storage capability of the 4–8 mesh AC layer with a water saturation of 70% was the highest among all systems, reaching 0.198 mol/mol. The saturation of the liquid phase induced the nucleation and growth of hydrates. Adherently growing hydrates of fully saturated liquid phase systems and mushroom-like hydrates of partially saturated liquid phase systems were found in turn. This research facilitates the commercialization of AC-based hydrate technology. [ABSTRACT FROM AUTHOR]

Due to the presence of liquid bridge forces, wet particles reveal totally different fluidization behaviours than dry fluidized beds. This paper studies the bubble dynamics of wet Geldart‐D particles in a wet 2D fluidized bed. A digital image analysis method is adopted to identify bubbles and extract bubble properties based on MATLAB software. During fluidization, the bubble feature analysis of optimized binary images captures the bubble size variation, bubble fraction, and bubble shape. The results show that the increasing liquid saturation promotes the gas holding capacity of the emulsion phase, leading to the decreasing bubble fraction. When the particle size grows, the stability of the emulsion phase is promoted and hence the average bubble diameter increases. As the liquid saturation increases, the enhancing liquid bridge forces limit the bubble growth and promote the bubble breakage, which contributes to the small bubbles. With respect to the wet particles, of which the diameter is 1 mm and the liquid saturation is over 15%, slugging is likely to be observed, resulting in the increasing equivalent bubble diameter. The growing liquid saturation increases the wide aspect ratio, decreases the shape factor, and disperses the dependence between the two parameters. This indicates that small and irregular bubbles are likely to be seen in wet fluidized beds. [ABSTRACT FROM AUTHOR]

• Using mineral wool wrapped around a pipe heated to over 100 °C, the heat flowing through the dry, water submerged, and drying insulation is measured and also predicted using CFD simulations. • In the submerged insulation, the liquid motion has a significant effect on the heat transfer rate. • The highest heat transfer rate occurs during the submerged period, followed by the drainage-drying period where it gradually falls back to the dry period rate. • During the drying period, evaporation dominates the heat transfer. To test the performance of wet mineral-wool insulation, a water submersion setup is used to monitor its heat transfer sequentially through dry, submerged, and drainage-drying periods. A cylindrical-shell insulation is wrapped around a pipe carrying a preheated (over 100 °C) oil stream. The temperature at various locations is monitored, and after a few hours in each period, steady-state conditions are reached. Numerical 2-D (with gravity) simulations of the transient, simultaneous heat, mass, and momentum transport are also performed, with the control of the insulation hydrophobicity through the insulation surface liquid saturation. The distributions of temperature, liquid saturation, liquid and vapor velocity, vapor mass fraction, and evaporation rate, are predicted as well as the total heat flow through the dry/wet insulation. The predicted heat flow rate and temperature distribution within the insulation, through the three periods, are in good agreement in heat flow rate and temperature distributions with the test results (maximum difference of 20 %). The predicted 2-D liquid saturation shows that gravity and capillary pressure play significant roles in the liquid distribution and the insulation hydrophobicity changes with temperature due to the dissolution of the hydrophobic fiber coating. The presence of a gap between the pipe and insulation plays a significant role in heat transfer during the submerged period, as it allows for continuous direct liquid contact with the pipe. During the drying period, the evaporation rate continuously decreases (with a decrease in the average liquid saturation), governed by the increasing resistances to the heat and liquid flow. [Display omitted] [ABSTRACT FROM AUTHOR]

A two-dimensional geometric model is developed for a polymer electrolyte based on the liquid water penetration mechanism in the membrane electrode assemblies under the action of capillary pressure. The effects of the diameter, number, and distribution of cracks in the micro-pore layers (MPLs) of the modeled MEA on the performance of the PEMFC are simulated to investigate the influence of mass transfer across the membrane. The results indicate that liquid water in the catalyst layer (CL) of the MEA can be discharged to gas channels through the cracks in MEA under the action of capillary pressure, thereby alleviating the flooding in the CL and enhancing the diffusion of oxygen to the CL. When the proportion of the total area of cracks in the active area of the MEA was 8%–12%, crack diameter was 20–30 µm, and cracks were distributed uniformly. MEAs with and without cracks were prepared, fuel cells were assembled, and their performance was measured. The effects of cracks on mass transfer were then verified. This study helps prepare MEAs with controllable cracks. [ABSTRACT FROM AUTHOR]

• Wet granulation induction growth has a severe negative effect of compactibility. • A novel method to quantify nucleation and induction growth was developed. • The transition from nucleation to induction growth was found to be gradual. • The growth regime can be controlled by liquid saturation and Stokes deformation number. • A granule porosity regime map is proposed to better depict the granulation mechanism. A methodological approach to define the regime map boundaries does not exist, and while some experimental data has been reported previously the boundaries have not been "validated". The present study explored and quantified the boundary between the nucleation and induction growth regimes for a model high drug load formulation. It was postulated that the induction period could be decreased by increasing the liquid saturation of the system, with the effect of introducing additional binder liquid to the surface of the granules, which facilitates consolidation and growth. An increase in liquid saturation was achieved by increasing the liquid to solid ratio, or by densifying the granules to squeeze intragranular liquid to the surface. The densification was achieved by subjecting the granules to an extended wet massing period and by varying the impeller speed. By characterizing the response in granule porosity as a function of liquid to solid ratio, wet massing time and impeller speed the boundary between the nucleation and induction growth regimes was defined. Granules formed in the induction growth regime were shown to suffer significantly reduced compactibility. Further analysis of liquid saturation and compactibility data allows a critical granule porosity to be established as a basis for developing a control strategy. The attention to granule porosity allows a mechanistic interpretation of the growth behavior, rather than simply relying upon size measurements. The porosity based approach was successfully superimposed onto the existing growth regime map framework, and established that the boundary between regimes is a gradual transition rather than the conventionally visualized abrupt binary state. [ABSTRACT FROM AUTHOR]

• Effects of saturation and dry density on gas or liquid permeability were analyzed. • Differences in gas and liquid permeability were investigated. • Applicability of vG-M model was examined to describe gas or liquid permeability. • A model was proposed to characterize the gas and liquid permeability relationship. Landfill final covers are crucial to mitigating both the emission of landfill gases and surface water infiltration into solid wastes. Loess that is widely distributed in Northwest China has been transformed into earthen final cover (EFC), although not yet on a large scale. There remains a need for laboratory bench-testing in assessing the performance of this loess as an EFC material, particularly with respect to gas and liquid permeability. To evaluate gas and liquid permeability in the variably saturated loess at the laboratory scale, loess specimens with various resultant dry densities and post-compaction water contents were prepared. Laboratory tests were performed including constant gas pressure-gradient tests and wetting tests in a step-wise fashion to measure gas and liquid permeability as well as water retention behaviours. Large differences were found between intrinsic permeability to gas and liquid, and an empirical power relationship was proposed to characterize the measured intrinsic permeability to gas and liquid relative to the void ratio. The data of relative gas permeability as a function of liquid saturation plotted nearly along a single curve for all dry densities; this was also the case for the relative liquid permeability. Based on the regression results of water retention curves, the measured relative gas permeability values were well described by the van Genuchten–Mulaem (vG-M) model, and a certain difference between the relative liquid permeability evaluated by the vG-M model and the modified Childs and Collis-George equation. An unique relationship was found between the ratio of liquid to gas permeability and liquid saturation for all dry densities, in a range of residual liquid to gas saturation. An empirical model was proposed to efficiently describe gas and liquid permeability of the compacted loess at various liquid saturations and dry densities as well as the data of the loam from another experimental program. [ABSTRACT FROM AUTHOR]

• A 1D two-phase multi-physics model is developed for a water-cooled PEMFC. • Two dimensionless indices are defined to represent the distribution features in MEA. • Porosity, permeability, and hydrophobicity are the main degradation parameters. • Component decay increases the chance of water flooding in the cathode electrode. • Water flooding increases the non-uniform distribution of physical fields in MEA. At present, a small number of studies aim at the influence of degraded components on the distribution characteristics in water-cooled proton exchange membrane fuel cells (PEMFCs). This study develops a one-dimensional (1D) two-phase multi-physics model for a water-cooled PEMFC and investigates the parameter distributions along the through-plane direction. The model is accomplished by self-written codes and validated by the experimental data. Two dimensionless indices (TUI, WUI) are defined to present the distribution uniformity in membrane electrode assembly (MEA). The current intensity and gaseous relative humidity (RH) are the primary factors affecting MEA temperature and membrane water. However, with the anode RH increasing from 0.2 to 1.0, the WUI value decreases from 0.123 to 0.104 despite the water content increasing from 8.9 to 14.4. The results through comparison indicate that, in case 2, severe degradation in porosity, hydrophobicity, and thickness of component materials leads to a high liquid saturation of 0.82 on CCL, which increases the hence of water flooding. In return, the water flooding further reduces voltage performance by 0.1 V compared with the normal cell, even accelerating the component degradation. [ABSTRACT FROM AUTHOR]

Summary: In this study, the effects of adding a microporous layer (MPL) as well as the impact of its physical properties on polymer electrolyte fuel cell (PEMFC) performance with serpentine flow channels were investigated. In addition, numerical simulations were performed to reveal the effect of relative humidity and operating temperature. It is indicated that adding an extra between the gas diffusion layer (GDL) and catalyst layer (CL), a discontinuity in the liquid saturation shows up at their interface because of differences in the wetting properties of the layers. In addition, results show that a higher MPL porosity causes the liquid water saturation to decrease and the cell performance is improved. A larger MPL thickness reduces the cell performance. The effects of MPL on temperature distribution and thermal transport of the membrane prove that the MPL in addition to being a water management layer also improves the thermal management of the PEMFC. Highlights The effect of adding an MPL in a PEMFC having serpentine type flow channels is analyzed.Effects of the MPL physical properties on liquid saturation and the PEMFC performance are revealed.Effects of the MPL on membrane temperature distribution and thermal transport are revealed.Reveals the effect of the MPL on the membrane at the different operating conditions. [ABSTRACT FROM AUTHOR]

Hydrodynamic data obtained from laboratory‐scale trickle‐beds often fail to accurately represent industrial‐scale systems with high packing aspect ratios and column‐to‐particle diameter ratios. In this study, pressure drop, liquid holdup, and flow regime transition were investigated in a pilot‐scale trickle‐bed column of 33 cm ID and 2.45 m bed height packed with 1.6 mm × 8.4 ± 1.4 mm cylindrical extrudates for air‐water mass superficial velocities of 0.0023 – 0.094 kg/m2s and 4.5 – 45 kg/m2s, respectively, at atmospheric pressure. Significant deviation was observed from pressure drop and liquid holdup correlations at low liquid flows rates, corresponding to gravity‐driven flow limit. Likewise, liquid saturation is overestimated by correlations at high liquid flow rates, owing to significantly reduced wall effects. Lastly, trickle‐to‐dispersed bubble flow and trickle‐to‐pulsing flow regime transitions are reported using a combination of visual observations and analysis of the magnitude of local pressure fluctuations within the column. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2560–2569, 2018 [ABSTRACT FROM AUTHOR]

The influence of floating vessel motions on the hydrodynamic behavior of multiphase flows in porous media was studied using a hexapod ship motion emulator with an embarked packed column. The response of gas-liquid distribution, pressure drop, liquid saturation, and flow regime transition to column inclinations and roll motions was compared to those of the corresponding static vertical and inclined configurations. Two-phase flow patterns in terms of local liquid saturation distribution were visualized online by means of a capacitance wire-mesh sensor positioned firmly on the floating packed bed. The hydrodynamic performance of packed beds under roll motion deviates strongly from that of the static beds, indicating that the known characteristics of the conventional land-based trickle-bed reactors cannot be transposed on a one-to-one basis for design and scale-up of the floating reactor configurations. [ABSTRACT FROM AUTHOR]

Gas purging has been a common and favorable strategy to improve the cold-start possibility of a polymer electrolyte membrane fuel cell (PEMFC). In the present study, water removal processes in a PEMFC during the gas purging prior to its shutdown are numerically investigated with a transient two-fluid model. The dryness in the sub-regions of the PEMFC is analyzed. Effects of the operation conditions (such as the gas flow rate, relative humidity and temperature) are also explored. The results indicate that water removal processes are not sequential and the main drying mechanism in the porous medium is the surface evaporation under the pendular stage. The overall dryness highly depends on the inlet water flux other than the convective water flux under the same gas flow rate. The operation parameters have a significant impact on the overall drying performance in terms of the purging effectiveness and the energy consumption. [ABSTRACT FROM AUTHOR]

Multiphase transport inside a proton exchange membrane electrolyzer cell (PEMEC) plays an important role in its performance and design. Most PEMEC modeling studies so far have mainly focused on its electrochemical performance prediction and analysis, and fundamental understanding of the effect of multiphase transport on the cell performance is still lacking. In this study, a two-phase mathematical model is developed to investigate the transport properties inside liquid/gas diffusion layers (LGDLs) and to explore their effects on the PEMEC voltage and efficiency. Sudden changes in the PEMEC voltage and efficiency are captured for the first time as the current density reaches a limiting value, and the limiting current density is greatly impacted by the LGDL contact angle, porosity, and thickness. In addition, the liquid water distribution and cell performance in PEMECs with different important operating and physical parameters are examined and discussed in detail. Increasing the LGDL porosity or/and decreasing its surface contact angle will improve the PEMEC performance especially at the high current density. The thickness changes of the LGDL and membrane also have significant impacts on the cell voltage and efficiency. The model can effectively examine two-phase transport properties and provide useful information for design optimization of a PEMEC. [ABSTRACT FROM AUTHOR]

The gas diffusion layers (GDLs) of polymer electrolyte fuel cells have been developed with applying microporous layers (MPLs) in their catalyst layer (CL) side to alleviate the accumulation of liquid water in the CL for oxygen transport to the cathode CL. A three-dimensional porous structure of our in-house hydrophobic MPL is numerically modeled with a pore network model (PNM). The convective air permeability and oxygen diffusivity, which depend on liquid saturation, are evaluated. To construct the PNM, focused ion beam scanning electron microscopy (FIB-SEM) is used to derive the pore size distribution (PSD). The model is ex-situ validated through air permeability and oxygen diffusivity tests with controlled saturation of non-volatile wetting liquid that is stable in the hydrophobic MPL. Oxygen diffusivity of the MPL is obtained by identifying the diffusion resistances of the concentration boundary layers and GDL substrate in the tests. The model predicts the effects of liquid water saturation in the MPL on the air and liquid water permeations, and the oxygen diffusion, and thus can be used to design optimal PSDs for practical cells. • A 3D porous structure of a microporous layer (MPL) is reconstructed with FIB-SEM. • Pore size distribution is thereby yielded for a pore network model (PNM). • The PNM is experimentally validated under dry and wet conditions of ex-situ tests. • The wet conditions for the hydrophobic MPL are given by non-volatile wetting liquid. • The PNM simulates the mass transport in the MPL with the saturation of liquid water. [ABSTRACT FROM AUTHOR]

Gas purging has been a common strategy to improve the possibility of cold start of a proton exchange membrane fuel cell (PEMFC) vehicles. In the present study, water removal in a PEMFC during gas purging after its shutdown is numerically investigated with a transient two-fluid model. The effect of purging gas flow rate on the liquid water and membrane water is explored. The results indicate that the water removal process is not sequential in the sub-regions of PEMFC and the purging gas flow rate affects the overall drying process. [ABSTRACT FROM AUTHOR]

Earthen constructions can experience severe damage under coupled action of moistening and freezing-thawing (F-T). This paper proposes an experimental study on the F-T behavior of earthen samples at different liquid saturation degrees (S L 0). Normal moisture conditions during winter were investigated using two values of relative humidity (RH): RH = 75% and RH = 95% which led to saturation levels lower than 35% for the tested earth. Pathological conditions of earthen walls were studied with samples at high liquid saturation (S L 0 = 75%, S L 0 = 85% and S L 0 = 95%). Axial strain and temperature of samples were measured during 12 F-T cycles conducted between +10 ∘C and − 20 ∘C. An experimental device was designed to maintain axial stress of 0 or 100 kPa on samples during the freezing-thawing test to reproduce loading conditions at the top and the basement of an earthen wall. The measurement of strains and temperature shows that earthen samples did not exhibit any freezing-thawing effect under the investigated normal moisture conditions. The results indicated that F-T deformations depend mainly on the initial liquid saturation. A microstructural approach can identify necessary moisture conditions for freezing-thawing in porous materials. Shrinkage during freezing and expansion during thawing were observed in the range of saturation investigated in this study. This mechanism was attributed to the drying shrinkage of clay particles due to the cryo-suction process during ice formation. The axial loading significantly increased residual strains developed at the first F-T cycles. Two creep tests at +20 ∘C and − 20 ∘C concluded that axial loading and freezing-thawing induced a combined effect on deformations in unsaturated earthen materials. • Freezing strains and temperature of unsaturated earthen materials were measured • Freeze-thaw and mechanical loading induced additional permanent strain with cycles • The microstructural analysis highlighted the role of initial liquid saturation • Freezing-shrinkage was attributed to cryo-suction process between unfrozen and frozen pores [ABSTRACT FROM AUTHOR]

We focus on the effect of cathode catalyst layer physical structure on the cell performance of proton exchange membrane fuel cell (PEMFC). At low polarization, high inlet humidification predicts better cell performance because of the more active surface in the CL. As polarization is extended near the mass transfer limited regime, high humidification only renders a flooded electrode and inferior cell performance. Catalyst layer with better capillary water transport parameters performs better than that with inferior water repulsion capability. Permeation in the gas diffusion layer (GDL) is important for efficient oxygen diffusion in mass transfer influenced regime. On the other hand, the permeability in catalyst layer only has secondary effect. The distribution of material properties in the CL is studied for the MEA fabrication strategy. The CL is divided into three sub-layers with changing material properties. With water effect considered, better performance is obtained for higher porosity near the GDL, higher electrolyte fraction in the agglomerate near the membrane. The effect of agglomerate particle size differs in the ohmic and mass transfer controlled regimes. Larger agglomerate size near GDL is preferred in the ohmic limited regime, while smaller size near GDL performs better if operated at mass transfer controlled regime. [ABSTRACT FROM AUTHOR]

A three-dimensional two-phase lattice Boltzmann model (LBM) is implemented and validated for qualitative study of the fundamental phenomena of liquid water transport in the porous layer of a proton exchange membrane fuel cell (PEMFC). In the present study, the three-dimensional microstructures of a porous layer are numerically reconstructed by a random generation method. The LBM simulations focus on the effects of the porous layer porosity and boundary liquid saturation on liquid water transport in porous materials. Numerical results confirm that liquid water transport is strongly affected by the microstructures in a porous layer, and the transport process prefers the large pores as its main pathway. The preferential transport phenomenon is more profound with a decreased porous layer porosity and/or boundary liquid saturation. In the transport process, the breakup of a liquid water stream can occur under certain conditions, leading to the formation of liquid droplets inside the porous layer. This phenomenon is related to the connecting bridge or neck resistance dictated by the surface tension, and happens more frequently with a smaller porous layer porosity. Results indicate that an optimized design of porous layer porosity and the combination of various pore sizes may improve both the liquid water removal and gaseous reactant transport in the porous layer of a PEMFC. [ABSTRACT FROM AUTHOR]

[Display omitted] • A multiphase, multidimensional proton exchange membrane water electrolyzer mode is established. • The simulation results are validated using the in-house experimental data, and the high fidelity of the developed model is confirmed. • High current density leads to low liquid saturation in the cathode diffusion layer. • High temperature increases and decreases liquid saturation in the cathode and anode, respectively. • High porosity and low contact angle promote the capillary transport of water in porous media. The water in a proton exchange membrane water electrolyzer (PEMWE) used for hydrogen production undergoes a multiphase flow, which makes it difficult to analyze the mass transfer phenomenon and predict the performance of the PEMWE. Therefore, the multiphase flow in the PEMWE requires a comprehensive study. This work theoretically investigates multiphase mass transfer along with the electrochemical reaction in a PEMWE using a three-dimensional computational modeling approach. First, the model was validated based on an in-house experiment, and a good agreement was achieved. Subsequently, the spatial distribution of species and the electrochemical process were investigated, and the liquid water saturation level was monitored. It was observed that the high current density results in low liquid saturation in the cathode. Oxygen is accumulated under the land owing to its longer diffusion path. Higher temperatures increase and decrease liquid saturation in the cathode and anode, respectively, because they weaken the capillary transport of water inside the porous layer. The concentration overpotential is significantly lower than the activation overpotential. Furthermore, it was found that the high porosity and low contact angle of the porous material aid the capillary transport of water inside the diffusion layer. [ABSTRACT FROM AUTHOR]

The dynamics of liquid drainage in inclined packed beds was studied experimentally using electrical capacitance tomography. The evolution of textural flow regimes and liquid saturation profiles were monitored as a function of bed tilt angle and bed height. Film and droplet textural regimes were discriminated during bed drainage tests. They consisted of a rapid step discharging, virtually at constant flow rate, ca. 80% of the poral dynamic liquid followed by as lower step of partially-saturated pores discharging the remaining 20%. The drainage time was markedly reduced upon tilting the column resulting ultimately in virtually bed-length independent drainage times. Bed inclination reduced the droplet paths to the vessel wall, stimulating migration and coalescence of liquid droplets towards the lower mostarea of the column cross-section. This ensured sufficient hydraulic pressure nearby the high-porosity wall area to maintain enhanced liquid outflows. As a prospective process intensification artifice, inclining packed beds way exhibit superior advantage in stimulating drainage of tall vessels especially if emergency circumstances arise. [ABSTRACT FROM AUTHOR]

A systematic method for the steady-state modeling of mass transport phenomena and electrochemical behavior focusing on the cathode side of a direct methanol fuel cell (DMFC) with an open-cathode design is presented and demonstrated using an experimental system. The architecture features an internal water-recovery technology that promotes back-convection of water from the cathode to the anode through a polymer electrolyte permeable membrane that includes laser-drilled holes. Eight adjustable modeling parameters are estimated from experimental data from a prototype system, and a model validation study shows that predictions of water venting rates and of cell voltage have errors better than 9 % when extrapolating to conditions not used for parameter estimation. Illustrations of the use of the model include an analysis of performance at various flow rates of feed air, operating temperatures and currents, and a prediction of the effect of alternative material designs for selected cell layers. The approach serves as a paradigm for the development of general models for open-cathode active-feed DMFC systems. [ABSTRACT FROM AUTHOR]

The axial evolution of gas–liquid distribution patterns in co-current downward gas–liquid two-phase flow through solid foam packings of different pore density expressed by pores per inch was experimentally studied. The experimental results are based on time-averaged capacitance measurements of the liquid phase with embedded wire-mesh sensors, positioned at different axial heights of the solid foam packing. The measurements revealed the spatial distribution of the liquid phase saturation, which was applied to quantify the degree of liquid maldistribution. Both the spray nozzle and the multipoint distributor provide rather uniform initial liquid distributions across the foam packings with low maldistribution factors at superficial liquid velocities above 0.009 m/s. However, the uniform initial irrigation deteriorates along the foam packing length, in particular for foams with low pore density. The gas flow rate does not significantly influence the liquid distribution. Furthermore, the foam’s ability to radially spread the liquid phase in the cross-section downstream from a single drip point distributor was studied and found to be low, independent from the pore density. [ABSTRACT FROM AUTHOR]

Highlights: [•] Experimental monitoring of carbon dioxide diffusion and sequestration. [•] Carbon dioxide breakthrough time across nickel mine residue is estimated. [•] Pores liquid saturation was a major factor influencing CO2 diffusion. [•] Partially saturated pores combined fast CO2 dissolution and sequestration. [ABSTRACT FROM AUTHOR]

A three-dimensional two-phase lattice Boltzmann model (LBM) is implemented and validated for qualitative study of the fundamental phenomena of liquid water transport in the porous layer of a proton exchange membrane fuel cell (PEMFC). In the present study, the three-dimensional microstructures of a porous layer are numerically reconstructed by a random generation method. The LBM simulations focus on the effects of the porous layer porosity and boundary liquid saturation on liquid water transport in porous materials. Numerical results confirm that liquid water transport is strongly affected by the microstructures in a porous layer, and the transport process prefers the large pores as its main pathway. The preferential transport phenomenon is more profound with a decreased porous layer porosity and/or boundary liquid saturation. In the transport process, the breakup of a liquid water stream can occur under certain conditions, leading to the formation of liquid droplets inside the porous layer. This phenomenon is related to the connecting bridge or neck resistance dictated by the surface tension, and happens more frequently with a smaller porous layer porosity. Results indicate that an optimized design of porous layer porosity and the combination of various pore sizes may improve both the liquid water removal and gaseous reactant transport in the porous layer of a PEMFC.

Abstract: The effects of inclination on the hydrodynamic behavior of a packed bed operating under gas–liquid cocurrent upflow were experimentally investigated in terms of liquid saturation, bed overall pressure drop and gas–liquid segregation. The non-invasive electrical capacitance tomography (ECT) imaging technique was applied to scrutinize local and axial phase distribution pattern and cross-sectionally averaged liquid saturation. The results indicate that bed inclination creates short circuits for the gas phase along the upper wall where it can flow in a segregated manner. Inception of transition from bubble to segregated flow regime was identified through monitoring a defined uniformity factor for ECT images. Phase segregation developed along the bed with minimum impact in the region close to the entrance. The removed bubbles were replaced by liquid phase resulting in higher liquid saturation values as complete segregation state was approached. The effect of operating conditions on axial profile of liquid saturation was examined. [Copyright &y& Elsevier]

Abstract: The flow dynamics of a unidirectional, spontaneous capillary flow of a wetting liquid into porous medium is investigated numerically and experimentally. With respect to whether the sessile liquid is present or absent at the porous medium surface, the flow within porous medium is referred to as primary and secondary flow, respectively. The primary flow ends once the sessile liquid is imbibed by porous medium, while the secondary flow ceases once the equilibrium liquid distribution in porous medium is achieved. Hence, the wetted volumes of porous medium at the end of the primary and secondary flow can differ significantly. Both primary and secondary flow are solved numerically using a dynamic capillary network model with a micro-force balance, which utilizes the general definition of liquid potential at the liquid free interface. The instantaneous liquid flow quantities, including saturation, flow rate, pressure and capillary pressure at the network pores, are calculated numerically and later averaged to obtain the macroscopic flow characteristics. Three distinct pairs porous medium/liquid: (i) and (ii) flow of two different liquids into the same porous medium, and (iii) flow of one of the previously used liquids into a new porous medium are studied experimentally, and the spatial and time saturation profiles are measured. The capillary network structure is adjusted such that the corresponding numerical and experimental saturation profiles agree. From the numerical solution, the multiphase flow parameters: capillary pressure and relative permeability dependences on the liquid saturation are deduced and referred to as intrinsic properties of the secondary flow. Throughout the secondary flow duration time, each pair of porous medium/liquid shows unique dependences of both capillary pressure and relative permeability as functions of the saturation. Finally, the changes in capillary pressure and relative permeability are compared for distinct pairs of porous medium/liquid. [Copyright &y& Elsevier]

Abstract: A two-phase, half cell, non-isothermal model of a proton exchange membrane fuel cell has been developed. The model geometry includes a gas diffusion layer, a micro-porous layer and a catalyst layer along with the interfaces for channel, land and membrane. The effect of channel and rib width on transport phenomena has been examined. The model was run with saturated gas feed at different operating current densities and cell temperatures. The results show that increasing the channel to rib width ratio does not have any effect on the total amount of liquid saturation, however, its distribution is significantly affected under the channel and rib region within the porous layers. The degree of supersaturation and undersaturation extends, but, the supersaturation region shrinks and the undersaturation region extends with increase in channel to rib width ratio. It is concluded that the transport mechanism within the cathode is a highly coupled phenomena which interlinks local distribution of temperature, liquid saturation and the relative humidity. [Copyright &y& Elsevier]

Abstract: In polymer electrolyte fuel cells (PEFCs), condensation of water within the pore network of the gas diffusion layers (GDL) can influence the gas transport properties and thus reduce the electrochemical conversion rates. The use of X-ray tomographic microscopy (XTM), which allows for a resolution in the order of one micrometer is investigated for studying ex situ the local saturation in GDL''s. The strength of XTM is the high spatial resolution with simultaneous contrast for water and carbon, allowing for non-destructive 3D-imaging of the solid and the contained water. The application of this method for imaging the ex situ water intrusion into the porous network of GDLs is explored using absorption and phase contrast methods. It is shown that the inhomogeneous filling behavior of GDL materials can indeed be visualized with sufficient resolution. For Toray paper TGP-H-060 the local saturation was measured as function of the water pressure. The results, evaluated in 1D, 2D and 3D show a liquid water retention effect at the denser layers near the surface. A comparison with established capillary pressure functions is presented. Altogether, the results show the potential of the XTM-method as a tool for studying the liquid water behavior in PEFC on a microscopic scale. [Copyright &y& Elsevier]

Abstract: Filtration and catalytic reaction studies of gas/(kaolin-kerosene) suspension flows through monolith reactors were conducted in isoflow-rate and in flow modulation modes in cocurrent downflow. Monoliths with small (1-mm) and large (4-mm) square channel openings were investigated. The effect of deposition of fines on the catalytic conversion in monolith reactors was assessed using the catalytic hydrogenation of α-methylstyrene as a model reaction on 1%Pd–alumina washcoated monolith blocks. The catalytic tests revealed that conversion was a decreasing function of monolith bed specific deposit. To reduce the impact of deposition on catalytic conversion, three feed flow rate modulation strategies (ON–OFF liquid-, ON–OFF gas- and gas/liquid alternating modes) for on-line self-cleaning of deposits were tested. The dynamics of liquid pulsations generated via these cyclic operation strategies was monitored using electrical capacitance tomography imaging. The corresponding liquid saturation, pressure drop and pulsation intensity were compared for clean fines-free two-phase flows. Under isoflow filtration, monoliths with large opening channels were found to favor detachment of kaolin deposit layers and to promote sieving filtration. Instabilities induced by the three flow modulation strategies favored detachment and removal of the deposits only for the large-opening channel monoliths. Conversely, detachment followed by re-deposition downstream in the bed in small-channel opening monoliths aggravated the deposition and pressure buildup in flow modulation. Hence, small-opening channels can be recommended for isoflow filtration of suspensions whereas large-diameter channels are preferable for cyclic operations. [Copyright &y& Elsevier]

Abstract: A two-dimensional, steady state, non-isothermal, multi-fluid, two-phase flow model is presented for a semi-passive direct methanol fuel cell at which air flows through a channel at the cathode side while liquid methanol solution is fed passively at the anode. The evaporation/condensation rates are formulated in a manner to capture the non-equilibrium effects between the liquid and gas phases. The model also accounts for the methanol and water crossover through the membrane. The behavior of a semi-passive system, including the temperature, oxygen and water vapor mass fractions, and the liquid saturation at different air flow rates is well described. Capturing the thermal effect in a semi-passive system is vital, since transport variables are highly dependent on temperature. While a higher flow rate of air at the cathode channel decreases the average cell temperature, it enhances the liquid removal from the porous media at the cathode side. The results also show that a higher flow rate results in a more uniform distribution of transport variables along the channel. For a cell that does not suffer from the flooding effect, a lower air flow rate through the channel results in a higher performance of the cell due to the temperature increase. [Copyright &y& Elsevier]

Abstract: The flow pattern in trickle bed reactors is of an inhomogeneous nature due to a random packing structure. Thus, the local values for liquid phase ratio and velocity are spread over the whole cross-section and a global determination of these values ignores the fact that local phenomena dominate mass and heat transfer and eventually the reactor efficiency. Only if both parameters can be locally quantified, can a realistic picture of the flow fields be drawn. A new wire-mesh sensor (WMS) setup based on the measurement of electrical permittivity of fluids was used, which can be applied to the packings of porous catalyst particles. A calibration method is proposed to get access to the liquid saturation. Axial velocity distributions are measured via a method based on the spatial tracer pulse time-of-flight between sensing points of two WMSs installed at a distance of few millimeters. The effects of gas and liquid flow rates on local liquid velocities were analyzed. The proposed technique was validated against liquid collector data, which showed very good agreement. For this purpose, distributions of the volumetric flow rate were calculated by applying the continuity equation to the saturation and velocity data obtained from WMS. [Copyright &y& Elsevier]

Abstract: The filtration of gas/(kaolin-kerosene) suspension flows in monolith reactors was monitored using electrical capacitance tomography. Two flow modulation modes (fast and semi-fast) were compared to constant-throughput feeds of the suspension in a quest for strategies for the reduction of fines accumulation. The effects of initial distribution of liquid suspension and superficial velocities, and base-pulse time splitting of flow modulation were found to affect both the structure of deposition and the amount of deposited fines. One major finding concerned dampening of liquid hydrodynamic waves generated in flow modulation which could severely limit implementation of self-cleaning fast-modulation strategies of monolith reactors subject to filtration of suspensions. [Copyright &y& Elsevier]

Abstract: In determining the liquid water distribution in the anode (or the cathode) diffusion medium of a liquid-feed direct methanol fuel cell (DMFC) with a conventional two-phase mass transport model, a current-independent liquid saturation boundary condition at the interface between the anode flow channel and diffusion layer (DL) (or at the interface between the cathode flow channel and cathode DL) needs to be assumed. The numerical results resulting from such a boundary condition cannot realistically reveal the liquid distribution in the porous region, as the liquid saturation at the interface between the flow channel and DL varies with current density. In this work, we propose a simple theoretical approach that is combined with the in situ measured water-crossover flux in the DMFC to determine the liquid saturation in the anode catalyst layer (CL) and in the cathode CL. The determined liquid saturation in the anode CL (or in the cathode CL) can then be used as a known boundary condition to determine the water distribution in the anode DL (or in the cathode DL) with a two-phase mass transport model. The numerical results show that the water distribution becomes much more realistic than those predicted with the assumed boundary condition at the interface between the flow channel and DL. [Copyright &y& Elsevier]

• Developed a two-phase thermo-fluidic transport model for boiling in thin porous evaporators. • Heat transfer is modeled by conduction and volumetric pore-scale evaporation. • Fluid flow is modeled using capillary pressure and relative permeability relationships. • The model is calibrated to experimental data on dryout heat flux and thermal resistance. • Model predicts dryout limit with a mean absolute percentage error of 29%. Two-phase passive heat transport devices such as vapor chambers, loop heat pipes, and capillary pumped loops utilize porous evaporators for phase change and to drive fluid transport. Nucleate boiling can occur within such capillary-fed porous evaporators, especially under high-heat-flux operation, as has been visually observed in various experimental studies in the literature. However, prior modeling efforts have typically only considered single-phase flow of liquid through a completely saturated porous medium for characterizing the dryout limit and thermal performance. The present work offers a new semi-empirical model for prediction of thermal resistance and dryout during boiling in capillary-fed evaporators. Thermal conduction across the solid and volumetric evaporation within the pores are solved to obtain the temperature distribution in the porous structure. Capillary-driven lateral liquid flow from the outer periphery of the evaporator to its center, with vapor flow across the thickness, is considered to obtain the local liquid and vapor pressures. The capillary pressure and the relative permeabilities (fraction of single-phase permeabilities) for two-phase flow in the porous medium are modeled as a function of the local liquid saturation. The heat flux at which the liquid saturation at the center of the evaporator becomes zero is defined as the dryout limit of the evaporator. Experiments are conducted on sintered copper particle evaporators of different particle sizes and heater areas to collect data for model calibration. To demonstrate the wider applicability of the model for other types of porous evaporators, the model is further calibrated against a variety of dryout limit and thermal resistance data collected from the literature. The model is shown to predict the experimentally observed trends in the dryout limit with mean particle/pore size, heater size, and evaporator thicknesses. [ABSTRACT FROM AUTHOR]

In order to know the mechanism of groundwater transport and the variation of ion concentrations in the near-field of the high-level radioactive waste repository, the whole process was simulated by EOS3 module of TOUGHREACT. Generally, the pH and cation concentrations vary obviously in the near-field saturated zone due to interaction between groundwater and bentonite. Moreover, the simulated results showed that calcite precipitation could not cause obvious variations in the porosity of media in the near-filed if the chemical components and their concentrations of groundwater and bentonite pore water are similar to those used in this study. [ABSTRACT FROM AUTHOR]

Abstract: Trickle bed reactor performance and safety may suffer from radial and axial liquid maldistribution and thus from non-uniform utilization of the catalyst packing. Therefore, experimental analysis and fluid dynamic simulation of liquid–gas flow in trickle bed reactors is an important topic in chemical engineering. In the present study for the first time a truly high-resolution gamma ray tomography technique was applied to the quantitative analysis of the liquid flow texture in a laboratory cold flow trickle bed reactor of 90mm diameter. The objective of this study was to present the comparative analysis of the liquid flow dynamics for two different initial liquid distributions and two different types of reactor configurations. Thus, the hydrodynamic behavior of a glass bead packing was compared to a porous Al2O3 catalyst particle packing using inlet flow from a commercial spray nozzle (uniform initial liquid distribution) and inlet flow from a central point source (strongly non-uniform initial liquid distribution), respectively. The column was operated in downflow mode at a gas flow rate of 180Lh−1 and at liquid flow rates of 15 and 25Lh−1. [Copyright &y& Elsevier]

Fully coupled simulations of two-phase transport in Polymer Electrolyte Fuel Cells (PEFCs) and heat transfer in coolant channels are performed in order to investigate the effects of cooling channel configuration on the distributions of temperature and water within PEFCs. When a practical coolant flow rate is applied to large-scale cells for automotive applications, a significant coolant temperature rise is expected from the coolant inlet to the outlet, particularly under high current density operations, creating a significant cell temperature gradient along the flow direction as well. Consequently, a two-phase water profile resulting from evaporation-condensation processes inside PEFCs is also strongly influenced by the cell temperature gradient from the hot coolant inlet toward the cold coolant outlet regions, demonstrating that both temperature and liquid saturation strongly depend on the thermal gradient along the coolant flow path. [ABSTRACT FROM AUTHOR]

Abstract: Flow distribution characteristics in a 0.05m diameter monolith reactor, operated co-current downward in the Taylor flow regime, was investigated and quantified using gamma ray computed tomography (CT). The results indicate that the flow distribution is a strong function of liquid distributor design, and gas and liquid superficial velocities. Additionally, within the range of gas and liquid superficial velocities investigated, a window of operating condition (gas and liquid velocities) was identified which resulted in a close to uniform phase distribution. [Copyright &y& Elsevier]