Pore Gas Research Articles

Ratcheting assessment of the catalyst layer in polymer electrolyte membrane fuel cells considering thermal-mechanical-humidity cycling

The properties of polymer electrolyte membrane fuel cells (PEMFCs) are highly affected by the catalyst layer (CL). However, the cyclic swelling and shrinking of the ionomer caused by the change of temperature and relative humidity (RH) combined with external random mechanical loads may lead to electrochemical activity degradation through cyclic plasticity accumulation. In this study, a theoretical model to assess the ratcheting limit of the CL for PEFMCs subjected to thermal-mechanical-humidity cycling is developed based on linear matching method (LMM). The influences of embedded depth of Pt/C particle, cyclic temperature or RH modes, as well as the random reaction force are considered by the proposed method. The ratcheting limits of CL are achieved under various thermal-mechanical-humidity cycling and verified by step-by-step simulation. Moreover, a simplified prediction model is established to evaluate the ratcheting limits under the combined temperature and RH cycling. It is of interest that the ratcheting limit under the pure normal force is almost 3 time of that under the pure tangential force under the same thermal-humidity cycling. Furthermore, the accumulated plastic strain occurs near the junction area causing the debonding of Pt/C particle from the ionomer under thermal-mechanical-humidity cycling, which agrees well with the microscopic observation. Based on ratcheting assessment results obtained by the proposed method, the reliability and durability of PEMFCs can be improved by increasing the affinity between Pt/C and ionomer, decreasing the size of gas pore and inhibiting the tangential force in the CL.

The impacts of scaled capillary pressure combined with coupled flow and geomechanics on gas hydrate deposits

In this study, we numerically analyze the effect of capillary pressure on gas hydrate deposits through coupled flow and geomechanics simulation, with a focus on the scaled capillary pressure. The scaled effect is predicated on sediment pore-size variations resulting from hydrate dissociation or formation, leading to non-monotonic capillary pressure curves influenced by two primary factors: alterations in pore space and gas saturation. Specifically, hydrate dissociation may increase pore space, thereby reducing capillary pressure. Conversely, enhanced gas saturation owing to dissociation can elevate capillary pressure. We employ a scaled capillary pressure model, accounting for porosity fluctuations caused by hydrate formation or dissociation. Additionally, equivalent pore pressure is utilized to ensure the numerical stability and accuracy in scenarios of strong capillarity. The numerical experiments incorporate two distinct methodologies for hydrate dissociation: heat injection and depressurization. In the heat injection scenario, sensitivity analyses are conducted using a range of model parameters, exhibiting characteristic non-monotonic capillary pressure behaviors attributable to the aforementioned competing factors. Regarding the depressurization approach, the UBGH2-6 site in the Ulleung Basin, East Sea, South Korea, is selected as a real-world field case. Over a 30-day gas production simulation, we observe notable enhancements in hydrate dissociation, signifying improved productivity, and distinctive geomechanical responses, under the influence of the scaled model. This investigation demonstrates that the scaled capillary pressure model, upon the hydrate or ice (i.e., solid) phase change, with coupled flow and geomechanics is crucial for accurate modeling of gas hydrate deposits.

The dynamics of Rn-222 cyclic flow within the shallow geological subsurface media as a daily temporal variated source for exhalation into the air

Extensive research on the dynamics of radon gas (Rn-222) originating from the radioactive decay of radium (Ra-226) in geological subsurface media, sheds light on its periodic release into the atmosphere. Radon is a product of the uranium-238 decay chain found within rock and soil grains. While only a fraction of the generated radon escapes (emanates) into porous spaces due to nuclear recoil, it serves as the source for subsurface gas flows and for cyclic exhalation into the soil-atmosphere interface.Ongoing study of radon movement in shallow and deep subsoil, and its emergence at the surface, reveals complete semi-diurnal, diurnal, and seasonal gas flow cycles in the subsoil. Complementary emissions occur nocturnally as radon is released into the atmosphere. Moreover, two natural driving forces govern complex semi-diurnal and diurnal flows below and above the surface. Subsurface gas movement in porous media exhibits nonlinear behavior influenced by surface temperature gradients, resulting in downward flow to depths of up to 100 m. This flow exhibits daily periodicity with depth-dependent time delays, correlating with the diurnal surface temperature cycle. Additionally, pore gas transport into and out of open boreholes responds linearly to semi-diurnal barometric pressure changes, known as barometric pumping.Beyond subsurface phenomena, Europe and Australia increasingly employ nocturnal radon measurements to study atmospheric stability and air quality, assuming that variations in local stationary near-surface radon concentrations reflect atmospheric mixing processes. Recognizing mechanisms governing radon's temporal changes within geological subsurface media highlights the need for continuous underground radon monitoring to validate variations in daily radon exhalation to the surface.On the other hand, monitoring radon at considerable depths minimizes climatic contributions and enhances the ability to discern non-periodic pre-seismic radon signals, independent of atmospheric compulsion. This research offers potential insights into seismic precursors and the complex interplay between subsurface geodynamics and atmospheric conditions.

On vibration response of heavy hammer self-falling pile driver in uneven unsaturated soil – Experimental and analytical study

The goal of this study is to determine the scope of the impact of pile driving construction vibration on surrounding residential houses and buildings during engineering construction. Firstly, introducing the gradient factor of unsaturated soil, proposed a model for non-uniform variation of pores with depth. Secondly, Consider the coupling between the model and soil skeleton density, relative fluid density, relative gas density, pore fluid, pore gas, and shear modulus. Finally, coupling the pile with the driving depth and propose a pile driving vibration model for non-uniform unsaturated soil. Proposed a new method for solving coupled equations using Hankel transform. Moreover, the analytical solution of this study was validated through on-site experiments. Results show that, deeper the pile is inserted into the soil, the greater the peak vertical velocity at different measuring points. The vibration of the pile is strongest at the last 1 m into the soil. When close to the vibration source, the amplitude values in the three directions are: maximum in vertical direction, followed by horizontal and radial direction, and minimum in horizontal and tangential direction. When pile driving construction is carried out outside the range of vibration influence, the seismic damage generated is equivalent to the seismic intensity below V (excluding V). The rate amplitude calculated by the model established in this study is closer to the measured value. Therefore, using the model established in this work can more accurately evaluate the impact of pile driving vibration during the construction process of highways and high-speed railways.

Experimental Study of the Generation of Pore Gas Pressure in Pyroclastic Density Currents Resulting From Eruptive Fountain Collapse

AbstractPyroclastic density currents formed through collapse of eruptive fountains commonly have runout distances of the order of tens of kilometers. A possible cause of this high flow mobility is elevated interstitial pore gas pressure, which may have various origins. We investigated experimentally the generation of pore pressure at the impact zone of an eruptive fountain, where concentrated pyroclastic density currents emerge from compaction of a free falling gas‐particle mixture. We simulated pyroclastic fountain collapse by releasing glass beads of mean sizes of 29–269 µm from a hopper at height of 3.27 m above a 5 m‐long horizontal channel, and we measured pore air pressure in the impact zone. During free fall, the granular mixtures accelerated and expanded to reach particle concentrations of 1.6–4.4 vol.% before they impacted the base of the channel. Upon impact, the particles accumulated to form concentrated granular flows with particle concentrations of 45–48 vol.% and pore air pressures that indicated almost full weight support for particle sizes ≤76 µm. Both the amount of pore pressure in the impact zone and the flow runout distance increased as we decreased the particle size and hence the hydraulic permeability of the concentrated granular mixtures. Our results suggest that pore gas pressure in concentrated pyroclastic density currents can be generated at the impact zone of collapsing fountains and that small particle size conferring low permeability and long pore pressure diffusion timescale is one of the main causes of long flow runout distances.

Single-track investigation of additively manufactured mold steel with larger layer thickness processing: Track morphology, melt pool characteristics and defects

Large layer thickness is essential to increase building rate, but excessive layer thickness easily lead to defects which negatively affect component quality. The evolution of track morphology, melt pool characteristics and defects of additively manufactured mold steel by using various larger layer thickness (60 and 90 μm) while also varying the laser energy density was investigated systematically. Results indicated that the morphologies of single tracks shifted sequentially from discontinuousness to continuous irregularity and continuous regularity with the increasing line energy density at 60 μm layer thickness. The width and depth of melt pool increased substantially with the line energy density increased from 0.4 to 1.3 J/mm, while the height of melt pool was nearly constant. Correspondently, transition of the melt pool based on the depth-to-width ratio from balling to conduction and keyhole modes was observed. When the layer thickness increased from 60 to 90 μm, the average build percentage of the single tracks decreased by 43 % with increasing line energy density from 0.26 to 0.51 J/mm, and more spatters around the surface of tracks were found at 0.8 J/mm. Interestingly, the average width of the melt pool decreased by 37 μm, while the height of melt pool increased by 45 μm. More small spherical gas pores (5–15 μm) occurred in bottom of keyhole due to trapped vapor at high energy density. The decrease in melt width with increased layer thickness is most likely due to the weakened Marangoni convection. Finally, formation mechanism of lack-of-fusion defects and gas pores with larger layer thickness processing were analyzed based on melt pool characteristics, such as spheroidization, keyhole and spatters.

Pore structure and gas adsorption characteristics in stress-loaded shale on molecular simulation

Since stress from overlying strata is one of the primary factors influencing pore structure and gas content in shale reservoirs, understanding its role is crucial for developing shale gas fields. In this study, Molecular Mechanics and Molecular Dynamics methods were employed to investigate the intrinsic relationship between pore structure, energy, and adsorption performance of the organic matter matrix under different stress conditions. The results show that the pore structure of the organic matrix can be categorized into three stages with the variation of external stress, with 0.2 and 0.8 GPa as the boundaries. The effect of external stress significantly impacts the methane adsorption capacity in the organic matrix. Changes in stress not only result in a reduction of adsorption space but also an increase in adsorption sites. These two factors have opposite effects on CH4 adsorption in the model. That is, the negative impact of the reduction in adsorption space outweighs the positive impact of the increase in adsorption sites, resulting in a decrease in the adsorption capacity. At 0.8 and 1 GPa, the maximum CH4 adsorption capacity was reduced by 70 % and 58 %, respectively. This study provides an important reference for understanding the pore structure of the organic matrix and the adsorption characteristics of CH4 in shale gas reservoirs.

Thermal Conductivity of Frozen and Unfrozen Gas-Saturated Soils

Arctic permafrost often contains gas-saturated horizons. The gas component in freezing and frozen soils can exist under different pressures, which are expected to affect their properties and behavior. The effect of pore gas pressure on the thermal conductivity of frozen and unfrozen silt loam saturated with methane or carbon dioxide at pressures below the hydrate formation conditions is observed in the current study. The variable gas pressure and temperature conditions are simulated in a specially designed pressure cell, which allows thermal conductivity measuring in pressurized samples at positive and negative temperatures. The experiments using natural samples collected near the gas emission crater (Yamal Peninsula) show that thermal conductivity is sensitive to pore gas pressure even at high moisture contents. The thermal conductivity of methane-saturated soil becomes 4% and 6% higher in frozen and unfrozen samples, respectively, as the gas pressure increases from 0.1 MPa to 2 MPa. In the case of CO2 saturation, the respective thermal conductivity increase in frozen and unfrozen samples reaches 25% and 15% upon pressure change from 0.1 to 0.9 MPa. The results stimulate further special studies of the effects of gas type and pressure on the thermal properties of closed gas-saturated taliks, of which the pore pressure is increasing during freezing up.

Competitive sorption of CO2/CH4 and CO2 capture on modified silica surfaces: A molecular simulation

The effect of functionalization of modified silica (FMS) surfaces on the adsorption behaviors for CH4, CO2 and the mixture have been studied by combining a grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods. The isotherms, gas distribution, sorption sites, isosteric heat, interaction energy and adsorption selectivity for CO2/CH4 mixture are computed and discussed. And the effects of pressure and temperature on adsorption behavior are also studied. The adsorption capacity (C) of surface with various functional groups for CH4 is as follows: C(SiO2-COOH) > C(SiO2-SH) > C(SiO2-OH) > C(SiO2-CH3) > C(SiO2-NH2) > C(SiO2-H) and for CO2 is as follows: C(SiO2-COOH) > C(SiO2-SH) > C(SiO2-NH2) > C(SiO2-OH) > C(SiO2-CH3) > C(SiO2-H). Increasing temperature is not advantageous for gas adsorption on all FMS surfaces. The CO2/CH4 selectivity (S) for FMS surfaces decrease in the following order of S(SiO2-COOH) > S(SiO2-NH2) > S(SiO2-OH) > S(SiO2-SH) > S(SiO2-CH3) > S(SiO2-H). As the pressure increase, the CO2/CH4 selectivity decrease early and then flatten out to a constant value. The isosteric heat of adsorption (Qst) for gas adsorbed on each FMS surface are also calculated, the value of isosteric heat for CO2 is much higher than that for CH4 due to the stronger interaction between CO2 molecules and functional groups of FMS surface. The result of the smallest self-diffusion coefficients of gas in SiO2-COOH slit pores is consistent with the adsorption capacity in the slit pore with corresponding functional group. The electronegative atoms in the functional group act as basic adsorption sites for gas adsorbate molecules, which generates a larger electrostatic contribution on the selectivity of CO2 over CH4.

Effect of oxygen shielding on the tensile and fatigue performance of 300M repaired through laser-directed energy deposition

AbstractLaser-directed energy deposition (L-DED) is a key enabling technology for the repair of high-value aerospace components, as damaged regions can be removed and replaced with additively deposited material. While L-DED repair improves strength and fatigue performance compared to conventional subtractive techniques, mechanical performance can be limited by process-related defects. To assess the role of oxygen on defect formation, local and chamber-based shielding methods were applied in the repair of 300M high strength steel. Oxidation between layers for locally shielded specimens is confirmed to cause large gas pores which have deleterious effects on fatigue life. Such pores are eliminated for chamber shielded specimens, resulting in an increased ductility of ∼15%, compared to ∼11% with chamber shielding. Despite this, unmelted powder defects are not affected by oxygen content and are found in both chamber- and locally shielded samples, which still have negative consequences for fatigue.

Shear strength degradation of gas hydrate-bearing sediment due to partial hydrate dissociation

Extraction of methane hydrate from subseafloor reservoir may potentially trigger seabed slides and induce subsidence. To address the problems, it is crucial to properly characterize the phase equilibrium condition of pore hydrate and the shear strength of the soil. As one of the key constitutive components, the phase equilibrium condition enforces a constraint over pore gas pressure, temperature and unhydrated water content. Such a constraint, however, has been traditionally ignored in analyzing the mechanical behavior of hydrate-bearing soil. In this paper, a series of stepwise hydrate dissociation tests was performed, and the phase equilibrium condition of pore hydrate was determined, providing an effective way to evaluate the unhydrated water content during hydrate dissociation. Meanwhile, a series of direct shear tests was also conducted to explore the shear strength characteristics of the soil. It is shown that the shear strength of the hydrate-bearing soil can be significantly influenced by pore gas pressure, unhydrated water content, hydrate saturation and several other factors. In particular, the measured shear strength depends upon the initial water content of the sample, pointing to a potential problem that the shear strength could be wrongly determined if not properly interpreted. A shear strength criterion, which enforces the equilibrium condition of pore hydrate, is developed for hydrate-bearing soil, establishing a link between the equilibrium condition and the shear strength. The proposed equation describes well the shear strength characteristics of hydrate-bearing soils, remarkably unifying the effects of pore pressure, temperature, water content and hydrate saturation.

Highly-efficient separation of SF6/N2 and NF3/N2 with record selectivity on one-step synthesized carbon nanosheet

The rich micropore development in porous adsorbents has been widely recognized as an indispensable prerequisite for effectively separating SF6/N2 and NF3/N2, while the fast adsorption kinetics is also essential to improve the overall adsorption-separation efficiency. In this work, the carbon nanosheet adsorbents with easily accessible micropores were synthesized via one-step self-activation method using potassium citrate as a precursor, and were systematically evaluated for the adsorptive separation of SF6/N2 and NF3/N2 mixtures. By tuning the carbonization temperatures in the range of 600–850 °C, the optimal PC-750 with dense and abundant micropores at 0.5–0.8 nm is obtained. Resultantly, PC-750 exhibits a high SF6 uptake of 4.09 mmol·g−1 and record SF6/N2 selectivity of 436.4 among the reported carbon adsorbents at 298 K and 100 kPa. Significantly, the NF3/N2 selectivity reaches up to 61.4 for PC-750 under the same conditions, surpassing all the reported adsorbents. Additionally, the correlation between volume of different pores sizes and gas uptake well explains the adsorption performance of SF6 and NF3. Furthermore, the adsorption kinetics and dynamic breakthrough experiments verify the rapid adsorption rate, excellent separation performance of SF6/N2 and NF3/N2, as well as recyclability of PC-750 adsorbent. These findings are beneficial for the rational design of more highly-efficient carbon adsorbents for separating SF6/N2 and NF3/N2.

Simulation of Porosity Regrowth during Heat Treatment after Hot Isostatic Pressing in Titanium Components

Abstract Additive manufacturing (AM) is driven by design freedom, having fewer process constraints than traditional manufacturing processes. It requires careful process control and qualified parameters to create dense metal parts. However, defects in the form of cavities can be detected in as-built specimens by computed tomography. Post-processing techniques such as hot isostatic pressing (HIP) are applied to eliminate porosity, but regrowth of argon gas pores is observed after additional heat treatment. In this work, a mesoscopic heat treatment simulation of an argon-filled gas pore in titanium components is presented. A combination of HIP and high-temperature heat treatment for β-annealing is simulated. Calculated pore regrowth is qualitatively consistent with experimental observation from the literature. Simulation results support the hypothesis of argon not dissolving in the titanium matrix by assuming a constant amount of argon particles in the pore. Mesoscopic heat treatment simulations may be a part of a simulation-driven optimization of thermal post-processing to improve the quality and performance of AM components.

Using digital cores and nuclear magnetic resonance to study pore-fracture structure and fluid mobility in tight volcanic rock reservoirs

Pore-fracture structure characterization and fluid mobility analysis are key to the effective development of tight reservoirs. However, it is difficult to accurately characterize the pore-fracture structure of tight volcanic rocks using a single method, and the mechanisms of fluid mobility are not fully understood. A combination of X-ray diffraction, casting thin section observations, scanning electron microscopy, high-pressure mercury injection, constant-speed mercury injection, nuclear magnetic resonance (NMR), X-ray computed tomography, and the advanced mathematical algorithms in the AVIZO visualization software was used to analyze the Permian tight volcanic rock in the ZhongGuai area of the Junggar Basin to investigate its overall pore-fracture structure characteristics. On this basis, NMR centrifugation experiments were conducted to monitor the fluid migration dynamics in the tight volcanic rocks, and the mobile fluid migration characteristics were studied based on the NMR T2 spectra. The results show that the porosity and permeability of the samples are 0.2–18.7 % (avg. of 8.3 %) and 0.001–6.680 × 10−3μm2 (avg. of 1.602 × 10−3μm2), respectively, and the various pore types are due to the differences in cementation and compaction. The distribution of the pore throats are mainly contiguous and isolated, and the connected pores are mainly distributed in enriched bands, which is due to the interconnection of gas pores, intergranular pores, and dissolution fractures; while the disconnected pores are mainly isolated, which is related to the development of inter-gravel dissolved pores and matrix dissolution pores. In addition, the contribution of the pore connectivity to seepage is greater than that at the pore scale. The pores in the tight volcanic rock have a wide range of sizes, from 3 nm to 120 μm, and are dominated by Gaussian and bimodal distribution patterns. The micron-scale pore radius in this area is mainly 4.47–31.56 μm. Also, the fractures can be divided into three types according to their occurrence and openings. They are mainly high-angle structural fractures and vertical fractures; the pore-fracture structures have strong heterogeneity, and the fractures play a larger role on the seepage of oil and gas. The connectivity of the pore throats in the tight matrix is poor, and the pore throat structure has a great influence on fluid mobility. Subsequently, the movable fluid saturation increases with increasing permeability, and the fractures and micropores have less flow resistance and are more conducive to water flow than small pores. This study provides new insights for the exploitation of similar tight reservoirs.

Thermal Conductivity Variations in Frozen Hydrate-Bearing Sand upon Heating and Dissociation of Pore Gas Hydrate

High-latitude permafrost, including hydrate-bearing frozen ground, changes its properties in response to natural climate change and to impacts from petroleum production. Of special interest is the behavior of thermal conductivity, one of the key parameters that control the thermal processes in permafrost containing gas hydrate accumulations. Thermal conductivity variations under pressure and temperature changes were studied in the laboratory through physical modeling using sand sampled from gas-bearing permafrost of the Yamal Peninsula (northern West Siberia, Russia). When gas pressure drops to below equilibrium at a constant negative temperature (about −6 °C), the thermal conductivity of the samples first becomes a few percent to 10% lower as a result of cracking and then increases as pore gas hydrate dissociates and converts to water and then to ice. The range of thermal conductivity variations has several controls: pore gas pressure, hydrate saturation, rate of hydrate dissociation, and amount of additionally formed pore ice. In general, hydrate dissociation can cause up to 20% thermal conductivity decrease in frozen hydrate-bearing sand. As the samples are heated to positive temperatures, their thermal conductivity decreases by a magnitude depending on residual contents of pore gas hydrate and ice: the decrease reaches ~30% at 20–40% hydrate saturation. The thermal conductivity decrease in hydrate-free saline frozen sand is proportional to the salinity and can become ~40% lower at a salinity of 0.14%. The behavior of thermal conductivity in frozen hydrate-bearing sediments under a pressure drop below the equilibrium and a temperature increase to above 0 °C is explained in a model of pore space changes based on the experimental results.

Exploring Li-CO2 batteries with electrospun PAN-derived carbon nanofibers and Li1.4Al0.4Ti1.6(PO4)3 solid-state electrolyte

This study introduces a highly stable and conductive sodium super ionic conductor-type solid electrolyte, Li1.4Al0.4Ti1.6(PO4)3 (LATP), to address safety concerns and mesoporous-structured polyacrylonitrile (PAN)-derived carbon nanofibers (CNFs) as a cathode material, which has the advantages of high affinity and adsorption of CO2 gas. LATP, an inorganic solid electrolyte, was synthesized using a solution-based method and its structural properties were assessed using X-ray diffraction and Raman spectroscopy. The ionic conductivity of LATP pellets was approximately 5.87 × 10−4 S cm−1. The gas adsorption properties and pore size distributions of the electrospun PAN-derived CNFs were analyzed using the Brunauer-Emmett-Teller method. A fabricated Li-CO2 battery successfully operated for 50 cycles with a current density of 50 mA g−1 and a cut-off capacity of 500 mAh g−1. Finally, a post-mortem analysis of the Li-CO2 battery was conducted using field-emission scanning electron microscopy and Raman and X-ray photoelectron spectroscopy to examine the electrode degradation mechanism. The analysis indicated that Li2CO3 was produced by the electrochemical reaction in the Li-CO2 batteries, causing electrode degradation. Therefore, improving the sluggish kinetics of Li2CO3 decomposition using a catalyst embedded in PAN-derived CNFs will realize high-performance Li-CO2 batteries.

Influence of temperature on deformation and damage mechanisms of wire + arc additively manufactured 2219 aluminum alloy

This work investigates the deformation and damage mechanisms of wire + arc additive manufacturing (WAAM) 2219 aluminum alloy over a broad temperature range of 77 K to 523 K. It is found that the strength and work hardening rate increase significantly with decreasing the temperature from 523 K to 77 K, and the fracture mode transitions from transgranular fracture mode to a mixture of intergranular and transgranular fracture ones. The large elongations to fracture are very similar for the specimens tested at 77 K and 523 K. This is mainly attributed to higher uniform elongation before necking at 77 K and higher post-necking elongation at 523 K. The cracking of α-Al + θ (Al2Cu) eutectics is the primary mechanism of micro-void nucleation, independent of the test temperature. The nucleated micro-voids, along with larger gas pore defects introduced by WAAM, extend along the tensile direction. In addition, as the temperature increases from 77 K to 523 K, the fracture mechanism transitions from high strain localization to extensive necking, leading to higher dislocation density, more grain fragmentation and micro-void nucleation, and more significant grain elongation and void growth. True stress and strain responses over the studied temperature range (77 ∼ 523 K) and strain rate range (0.0001 ∼ 0.01 s−1) are well predicted using an improved Arrhenius-type constitutive model, by considering the activation energy of deformation as a function of temperature.

Gas entrapment and pore formation in metal droplet-based 3D printing

Gas entrapment behavior during metal droplet deposition tends to form pore defects and discourages interfacial bonding between droplets, resulting in lower densities and poorer performance of printed components. Moreover, the gas entrapment process is influenced by the interaction of gas-liquid two-phase flow and non-isothermal solidification, which makes the interfacial behavior of droplets challenging to identify. Therefore, gaining insight into the gas entrapment behavior and pore formation mechanism at the bottom of molten droplets is essential. This paper established a model of two-dimensional axisymmetric numerical by combining the fluid volume tracking method and heat transfer solidification model. Single metal molten droplet deposition experiments verified the accuracy of the model. The evolution of solid phase fraction, velocity field, pressure field, and contact surface heat flux during the deposition of the molten droplet at low Weber numbers (0.1-10) was investigated comprehensively. The mechanisms of gas entrapment and pore formation were revealed. The effects of molten droplet impact velocity, contact angle, and surface tension on the gas film evolution process and pore morphology were further investigated. The results show that decreasing the droplet impact velocity causes capillary waves to contact the substrate surface and form local solidification, allowing the gas disk to evolve into central and two peripheral pores. Moreover, the pore breaks away from the substrate and remains inside the droplet when decreasing the droplet contact angle to 50°. Furthermore, the rate of gas disk retraction decreases as surface tension reduces, which may accentuate the resistance effect of the contact surface solidification layer on the gas disk retraction, thus changing the final morphology of the pores. These findings elucidate the mechanism of pore formation at the bottom of molten metal droplets and further understand the interfacial behavior of droplets in 3D-printed metal structures.

Isotopic Characterization of Mercury Atmosphere-Foliage and Atmosphere-Soil Exchange in a Swiss Subalpine Coniferous Forest.

To understand the role of vegetation and soil in regulating atmospheric Hg0, exchange fluxes and isotope signatures of Hg were characterized using a dynamic flux bag/chamber at the atmosphere-foliage/soil interfaces at the Davos-Seehornwald forest, Switzerland. The foliage was a net Hg0 sink and took up preferentially the light Hg isotopes, consequently resulting in large shifts (-3.27‰) in δ202Hg values. The soil served mostly as net sources of atmospheric Hg0 with higher Hg0 emission from the moss-covered soils than from bare soils. The negative shift of δ202Hg and Δ199Hg values of the efflux air relative to ambient air and the Δ199Hg/Δ201Hg ratio among ambient air, efflux air, and soil pore gas highlight that Hg0 re-emission was strongly constrained by soil pore gas evasion together with microbial reduction. The isotopic mass balance model indicates 8.4 times higher Hg0 emission caused by pore gas evasion than surface soil photoreduction. Deposition of atmospheric Hg0 to soil was noticeably 3.2 times higher than that to foliage, reflecting the high significance of the soil to influence atmospheric Hg0 isotope signatures. This study improves our understanding of Hg atmosphere-foliage/soil exchange in subalpine coniferous forests, which is indispensable in the model assessment of forest Hg biogeochemical cycling.

Numerical gas flow simulation performed to analyze advective gas flow in a compacted clay material

Burial of nuclear waste in a clay formation can result in the generation of significant amount of hydrogen and other gases due to corrosion of metallic materials under anoxic conditions, radioactive decay of waste, and radiolysis of water. If the rate of gas production exceeds the rate of gas diffusion within the clay porewater, formation and accumulations of a discrete gas phase will occur until the gas pressure becomes large enough to surpass the surrounding material gas-entry pressure; at that critical point occurrence of dilating and advective gas flow is expected [1]. Precompacted bentonite is suggested as a sealing material for the isolation of boreholes, disposal galleries, and deposition holes in a deep geological disposal facility for radioactive waste. Both formation of new porosity and the spread of dilatant pathways have been related to the advective flow of gas in bentonite [2]. Understanding of the processes and mechanisms involved is a key aspect when evaluating the impact of gas flow on repository design and construction of any future facility. A series of gas injection tests on compacted bentonite were carried out at the British Geological Survey within DECOVALEX-2023 (D-23) Task B: MAGIC and LASGIT. The project focuses on the development of new numerical techniques for the quantitative prediction of gas flow in repository systems.
 DECOVALEX-2023 Task B started with Modelling Advection of Gas In Clays (MAGIC). The planned approach of the task includes different stages, with an initial conceptual model development phase. In this work, the D-23 MAGIC has been studied based on the same methodology as previously applied in the work for DECOVALEX-2019 Task A - stage 1A (ENGINEER) using the same material properties and modelling strategy [3, 4]. Based on the actual dimensions of the sample, the 3D finite element model includes the geometry of the sample, the injection rod, the shape of the injection tip, the back pressure and the two filters associated to the location of the sensors. Hexahedral elements have been generated utilizing the CODE_BRIGHT software [5], and the random heterogeneity have been defined considering 3 different materials (M1-M2-M3). Layer-by-layer random permeability distribution was assumed (see Figure 1) applying 2/3–1/6–1/6 weighting for intrinsic permeabilities (ki) equal to 1×10-21, 1×10-20 and 1×10-19 m2, for materials M1, M2 and M3, respectively, as well as incorporating different embedded fracture parameters.
 LASGIT (LArge Scale Gas Injection Tests modelling) is a full-scale demonstration experiment operated by SKB at the Äspö Hard Rock Laboratory Hard Rock Laboratory at a depth of 420 m [6]. The 3D finite element model geometry is generated according to the real dimension of the test. Tests measurements included pressure and rate of gas inflow, gas outflow volume and pore pressure observed at various points of the sample. The gas test 1 of the LASGIT experiment is modeled in this study using a multiphase flow in porous media approach with the aim of facilitating the creation of favorable gas migration routes assuming permeability heterogeneity (Figure 1(b)), the same strategy as the MAGIC random permeability distribution outlined above. In this case, a coupled hydro-gas 3D FEM numerical model has been developed by the UPC/Andra research team to simulate the gas flow tests using the computer software CODE_BRIGHT, including initial permeability heterogeneity throughout the model. The numerical formulation also includes embedded fractures. The proposed model has been able to reproduce satisfactorily the observed behavior of the test measurements.