Gas-water Interface Research Articles

Observation of In Situ Growth and Decomposition of Carbon Dioxide Hydrate at Gas–Water Interfaces Using Magnetic Resonance Imaging

The kinetics of gas hydrate formation and decomposition at the gas–water interface are of crucial importance to the application of the hydrate-based technique and the gas recovery process from marine sediments. Herein, the behaviors of in situ CO2 hydrate growth and dissociation at the interface of liquid CO2 and water were observed using magnetic resonance imaging. The results indicated that the growth of the hydrate film was primarily controlled by the mass transfer of CO2 into the water phase. Notably, the stepwise depressurization beyond the equilibrium pressure was found to facilitate the thickening of hydrate films as a result of the enhanced evolution of dissolved gas out of water for hydrate formation. The addition of surfactants (sodium dodecyl sulfate) could contribute to a shorter induction time and a thinner hydrate film. The dissolution of the gas molecule and bubbles in the water phase were suggested as the crucial factor impacting the thickening of CO2 hydrate films.

Characteristics of N2O Release and Influencing Factors in Grass-type and Algae-type Zones of Taihu Lake During Summer

Spatial heterogeneity of N2O generation and emissions in multi-ecotype lakes limited the accurate estimation of the N2O fluxes in lakes, but few studies on the characteristics of N2O generation and emissions have been conducted. In this study, N2O flux at the water-gas interface, dissolved N2O concentration in the water column, and N2O flux at the sediment-water interface in typical grass-type and algal-type zones of Taihu Lake were analyzed during summer, and indoor micro-environment experiments were conducted to illustrate the main factors affecting the generation and emissions of N2O. The results showed that the N2O fluxes at the water-gas interface, dissolved N2O concentration, and N2O fluxes at the sediment-water interface of the emergent macrophyte type area was higher than the algae-type area and submerged macrophyte area during the summer., with N2O fluxes at the water-gas interface of (115.807±7.583), (79.768±1.842), and (3.685±0.295) μmol ·(m2 ·h)-1, respectively. The dissolved N2O concentration in the water column were (0.051±0), (0.029±0.001), and (0.018±0) μmol ·L-1, respectively; and the N2O fluxes at the sediment-water interface were (178.275±3.666), (160.685±0.642), and (75.665±1.016) μmol ·(m2 ·h)-1, respectively. The spatial difference could be attributed to dominant plants and the concentration of inorganic nitrogen in the water column. The results of micro-environment experiments showed that nitrate and organic carbon sources could significantly increase the N2O production potential of sediments, the high concentration of NH4+-N in the water column might inhibit the N2O production in sediments, and the production rates of N2O in the sediment increased remarkably when the incubation temperature increased, suggesting that the generation and emissions of N2O were mainly restricted by nitrate, organic carbon, and temperature in summer.

In situ Raman spectroscopy study of synthetic gas hydrate formed by cold seep flow in the South China Sea

Properties of gas hydrate are commonly investigated in laboratory by simulators or studied using the hydrate samples recovered from permafrost and marine sediments. Here synthetic gas hydrate (SGH) is quickly formed for the first time by fluids and bubbles erupted from the active cold seeps at the Formosa Ridge in the South China Sea (SCS). SGH samples are in situ detected by a Raman insertion probe for gas hydrate (RiP-Gh) which is deployed by the remotely operated vehicle (ROV) Faxian. Both spatial and temporal in situ Raman spectra of the SGH samples are acquired in order to determine the structure and the evolution of the synthetic gas hydrate. Authigenic carbonate debris or other debris serving as nucleation particles, which is found in the in situ Raman spectra of SGH samples for the first time, may be one factor that can promote the formation of synthetic gas hydrate. The gas-water interface demonstrated by the observation results, also contributes to the quick formation of SGH samples, which agrees well with the previous study. In situ Raman spectra of three SGH samples which have been placed on the seafloor for 0 h, 4 h and 21 h respectively are acquired. Laboratory Raman spectra of one sample which has been surprisingly found and recovered after 9840 h (410 days) since formed on the seafloor are also acquired. The Raman spectra of the four SGH samples indicate that the methane large-to-small cage occupancy ratios of the hydrates vary from 1.01 to 1.39, and the ratios of methane large-to-gas of the hydrates increase from 0.53 to 1.55. Overall, our work suggests a new explanation for the quick formation of gas hydrate compared to that in laboratory simulation work, and reveals the evolution of synthetic gas hydrate after quick formation, which provides a new insight for the study of natural gas hydrate.

CFD analysis of steam jet injection in a hybrid SIT

In the safety water injection system of an advanced nuclear power plant, APR+, steam is injected onto water inside a tank called a hybrid SIT. Depending on the condensation rate of steam and the rising time of pressure inside the tank, the performance of the system is determined. To understand the phenomena, an experiment with visualization and finely installed instrumentation was conducted. In parallel, simulation of the steam jet development and subsequent condensation was performed using a CFD code, STAR-CCM+. Each phenomenon was first validated by comparing with existing experiments; then, simulation on our hybrid SIT experiment was conducted. In the simulation, three domains of gas (steam and air), water, and wall were considered with fixed gas-water interface for convergence. As a result, pressure and temperature trends were compared between experiments and simulation. Pressure was over-predicted in simulation because of reduced water circulation under the fixed interface. The additional momentum source in the water interface enhanced the direct contact condensation and retarded the pressure rise, narrowing the gap between the experiment and the calculation.

Effect of Electric Field on Gas Hydrate Nucleation Kinetics: Evidence for the Enhanced Kinetics of Hydrate Nucleation by Negatively Charged Clay Surfaces.

Natural gas hydrates are found widely in oceanic clay-rich sediments, where clay-water interactions have a profound effect on the formation behavior of gas hydrates. However, it remains unclear why and how natural gas hydrates are formed in clay-rich sediments in spite of factors that limit gas hydrate formation, such as small pore size and high salinity. Herein, we show that polarized water molecules on clay surfaces clearly promote gas hydrate nucleation kinetics. When water molecules were polarized with an electric field of 104 V/m, gas hydrate nucleation occurred significantly faster with an induction time reduced by 5.8 times. Further, the presence of strongly polarized water layers at the water-gas interface hindered gas uptake and thus hydrate formation, when the electric field was applied prior to gas dissolution. Our findings expand our understanding of the formation habits of naturally occurring gas hydrates in clay-rich sedimentary deposits and provide insights into gas production from natural hydrate deposits.

Surfactant Interactions and Organization at the Gas-Water Interface (CTAB with Added Salt).

We have studied adsorbed layers of cetyltrimethylammonium bromide (CTAB) at air-water interfaces in the presence of added electrolyte. Fast bubble compression/expansion measurements were used to obtain the surface equation of state, i.e., the surface tension vs CTAB surface concentration dependence. We show that while a simple model where the surfactant molecules are assumed to be noninteracting is insufficient to describe the measured response of the surfactant layer, a modified Frumkin equation where the local interactions between the molecular components depend on their surface concentration captures the response. The variation of the effective interactions in the surfactant layer in the model shows that the interactions in the surfactant layer change from effectively repulsive to attractive with increasing surface concentration. Molecular dynamics simulations are performed to probe the origins of the change in the interactions. The simulations indicate that already at low surface concentrations the surfactants aggregate as highly dynamic rafts with surfactant orientation parallel to the interface. Increasing the concentration leads to a change in the assembly morphology at the interface: the surfactant layer thickens and the surfactants sample a range of tilted orientations with respect to the interfacial plane. The change from transient raftlike assemblies to dynamical aggregates at the interface involves a clear increase in the degree of counterion binding: we speculate that the flip of the effective interaction parameter in the model used to interpret the experimental results could result from this. The work here presents basic steps toward a proper understanding of the molecular organization and interactions of surfactants at an air-water interface. This is crucially important in understanding macroscopic properties of surfactant-stabilized systems such as foams, emulsions, and colloidal dispersions.

Effect of Mixing Gas for Machining Surface Property of Micro-bubble Entrained EDM

Electrical discharge machining (EDM) is the process of removing a material by means of electrical discharge inside an insulating working fluid. The material is melted by the discharge and is blown off by the vaporizing explosion of the working fluid. Expansion and contraction of the insulated fluid bubbles affect the process of removing a material. The stronger the vaporizing explosion, the larger is the quantity of material removed and the worse is the surface roughness, and vice versa. The power of the vaporizing explosion is determined by the input energy and quantity of material removed is determined by the power of the vaporizing explosion. Recently, there have been some studies on micro bubbles that are 50μm or less in diameter. Micro-bubbles possess special characteristics, such as an increased interior gas pressure and increased ion concentration around the gas–water interface. We considered changes in the characteristics of EDM by using a fluid mixed with micro bubbles. The vaporizing explosion was considered to be weakened by using fluid containing micro-bubbles, and the surface roughness seemed to improve Therefore, we performed EDM using pure water and micro-bubble mixed fluid, and researched the characteristics of EDM. Initially, We were expecting a change in the machining removal rate. Indeed, in oil dielectric fluid, there was no change, and there was little difference with the machining rate achieved using pure water dielectric fluid. However, a clear difference was observed in terms of the skewness (Rsk) of surface roughness. In this paper, this difference is described as a change in surface roughness in micro-bubble mixed EDM.

Global significance of the carbon cycle in the karst dynamic system: evidence from geological and ecological processes

ABSTRACT On the basis of proposing the existence of a karst carbon cycle and carbon sink at a watershed scale, this paper provides four pieces of evidence for the integration of geology and ecology during the carbon cycle processes in the karst dynamic system, and estimated the karst carbon sink effect using the methods of comparative monitoring of paired watersheds and the carbon stable isotope tracer technique. The results of the soil carbon cycle in Maocun, Guilin, showed that the soil carbon cycle in the karst area, the weathering and dissolution of carbonate rocks under the soil, resulted in a lower soil respiration of 25% in the karst area than in a non-karst area (sandstone and shale), and the carbon isotope results indicated that 13.46% of the heavy carbon of the limestone is involved in the soil carbon cycle. The comparative monitoring results in paired watersheds, suggesting that the HCO3- concentration in a karst spring is 10 times that of a rivulet in a non-karst area, while the concentration of inorganic carbon flux is 23.8 times. With both chemical stoichiometry and carbon stable isotopes, the proportion of carbon in karst springs derived from carbonate rocks was found to be 58.52% and 37.65% respectively. The comparison on carbon exchange and isotopes at the water-gas interface between the granite and carbonate rock basins in the Li River showed that the CO2 emission of the karst water is 10.92 times that of the allogenic water from the non-karst area, while the carbon isotope of HCO3- in karst water is lighter by 8.62‰. However, this does not mean that the karst water body has a larger carbon source effect. On the contrary, it means the karst water body has a greater karst carbon sink effect. When the karst subterranean stream in Zhaidi, Guilin, is exposed at the surface, carbon-rich karst water stimulated the growth of aquatic plants. The values of carbon stable isotopes in the same species of submerged plants gradually becomes heavier and heavier, and the 512 m flow process has a maximum range of 15.46‰. The calculation results showed that 12.52% of inorganic carbon is converted into organic carbon. According to the data that has been published, the global karst carbon sink flux was estimated to be 0.53-0.58 PgC/a, equivalent to 31.18%-34.41% of the global forest carbon sink flux. In the meanwhile, the karst carbon sink flux in China was calculated to be 0.051 PgC/a, accounting for 68% of its forest carbon sink flux.

Experimental study on submerged supersonic gaseous jet induced tail cavity

Multiphase jets and cavitation problems are inevitable for high-speed underwater vehicles propelled by jet engines. Unlike being injected into stagnant water, the gaseous jet behind a underwater vehicle is usually conjugated with a tail cavity. The pulsation and collapse of such cavities can seriously affect the vehicle performance. In this study, the shape character, forming mechanism and control conditions for the supersonic gaseous jet induced tail cavity at the wake of a revolution body are experimentally investigated in a water tunnel. The induced cavity is ventilated only by a convergent-divergent nozzle with a designed Mach number of 2.45. The form of the cavity is recorded through two high-speed cameras both horizontally and vertically under different Froude numbers and ventilation rates. The time averaged form is thus obtained through digital image processing to eliminate the transient characteristics of the cavity. The experiment is conducted with the Froude number ranging from 3.2 to 16.2, and the ventilation rate 0 to 0.5. Due to the high density and velocity ratio between water and gas, the structure of such flow is usually very complicated. Many novel phenomena of the jet-cavity interaction are observed. With increasing stagnation pressure of the central jet, the induced cavities evolves form foamy, intact, partially break, to pulsating foamy closure type. The foamy and intact tail cavities share the same profile and characteristics that of a supercavity. And the pulsating foamy closure type was never observed before in a traditional supercavitating flow. The outline of the pulsating foamy cavity is the same as the foamy cavity's, indicating that they have the similar forming mechanism. A comparison with the jet-cavity interaction model is made and the following conclusions are obtained:the real ventilation rate, which corresponds to the re-entrant jet gas blocked by the cavity boundary, is the key factor in controlling the cavity form. When the gaseous jet is completely blocked by the water-gas interface, an intact or foamy cavity will be formed. A partially break cavity appears only when some fraction of the jet is blocked and this is when some of the strongest interactions between jet and cavity occurs. When little gas was blocked by the interface, a pulsating foamy cavity forms. With the structure of gaseous jet considered, the transition of the induced cavity closure between different types is in favour of the prediction from Paryshev's model of cavity closure to a central jet. The variation of the cavity form, thus the interaction strength between jet and cavity, coincides with the real ventilation rate estimated through the theoretical model.

A three-phase solid-liquid-gas slug flow mechanistic model coupling hydrate dispersion formation with heat and mass transfer

Pipe blockage due to gas hydrate formation is a main concern in the oil and gas industry due to the revenue losses caused by either production impairments or interruptions, and to the high costs associated to the elimination of such blockages. Assuming a hydrate formation rate in the gas-water interface based on the system subcooling, the present work models the transition from two-phase liquid-gas to three-phase solid-liquid-gas flows when hydrates form. The multiphase flow is assumed to be within the slug flow pattern region, as this is the prevailing flow regime in offshore production scenarios. The model couples mass, momentum and energy balances for the slug flow unit cell. The hydrate phase is assumed as homogeneously dispersed in the water. The gas and water consumption rates due to hydrate formation are modeled as source terms in the mass balance equations. The exothermic characteristic of the hydrate formation is taken into account in the energy conservation equation. The model provides analytic expressions for temperature and pressure distributions along the pipeline. However, the unit cell geometry is solved by numerical integration and the model closure is achieved only when empirical correlations for the slug frequency, the unit cell translational velocity and the slug aeration are used. The results from the numerical simulations are presented for the same input parameters for cases with and without hydrate formation. The discussion focuses on the influence of hydrate formation in the slug flow hydrodynamics and heat transfer. The main mechanisms affecting the mixture temperature and pressure distributions, the mixture heat transfer coefficient, the superficial velocities of the phases, the liquid loading, the slug flow frequency and the unit cell geometry are presented.

Measurements of water molecule density by tunable diode laser absorption spectroscopy in dielectric barrier discharges with gas–water interface

We measured water molecule (H2O) density by tunable diode-laser absorption spectroscopy (TDLAS) for applications in dielectric barrier discharges (DBDs) with a gas–water interface. First, the effects of water temperature and presence of gas flow were tested using a Petri dish filled with water and a gas injection nozzle. Second, the TDLAS system was applied to the measurements of H2O density in two types of DBDs; one was a normal (non-inverted) type with a dielectric-covered electrode above a water-filled counter electrode and the other was an inverted type with a water-suspending mesh electrode above a dielectric-covered counter electrode. The H2O density in the normal DBD was close to the density estimated from the saturated vapor pressure, whereas the density in the inverted DBD was about half of that in the former type. The difference is attributed to the upward gas flow in the latter type, that pushes the water molecules up towards the gas–water interface.

Ostwald ripening changes the pore habit and spatial variability of clathrate hydrate

Clathrate hydrate is a solid crystalline compound that consists of water cages with trapped gas molecules. Methane hydrate in permafrost and marine sediments is estimated to contain 2 × 103 to 4 × 106 Gt of carbon, which makes it both a potential energy source and a liability to global warming. The transport and mechanical properties of hydrate bearing sediments (HBS), and therefore hydrate dissociation through depressurization, thermal stimulation, or CO2 injection, depend on hydrate pore habit and spatial distribution in sediments. However, the evolution of hydrate pore habit and spatial distribution in HBS are not well understood yet. Here we show experimental evidence of Ostwald ripening of gas hydrate crystals in pores and porous media. We find that (1) hydrate growth rate (under nearly isothermal conditions) versus the degree of overpressurization fits an Arrhenius-type equation, and (2) Ostwald ripening gradually changes hydrate pore habit from grain-attaching to pore-filling and increases hydrate saturation heterogeneity at both pore and core scales. The latter ultimately creates significant spatial variations of permeability and sediment strength. Our results provide insights into kinetics and pore-scale physics that determine the geophysical signature from HBS at various scales. Based on our findings of hydrate occurrence in both gas and gas-water interface, we argue that hydrate growth in natural HBS is not limited to the water liquid phase. Our observation of hydrate pore habit alteration by Ostwald ripening explains patchy saturation and heterogeneous distribution of hydrate in natural HBS, which gives physical insight to improve geophysical hydrate models in porous media.

Characteristics and Influencing Factors of CH4 Emissions from the Drawdown Area of the Three Gorges Reservoir

Five levels (180 m, 175 m, 165 m, 155 m, and 140 m) in a typical drawdown area in Wangjiagou in the Three Gorges Reservoir were selected to study CH4 emissions from subtropical reservoirs. The experimental period lasted two years from September 2010 to August 2012. The methods of static opaque chambers during the drainage period and floating chambers during flooding period were used in this study. The elevations of 175 m, 165 m, and 155 m were all located in the drawdown area, whereas the 180 m elevation was located in the land and never flooded. The 140 m elevation was permanently flooded and used as a control area. The results showed that the CH4 fluxes showed no significant trends at 175 m and 165 m in the first year of the experiment, while the fluxes showed a single peak pattern with the climax in the summer at 155 m and 140 m. At 175 m, the CH4 emissions showed a single peak pattern with the climax during its flooding period, and then showed not regular CH4 emission sources or sinks alternately in the second year, whereas the CH4 fluxes at 165 m, 155 m, and 140 m presented a single-peak shape with winter climax. During the entire observation period, the CH4 emission fluxes at 180 m were stable and showed no obvious peaks. In addition, CH4 fluxes were higher during the flooding period than in the drainage period at 175 m, 165 m, and 155 m.The order of the annual CH4 cumulative emissions at the five elevations was 140 m (99.58 kg·hm-2) > 155 m (82.98 kg·hm-2) > 165 m (65.38 kg·hm-2) > 180 m (6.32 kg·hm-2) > 175 m (4.27kg·hm-2), suggesting that the soil was more conducive to CH4 production when the flooding period was longer. Correlation analysis indicated that there were no significant correlations between CH4 fluxes and the soil carbon component and pH on land and during the drainage period but CH4 fluxes increased with the increase in soil water content. There was a significant linear negative correlation between CH4 emissions from the gas-water interface at 140 m and in water. The soil moisture content was one of the key factors affecting the CH4 fluxes during the drainage period, while during flooding period, the CH4 fluxes were regulated by flooding depth.

Formation, physicochemical and interfacial study of carbamate surfactants

Carbon dioxide is commonly used as pH regulator in switchable surfactant systems and in the formation of alkyl ammonium-alkyl carbamate ion-pair. Its use to form a meta-stable anionic surfactant has been less explored and can impart a cleavable character to the amphiphile.The reaction between CO2 and an alkylamine, N,N-di(propylamino)dodecylamine (Y12-amine), under alkaline pH conditions, produced a stable anionic carbamate-based surfactant (Y12-carbamate). By heating and exposure to N2, anionic Y12-carbamate could slowly be reverted into Y12-amine. The surface activity of Y12-amine and Y12-carbamate was investigated by surface tension measurements. To study the behavior of Y12-amine at the gas-water interface during CO2 exposure, we used the pendant drop technique with a sealed chamber where the gas composition could be controlled.The Y12-carbamate had a higher CMC than Y12-amine at pH 12, and was also less surface active. The ion pair Y12-ammonium – Y12-carbamate, obtained at neutral pH, exhibited the lowest CMC and the highest surface activity. The interfacial formation of anionic Y12-carbamate induced an increase in surface tension. When CO2 was exchanged to N2, the migration from the bulk to the interface of Y12-amine induced a decrease in surface tension. The rate was dependent on the concentration of Y12-amine.

Numerical study of two-dimensional cylindrical underwater explosion by a modified method of characteristics

A modified method of characteristics (mMOC) is introduced for solution of 2D axisymmetric supersonic flow, as well as its application to simulation of a cylindrical underwater explosion, which is a typical non-homentropic flow problem. The modifications include a family of compatibility equations which contains entropy term in streamline direction, an improved backward scheme with second-order accuracy, a modified forward scheme that uses adaptive technique, which in total make this mMOC be appropriate for a branch fluid dynamics with isentropic or non-isentropic flows. The numerical results show good agreement between the mMOC and experiments. It is established that this mMOC yields shock location, shock peak pressure, and gas-water interface location with an acceptable accuracy. Compared with an explicit dynamics program AUTODYN, which needs to select an appropriate artificial viscosity to handle shock discontinuity properly, the mMOC typically requires far less computer time with the same accuracy in the calculation of the underwater shock wave. Besides, the formation of a secondary shock is well revealed in the pressure nephogram, which suggests the mMOC is able to capture small variation in flow field. In addition, this mMOC can clearly display the domain of dependence in characteristic net, which makes it possible to quantitatively describe the connection between near field shock and detonation products expansion.

Numerical simulation of migration characteristics of the two-phase interface in water–gas displacement

The process of water–gas displacement in water–drive gas reservoirs has been always an important research topic. However, knowledge about the flow patterns of water–gas flow under different conditions is insufficient at present. To investigate the variation of the geometry and position of the water–gas interface as well as the water flooding efficiency with time during water–gas displacement, the distribution law of water and gas at the microcosmic scale is simulated and analysed based on the geometric model of a single pore channel abstracted from the parallel tube bundle model and continuous tube bundle model of a porous medium reservoir, with the help of laminar two-phase flow and the level set module of COMSOL Multiphysics. The simulation results show the following: (1) Under the combined effects of the boundary layer effect, interfacial tension and pore diameter changes in water–gas displacement, the shape of the water–gas interface changes from a tongue-shape to a U-shape, during which a series of interface shapes, including a piston shape, finger shape, W-shape and Ω-shape, are observed. In the pore channel, the area of the water phase below the pore channel axial is larger than that above the pore channel axial. (2) The pore-throat ratio has a considerable impact on the shape and location of the water–gas interface as well as the displacement efficiency. When the water–gas displacement is from a small pore-channel to a large one, the displacement efficiency increases as the pore-throat ratio decreases. On the contrary, if the water–gas displacement is from a large pore channel to a small one, the displacement efficiency increases as the pore-throat ratio increases. Numerical simulation can be used to study the shape and location of the water–gas interface as well as the displacement efficiency in pore channels with different scales of diameters.

Simulation of Underwater Explosions Initiated by High-Pressure Gas Bubbles of Various Initial Shapes

UNDerwater EXplosions (UNDEXs) are widely used in many areas of applied engineering including oil production and warship protection. However, the three-dimensional computations of UNDEXs, especially for explosives with complex initial shapes are still lacking, which is mainly due to the difficulty in capturing the multi-medium interface with high pressure ratio. In this study, we conducted a series of three-dimensional numerical simulations of UNDEXs with different initial shapes of a high-pressure gas bubble surrounded with water, to investigate the dynamics of the explosion caused by the shape change of the gas bubble. The movement of the interface was traced with the level-set method, and the conditions at the gas–water interface were treated using the Real Ghost Fluid Method (RGFM). As a result, the temporal evolution of the pressure field during the explosion and the pressure exerted at the boundaries of the computational domain in each simulation scenario were obtained. It was found that an initial shock wave is generated by the explosion and transmitted in the water, leading to an increase of the pressure and density. Meanwhile, inside the gas bubble, a rarefaction wave is formed, causing a pressure drop of the explosive gas. The results also show that if the initial shape of the bubble filled with the explosive gas is simple (e.g., spherical, cylindrical, cuboidal), the peak pressure of the shock wave is dominated by the cross-sectional area of the initial bubble along each direction. In addition, the duration of the high pressure phase of the shock wave is dictated by the thickness of the bubble. Moreover, the simulation of a bubble with an initially bullet-like shape revealed that this specific shape enables a concentration of the energy in a well-defined direction. The peak of the pressure generated by the gas bubble of this more complex shape is approximately twice than that of the other scenarios. However, the high pressure was found to drop more rapidly than that of the other cases, which might be attributed to the comparably small thickness of the initial bubble.

Experimental and pore-level numerical investigation of water evaporation in gas diffusion layers of polymer electrolyte fuel cells

The liquid water generated by the electrochemical reaction in a polymer electrolyte fuel cell (PEFC) can greatly affect the cell performance and/or efficiency. Phase change of the water due to evaporation is therefore an important process in understanding the water and flow behavior in the porous gas diffusion layers (GDLs) of PEFCs. In this work a 3D lattice Boltzmann (LB) methodology is adopted to model the flow and mass diffusion in a binary air/water mixture. A modeling framework is designed so as to focus on the understanding of the phase change processes of stationary water inside a GDL with a thickness less than 0.25mm. Consistent models are employed to simulate evaporation at pore scale and on the irregular water-gas interface. The numerically obtained evaporation fluxes are compared against experimental data for the exact same 3D porous GDL sample. The pore-scale simulations are first shown to be in good agreement with the measurements for global evaporation rates in GDLs. Numerical simulations are then performed inside and outside the GDL to investigate the physics of evaporation, including the interaction between the diffusion at pore scale and the external convection in the channel of the PEFC. The role of evaporation-induced flows is finally studied by comparing the computed magnitudes of the vertical velocities obtained with and without inclusion of evaporation.

Dynamic Modeling, Analysis, and Testing of a Variable Buoyancy System for Unmanned Multidomain Vehicles

This paper presents the system design and dynamic model of an active variable buoyancy system (VBS) actuator with applications to unmanned multidomain vehicles. Unmanned multidomain vehicles have a unique concept of operations that require nontraditional VBS designs. We present a VBS actuator design that focuses on vehicle design objectives of high endurance, stealth, and loitering while underwater. The design consists of an elastic bladder housed within a rigid ballast tank, hydraulic pump, and proportionally controlled vent valve. Ambient surrounding water is the system working fluid and the elastic bladder serves to separate the gas–water interface, eliminating the risk of the compressed gas escaping when venting the water during extreme pitch maneuvers. A nonlinear analytic model of the VBS is derived and used to examine the parameter design space and the effects on water flow rate, actuation force, and energy efficiency. The VBS actuator design is shown to require a smaller, denser energy storage device than a comparable buoyancy system that uses consumable compressed air. A vehicle model is studied that features forward and aft VBS actuators, which enables vehicle pitch control by shifting the center of gravity along the vehicle's longitudinal axis. The coupling between the VBS actuator dynamics and vehicle dynamics is presented and discussed. A proof-of-concept demonstration is presented and compared to the analytical system model.

Changes in mineral reactivity driven by pore fluid mobility in partially wetted porous media

Microfluidics experiments were used to examine mineral dissolution-precipitation reactions under evaporative conditions and identify pore-scale processes that control reaction rate. The entrainment of reacting mineral particles by a mobile water-gas interface driven by evaporation dramatically altered the relative abundance of reactive mineral surface area to fluid reservoir volume. This ratio, which directly influences reaction rate and reaction progress, was observed to vary by nearly two orders of magnitude as evaporation progressed in the experiments. Its dynamic evolution may have a correspondingly large impact on mineral-fluid reaction in Earth's shallow subsurface. We predict that the spatial and temporal variability of pore-scale reaction rates will be significant during evaporation, imbibition, or drainage in the vadose zone, with implications for chemical weathering, soil quality, and carbon cycling. Variable reaction rates during particle mobility are likely to be of increased significance as global rainfall patterns and soil moisture contents evolve in response to climate change.