Effects Of Molecular Diffusion Research Articles

A novel analytical model of solute transport in a layered aquifer system with mixing processes in the reservoirs.

Analytical models of solute transport have been widely used to aid the understanding of the physical and chemical processes undergone by substances introduced in a layered aquifer system. However, in previous studies, the advection component of transport was assumed to be one dimensional, while also ignoring the mixing processes that occur in the inlet and the outlet reservoirs. In this study, new sets of models describing those mixing processes are presented. Beyond that, these models were integrated into already existing models and the result is a novel analytical model of solute transport in aquifer-aquitard systems. The novel analytical solution was derived by the Laplace transform method and the finite-cosine Fourier transform method under the mobile-immobile (MIM) framework. The calculations take into account: the longitudinal and vertical dispersion, the molecular diffusion and the horizonal and vertical advection components of solute transport, as well as first-order chemical reaction, in both the aquifer and the aquitard. A finite-difference solution of the model is tested against experimental data in order to critique its reliability. Results indicate that the numerical and analytical solutions of the new model match well with experimental data. This new model outperforms the previous models in terms of interpreting experimental data. The mixing old and new water in the reservoirs during solute transport in aquifer-aquitard systems is important. Global sensitivity analysis demonstrates that the output concentration of solute in the aquifer-aquitard system is most sensitive to the volume of water in the inlet reservoir. The contribution of the molecular diffusion effect to the total mass flux of the tracer cross the aquifer-aquitard interface is much smaller than the contribution of the dispersive and advective effects.

Experimental and numerical studies of rich gas Huff-n-Puff injection in tight formation

Huff-n-Puff technology is considered one of the most effective methods to increase oil recovery in shale reservoirs. Previous works were mainly focused on Bakken and Eagle Ford shales. This paper is devoted to experimental and numerical studies of rich gas injection in core samples with high organic matter content from a tight oil reservoir in Russia. Eighteen core samples from different well intervals and different sizes were used in a Huff-n-Puff experimental study. Eight of the core samples were investigated at an injection pressure of 15 MPa, while ten core samples were studied at near-miscible pressure conditions of 30 MPa. The experiment results indicate that Huff-n-Puff injection of rich gas could give an oil recovery average of 63.03% and a maximum of 88.40% at 30 MPa injection pressure. A compositional model was created utilizing a commercial simulator and history matched to experimental results. The validated model was used to investigate the effect of molecular gas diffusivity, injection pressure, injection duration, shut-in duration, production duration, and other reservoir properties on the recovery factor. Sensitivity analysis by numerical modeling shows that the molecular gas diffusivity was critical in the history matching of the model. The numerical simulation results imply that the recovery factor could be increased by extending the injection period. In contrast, the duration of a soak period does not significantly influence the recovery factor. Furthermore, the recovery factor could be increased by extension of the production due to a slow decline in the pressure gradient between the matrix and fracture at the later stage of the production. The results of the present study imply that the application of the Huff-n-Puff injection of rich gas could increase oil recovery from tight reservoirs.

Mechanistic simulation of cushion gas and working gas mixing during underground natural gas storage

One of the approaches to sustainable energy supply is underground natural gas storage (UGS). In UGS, the gas in the underground storage is made up of two fractions, namely base gas (cushion gas) and working gas. The base gas is injected into the reservoir to maintain the pressure and stays there until the end of the storage period. Depending on the underground structure of the storage reservoir, base gas constitutes 15 – 75% of total gas stored in the storage reservoir. This implies that a large fraction of the required gas for storage into a depleted hydrocarbon reservoir, which is the most popular form of structures for UGS, is the base gas. Therefore, supplying the base gas incurs high costs for a UGS project. In order to reduce the cost of base gas, alternative less-expensive gasses are practically used as base gas; such as nitrogen and carbon dioxide. The main challenge ahead of developing this replacement is the cushion and working gas mixing and the resultant decrease in the heat value of the produced gas. In this paper, effects of molecular diffusion, operating conditions, reservoir, and rock properties on the replacement of cushion gas in conventional reservoirs were investigated, using mechanistic numerical simulation. Effect of cushion and working gas mixing on the gas quality profile during production cycle were addressed as a first time. Based on the results, we found that, with increasing the vertical permeability and compressibility of the reservoir rock, the cushion and operating gas mixing increases, thereby degrading the heat value of the produced gas. In general, the cushion and operating gas mixing was seen to decrease with porosity but rather increase with temperature. An increase in initial reservoir pressure increases the gas mixing in the early stages of production while decreasing it at later stages of production. In early stage of production from the reservoir, an increase in the stored volume of gas decreases the gas mixing effect. As the time passes and the cushion gas reaches the near-well zone, however, the same change in stored volume tends to increase the gas mixing effect. Comparing carbon dioxide with nitrogen as alternative cushion gas, the carbon dioxide was seen to result in lower gas mixing.

Fractal microstructure effects on effective gas diffusivity of a nanoporous medium based on pore-scale numerical simulations with lattice Boltzmann method

The interaction between gas diffusion and complex microstructures of a nanoporous medium is important to gas diffusion in shale formation, but this interaction is complex and has not been fully understood. Those approaches based on fractal theory can well describe the fractal multiscale microstructure with simple boundaries. The lattice Boltzmann method (LBM) can consider the effects of complex microstructures at one scale. This study proposes a combination approach of fractal theory and LBM to simulate the gas diffusion process in fractal multiscale microstructures of a nanoporous medium. In this study, the gas diffusion in a complex microstructure at base scale is numerically simulated by the LBM and the gas diffusion in finer microstructures is described through a diffusion equation with a local gas diffusion coefficient. First, a microstructure of a nanoporous medium is reconstructed by a random reconstruction algorithm. Then, a local gas diffusion coefficient is proposed based on fractal theory to consider the effects of coupling molecular diffusion and Knudsen diffusion with the fractal structures of a nanoporous medium on the gas diffusion in finer microstructures. This local gas diffusion coefficient is validated by experimental data and introduced into the governing equation of gas diffusion. The LBM simulation is verified with experimental data and two other model results on the effective gas diffusivity of a nanoporous medium. Finally, key parameters for the effective gas diffusivity of a nanoporous medium are identified through sensitivity analysis. It is found that the effective gas diffusivity in a microstructure is higher for a smaller range of Knudsen number. Bigger pore diameter fractal dimension and smaller tortuosity fractal dimension represent lower gas diffusion resistance and have higher effective gas diffusivity. This combination approach provides a powerful tool to estimate the effective gas diffusivity in a complex nanoporous medium.

Enhanced burning rates in hydrogen-enriched turbulent premixed flames by diffusion of molecular and atomic hydrogen

Direct numerical simulations (DNS) of fuel-lean turbulent premixed hydrogen-enriched flames are analyzed to improve understanding of local effects of molecular diffusion of fast-diffusing, low-Lewis-number hydrogen species (H2 and H) on the overall burning rates of reactant mixtures. Although it is often assumed that the importance of molecular diffusion decreases with increasing turbulence intensity, our analysis reveals that, on the contrary, diffusion of molecular and atomic hydrogen can remain the rate-controlling processes even at high Karlovitz numbers (Ka≫1). Three-dimensional DNS with detailed chemical kinetics and species transport of turbulent premixed hydrogen-air and hydrogen/ammonia-air flames in the regimes of thin and distributed reaction zones are performed and analyzed at different reactant temperatures and pressure levels. The DNS data reveal a significant impact of H2 and H diffusion in all flames analyzed. In particular, the magnitude of H2 diffusion, occurring from low progress variable near the flame elements of the turbulent reaction front that exhibit convex (positive) curvature towards the fresh mixture, is greatly enhanced at high pressure through increased spatial gradients, resulting in localized equivalence-ratio enrichment and super-adiabatic conditions that accelerate and strengthen the flame front. Moreover, for higher reactant temperatures, back diffusion of atomic hydrogen, transported from the reaction zone into unburnt regions from negatively curved reaction layers surrounding them, facilitates the occurrence of localized spontaneous ignition events at the trailing edges of reaction fronts. For certain conditions these events, induced by differential diffusion, can become the dominant process controlling the overall rate of reactants consumption.

Machine learning assisted modeling of mixing timescale for LES/PDF of high-Karlovitz turbulent premixed combustion

Accurate modeling of mixing in the transported probability density function (PDF) method remains a great challenge, especially for turbulent premixed combustion under extreme conditions such as high Karlovitz number Ka. Recently, a power-law based mixing timescale model was developed (Zhang et al., Proceedings of the Combustion Institute, 2021, 38(2): 2917–2927) for the large-eddy simulations (LES)/PDF modeling of high-Ka number turbulent premixed flames. It is found in this work that the power-law mixing timescale model is highly sensitive to the model parameters. It is thus critically needed to develop accurate calibration of these model parameters. The empirical specification of the model parameters developed in Zhang et al. is found to be inadequate for accurate modeling of the mixing timescale. Machine learning is introduced as an attractive alternative in this work for the specification of the model parameters. A high-Ka number DNS jet flame is used as the training and validation of the machine learning models. The choices of the input parameters are discussed and compared for the machine learning models. The effect of differential molecular diffusion on mixing is examined by including the effect of the Lewis number in the training of the machine learning models. The performance of different machine learning algorithms is compared for the specification of the mixing model parameters. Overall, excellent performance of the machine learning models is observed for assisting the mixing modeling. The feasibility, interpretability, applicability, generality, and portability of using machine learning are discussed in general to provide a perspective on applying data-driven machine learning for turbulent combustion modeling studies.

Implementation of the Scalar Dissipation Rate in the REDIM Concept and its Validation for the Piloted Non-Premixed Turbulent Jet Flames

In order to address the impact of the concentration gradients on the chemistry – turbulence interaction in turbulent flames, the REDIM reduced chemistry is constructed incorporating the scalar dissipation rate, which is a key quantity describing the turbulent mixing process. This is achieved by providing a variable gradient estimate in the REDIM evolution equation. In such case, the REDIM reduced chemistry is tabulated as a function of the reduced coordinates and the scalar dissipation rate as an additional progress variable. The constructed REDIM is based on a detailed transport model including the differential diffusion, and is validated for a piloted non-premixed turbulent jet flames (Sandia Flame D and E). The results show that the newly generated REDIM can reproduce the thermo-kinetic quantities very well, and the differential molecular diffusion effect can also be well captured.

Effects of molecular diffusivity on shock-wave structures in monatomic gases.

We present a full investigation into shock-wave profile description using hydrodynamics models. We identified constitutive equations that provide better agreement for all parameters involved in testing hydrodynamic equations for the prediction of shock structure in a monatomic gas in the Mach number range 1.0-11.0. The constitutive equations are extracted from a previously derived thermomechanically consistent Burnett regime continuum flow model. The numerical computations of the resulting hydrodynamic equations along with classical ones are performed using a finite difference global solution (FDGS) scheme. Compared to previous studies that focused mainly on the density profile across the shock, here we also include temperature profiles as well as non-negativity of entropy production throughout the shock. The results obtained show an improvement upon those obtained previously in the bivelocity (or volume and mass diffusion) hydrodynamics and are more accurate than in the hydrodynamic models from expansions method solutions to the Boltzmann equation.

A dry deposition scheme for particulate matter coupled with a well-known Lagrangian Stochastic model for pollutant dispersion

A 3D dry deposition scheme for particulate matter (PM) is presented as a Free-Libre and Open-Source Software (FOSS) library, DePaSITIA (RSE SpA). This combines some advanced formulations for the deposition mechanisms of sedimentation, inertial impaction, turbulent impaction and interception. The scheme also considers bouncing effects. The input quantities relate to the canopy elements (Leaf Area Density, leaf equivalent diameter, leaf shape, orientation and roughness parameters), the transporting fluid (local mean velocity, friction velocity) and the particulate matter (PM local mean concentration, median diameter and density). The deposition scheme is coupled with a well-known Lagrangian Stochastic model for pollutant dispersion, the Open-Source code SPRAY-WEB (Universita del Piemonte Orientale et al.). The coupled numerical solution is validated on a laboratory test case representing the dispersion of particulate matter from two line sources within a canopy atmospheric boundary layer. The deposition interfaces are represented by the trees of a scaled spruce forest. Validation refers to the average vertical profile of the deposited mass (not the mean concentration) normalized by the above-canopy mean concentration. Some inter-comparisons are also reported considering uniform Leaf Area Density, the additional effects of molecular diffusion, the height-dependent relative contribution of each deposition mechanism and an alternative deposition scheme. The results of this test case are available as a FOSS tutorial. Considering the Fractional Bias obtained for the deposited mass (FB = 27%), this numerical solution seems suitable to simulating stationary dispersion phenomena within complex canopy boundary layers, assessing the height-dependent dry deposition fluxes of atmospheric PM. The current numerical solution might be improved and applied to elevated obstacles such as electric insulators.

Comprehensive numerical study of molecular diffusion effects and Eddy Dissipation Concept model in MILD combustion

Moderate or Intense Low-oxygen Dilution (MILD) combustion is a clean combustion technology with high thermal efficiency and low levels of emissions. In this paper, by employing Adelaide Jet-in-Hot-Co-flow (AJHC), several approaches are examined to increase the numerical solution accuracy. First, molecular diffusion effects are investigated in MILD combustion. Second, adjusting the Eddy Dissipation Concept (EDC) coefficients is comprehensively discussed, and finally, the reaction fraction coefficient and EDC formulation are investigated. The results show that the effect of enthalpy transport caused by molecular diffusion on the energy equation must be considered in the low oxygen concentration regions. Also, the maximum temperature in the MILD region can be kept constant by adjusting EDC coefficients. Furthermore, it is shown that applying the reaction fraction factor increases the accuracy of the numerical solution in the MILD region.

Detailed Numerical Comparison of Laminar Burning Speed of Stratified Hydrogen–Air and Methane–Air Mixture With Corresponding Homogeneous Mixture Using Open-Source Code

Abstract A detailed numerical study of laminar burning speed for fuel–air mixture is conducted using laminarReactingLMFoam solver which is a modified version of reactingFoam solver based on openfoam code. It accounts for detailed mixture-averaged transport property calculation for reacting flow using low-Mach number governing equations. The effect of various equivalence ratio gradients is studied on stratified hydrogen–air and methane–air mixture with mixture-averaged transport model and unity Lewis number for all species, and corresponding laminar burning speed is compared with homogeneous mixture. For both the fuel–air mixture, rich to lean stratified mixture resulted in a higher laminar burning speed and no significant difference was noticed for lean to rich stratified mixture when compared with homogeneous mixture at same local equivalence ratio. Increased burning speed is explained based on higher burnt gas temperature and molecular diffusion of lighter species from burnt gas referred to “Chemical Effect” in this study. The effect of thermal and molecular diffusion from the burnt gas on laminar burning speed is studied for stratified and homogeneous mixture using mixture-averaged transport model and unity Lewis number for all species. It is shown that the molecular diffusion effect from burnt gas (“Chemical Effect”) is more prominent as compared with the thermal diffusion effect. Extension in lean flammability limit for stratified mixture of both the fuel is shown based on higher heat release rate as compared with homogeneous mixture and extension in flammability limit for stratified mixture is explained based on higher Chemical Effect from burnt gas.

Numerical modeling of molecular diffusion and convection effects during gas injection into naturally fractured oil reservoirs

Gas injection into a naturally fractured oil reservoir keeps the reservoir pressure and increments the initial recovery from the reservoir. The main aim of this work was to develop a numerical model to calculate the mass transfer (molecular diffusion and convection) between a gas injected in the fracture and residual fluid (gas and oil) in a matrix block. The dual continuum model is applied to describe flow behaviour and fluid recovery in porous media. Finally, the model is validated by comparing the outcomes with the results of two experimental works available in the literature. The mathematical model results are in agreement with the laboratory data including recovery of each component, saturation profile, and the pressure gradient between matrix and fracture. Modeling results show that after 25 days of N2 injection, the lighter and heavier components (C1 and C5) are recovered about 51% and 39%, respectively. These amounts for CO2 injection are 49% and 27%. It is found that the convection mechanism has a great effect on preventing the pressure drop of the reservoir during injection operations. In the nitrogen injection, without considering the convection, after 30 days, the matrix pressure reaches 1320 Psi from 1479 Psi but after 30 days, considering the convection, the pressure reaches 1473 Psi from 1479 Psi.

Experimental and modeling study on the effect of molecular diffusion during CO2 injection

In this work, a new approach based on the effect of molecular diffusion on oil recovery during CO2 injection in naturally fractured reservoirs has been proposed to examine the best condition for oil recovery in the matrix block. A homogeneous matrix block from one of Iran’s naturally fractured reservoirs has been investigated. The matrix block is filled with oil and connate water surrounded by fractures filled with injection gas from the top, bottom and right side. In which matrix block oil saturation and behavior of matrix block from inside for specific time are examined simultaneously. Sensitivity analyses on the matrix block at different saturation pressures showed first the positive effect of increasing saturation pressure could be seen when diffusion was involved. Secondly, as the pressure increased, the side of recovery changed from the top to the bottom just absolutely more effective when we had diffusion in the system. Third, gravity drainage at lower saturation pressures was dominant for 2 cases with and without diffusion and at higher saturation pressures capillary force becomes more involved.

Effective dispersion coefficients for the upscaling of pore-scale mixing and reaction

We upscale reactive mixing using effective dispersion coefficients to capture the combined effect of pore-scale heterogeneity and molecular diffusion on the evolution of the mixing interface between two initially segregated dissolved species. Effective dispersion coefficients are defined in terms of the average spatial variance of the solute distribution evolving from a pointlike injection, that is, the transport Green function. We numerically investigate the temporal behavior of the longitudinal effective dispersion coefficients for two porous media of different pore-scale heterogeneity as measured by the statistics of the flow speed, and different Péclet numbers. We find that the effective dispersion coefficients evolve with time, or equivalently travel distance. As the solute samples the pore-scale flow heterogeneity due to advection and transverse diffusion, the effective dispersion coefficients evolve from the value of molecular diffusion to the corresponding hydrodynamic dispersion coefficients. Thus, at times smaller than the diffusion time over a characteristic pore length, the effective dispersion coefficients can be significantly smaller than the hydrodynamic dispersion coefficients. This difference can explain frequently observed mismatches between pore-scale reactive mixing data, and predictions using Darcy scale transport descriptions based on hydrodynamic dispersion coefficients that are constant in time. This suggests that the notion of incomplete mixing on the support scale can be quantified in terms of effective pore-scale dispersion coefficients. We use effective dispersion in order to approximate the transport Green function in terms of a Gaussian-shaped distribution that is characterized by the effective variance. This is approximation is termed dispersive lamella. Based on this representation, we study reactive mixing between two initially segregated solutes. The dispersive lamella approach accurately predicts the evolution of the product mass of an instantaneous bimolecular reaction obtained from direct numerical simulations. This demonstrates that effective dispersion is an accurate measure for width of the mixing interface between the two reacting species. These results shed some new light on pore-scale mixing, the notion of incomplete mixing, and its prediction and upscaling in terms of an effective mixing model.

Synthesis of polymers in nanoreactors: A tool for manipulating polymer properties

The use of nanoreactors to confine monomers and synthesize polymers results in changes in the reaction kinetics and polymer properties making nanoconfinement a potential tool for manipulating and engineering polymer properties. In this perspective, we cover conventional nanoconfinement hosts, nanopore-confined free radical, step-growth, and ring-opening polymerizations, and changes in molecular weight, tacticity, glass transition temperature (Tg), thermal stability, and electrical properties. We use examples from research in our laboratory, as well as comparisons of the work in the literature, to illustrate the competing forces that drive these changes, namely molecular layering or orientation at the nanopore surface, decreased molecular and segmental diffusion, and catalytic or inhibitory effects caused by chemical moieties on the native or surface-functionalized nanopore surface. The majority of nanoconfined polymerizations are found to be accelerated, and in the case of free radical polymerizations to generally yield higher molecular weights and higher isotacticity. Tgs for the nanoconfined polymers tend to increase if strong interactions exist between the polymer and the confinement surface, but depressions are observed for confined polycyanurates; the importance of removing unreacted monomer and comparing results to the bulk material of same molecular weight and structure is emphasized. Examples are also provided of enhanced thermal stability and conductivity of polymers synthesized under nanoconfinement.

Flame speed scaling in autoignition-assisted freely propagating n-heptane/air flames

Autoignition and flame propagation embody different controlling mechanisms for flame stabilization in various modern combustion devices such as the internal combustion engines with hot air and elevated pressure environments. In this study, the flame structure and propagation speed scaling for n-heptane/air mixtures are numerically investigated over a wide range of unburnt temperatures, i.e., 600–1000 K. Under these autoignitive conditions, the flame structure and propagation speed depend not only on the unburnt mixture properties but also on the residence time, which is further complicated by the two-stage ignition processes associated with the negative temperature coefficient (NTC) behavior. The budget analysis of different physical processes for species evolution, together with manifold examination, demonstrates the diminishing effect of molecular diffusion and the increasing contribution of autoignition when the flame is in transition from flame propagation to autoignition. It is shown that, for complex hydrocarbon fuels with NTC, the enhancement of flame propagation due to autoignition assistance cannot be fully characterized by the scaling which shows the flame speed (normalized with the reference flame speed, i.e., Sl/Sl0) only depends on the residence time (normalized with the corresponding ignition delay time, i.e., τres/τign). Sensitivity analysis is conducted and results illustrate the crucial roles of the important radicals H, OH and CO in n-heptane/air flames. Two distinct correlations are revealed in the transition from flame propagation to autoignition: a near linearity and an exponential growth between the propagation speed and the maximum mole fraction of the (H+OH) radicals, respectively. Finally for complex hydrocarbons with NTC, a general scaling is for the first time identified between the normalized flame speed and the maximum mole fraction of (H+OH)/CO radicals (normalized with the maximum (H+OH)/CO mole fraction from corresponding autoignition process, i.e., Xmax(H+OH)/Xmax(H+OH),ign and Xmax(CO)/Xmax(CO),ign), with one branch representing flame propagation and the other representing autoignition-assisted flame propagation.

Unsteady Pressure Dynamics of Polymer Flooding Reservoirs Considering Concentration Changes

In this paper a model is established for unstable seepage flow with polymer concentration and pressure diffusion coupling, considering the effects of polymer molecular diffusion, adsorption, and viscoelasticity of polymer solution in the formation. The factors are close to the actual seepage parameters of the injected reservoir. For the nonlinear adsorption, the combined variable and the analytical iterative method are used to obtain the approximate analytical solution of the model. According to the concentration model, the relationship between concentration and pressure distribution is obtained. Using the model theory curve to fit the well test data, the seepage parameters of the formation are obtained, and the reflection characteristics of the unstable wellbore pressure derivative curve are analyzed.

Hydrodynamic Dispersion in Porous Media and the Significance of Lagrangian Time and Space Scales

Transport in porous media is critical for many applications in the environment and in the chemical process industry. A key parameter for modeling this transport is the hydrodynamic dispersion coefficient for particles and scalars in a porous medium, which has been found to depend on properties of the medium structure, on the dispersing compound, and on the flow field characteristics. Previous studies have resulted in suggestions of different equation forms, showing the relationship between the hydrodynamic dispersion coefficient for various types of porous media in various flow regimes and the Peclet number. The Peclet number is calculated based on a Eulerian length scale, such as the diameter of the spheres in packed beds, or the pore diameter. However, the nature of hydrodynamic dispersion is Lagrangian, and it should take the molecular diffusion effects, as well as the convection effects, into account. This work shifts attention to the Lagrangian time and length scales for the definition of the Peclet number. It is focused on the dependence of the longitudinal hydrodynamic dispersion coefficient on the effective Lagrangian Peclet number by using a Lagrangian length scale and the effective molecular diffusivity. The lattice Boltzmann method (LBM) was employed to simulate flow in porous media that were constituted by packed spheres, and Lagrangian particle tracking (LPT) was used to track the movement of individual dispersing particles. It was found that the hydrodynamic dispersion coefficient linearly depends on the effective Lagrangian Peclet number for packed beds with different types of packing. This linear equation describing the dependence of the dispersion coefficient on the effective Lagrangian Peclet number is both simpler and more accurate than the one formed using the effective Eulerian Peclet number. In addition, the slope of the line is a characteristic coefficient for a given medium.

A multi-scale framework for modelling effective gas diffusivity in dry cement paste: Combined effects of surface, Knudsen and molecular diffusion

Owing to the hierarchical pore structures of cement paste, varying gas diffusion mechanisms, i.e. surface, Knudsen and molecular diffusion, coexist. However, their combined effects have not been fully investigated. To this end, a multi-scale model which considers the combined effects is developed to simulate and analyse the effective gas diffusivity in dry cement paste. The multi-scale structures of cement paste using representative volume elements are reconstructed, capturing the structural characteristics at each scale. Afterwards, based on classical adsorption theory, random walk method and lattice Boltzmann method, the gas diffusivity at each scale is predicted and analysed. The results indicate that surface diffusion plays an important role in the gas diffusion in C-S-H, while the main diffusion regime in C-S-H gradually becomes Knudsen diffusion as the porosity increases. The surface and Knudsen diffusion cannot be ignored or simplified as molecular diffusion in the prediction of gas diffusivity in cement paste.

Air/Sea Transfer of Highly Soluble Gases Over Coastal Waters

AbstractThe deposition of soluble trace gases to the sea surface is not well studied due to a lack of flux measurements over the ocean. Here we report simultaneous air/sea eddy covariance flux measurements of water vapor, sulfur dioxide (SO2), and momentum from a coastal North Atlantic pier. Gas transfer velocities were on average about 20% lower for SO2 than for H2O. This difference is attributed to the difference in molecular diffusivity between the two molecules (DSO2/DH2O = 0.5), in reasonable agreement with bulk parameterizations in air/sea gas models. This study demonstrates that it is possible to observe the effect of molecular diffusivity on air‐side resistance to gas transfer. The slope of observed relationship between gas transfer velocity and friction velocity is slightly smaller than predicted by gas transfer models, possibly due to wind/wave interactions that are unaccounted for in current models.