Articles de revues sur le sujet « Transport/Diffusion Equivalence »
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1
Liu, Zhi Bin, Jin Ma, Bing Shu Wang et Xin Hui Duan. « The Study on Homogeneous Parameters of Light Water Reactor by the Nodal Diffusion Method ». Applied Mechanics and Materials 666 (octobre 2014) : 144–48. http://dx.doi.org/10.4028/www.scientific.net/amm.666.144.
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Recent progress in the development of coarse-mesh nodal methods for the numerical solution of the neutron diffusion and transport equations is reviewed. Compared with earlier nodal simulators, more recent nodal diffusion methods are characterized by the systematic derivation of spatial coupling relationships which are entirely consistent with the multi-group diffusion equation. These relationships most often are derived by developing approximations to the one-dimensional equations obtained by integrating the multidimensional diffusion equation over directions transverse to each coordinate axis. The objective of this research is to develop accurate and efficient spatial homogenization method for coarse mesh analysis of light water reactors. More sophisticated methods for computing approximate equivalence parameters are also introduced and make use of nonlinear iterations between homogenized reactor calculations and local fixed-source calculations to compute equivalence parameters. This special feature induces the need for the study on homogeneous parameters of light water reactor which takes phenomena of different scale and their interaction into account by means of the nodal diffusion method.
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Rapaport, Alain, Alejandro Rojas-Palma, Jean-Raynald de Dreuzy et Hector C. Ramirez. « Equivalence of Finite Dimensional Input–Output Models of Solute Transport and Diffusion in Geosciences ». IEEE Transactions on Automatic Control 62, no 10 (octobre 2017) : 5470–77. http://dx.doi.org/10.1109/tac.2017.2701150.
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Buonocore, Salvatore, Mihir Sen et Fabio Semperlotti. « Stochastic scattering model of anomalous diffusion in arrays of steady vortices ». Proceedings of the Royal Society A : Mathematical, Physical and Engineering Sciences 476, no 2238 (juin 2020) : 20200183. http://dx.doi.org/10.1098/rspa.2020.0183.
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We investigate the occurrence of anomalous transport phenomena associated with tracer particles propagating through arrays of steady vortices. The mechanism responsible for the occurrence of anomalous transport is identified in the particle dynamic, which is characterized by long collision-less trajectories (Lévy flights) interrupted by chaotic interactions with vortices. The process is studied via stochastic molecular models that are able to capture the underlying non-local nature of the transport mechanism. These models, however, are not well suited for problems where computational efficiency is an enabling factor. We show that fractional-order continuum models provide an excellent alternative that is able to capture the non-local nature of anomalous transport processes in turbulent environments. The equivalence between stochastic molecular and fractional continuum models is demonstrated both theoretically and numerically. In particular, the onset and the temporal evolution of heavy-tailed diffused fields are shown to be accurately captured, from a macroscopic perspective, by a fractional diffusion equation. The resulting anomalous transport mechanism, for the selected ranges of density of the vortices, shows a superdiffusive nature.
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Wassmer, Dominik, Bruno Schuermans, Christian Oliver Paschereit et Jonas P. Moeck. « Measurement and modeling of the generation and the transport of entropy waves in a model gas turbine combustor ». International Journal of Spray and Combustion Dynamics 9, no 4 (24 avril 2017) : 299–309. http://dx.doi.org/10.1177/1756827717696326.
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Indirect combustion noise is caused by entropy spots that are accelerated at the first turbine stage. These so-called entropy waves originate from the equivalence ratio fluctuations in the air–fuel mixture upstream of the flame. As entropy waves propagate convectively through the combustion chamber, they are subject to diffusion and dispersion. Because of the inherent difficulty of accurately measuring the burned gas temperature with sufficient temporal resolution, experimental data of entropy waves are scarce. In this work, the transfer function between equivalence ratio fluctuations and entropy fluctuations is modeled by a linearized reactor model, and the transport of entropy waves is investigated based on a convection-diffusion model. Temperature fluctuations are measured by means of a novel measurement technique at different axial positions downstream of the premixed flame, which is forced by periodic fuel injection. Experiments with various flow velocities and excitation frequencies enable model validation over a wide range of parameters.
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Kather, Vincent, Finn Lückoff, Christian O. Paschereit et Kilian Oberleithner. « Interaction of equivalence ratio fluctuations and flow fluctuations in acoustically forced swirl flames ». International Journal of Spray and Combustion Dynamics 13, no 1-2 (juin 2021) : 72–95. http://dx.doi.org/10.1177/17568277211015544.
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The generation and turbulent transport of temporal equivalence ratio fluctuations in a swirl combustor are experimentally investigated and compared to a one-dimensional transport model. These fluctuations are generated by acoustic perturbations at the fuel injector and play a crucial role in the feedback loop leading to thermoacoustic instabilities. The focus of this investigation lies on the interplay between fuel fluctuations and coherent vortical structures that are both affected by the acoustic forcing. To this end, optical diagnostics are applied inside the mixing duct and in the combustion chamber, housing a turbulent swirl flame. The flame was acoustically perturbed to obtain phase-averaged spatially resolved flow and equivalence ratio fluctuations, which allow the determination of flux-based local and global mixing transfer functions. Measurements show that the mode-conversion model that predicts the generation of equivalence ratio fluctuations at the injector holds for linear acoustic forcing amplitudes, but it fails for non-linear amplitudes. The global (radially integrated) transport of fuel fluctuations from the injector to the flame is reasonably well approximated by a one-dimensional transport model with an effective diffusivity that accounts for turbulent diffusion and dispersion. This approach however, fails to recover critical details of the mixing transfer function, which is caused by non-local interaction of flow and fuel fluctuations. This effect becomes even more pronounced for non-linear forcing amplitudes where strong coherent fluctuations induce a non-trivial frequency dependence of the mixing process. The mechanisms resolved in this study suggest that non-local interference of fuel fluctuations and coherent flow fluctuations is significant for the transport of global equivalence ratio fluctuations at linear acoustic amplitudes and crucial for non-linear amplitudes. To improve future predictions and facilitate a satisfactory modelling, a non-local, two-dimensional approach is necessary.
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Takasugi, Cole, Nicolas Martin, Vincent Labouré, Javier Ortensi, Kostadin Ivanov et Maria Avramova. « Preservation of kinetics parameters generated by Monte Carlo calculations in two-step deterministic calculations ». EPJ Nuclear Sciences & ; Technologies 9 (2023) : 15. http://dx.doi.org/10.1051/epjn/2022056.
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The generation of accurate kinetic parameters such as mean generation time Λ and effective delayed neutron fraction βeff via Monte Carlo codes is established. Employing these in downstream deterministic codes warrants another step to ensure no additional error is introduced by the low-order transport operator when computing forward and adjoint fluxes for bilinear weighting of these parameters. Another complexity stems from applying superhomogenization (SPH) equivalence in non-fundamental mode approximations, where reference and low-order calculations rely on a 3D full core model. In these cases, SPH factors can optionally be computed for only part of the geometry while preserving reaction rates and K-effective, but the impact of such approximations on kinetics parameters has not been thoroughly studied. This paper aims at studying the preservation of bilinearly-weighted quantities in the Serpent–Griffin calculation procedure. Diffusion and transport evaluations of IPEN/MB-01, Godiva, and Flattop were carried out with the Griffin reactor physics code, testing available modeling options using Serpent-generated multigroup cross sections and equivalence data. Verifying Griffin against Serpent indicates sensitivities to multigroup energy grid selection and regional application of SPH equivalence, introducing significant errors; these were demonstrated to be reduced through the use of a transport method together with a finer energy grid.
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Heinz, Stefan, Jakob Heinz et Jonathan A. Brant. « Mass Transport in Membrane Systems : Flow Regime Identification by Fourier Analysis ». Fluids 7, no 12 (30 novembre 2022) : 369. http://dx.doi.org/10.3390/fluids7120369.
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The numerical calculation of local mass distributions in membrane systems by computational fluid dynamics (CFD) offers indispensable benefits. However, the concept to calculate such distributions in response to separate variations of operation conditions (OCs) makes it difficult to address overall, flow-physics-related questions, which require the consideration of the collective interaction of OCs. It is shown that such understanding-related relationships can be obtained by the analytical solution of the advection–diffusion equation considered. A Fourier series model (FSM) is presented, which provides exact solutions of an advection–diffusion equation for a wide range of OCs. On this basis, a new zeroth-order model is developed, which is very simple and as accurate as the complete FSM for all conditions of practical relevance. Advection-dominated blocked and diffusion-dominated unblocked flow regimes are identified (depending on a Péclet number which compares the flow geometry with a length scale imposed by the flow), which implies relevant requirements for the use of lab results for pilot- and full-scale applications. Analyses reveal the equivalence of variations of OCs, which offers a variety of options to accomplish desired flow regime changes.
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Rahnema, Farzad, et Piero Ravetto. « On the Equivalence of Boundary and Boundary Condition Perturbations in Transport Theory and Its Diffusion Approximation ». Nuclear Science and Engineering 128, no 2 (février 1998) : 209–23. http://dx.doi.org/10.13182/nse98-a1952.
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Gazizov, R. K., A. A. Kasatkin et S. Yu Lukashchuk. « Symmetry properties of fractional order transport equations ». Proceedings of the Mavlyutov Institute of Mechanics 9, no 1 (2012) : 59–64. http://dx.doi.org/10.21662/uim2012.1.010.
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In the paper some features of applying Lie group analysis methods to fractional differential equations are considered. The problem related to point change of variables in the fractional differentiation operator is discussed and some general form of transformation that conserves the form of Riemann-Liouville fractional operator is obtained. The prolongation formula for extending an infinitesimal operator of a group to fractional derivative with respect to arbitrary function is presented. Provided simple example illustrates the necessity of considering both local and non-local symmetries for fractional differential equations in particular cases including the initial conditions. The equivalence transformation forms for some fractional differential equations are discussed and results of group classification of the wave-diffusion equation are presented. Some examples of constructing particular exact solutions of fractional transport equation are given, based on the Lie group methods and the method of invariant subspaces.
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Yan, Sheng, Zhili Zou et Zaijin You. « Eulerian Description of Wave-Induced Stokes Drift Effect on Tracer Transport ». Journal of Marine Science and Engineering 10, no 2 (12 février 2022) : 253. http://dx.doi.org/10.3390/jmse10020253.
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The wave-induced Stokes drift plays a significant role on mass/tracer transport in the ocean and the evolution of coastal morphology. The tracer advection diffusion equation needs to be modified for Eulerian ocean models to properly account for the surface wave effects. The Eulerian description of Stokes drift effect on the tracer transport is derived in this study to show that this effect can be accounted for automatically in the wave-averaged advection-diffusion equation. The advection term in this equation is the wave-averaged concentration flux produced by the interaction between fluctuations of linear wave orbital velocity and tracer concentration, and the advection velocity is the same as the Stokes drift velocity. Thus, the effective dispersion of tracers by surface gravity waves is calculated due to the Stokes drift effect and the corresponding dispersion coefficient in the depth-integrated equation is then derived. The Eulerian description of Stokes drift effect of tracer concentration is illustrated by the direct numerical simulation of the advection–diffusion equation under simple linear waves. The equivalence between both the Eulerian and Lagrangian descriptions is also verified by particle tracking method. The theoretical analysis is found to agree well with the wave-induced dye drift velocity observed outside the surf zone in a longshore current experiment.
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Jost, Andreas, Michel Bendias, Jan Böttcher, Ewelina Hankiewicz, Christoph Brüne, Hartmut Buhmann, Laurens W. Molenkamp et al. « Electron–hole asymmetry of the topological surface states in strained HgTe ». Proceedings of the National Academy of Sciences 114, no 13 (9 mars 2017) : 3381–86. http://dx.doi.org/10.1073/pnas.1611663114.
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Topological insulators are a new class of materials with an insulating bulk and topologically protected metallic surface states. Although it is widely assumed that these surface states display a Dirac-type dispersion that is symmetric above and below the Dirac point, this exact equivalence across the Fermi level has yet to be established experimentally. Here, we present a detailed transport study of the 3D topological insulator-strained HgTe that strongly challenges this prevailing viewpoint. First, we establish the existence of exclusively surface-dominated transport via the observation of an ambipolar surface quantum Hall effect and quantum oscillations in the Seebeck and Nernst effect. Second, we show that, whereas the thermopower is diffusion driven for surface electrons, both diffusion and phonon drag contributions are essential for the hole surface carriers. This distinct behavior in the thermoelectric response is explained by a strong deviation from the linear dispersion relation for the surface states, with a much flatter dispersion for holes compared with electrons. These findings show that the metallic surface states in topological insulators can exhibit both strong electron–hole asymmetry and a strong deviation from a linear dispersion but remain topologically protected.
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Seibert, P., et A. Frank. « Source-receptor matrix calculation with a Source-receptor matrix calculation with a backward mode ». Atmospheric Chemistry and Physics Discussions 3, no 4 (29 août 2003) : 4515–48. http://dx.doi.org/10.5194/acpd-3-4515-2003.
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Abstract. The possibility to calculate linear-source receptor relationships for the transport of atmospheric trace substances with a Lagrangian particle dispersion model (LPDM) running in backward mode is shown and presented with many tests and examples. The derivation includes the action of sources and of any first-order processes (transformation with prescribed rates, dry and wet deposition, radioactive decay, ...). The backward mode is computationally advantageous if the number of receptors is less than the number of sources considered. The combination of an LPDM with the backward (adjoint) methodology is especially attractive for the application to point measurements, which can be handled without artificial numerical diffusion. Practical hints are provided for source-receptor calculations with different settings, both in forward and backward mode. The equivalence of forward and backward calculations is shown in simple tests for release and sampling of particles, pure wet deposition, pure convective redistribution and realistic transport over a short distance. Furthermore, an application example explaining measurements of Cs-137 in Stockholm as transport from areas contaminated heavily in the Chernobyl disaster is included.
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Lee, Hsu Chew, Peng Dai, Minping Wan et Andrei N. Lipatnikov. « Lewis number and preferential diffusion effects in lean hydrogen–air highly turbulent flames ». Physics of Fluids 34, no 3 (mars 2022) : 035131. http://dx.doi.org/10.1063/5.0087426.
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Unsteady three-dimensional direct numerical simulations of highly turbulent, complex-chemistry, lean hydrogen-air flames were performed by changing the equivalence ratio [Formula: see text], root mean square velocity [Formula: see text], and turbulence length scale [Formula: see text]. For each set of [Formula: see text], to explore the influence of molecular transport coefficients on the turbulent burning velocity [Formula: see text], four cases were designed: (i) mixture-averaged diffusivities; (ii) diffusivities equal to the heat diffusivity [Formula: see text] of the mixture for all species; (iii) mixture-averaged diffusivities for all species with the exception of O2, whose diffusivity was equal to the diffusivity [Formula: see text] of H2 to suppress preferential diffusion effects; and (iv) mixture-averaged diffusivities multiplied with [Formula: see text] to suppress Lewis number effects but retain preferential diffusion effects. The computed results show a significant increase in [Formula: see text] due to differences in molecular transport coefficients even at Karlovitz number [Formula: see text] as large as 565. The increase is documented in cases (i) and (iii) but is not observed in case (iv)—indicating that this phenomenon is controlled by Lewis number effects, whereas preferential diffusion effects play a minor role. The phenomenon is more pronounced in leaner flames, with all other things being equal. While the temperature profiles [Formula: see text] conditionally averaged at the local value of the combustion progress variable [Formula: see text] and sampled from the entire flame brushes are not sensitive to variations in molecular transport coefficients at high [Formula: see text], the [Formula: see text]-profiles sampled from the leading edges of the same flame brushes show significant increase in the local temperature in cases (i) and (iii) characterized by a low Lewis number.
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Benoist, P., M. Carta, G. Palmiotti, M. Salvatores et J. Tullett. « A Simple Calculation of Control Assembly Effectiveness in a Liquid-Metal Fast Breeder Reactor by a Transport-Diffusion Equivalence Method ». Nuclear Science and Engineering 103, no 3 (novembre 1989) : 254–64. http://dx.doi.org/10.13182/nse89-a23676.
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Vrhovac, Slobodan B., et Zoran Lj Petrovic. « Boltzmann Equation Theory of Charged Particle Transport in Neutral Gases : Perturbation Treatment ». Australian Journal of Physics 52, no 6 (1999) : 999. http://dx.doi.org/10.1071/ph99053.
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This paper examines the formal structure of the Boltzmann equation (BE) theory of charged particle transport in neutral gases. The initial value problem of the BE is studied by using perturbation theory generalised to non-Hermitian operators. The method developed by R�sibois was generalised in order to be applied for the derivation of the transport coecients of swarms of charged particles in gases. We reveal which intrinsic properties of the operators occurring in the kinetic equation are sucient for the generalised diffusion equation (GDE) and the density gradient expansion to be valid. Explicit expressions for transport coecients from the (asymmetric) eigenvalue problem are also deduced. We demonstrate the equivalence between these microscopic expressions and the hierarchy of kinetic equations. The establishment of the hydrodynamic regime is further analysed by using the time-dependent perturbation theory. We prove that for times t ? τ0 (τ0 is the relaxation time), the one-particle distribution function of swarm particles can be transformed into hydrodynamic form. Introducing time-dependent transport coecients ? *(p) (?q,t), which can be related to various Fourier components of the initial distribution function, we also show that for the long-time limit all ? *(p) (?q,t) become time and ?q independent in the same characteristic time and achieve their hydrodynamic values.
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Seibert, P., et A. Frank. « Source-receptor matrix calculation with a Lagrangian particle dispersion model in backward mode ». Atmospheric Chemistry and Physics 4, no 1 (23 janvier 2004) : 51–63. http://dx.doi.org/10.5194/acp-4-51-2004.
Texte intégralRésumé :
Abstract. The possibility to calculate linear-source receptor relationships for the transport of atmospheric trace substances with a Lagrangian particle dispersion model (LPDM) running in backward mode is shown and presented with many tests and examples. This mode requires only minor modifications of the forward LPDM. The derivation includes the action of sources and of any first-order processes (transformation with prescribed rates, dry and wet deposition, radioactive decay, etc.). The backward mode is computationally advantageous if the number of receptors is less than the number of sources considered. The combination of an LPDM with the backward (adjoint) methodology is especially attractive for the application to point measurements, which can be handled without artificial numerical diffusion. Practical hints are provided for source-receptor calculations with different settings, both in forward and backward mode. The equivalence of forward and backward calculations is shown in simple tests for release and sampling of particles, pure wet deposition, pure convective redistribution and realistic transport over a short distance. Furthermore, an application example explaining measurements of Cs-137 in Stockholm as transport from areas contaminated heavily in the Chernobyl disaster is included.
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Zhang, Lili, Yongzhang Cui, Pengfei Yin, Wenlong Mao et Pengzhao Zhang. « Numerical Investigation on the Flame Characteristics of Lean Premixed Methane Flame Piloted with Rich Premixed Flame ». Energies 17, no 14 (12 juillet 2024) : 3430. http://dx.doi.org/10.3390/en17143430.
Texte intégralRésumé :
The introduction of the pilot flame can stabilize the lean premixed flame and promote its industrial application. However, the interaction mechanism between the pilot and main flames is complicated. To reveal the effect of the pilot flame on the main flame, a laminar lean premixed flame adjacent to a rich premixed pilot flame on one side and a similar lean premixed flame on the other side was considered. A two-dimensional numerical model was adopted with detailed chemistry and species transport, also with no artificial flame anchoring boundary conditions. The results show that the pilot flame could promote the main flame stabilized in different locations with various shapes, by adjusting the stretch, heat transfer, and preferential diffusion in a complicated manner. The pilot flame improves the local equivalence ratio and transfer more heat to the main flame. The growth of the pilot flame equivalence ratio and inlet velocity enhances the combustion on the rich side of the main flame and helps it anchor closer to the flame wall. Both the curvature and strain rate show a significant effect on the flame root, which contributes to the main flame bending towards the pilot flame.
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Hadi, Jihad M., Shujahadeen B. Aziz, Salah R. Saeed, Mohamad A. Brza, Rebar T. Abdulwahid, Muhamad H. Hamsan, Ranjdar M. Abdullah, Mohd F. Z. Kadir et S. K. Muzakir. « Investigation of Ion Transport Parameters and Electrochemical Performance of Plasticized Biocompatible Chitosan-Based Proton Conducting Polymer Composite Electrolytes ». Membranes 10, no 11 (21 novembre 2020) : 363. http://dx.doi.org/10.3390/membranes10110363.
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In this study, biopolymer composite electrolytes based on chitosan:ammonium iodide:Zn(II)-complex plasticized with glycerol were successfully prepared using the solution casting technique. Various electrical and electrochemical parameters of the biopolymer composite electrolytes’ films were evaluated prior to device application. The highest conducting plasticized membrane was found to have a conductivity of 1.17 × 10−4 S/cm. It is shown that the number density, mobility, and diffusion coefficient of cations and anions fractions are increased with the glycerol amount. Field emission scanning electron microscope and Fourier transform infrared spectroscopy techniques are used to study the morphology and structure of the films. The non-Debye type of relaxation process was confirmed from the peak appearance of the dielectric relaxation study. The obtained transference number of ions (cations and anions) and electrons for the highest conducting sample were identified to be 0.98 and 0.02, respectively. Linear sweep voltammetry shows that the electrochemical stability of the highest conducting plasticized system is 1.37 V. The cyclic voltammetry response displayed no redox reaction peaks over its entire potential range. It was discovered that the addition of Zn(II)-complex and glycerol plasticizer improved the electric double-layer capacitor device performances. Numerous crucial parameters of the electric double-layer capacitor device were obtained from the charge-discharge profile. The prepared electric double-layer capacitor device showed that the initial values of specific capacitance, equivalence series resistance, energy density, and power density are 36 F/g, 177 Ω, 4.1 Wh/kg, and 480 W/kg, respectively.
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Bykov, V., V. V. Gubernov et U. Maas. « Mechanisms performance and pressure dependence of hydrogen/air burner-stabilized flames ». Mathematical Modelling of Natural Phenomena 13, no 6 (2018) : 51. http://dx.doi.org/10.1051/mmnp/2018046.
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The kinetic mechanism of hydrogen combustion is the most investigated combustion system. This is due to extreme importance of the mechanism for combustion processes, i.e. it is present as a sub-mechanism in all mechanisms for hydrocarbon combustion systems. Therefore, detailed aspects of hydrogen flames are still under active investigations, e.g. under elevated pressure, under conditions of different heat losses intensities and local equivalence ratios etc. For this purpose, the burner stabilized flame configuration is an efficient tool to study different aspects of chemical kinetics by varying the stand-off distance, pressure, temperature of the burner and mixture compositions. In the present work, a flat porous plug burner flame configuration is revisited. A hydrogen/air combustion system is considered with detailed molecular transport including thermo-diffusion and with 8 different chemical reaction mechanisms. Detailed numerical investigations are performed to single out the role of chemical kinetics on the loss of stability and on the dynamics of the flame oscillations. As a main outcome, it was found/demonstrated that the results of critical values, e.g. critical mass flow rate, weighted frequency of oscillations and blow-off velocity, with increasing the pressure scatter almost randomly. Thus, these parameters can be considered as independent and can be used to improve and to validate the mechanisms of chemical kinetics for the unsteady dynamics.
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Amin, E. M., G. E. Andrews, M. Pourkashnian, A. Williams et R. A. Yetter. « A Computational Study of Pressure Effects on Pollutant Generation in Gas Turbine Combustors ». Journal of Engineering for Gas Turbines and Power 119, no 1 (1 janvier 1997) : 76–83. http://dx.doi.org/10.1115/1.2815565.
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A numerical study of the effect of pressure on the formation of NOx and soot in an axisymmetric 30 deg counterrotating axial swirler lean low-NOx gas turbine combustor has been conducted. This has previously been studied experimentally and this CFD investigation was undertaken to explain the higher than expected NOx emissions. The combustion conditions selected for the present study were 300 K inlet air, 0.4 overall equivalence ratio, and pressures of 1 and 10 bar. The numerical model used here involved the solution of time-averaged governing equations using an elliptic flow-field solver. The turbulence was modeled using algebraic stress modeling (ASM). The thermochemical model was based on the laminar flame let formulation. The conserved scalar/assumed pdf approach was used to model the turbulence chemistry interaction. The study was for two pressure cases at 1 and 10 bar. The turbulence–chemistry interaction is closed by assumption of a clipped Gaussian function form for the fluctuations in the mixture fraction. The kinetic calculations were done separately from the flowfield solver using an opposed laminar diffusion flame code of SANDIA. The temperature and species profiles were made available to the computations through look-up tables. The pollutants studied in this work were soot and NO for which three more additional transport equations are required, namely: averaged soot mass fraction, averaged soot particle number density, and finally averaged NO mass fraction. Soot oxidation was modeled using molecular oxygen only and a strong influence of pressure was predicted. Pressure was shown to have a major effect on soot formation.
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MUESCHKE, NICHOLAS J., OLEG SCHILLING, DAVID L. YOUNGS et MALCOLM J. ANDREWS. « Measurements of molecular mixing in a high-Schmidt-number Rayleigh–Taylor mixing layer ». Journal of Fluid Mechanics 632 (27 juillet 2009) : 17–48. http://dx.doi.org/10.1017/s0022112009006132.
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Molecular mixing measurements are reported for a high-Schmidt-number (Sc~ 103), small-Atwood-number (A≈ 7.5 × 10−4) buoyancy-driven turbulent Rayleigh–Taylor (RT) mixing layer in a water channel facility. Salt was added to the top water stream to create the desired density difference. The degree of molecular mixing was measured as a function of time by monitoring a diffusion-limited chemical reaction between the two fluid streams. The pH of each stream was modified by the addition of acid or alkali such that a local neutralization reaction occurred as the two fluids molecularly mixed. The progress of this neutralization reaction was tracked by the addition of phenolphthalein – a pH-sensitive chemical indicator – to the acidic stream. Accurately calibrated backlit optical techniques were used to measure the average concentration of the coloured chemical indicator. Comparisons of chemical product formation for pre-transitional buoyancy- and shear-driven mixing layers are given. It is also shown that experiments performed at different equivalence ratios (acid/alkali concentrations) can be combined to obtain a mathematical relationship between the coloured product formed and the density variance. This relationship was used to obtain high-fidelity quantitative measures of the degree of molecular mixing which are independent of probe resolution constraints. The dependence of molecular mixing on the Schmidt and Reynolds numbers is examined by comparing the currentSc~ 103measurements with previousSc= 0.7 gas-phase andPr= 7 (wherePris the Prandtl number) liquid-phase measurements. This comparison indicates that the Schmidt number has a large effect on the quantity of mixed fluid at small Reynolds numbersReh< 103. At larger Reynolds numbers, corresponding to later times in this experiment, all mixing parameters indicated a greater degree of molecular mixing and a decreased Schmidt number dependence. Implications for the development and quantitative assessment of turbulent transport and mixing models appropriate for RT instability-induced mixing are discussed.
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Burbea, Z. H., S. R. Gullans et S. Ben-Yaakov. « Delta alkalinity : a simple method to measure cellular net acid-base fluxes ». American Journal of Physiology-Cell Physiology 253, no 4 (1 octobre 1987) : C525—C534. http://dx.doi.org/10.1152/ajpcell.1987.253.4.c525.
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Acid-base transport across cell plasma membranes is important for cell homeostasis and growth. Current techniques for quantitatively measuring net acid-base fluxes are generally limited by either assumptions concerning properties of intracellular compartments or use of poorly buffered, nonphysiological solutions. We adapted an approach from marine chemistry to quantitate net acid-base changes in standard physiological media that obviates these problems. This method is based on conservation of charge and involves a simple acid titration of the extracellular medium to an end point, the equivalence point, pHe. For standard physiological solutions containing buffers such as bicarbonate and phosphate, pHe exists in the range of pH 4.0-4.7 and is identified as the pH where dpH/dH+ is maximal in an HCl titration. By determining the quantity of H+ required to reach pHe, one can determine precisely total quantity of proton acceptors (alkalinity) present in physiological pH range. Alkalinity (in meq) is a relative measure of the charge capable of interacting with protons. We show that, unlike pH, changes in alkalinity (delta alkalinity) result only from net acid-base changes in medium. Therefore, by monitoring extracellular delta alkalinity associated with cell function, it is possible to quantitate precisely net acid-base fluxes. Moreover, through a second titration procedure, delta alkalinity can be divided into bicarbonate and nonbicarbonate fractions. As an example, we performed the first direct measurement of net Cl(-)-HCO3- exchange in intact human erythrocytes and observed a Cl(-)-HCO3- exchange ratio of 1.01 +/- 0.03. Overall, delta alkalinity measurements are applicable to numerous cell systems, can be performed with solutions containing a mixture of buffers at normal physiological concentrations (e.g., 25 mM HCO3- and 2 mM HPO4(-2), do not require corrections for CO2 diffusion or loss of CO2 from the solution, and avoid assumptions about intracellular or extracellular buffer properties.
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Huang, Tao, Zhongqi Peng, Mengge Wang et Shuang Feng. « Study on the Ionic Transport Properties of 3D Printed Concrete ». Buildings 14, no 5 (24 avril 2024) : 1216. http://dx.doi.org/10.3390/buildings14051216.
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Three-dimensional printed concrete (3DPC) is an anisotropic heterogeneous material composed of a concrete matrix and the interfaces between layers and filaments that form during printing. The overall ion transport properties can be characterized by the equivalent diffusion coefficient. This paper first establishes a theoretical model to calculate the equivalent diffusion coefficient of 3DPC. Verification through numerical calculations shows that this theoretical model is highly precise. Based on this, the model was used to analyze the effects of dimensionless interface parameters on the equivalent diffusion coefficients in different directions of 3DPC. Finally, the dynamic ionic transport properties of 3DPC were investigated through finite element numerical simulation. The results of the dynamic study indicate that interfaces have a significant impact on the ion distribution and its evolution within 3DPC. The product of the interface diffusion coefficient and interface size can represent the ionic transport capacity of an interface. The stronger the ionic transport capacity of an interface, the higher the ion concentration at that interface. Due to the “drainage” effect of lateral interfaces, the ion concentration in the middle of 3DPC with a smaller equivalent diffusion coefficient is higher than that in 3DPC with a larger equivalent diffusion coefficient.
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Dannoun, Elham M. A., Shujahadeen B. Aziz, Rebar T. Abdulwahid, Sameerah I. Al-Saeedi, Muaffaq M. Nofal, Niyaz M. Sadiq et Jihad M. Hadi. « Study of MC:DN-Based Biopolymer Blend Electrolytes with Inserted Zn-Metal Complex for Energy Storage Devices with Improved Electrochemical Performance ». Membranes 12, no 8 (8 août 2022) : 769. http://dx.doi.org/10.3390/membranes12080769.
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Stable and ionic conducting electrolytes are needed to make supercapacitors more feasible, because liquid electrolytes have leakage problems and easily undergo solvent evaporation. Polymer-based electrolytes meet the criteria, yet they lack good efficiency due to limited segmental motion. Since metal complexes have crosslinking centers that can be coordinated with the polymer segments, they are regarded as an adequate method to improve the performance of the polymer-based electrolytes. To prepare plasticized proton conducting polymer composite (PPC), a simple and successful process was used. Using a solution casting process, methylcellulose and dextran were blended and impregnated with ammonium thiocyanate and zinc metal complex. A range of electrochemical techniques were used to analyze the PPC, including transference number measurement (TNM), linear sweep voltammetry (LSV), cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). The ionic conductivity of the prepared system was found to be 3.59 × 10−3 S/cm using the EIS method. The use of glycerol plasticizer improves the transport characteristics, according to the findings. The carrier species is found to have ionic mobility of 5.77 × 10−5 cm2 V−1 s−1 and diffusion coefficient of 1.48 × 10−6 cm2 s−1 for the carrier density 3.4 × 1020 cm−3. The TNM revealed that anions and cations were the predominant carriers in electrolyte systems, with an ionic transference value of 0.972. The LSV approach demonstrated that, up to 2.05 V, the film was stable, which is sufficient for energy device applications. The prepared PPC was used to create an electrical double-layer capacitor (EDLC) device. The CV plot exhibited the absence of Faradaic peaks in the CV plot, making it practically have a rectangular form. Using the GCD experiment, the EDLC exhibited low equivalence series resistance of only 65 Ω at the first cycle. The average energy density, power density, and specific capacitance values were determined to be 15 Wh/kg, 350 W/kg, and 128 F/g, respectively.
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TAHARA, Yoshihisa, et Hiroshi SEKIMOTO. « Transport Equivalent Diffusion Constants for Reflector Region in PWRs ». Journal of Nuclear Science and Technology 39, no 7 (juillet 2002) : 716–28. http://dx.doi.org/10.1080/18811248.2002.9715253.
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Hsia, Connie C. W., Robert L. Johnson, D. Merrill Dane, Eugene Y. Wu, Aaron S. Estrera, Harrieth E. Wagner et Peter D. Wagner. « The canine spleen in oxygen transport : gas exchange and hemodynamic responses to splenectomy ». Journal of Applied Physiology 103, no 5 (novembre 2007) : 1496–505. http://dx.doi.org/10.1152/japplphysiol.00281.2007.
Texte intégralRésumé :
In athletic animals the spleen induces acute polycythemia by dynamic contraction that releases red blood cells into the circulation in response to increased O2 demand and metabolic stress; when energy demand is relieved, the polycythemia is rapidly reversed by splenic relaxation. We have shown in adult foxhounds that splenectomy eliminates exercise-induced polycythemia, thereby reducing peak O2 uptake and lung diffusing capacity for carbon monoxide (DLCO) as well as exaggerating preexisting DLCO impairment imposed by pneumonectomy (Dane DM, Hsia CC, Wu EY, Hogg RT, Hogg DC, Estrera AS, Johnson RL Jr. J Appl Physiol 101: 289–297, 2006). To examine whether the postsplenectomy reduction in DLCO leads to abnormalities in O2 diffusion, ventilation-perfusion inequality, or hemodynamic function, we studied these animals via the multiple inert gas elimination technique at rest and during exercise at a constant workload equivalent to 50% or 80% of peak O2 uptake while breathing 21% and 14% O2 in balanced order. From rest to exercise after splenectomy, minute ventilation was significantly elevated with respect to O2 uptake compared with exercise before splenectomy; cardiac output, O2 delivery, and mean pulmonary and systemic arterial blood pressures were 10–20% lower, while O2 extraction was elevated with respect to O2 uptake. Ventilation-perfusion inequality was unchanged, but O2 diffusing capacities of lung (DLO2) and peripheral tissue during exercise were lower with respect to cardiac output postsplenectomy by 32% and 25%, respectively. The relationship between DLO2 and DLCO was unchanged by splenectomy. We conclude that the canine spleen regulates both convective and diffusive O2 transport during exercise to increase maximal O2 uptake.
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Reyes, J. G., M. V. Velarde, R. Ugarte et D. J. Benos. « Glycolytic component of rat spermatid energy and acid-base metabolism ». American Journal of Physiology-Cell Physiology 259, no 4 (1 octobre 1990) : C660—C667. http://dx.doi.org/10.1152/ajpcell.1990.259.4.c660.
Texte intégralRésumé :
The impact of glycolysis on rat spermatid energy metabolism is made apparent by the simultaneous occurrence of the following three events upon glucose addition to the extracellular medium of a rat spermatid cell suspension: decrease in ATP content, exit of acid equivalents, and increased lactate production and efflux. In this work, we have studied the interrelations between these three phenomena. By measuring ATP content, net acid transport, lactate exit, oxygen consumption, intracellular pH, CO2 production, and glycolytic intermediates in the presence of glucose and glucose analogues, we conclude that 1) lactate production, decrease in ATP content, and acid equivalent exit are dependent on the metabolism of glucose up to different stages in glycolysis. 2) The decrease in ATP content is not directly related to the exit of acid equivalents from rat spermatids. 3) Glucose metabolism is a net ATP-consuming process at high intracellular ATP content but is a net ATP-producing process at low intracellular ATP concentration in rat spermatids. 4) Acid equivalent production arises from the metabolism of glucose beyond glyceraldehyde 3-phosphate dehydrogenase. 5) Lactic acid diffusion and/or lactate transport and CO2 production and exit could account for the glucose-dependent acid equivalent efflux in rat spermatids.
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Chen, Gang, et Alan Plumb. « Effective Isentropic Diffusivity of Tropospheric Transport ». Journal of the Atmospheric Sciences 71, no 9 (28 août 2014) : 3499–520. http://dx.doi.org/10.1175/jas-d-13-0333.1.
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Abstract Tropospheric transport can be described qualitatively by the slow mean diabatic circulation and rapid isentropic mixing, yet a quantitative understanding of the transport circulation is complicated, as nearly half of the isentropic surfaces in the troposphere frequently intersect the ground. A theoretical framework for the effective isentropic diffusivity of tropospheric transport is presented. Compared with previous isentropic analysis of effective diffusivity, a new diagnostic is introduced to quantify the eddy diffusivity of the near-surface isentropic flow. This diagnostic also links the effective eddy diffusivity directly to a diffusive closure of eddy fluxes through a finite-amplitude wave activity equation. The theory is examined in a dry primitive equation model on the sphere. It is found that the upper troposphere is characterized by a diffusivity minimum at the jet’s center with enhanced mixing at the jet’s flanks and that the lower troposphere is dominated by stronger mixing throughout the baroclinic zone. This structure of isentropic diffusivity is generally consistent with the diffusivity obtained from the geostrophic component of the flow. Furthermore, the isentropic diffusivity agrees broadly with the tracer equivalent length obtained from either a spectral diffusion scheme or a semi-Lagrangian advection scheme, indicating that the effective diffusivity of tropospheric transport is largely dictated by large-scale stirring rather than details of the small-scale diffusion acting on the tracers.
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Moya, Antonio Angel. « Identifying Characteristic Frequencies in the Electrochemical Impedance of Ion-Exchange Membrane Systems ». Membranes 12, no 10 (16 octobre 2022) : 1003. http://dx.doi.org/10.3390/membranes12101003.
Texte intégralRésumé :
In this study, the characteristic frequencies of the electrochemical impedance of ion-exchange membrane systems constituted by the membrane and two diffusion boundary layers adjacent to the membrane were investigated. Approximations of the impedance of the Randles equivalent electric circuit in multiple frequency ranges were considered, and the characteristic frequencies of the zeros and poles of orders ½ and 1 were derived. The characteristic geometric frequencies, those associated with the interfacial charge transfer and the diffusive transport processes, as well as those associated with the transitions between processes, were identified by means of analytical expressions.
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Jaroš, P., et M. Vertaľ. « Water vapor transmission parameters of the Kežmarok sandstone ». IOP Conference Series : Materials Science and Engineering 1252, no 1 (1 septembre 2022) : 012038. http://dx.doi.org/10.1088/1757-899x/1252/1/012038.
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Abstract Dynamic complex heat and water transport in building constructions can be calculated using modern simulation software such as Wufi, Delphin, Math, Comsol Multiphysics and other. This software is suitable for the evaluation of the thermal and hygric processes in constructions of historical buildings. Because it includes many important factors such as dynamic boundary conditions, solar radiation, driven rain, ground water, diffusion and others. The heat and water transport parameters of historical materials are needed for simulation of coupled heat and water transport in building materials. The Kežmarok sandstone was chosen for analysis. One of the important material parameters for evaluation of complex heat and water transport in building constructions are water vapor transmission parameters. Vapor transport can be described as water vapor diffusion resistance factor, water vapor diffusion coefficient and diffusion equivalent air layer thickness. The dry cup and wet cup method were used to determine the water vapor transmission parameters of Kežmarok sandstone.
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Huo, Guanping, et Xueyan Guo. « Numerical Analyses of Heterogeneous CLC Reaction and Transport Processes in Large Oxygen Carrier Particles ». Processes 9, no 1 (8 janvier 2021) : 125. http://dx.doi.org/10.3390/pr9010125.
Texte intégralRésumé :
Heterogeneous chemical looping combustion (CLC) reactions and conjugate transports in large oxygen carrier particles were numerically investigated with computational fluid dynamics (CFD) approaches, in which a simplified noncatalytic reaction model was implemented for reducing intraparticle modelling computation. Volumic gas-solid reactions were treated as surface reactions based on the equivalent internal surface in the particle model. In large porous particles such as fixed bed CLC reactors, the heterogeneous reactions are often limited by intraparticle diffusion. Comprehensive analyses were conducted on transports across the particle surface and their influences on reactions inside the single particles. A threshold Reynolds number of external convections was found for the enhancement of intraparticle reactions. The heterogeneous reactions, intraparticle diffusions and interstitial transports in a fixed bed CLC reactor randomly packed with 597 spheres were thoroughly analysed with the same numerical approaches. Comprehensive insights of the temporal evolution and spatial distribution of scalars in the packed bed reactor were presented.
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Huo, Guanping, et Xueyan Guo. « Numerical Analyses of Heterogeneous CLC Reaction and Transport Processes in Large Oxygen Carrier Particles ». Processes 9, no 1 (8 janvier 2021) : 125. http://dx.doi.org/10.3390/pr9010125.
Texte intégralRésumé :
Heterogeneous chemical looping combustion (CLC) reactions and conjugate transports in large oxygen carrier particles were numerically investigated with computational fluid dynamics (CFD) approaches, in which a simplified noncatalytic reaction model was implemented for reducing intraparticle modelling computation. Volumic gas-solid reactions were treated as surface reactions based on the equivalent internal surface in the particle model. In large porous particles such as fixed bed CLC reactors, the heterogeneous reactions are often limited by intraparticle diffusion. Comprehensive analyses were conducted on transports across the particle surface and their influences on reactions inside the single particles. A threshold Reynolds number of external convections was found for the enhancement of intraparticle reactions. The heterogeneous reactions, intraparticle diffusions and interstitial transports in a fixed bed CLC reactor randomly packed with 597 spheres were thoroughly analysed with the same numerical approaches. Comprehensive insights of the temporal evolution and spatial distribution of scalars in the packed bed reactor were presented.
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Chebbo, Ghassan, Nicolas Forgues, Emmanuelle Lucas-Aiguier et Stéphane Berthebaud. « A stochastic approach to modelling solid transport in settling tanks ». Water Science and Technology 37, no 1 (1 janvier 1998) : 277–84. http://dx.doi.org/10.2166/wst.1998.0066.
Texte intégralRésumé :
Dispersion and settling models for suspended solids (SS) in turbulent flow are in the majority of cases Eulerian and deterministic. They simulate SS transport using a convection-diffusion equation. When the diffusion term is supposed not to be zero at the boundary of a settling tank, we show that an alternative (or equivalent) description of the basic convection-diffusion equation for the concentration of suspended solids consists in modelling the particles trajectories as a stochastic diffusion process. This paper has two aims, firstly to present the proposed stochastic approach and secondly to apply this new method to the calculation of a settling tank's efficiency.
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Ramanan, B., W. M. Holmes, W. T. Sloan et V. R. Phoenix. « Application of Paramagnetically Tagged Molecules for Magnetic Resonance Imaging of Biofilm Mass Transport Processes ». Applied and Environmental Microbiology 76, no 12 (30 avril 2010) : 4027–36. http://dx.doi.org/10.1128/aem.03016-09.
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ABSTRACT Molecules become readily visible by magnetic resonance imaging (MRI) when labeled with a paramagnetic tag. Consequently, MRI can be used to image their transport through porous media. In this study, we demonstrated that this method could be applied to image mass transport processes in biofilms. The transport of a complex of gadolinium and diethylenetriamine pentaacetic acid (Gd-DTPA), a commercially available paramagnetic molecule, was imaged both in agar (as a homogeneous test system) and in a phototrophic biofilm. The images collected were T 1 weighted, where T 1 is an MRI property of the biofilm and is dependent on Gd-DTPA concentration. A calibration protocol was applied to convert T 1 parameter maps into concentration maps, thus revealing the spatially resolved concentrations of this tracer at different time intervals. Comparing the data obtained from the agar experiment with data from a one-dimensional diffusion model revealed that transport of Gd-DTPA in agar was purely via diffusion, with a diffusion coefficient of 7.2 × 10−10 m2 s−1. In contrast, comparison of data from the phototrophic biofilm experiment with data from a two-dimensional diffusion model revealed that transport of Gd-DTPA inside the biofilm was by both diffusion and advection, equivalent to a diffusion coefficient of 1.04 × 10−9 m2 s−1. This technology can be used to further explore mass transport processes in biofilms, either by using the wide range of commercially available paramagnetically tagged molecules and nanoparticles or by using bespoke tagged molecules.
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Conhaim, Robert L., Kal E. Watson, Stephen J. Lai-Fook et Bruce A. Harms. « Transport properties of alveolar epithelium measured by molecular hetastarch absorption in isolated rat lungs ». Journal of Applied Physiology 91, no 4 (1 octobre 2001) : 1730–40. http://dx.doi.org/10.1152/jappl.2001.91.4.1730.
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To evaluate the transport properties of the alveolar epithelium, we instilled hetastarch (Het; 6%, 10 ml, 1 − 1 × 104kDa) into the trachea of isolated rat lungs and then measured the molecular distribution of Het that entered the lung perfusate from the air space over 6 h. Het transport was driven by either diffusion or an oncotic gradient. Perfusate Het had a unique, bimodal molecular weight distribution, consisting of a narrow low-molecular-weight peak at 10–15 kDa (range, 5–46 kDa) and a broad high-molecular-weight band (range 46–2,000 kDa; highest at 288 kDa). We modeled the low-molecular-weight transport as (passive) restricted diffusion or osmotic flow through a small-pore system and the high-molecular-weight transport as passive transport through a large-pore system. The equivalent small-pore radius was 5.0 nm, with a distribution of 150 pores per alveolus. The equivalent large-pore radius was 17.0 nm, with a distribution of one pore per seven alveoli. The small-pore fluid conductivity (2 × 10−5ml · h−1 · cm−2 · mmHg−1) was 10-fold larger than that of the large-pore conductivity.
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Tárnoki-Zách, Júlia, Elod Mehes, Zsófia Varga-Medveczky, Dona Greta Isai, Nandor Barany, Edina Bugyik, Zsolt Revesz, Sándor Paku, Franciska Erdo et Andras Czirok. « Development and Evaluation of a Human Skin Equivalent in a Semiautomatic Microfluidic Diffusion Chamber ». Pharmaceutics 13, no 6 (20 juin 2021) : 910. http://dx.doi.org/10.3390/pharmaceutics13060910.
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There is an increasing demand for transdermal transport measurements to optimize topical drug formulations and to achieve proper penetration profile of cosmetic ingredients. Reflecting ethical concerns the use of both human and animal tissues is becoming more restricted. Therefore, the focus of dermal research is shifting towards in vitro assays. In the current proof-of-concept study a three-layer skin equivalent using human HaCaT keratinocytes, an electrospun polycaprolactone mesh and a collagen-I gel was compared to human excised skin samples. We measured the permeability of the samples for 2% caffeine cream using a miniaturized dynamic diffusion cell (“skin-on-a-chip” microfluidic device). Caffeine delivery exhibits similar transport kinetics through the artificial skin and the human tissue: after a rapid rise, a long-lasting high concentration steady state develops. This is markedly distinct from the kinetics measured when using cell-free constructs, where a shorter release was observable. These results imply that both the established skin equivalent and the microfluidic diffusion chamber can serve as a suitable base for further development of more complex tissue substitutes.
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Orre, Steinar, Yongqi Gao, Helge Drange et Eric Deleersnijder. « Diagnosing ocean tracer transport from Sellafield and Dounreay by equivalent diffusion and age ». Advances in Atmospheric Sciences 25, no 5 (septembre 2008) : 805–14. http://dx.doi.org/10.1007/s00376-008-0805-y.
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Luo, Gang, Yan Ji, Chao-Yang Wang et Puneet K. Sinha. « Modeling liquid water transport in gas diffusion layers by topologically equivalent pore network ». Electrochimica Acta 55, no 19 (juillet 2010) : 5332–41. http://dx.doi.org/10.1016/j.electacta.2010.04.078.
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GOSS, GREG G., et CHRIS M. WOOD. « Na+ and Cl− Uptake Kinetics, Diffusive Effluxes and Acidic Equivalent Fluxes Across the Gills of Rainbow Trout : I. Responses to Environmental Hyperoxia ». Journal of Experimental Biology 152, no 1 (1 septembre 1990) : 521–47. http://dx.doi.org/10.1242/jeb.152.1.521.
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Endogenous respiratory acidosis and metabolic alkalosis were induced in bladder-catheterized freshwater rainbow trout by exposure to environmental hyperoxia (72 h) and its subsequent removal. Unidirectional and net fluxes of Na+, Cl− and acidic equivalents across the gills were examined over 0.5 h intervals. Hyperoxia resulted in a positive Na+ balance, negative Cl− balance and net acidic equivalent excretion. Return to normoxia caused a negative Na+ balance, a positive Cl− balance and net basic equivalent excretion (=acidic equivalent uptake). Cl−/basic equivalent exchange was more important than Na+/acidic equivalent exchange in the homeostatic responses, and alkalosis was a more potent stimulus than acidosis for change in branchial ion fluxes. Kinetic analysis demonstrated that alterations in ion fluxes were achieved by complex changes in both the Km (inverse of affinity) and the Jmax (maximal transport rate) of the branchial Cl−/HCO3−(OH−) and Na+/H+(NH4+) transporters. KmCl (control=165 μequiv l−1) and KmNa (114 μequiv l−1) were increased during hyperoxic acidosis to 250 and 445 μequiv l−1, respectively. JmaxCl, (291 μequiv kg−1 h−1) and JmaxNa (456 μequiv kg−1 h−1) did not change significantly. During post-hyperoxic alkalosis, KmNa was further increased to 559 μequiv l−1, JmaxCl increased to 445 μequiv kg−1 h−1, while KmCl and JmaxNa decreased to 137 μequiv l−1 and 309 μequiv kg−1 h−1, respectively. Diffusive efflux was examined using a novel method. There was no significant differential diffusive efflux of Na+ and Cl− during hyperoxia but diffusive Na+ efflux exceeded Cl− efflux during posthyperoxic alkalosis, thereby serving as an additional mechanism for basic equivalent excretion.
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Huang, Xiang, Wei Zhou et Daxiang Deng. « Effective Diffusion in Fibrous Porous Media : A Comparison Study between Lattice Boltzmann and Pore Network Modeling Methods ». Materials 14, no 4 (5 février 2021) : 756. http://dx.doi.org/10.3390/ma14040756.
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The understanding of the correlation between a pore-scale structure and its coupled diffusion transport property is crucial in the virtual design and performance optimization of porous fibrous material for various energy applications. Two most common and widely employed pore-scale modeling techniques are the lattice Boltzmann method (LBM) and the pore network modeling (PNM). However, little attention has been paid to the direct comparison between these two methods. To this end, stochastic porous fibrous structures are reconstructed reflecting the structural properties of the fibrous porous material on a statistical level with structural properties obtained from X-ray computed microtomography. Diffusion simulation through the porous phase was subsequently conducted using LBM of D3Q7 lattice and topological equivalent PNM derived from the watershed method, respectively. It is detected that the effective diffusion coefficients between these two methods are in good agreement when the throat radius in the pore network is estimated using the cross-section area equivalent radius. Like most literature, the diffusivity in the in-plane (IP) direction is larger than in the through-plane (TP) direction due to the laid fiber arrangement, but the values are very close. Besides, tortuosity was evaluated from both geometry and transport measurements. Tortuosity values deduced from both methods are in line with the anisotropy of the diffusion coefficients.
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Ramírez Sabag, Jetzabeth, et Dennys Armando López Falcón. « How to use solutions of Advection-Dispersion Equation to describe reactive solute transport through porous media ». Geofísica Internacional 60, no 3 (1 juillet 2021) : 229–40. http://dx.doi.org/10.22201/igeof.00167169p.2021.60.3.2024.
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ResumenLas soluciones de la Ecuación de Advección-Dispersión son usadas frecuentemente para describir el transporte de solutos a través de medios porosos, considerando adsorción en equilibrio, de tipo lineal y reversible. Para indicar algunas sugerencias acerca de este tema, se hizo una revisión de las soluciones analíticas disponibles. Hay soluciones para Problemas con Condiciones de Frontera, de primer y tercer-tipo en la entrada así como de primer y segundo-tipo a la salida. Se analiza el comportamiento de las soluciones equivalentes, para sistemas finitos y semi-infinitos, observando que las soluciones de los sistemas semi-infinitos se aproximan a las correspondientes de los sistemas finitos conforme la condición de frontera de salida en el infinito se aproxima a la ubicación de medición del sistema finito. Solamente se presentan las soluciones analíticas con condiciones de frontera de segundo-tipo a la salida, ya que son iguales a las correspondientes soluciones analíticas con frontera de primer-tipo a la salida, para ambos tipos de condiciones de frontera de entrada usadas. Un análisis paramétrico, basado en el número de Peclet, muestra que todas las soluciones convergen cuando el número de Peclet es mayor que veinte. Los sistemas investigados deben tener un número de Peclet mayor que cinco para usar con confianza las soluciones de la Ecuación de Advección-Dispersión para describir el transporte de soluto en medios porosos.Palabras Clave: Ecuación de Advección-Difusión, Soluciones Analíticas, Transporte de Solutos Reactivos, Medios Porosos.AbstractThe solutions of Advection-Dispersion Equation are frequently used to describe solute transport through porous media when considering lineal and reversible equilibrium adsorption. To notice some warnings about this item, a review of analytical solutions available was done. There are solutions for Boundary Value Problems with first and third-type inlet boundary conditions as well as first and second-type outlet boundary condition. The behavior of equivalent solutions for finite and semi-infinite systems are analyzed, observing that semi-infinite system solutions approximates to the corresponding finite ones as the “infinite” outlet boundary condition approach to the finite measurement location. Because the analytical solutions with a first-type outlet boundary condition are equal to the corresponding analytical solutions with a second-type one, for both inlet boundary condition type used, only the latter is presented. A parametric analysis based on Peclet number shows that all solutions converge for Peclet number greater than twenty. Systems under research must have Peclet number greater than five to use confidently the solutions of Advection-Dispersion Equation to describe reactive solute transport through porous media.Keywords: Advection-Diffusion Equation, Analytical solutions, Reactive Solute Transport, Porous Media.
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Sieniutycz, Stanisław. « Thermodynamic Structure of Nonlinear Macrokinetics in Reaction-Diffusion Systems ». Open Systems & ; Information Dynamics 11, no 02 (juin 2004) : 185–202. http://dx.doi.org/10.1023/b:opsy.0000034196.56929.71.
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Affinity picture — new for transport phenomena — and the traditional Onsagerian picture are shown to constitute two equivalent representations for kinetics of chemical reactions and transfer processes. Two competing directions in elementary chemical or transport steps are analyzed. Nonequilibrium systems are described by equations of nonlinear kinetics of Marcelin-Kohnstamm-de Donder type that contain terms exponential with respect to the Planck potentials and temperature reciprocal. Simultaneously these equations are analytical expressions characterizing the transport of the substance or energy through the energy barrier. We regard kinetics of this sort as potential representations of a generalized law of mass action that includes the effect of transfer phenomena and external fields. We also consider physical consequences of these kinetics closely and far from equilibrium, and show how diverse processes can be described. In these developments we point out the significance of nonlinear symmetries and generalized affinity. Correspondence with the Onsager's theory is shown in the vicinity of thermodynamic equilibrium. Yet, the theory shows that far from equilibrium the rates of transport processes and chemical reactions cannot be determined uniquely in terms of their affinities because these rates depend on all state coordinates of the system.
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Macey, R. I., et L. W. Yousef. « Osmotic stability of red cells in renal circulation requires rapid urea transport ». American Journal of Physiology-Cell Physiology 254, no 5 (1 mai 1988) : C669—C674. http://dx.doi.org/10.1152/ajpcell.1988.254.5.c669.
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Urea transport by the human erythrocyte occurs via an asymmetric-facilitated diffusion system with high Michaelis constants and high maximal velocities; the equivalent permeability in the limit of zero urea concentration is approximately 10(-3) cm/s (J. Gen. Physiol. 81: 221-237, 239-253, 1983). A physiological role for this system is revealed by numerical integration of the appropriate equations that show that rapid urea transport is essential for red cell stability in passing through the renal medulla. The calculation compares two cells. Cell A transports urea with permeability characteristics of normal red cells; cell B has urea permeability similar to lipid bilayers. On entering the hypertonic medulla, both cells shrink, but only B swells on leaving the medulla. The osmotic stress for cell B is greater than for A. Cell B is close to hypertonic hemolysis in the medulla and to hypotonic hemolysis in the cortex. Cell B remains swollen for some time after its exit; the resulting decreased deformability presents a hazard if B reenters the microcirculation. Furthermore, cell B removes a significant fraction of the filtered load of urea and compromises the osmotic gradients in the medulla.
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Pediz, Gökhan, Ayhan Yılmazer et Mehmet Tombakoğlu. « Transport equivalent full core neutronics calculations via finite volume method and optimized diffusion parameters ». Progress in Nuclear Energy 165 (novembre 2023) : 104935. http://dx.doi.org/10.1016/j.pnucene.2023.104935.
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Wu, Lipeng, Peng Dai et Yong Li. « Determination of the Transport Properties of Structural Concrete Using AC Impedance Spectroscopy Techniques ». Journal of Engineering 2016 (2016) : 1–8. http://dx.doi.org/10.1155/2016/2630186.
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All over the world, particularly in severe environmental conditions, there are reinforced concrete structures that develop nonnegligible phenomena of durability problems. Most of the durability problems are related to hazardous substances invasion. Both engineering practice and scientific studies have revealed that the transport property of near-surface concrete is a main factor in the durability of concrete structures. Among many transport parameters, the chloride ion diffusion coefficient is the most important one, which provides important information on material design and service life prediction. In this paper, AC impedance spectroscopy technology was employed in the measurement of chloride ion diffusion coefficient. The relationship between mesostructure parameters and chloride ion diffusion coefficient was deduced by introducing a reasonable equivalent circuit model. Taking into account the conductivity difference caused by various cementitious material systems, the diffusion coefficient can be corrected, and a diffusion coefficient determination method based on AC impedance spectroscopy technique was established. For the convenience of application, a relationship between the newly proposed method and a widely recognized standard method was obtained. The proposed method can be applied to laboratory testing and establishes the theoretical basis for field tests.
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Dehghan, Mehdi. « On the numerical solution of the one-dimensional convection-diffusion equation ». Mathematical Problems in Engineering 2005, no 1 (2005) : 61–74. http://dx.doi.org/10.1155/mpe.2005.61.
Texte intégralRésumé :
The numerical solution of convection-diffusion transport problems arises in many important applications in science and engineering. These problems occur in many applications such as in the transport of air and ground water pollutants, oil reservoir flow, in the modeling of semiconductors, and so forth. This paper describes several finite difference schemes for solving the one-dimensional convection-diffusion equation with constant coefficients. In this research the use of modified equivalent partial differential equation (MEPDE) as a means of estimating the order of accuracy of a given finite difference technique is emphasized. This approach can unify the deduction of arbitrary techniques for the numerical solution of convection-diffusion equation. It is also used to develop new methods of high accuracy. This approach allows simple comparison of the errors associated with the partial differential equation. Various difference approximations are derived for the one-dimensional constant coefficient convection-diffusion equation. The results of a numerical experiment are provided, to verify the efficiency of the designed new algorithms. The paper ends with a concluding remark.
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Halaihel, Nabil, Daniele Gerbaud, Monique Vasseur et Francisco Alvarado. « Heterogeneity of pig intestinald-glucose transport systems ». American Journal of Physiology-Cell Physiology 277, no 6 (1 décembre 1999) : C1130—C1141. http://dx.doi.org/10.1152/ajpcell.1999.277.6.c1130.
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Heterogeneity of intestinald-glucose transport is demonstrated using pig jejunal brush-border membrane vesicles in the presence of 100/0 (out/in) mM gradients each of NaCl, NaSCN, and KSCN. Two d-glucose transport systems are kinetically distinguished: high-affinity, low-capacity system 1, which is equivalent to the symporter SGLT1; and low-affinity, high-capacity system 2, which is not a member of the SGLT family but is a d-glucose and d-mannose transporter exhibiting no preference for Na+over K+. A nonsaturabled-glucose uptake component has also been detected; uptake of this component takes place at rates 10 times the rate of components characterizing the classical diffusion marker l-glucose. It is also shown that, in this kinetic work, 1) use of d-glucose-contaminatedd-sorbitol as an osmotic replacement cannot cause the spurious appearance of nonexistent transport systems and 2) a large range (≥50 mM) of substrate concentrations is required to correctly fit substrate saturation curves to distinguish between low-affinity transport systems and physical diffusion.
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Landel, Julien R., A. L. Thomas, H. McEvoy et Stuart B. Dalziel. « Convective mass transfer from a submerged drop in a thin falling film ». Journal of Fluid Mechanics 789 (25 janvier 2016) : 630–68. http://dx.doi.org/10.1017/jfm.2015.742.
Texte intégralRésumé :
We investigate the fluid mechanics of removing a passive tracer contained in small, thin, viscous drops attached to a flat inclined substrate using thin gravity-driven film flows. We focus on the case where the drop cannot be detached either partially or completely from the surface by the mechanical forces exerted by the cleaning fluid on the drop surface. Instead, a convective mass transfer establishes across the drop–film interface and the dilute passive tracer dispersed in the drop diffuses into the film flow, which then transports them away. The Péclet number for the passive tracer in the film phase is very high, whereas the Péclet number in the drop phase varies from $\mathit{Pe}_{d}\approx 10^{-2}$ to $1$. The characteristic transport time in the drop is much larger than in the film. We model the mass transfer of the passive tracer from the bulk of the drop phase into the film phase using an empirical model based on an analogy with Newton’s law of cooling. This simple empirical model is supported by a theoretical model solving the quasi-steady two-dimensional advection–diffusion equation in the film, coupled with a time-dependent one-dimensional diffusion equation in the drop. We find excellent agreement between our experimental data and the two models, which predict an exponential decrease in time of the tracer concentration in the drop. The results are valid for all drop and film Péclet numbers studied. The overall transport characteristic time is related to the drop diffusion time scale, as diffusion within the drop is the limiting process. This result remains valid even for $\mathit{Pe}_{d}\approx 1$. Finally, our theoretical model predicts the well-known relationship between the Sherwood number and the Reynolds number in the case of a well-mixed drop $\mathit{Sh}\propto \mathit{Re}_{L}^{1/3}=({\it\gamma}L^{2}/{\it\nu}_{f})^{1/3}$, based on the drop length $L$, film shear rate ${\it\gamma}$ and film kinematic viscosity ${\it\nu}_{f}$. We show that this relationship is mathematically equivalent to a more physically intuitive relationship $\mathit{Sh}\propto \mathit{Re}_{{\it\delta}}$, based on the diffusive boundary-layer thickness ${\it\delta}$. The model also predicts a correction in the case of a non-uniform drop concentration. The correction depends on $Re_{{\it\delta}}$, the film Schmidt number, the drop aspect ratio and the diffusivity ratio between the two phases. This prediction is in remarkable agreement with experimental data at low drop Péclet number. It continues to agree as $\mathit{Pe}_{d}$ approaches $1$, although the influence of the Reynolds number increases such that $\mathit{Sh}\propto \mathit{Re}_{{\it\delta}}$.
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ZHENG, QIAN, JINTU FAN, XIANGPENG LI et SHIFANG WANG. « FRACTAL MODEL OF GAS DIFFUSION IN FRACTURED POROUS MEDIA ». Fractals 26, no 03 (juin 2018) : 1850035. http://dx.doi.org/10.1142/s0218348x18500354.
Texte intégralRésumé :
Understanding gas transport behavior though fractured porous media is essential in many fields including fiber science, energy science, soil science, environmental engineering, chemical engineering, etc. In this paper, a fractal model is developed to characterize gas diffusion through fractured porous media, where a bundle of fractal-like tree branching networks is used to represent the fracture system according to fractal scaling laws. The analytical expression for relative gas diffusion coefficient of fractured porous media is derived. The proposed fractal model has been validated by the available experimental data and empirical correlations. From the parametrical study, it can be seen that structural parameters of fractured porous media (for example porosity, the fractal dimension, the diameter ratio, the length ratio and the branching angle) have a significant effect on equivalent gas transport properties. Gas relative diffusion coefficient has a positive correlation with the porosity, the pore size fractal dimension, or the diameter ratio, whereas it has a negative correlation with the length ratio, the branching levels, or the branching angle. The proposed fractal model does not only shed light on gas transport physics of fractured porous media, but also reveals more mechanisms than experimental measurements.
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Berg, M. M., K. J. Kim, R. L. Lubman et E. D. Crandall. « Hydrophilic solute transport across rat alveolar epithelium ». Journal of Applied Physiology 66, no 5 (1 mai 1989) : 2320–27. http://dx.doi.org/10.1152/jappl.1989.66.5.2320.
Texte intégralRésumé :
Diffusional fluxes of a series of hydrophilic nonelectrolytes (molecular radii ranging from 0.15 to 0.57 nm) were measured across the alveolocapillary barrier in the isolated perfused fluid-filled rat lung. Radiolabeled solutes were lavaged into the distal air spaces of isolated Ringer-perfused lungs, and apparent permeability-surface area products were calculated from the rates of isotope appearance in the recirculating perfusate. These data were used to estimate theoretical equivalent pore radii in the alveolar epithelium, with the assumption of diffusive flow through water-filled cylindrical pores. The alveolar epithelium is best characterized by two pore populations, with small pores (radius 0.5 nm) occupying 98.7% of total pore area and larger pores (radius 3.4 nm) occupying 1.3% of total pore area. Net water flow out of the alveolar space was measured by including an impermeant solute (dextran) in the lavage fluid and measuring its concentration in the alveolar space as a function of time. Under control conditions, net water flow averaged 167 nl/s. When 24 microM terbutaline was added to the perfusate, net water flow increased significantly to 350 nl/s (P less than 0.001). Terbutaline had no effect on the fluxes of either glycerol (which traverses the small pore pathway) or sucrose (which traverses the large pore pathway). These findings indicate that the intact mammalian alveolar epithelium is complex and highly resistant to the flow of solutes and water.