Effects Of Diffusive Transport Research Articles

Generalised radiating fields in Einstein\u2013Gauss\u2013Bonnet gravity

A five-dimensional spherically symmetric generalised radiating field is studied in Einstein–Gauss–Bonnet gravity. We assume the matter distribution is an extended Vaidya-like source and the resulting Einstein–Gauss–Bonnet field equations are solved for the matter variables and mass function. The evolution of the mass, energy density and pressure are then studied within the spacetime manifold. The effects of the higher order curvature corrections of Einstein–Gauss–Bonnet gravity are prevalent in the analysis of the mass function when compared to general relativity. The effects of diffusive transport are then considered and we derive the specific equation for which diffusive behaviour is possible. Gravitational collapse is then considered and we show that collapse ends with a weak and conical singularity for the generalised source, which is not the case in Einstein gravity.

Spatiotemporal Decoupling of Water Electrolysis for Dual-Use Grid Energy Storage and Hydrogen Generation

Summary The implementation of electrolysis systems for electrochemical hydrogen production has continued to grow as the paradigm shift toward renewable energy and fuels progresses. However, issues regarding conventional polymer electrolyte membrane (PEM) electrolyzers remain; their performance can be affected when operated with intermittent energy sources due to gas crossover, while the high cost of electricity continues to hinder large-scale adoption of the technology. To make electrochemical hydrogen production more competitive, renewable energy sources need to be used with new strategies for electrochemical hydrogen production. Here, we show a cerium-mediated decoupled electrolysis system that produces hydrogen and stores energy in the redox couples. We present electrochemical studies to observe the effects of diffusive transport, convective transport, and thermal effects. Following this, a technoeconomic analysis is done, focusing on the optimization of the system operation and the identification of target operation parameters to achieve hydrogen production at a competitive price.

A fast regression model for the interpretation of electrochemical impedance spectra of Intermediate Temperature Solid Oxide Fuel Cells

A frequency domain transformation method is applied to a physically consistent model of an IT-SOFC with MIEC electrolyte. Fast computation of the impedance spectra is achieved, which allows to apply the model to kinetic studies using traditional regressive techniques. The model includes conservation equations of mass and charge, gas diffusive transport effects and leakage current effects due to the semiconductor electrolyte. Global rate equations for the electrocatalytic semi-reactions guarantee economy of calibrated parameters. Two sets of impedance experiments collected on electrolyte-supported Cu-Pd-CZ80-SDC/SDC/LSCF-GDC cells are analyzed. The description of the H2 oxidation process is improved based on experiments with H2/N2 mixtures at fixed temperature and increasing H2 molar fraction (700 °C, 30–100% H2). The introduction of constant phase elements proves essential to closely predict Nyquist and Bode plots: strong inhomogeneity of the anode structure emerges, while the cathodic determining process is better identified via refinement of the double layer capacitance. Novel impedance experiments, performed with a CO-rich mixture (97% CO, 3% CO2) between 600 °C and 700 °C, allow to estimate the kinetic constant for CO electro-oxidation. The variation of the CPE parameters suggests that carbon deposition during exposure to CO simultaneously reduces the anodic active area and double layer capacitance.

Incorporating electrical double layers into reactive-transport simulations of processes in clays by using the Nernst-Planck equation: A benchmark revisited

Owing to their low permeability clay formations are considered as potential host rocks for nuclear waste or as seals capping permeable reservoirs for storing unconventional gases. Clay materials such as bentonite are considered as backfill or buffer material in nuclear waste repositories forming barriers to fluid flow owing to their hydraulic and swelling properties. The low permeability of clays implies that solute transport in the pore water is dominated by diffusion. Another important characteristic of clays is the negative surface charge of clay minerals which affects transport and the distribution of ions in the pore space: cations are attracted to while anions are repelled from clay mineral surfaces. Models of reactive transport need to consider these electrostatic effects to be able to accurately simulate the transport of ions through clay. Here we use a new approach which is entirely based on the solution of the Nernst-Planck equation to incorporate the effect of diffuse layers (DLs) into reactive transport simulations. A simulation benchmark is used to validate this new approach. In variants of this benchmark problem the impact of different activity models on the pore water composition, Donnan equilibrium versus a kinetic exchange between the DL and free pore water and the effect of diffusive transport in the DL are explored.

Diffusive and dynamical radiating stars with realistic equations of state

We model the dynamics of a spherically symmetric radiating dynamical star with three spacetime regions. The local internal atmosphere is a two-component system consisting of standard pressure-free, null radiation and an additional string fluid with energy density and nonzero pressure obeying all physically realistic energy conditions. The middle region is purely radiative which matches to a third region which is the Schwarzschild exterior. A large family of solutions to the field equations are presented for various realistic equations of state. We demonstrate that it is possible to obtain solutions via a direct integration of the second order equations resulting from the assumption of an equation of state. A comparison of our solutions with earlier well known results is undertaken and we show that all these solutions, including those of Husain, are contained in our family. We then generalise our class of solutions to higher dimensions. Finally we consider the effects of diffusive transport and transparently derive the specific equations of state for which this diffusive behaviour is possible.

Mathematical modelling of the Phloem: the importance of diffusion on sugar transport at osmotic equilibrium.

Plants rely on the conducting vessels of the phloem to transport the products of photosynthesis from the leaves to the roots, or to any other organs, for growth, metabolism, and storage. Transport within the phloem is due to an osmotically-generated pressure gradient and is hence inherently nonlinear. Since convection dominates over diffusion in the main bulk flow, the effects of diffusive transport have generally been neglected by previous authors. However, diffusion is important due to boundary layers that form at the ends of the phloem, and at the leaf-stem and stem-root boundaries. We present a mathematical model of transport which includes the effects of diffusion. We solve the system analytically in the limit of high Münch number which corresponds to osmotic equilibrium and numerically for all parameter values. We find that the bulk solution is dependent on the diffusion-dominated boundary layers. Hence, even for large Péclet number, it is not always correct to neglect diffusion. We consider the cases of passive and active sugar loading and unloading. We show that for active unloading, the solutions diverge with increasing Péclet. For passive unloading, the convergence of the solutions is dependent on the magnitude of loading. Diffusion also permits the modelling of an axial efflux of sugar in the root zone which may be important for the growing root tip and for promoting symbiotic biological interactions in the soil. Therefore, diffusion is an essential mechanism for transport in the phloem and must be included to accurately predict flow.

The Impact of Noncontinuum Thermal Transport on the Temperature of AlGaN/GaN HFETs

The effects of power density and heat generation zone size on the hotspot temperature of AlGaN/GaN HFET devices were predicted using an electrothermal modeling approach. The thermal response was modeled using a multiscale model that accounted for ballistic-diffusive phonon transport effects in the heat generation zone near the gate and diffusive transport effects outside of this zone. The Joule heating distribution was calculated using a hydrodynamic model in Sentaurus Device. The hotspot temperatures at different biasing conditions were determined using the multiscale thermal model and compared with a fully diffusive transport model. The results show that the hotspot temperature is higher when ballistic-diffusive transport effects are considered and this difference increases with increasing power density in the AlGaN/GaN HFETs.

CFD Simulation of Contaminant Decay for High Reynolds Flow in a Controlled Environment

This study examines the usage of computational fluid dynamics (CFDs) for estimating the time-elapsed decay of contaminants within a chamber experiencing high Reynolds flow. CFD results were compared with measurements taken at a controlled facility. In addition, parameters of the CFD simulation were examined; namely the effects of turbulence and inertial transport at high Reynolds number ventilating flows, as well as inlet duct configuration and its effect on the inlet velocity profile. The agreement between the computational and experimental clearance times was quite good, with percent errors as low as -5.32% at high flow rate and -11.8% at the lower flow rate. This study determined that for high Reynolds flow, diffusive transport effects may be ignored as the majority of mass is transported via the bulk stream, i.e. momentum transport. In addition, resolving the inlet velocity profile was of prime importance for accurate simulation of ventilating flows and prediction of contaminant washout. This was done by including the inlet duct geometry in the computational domain. In addition, it was found that despite different flow rates, the predicted contaminant washout took approximately 12-13% longer than predicted assuming instantaneous mixing. Furthermore, percent error between computational and experimental data as low as -5.32% shows that CFD is a useful tool for studying ventilation phenomena.

Extension of a Multiscale Particle Scheme to Near-Equilibrium Viscous Flows

A recently proposed low diffusion (LD) equilibrium particle method based on the direct simulation Monte Carlo (DSMC) method is modified for use with near-equilibrium viscous flows. A finite volume discretization of the viscous terms in the compressible Navier-Stokes equations is used to incorporate diffusive transport effects into the LD particle method, and a velocity and temperature slip wall boundary condition is employed for improved accuracy in the slip flow Knudsen number regime. The modified method is compared with both DSMC and theory for a series of unsteady boundary layer problems, and excellent agreement is observed. The computational cost of the modified LD particle method is shown for a representative near-equilibrium case to be roughly four orders of magnitude lower than that of DSMC, due to reduced scatter as well as less stringent cell size and time step requirements in the LD method. Simulation procedures are outlined for a strongly coupled hybrid algorithm, where the LD particle method is used in continuum flowfield regions and DSMC is employed in nonequilibrium regions. The hybrid scheme is evaluated through a comparison with numerical and experimental data for a flow of N2 through a small convergent-divergent nozzle into a near vacuum, and hybrid simulation results are generally found to agree very well with other available data. I. Introduction AS flows involving a wide range of characteristic length scales appear in a number of different engineering applications, including those related to atmospheric flow around reentry or hypersonic vehicles, high-altitude rocket plumes, flows within and around micro-electro-mechanical systems, and any supersonic flow where internal shock structures are of interest. In these types of flows, a near-equilibrium gas velocity distribution may exist through much of the flowfield, as the equilibrating effect of intermolecular collisions dominates over other processes (such as inhomogeneous diffusive transport or gas-surface interaction) which tend to pull the velocity distribution away from equilibrium. However, some flowfield regions may have characteristic length scales comparable to or smaller than the local mean free path, so that the influence of collisions does not dominate and the velocity distribution diverges considerably from the equilibrium limit. While simulation of near-equilibrium flowfield regions may be efficiently performed using computational fluid dynamics (CFD) techniques based on the Navier-Stokes equations, nonequilibrium regions must be simulated using more expensive techniques based on the Boltzmann equation. The Boltzmann equation is the governing equation for dilute gas flows at arbitrary Knudsen numbers, and its derivation follows from assumptions of molecular chaos and binary intermolecular collisions with no approximations regarding the shape of the velocity distribution. The most mature and commonly used simulation method for the Boltzmann equation is the direct simulation Monte Carlo (DSMC) method, first introduced by Bird. 1 In a DSMC simulation, a large number of representative particles are tracked through a computational grid, and move and collide in a manner consistent with physical arguments underlying the Boltzmann equation. While this method may be applied to both nonequilibrium and continuum flowfield regions, cell size and time step limitations often make it prohibitively expensive for simulating continuum flows where global characteristic length scales are far larger than the mean free path. In practice, such multiscale flows are usually simulated using both DSMC and CFD methods, by applying CFD techniques in near-equilibrium regions where the Navier-Stokes equations are valid, and using DSMC elsewhere in the flow. In the simplest type of hybrid CFD-DSMC approach, a CFD simulation is performed on a domain which 1

Physical controls on the isotopic composition of soil‐respired CO2

Measurement of the isotopic composition of soil and soil‐respired CO2 (δ13CO2) has become an invaluable tool in understanding ecosystem carbon‐cycling processes. While steady state work has been indispensable in understanding the effects of diffusive transport on soil CO2 isotopic composition, it is crucial that researchers studying temporally dependent processes, such as soil CO2 efflux, realize that these systems are rarely at steady state. Non‐steady‐state effects could result in misinterpretation of isotopic data, but have not been addressed in the literature, despite their fundamental importance to researchers who use isotopes in diffusive, non‐steady‐state environments. Here, we use an isotopologue‐based model to study dynamic fractionation, which we propose is a byproduct of transient changes in environmental variables. Time varying soil characteristics and processes such as biological production rate, soil pore space, diffusivity and atmospheric concentration were all found to induce non‐steady‐state gas transport conditions in the soil leading to transient changes in the isotopic composition of soil CO2 flux. The main driving force behind this transport related fractionation of CO2 is the rate of the change in 12CO2 gradient compared to that of 13CO2. These numerical simulations show that dynamic fractionation exists under non‐steady‐state diffusive conditions and suggest that isotopic data collected in non‐steady‐state, natural environments, cannot be properly interpreted without considering dynamic fractionation effects.

Anomaly in temperature dependence of thermal transport of two hydrogen-bonded glass-forming liquids

The thermal conductivity of two molecular glasses (ethanol and 1-propanol) decrease with increasing temperature up to their glass transitions at ${T}_{g}\ensuremath{\approx}97$ and $98\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively. Within their supercooled liquid phases, the conductivity increases with rising temperature up to a maximum which roughly coincides with the liquidus (or melting temperatures ${T}_{m}\ensuremath{\approx}159\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{m}\ensuremath{\approx}149\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively). From there on, the conductivity decreases with increasing temperature, a behavior common to most liquids examined so far, exception made of liquid water. The origin of the rather different dependencies with temperature of thermal transport is understood as a competition between phonon-assisted and diffusive transport effects which are amenable to experiments using high resolution quasielastic neutron scattering and visible and ultraviolet Brillouin light-scattering spectroscopies.

Signaling in morphogenesis: transport cues in morphogenesis

Extracellular transport processes play critical roles in morphogenesis. While diffusive transport effects on morphogenesis are well illustrated in examples like blood capillary architecture and in cell morphogenetic responses to the local extracellular protein environment, the effects of fluid convection, although important in many developing and regenerating tissues, are not well understood. Convective forces are present whenever a hydrated tissue undergoes dynamic mechanical strain, and so convection could not only dominate the transport of large molecules like proteins, but might also serve as a mechanism for mechanosensing. The complex interdependence of mechanical forces, protein transport and extracellular morphogen gradients needs to be elucidated in a comprehensive way in order for the importance of transport on morphogenesis to be fully appreciated.

Convective and diffusive transport effects in a high pressure induced inactivation process of packed food

The present contribution investigates the influence of heat and mass transport effects on the uniformity of a high-pressure induced inactivation. It is based on mathematical modelling and numerical simulation. The inactivation of E. coli suspended in packed UHT-milk carried out in a batch process with water as pressure medium is taken as a model process. The principal results show, that the effective inactivation rate increases with the geometrical scale of the high-pressure vessel. Significant non-uniformities of more than one log cycle in the residual surviving cell concentration can be observed depending on the package material parameters, on the position and on the arrangement of the packages in the vessel. Furthermore, the process cycle can be shortened since the effective inactivation rate is larger.

Numerical simulation of convective and diffusive transport effects on a high‐pressure‐induced inactivation process

High-pressure-induced conversions, such as the inactivation of enzymes or of microorganisms, are dependent on the applied pressure and the temperature of the process. The former can be considered to be a spatially homogeneous quantity, while the latter, being a transport quantity, varies over space and time. Here we question whether the uniformity of a high-pressure conversion can be disturbed by convective and conductive heat and mass transport conditions. Enzyme inactivation is taken as a model process for a high-pressure conversion. To cover a broad range of parameters and to consider scale-up effects, the investigation is based on mathematical modeling and numerical simulation for different sizes of the pressure chamber and different solvent viscosities. Apart from viscosity, the physical properties of the enzyme solutions are assumed to be identical in all cases. Therefore, matrix effects other than that of viscosity are excluded. Moreover, the authors postulate that viscosity solely acts on the continuum mechanical scale of momentum exchange but not on the molecular scale on the inactivation kinetics. It has been found that nonuniform thermal conditions can strongly influence the result of a high-pressure process. A variation of the activity retention between 28% and 48% can be observed after 20 minutes for a 0.8-L high-pressure chamber and a matrix fluid with a viscosity comparable to that of edible oils. The same process carried out in a 6.3-L device leads to an activity retention that varies between 16% and 40%. From the analysis of the time scales for the inactivation and for hydrodynamic and thermal compensation, it can be deduced that a nonuniform activity retention has to be expected if the inactivation time scale is larger than the hydrodynamic time scale and smaller than the thermal compensation time scale.

New Efficient Treatment of Impact Ionization in Submicron Metal-Oxide-Semiconductor Field-Effect Transistors

Shockley's lucky electron model has been widely used for modeling impact ionization. Various nonlocal generalizations of this model have been proposed in order to model impact ionization more accurately for strongly inhomogeneous material and field conditions which exist, e.g., in n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs). Recently it has been shown that the spatial distribution of impact ionization events resulting from the nonlocal variant is in close agreement with corresponding results derived from an advanced Monte Carlo model. However, the former lucky electron model fails to closely reflect the underlying microscopic processes and is based on the restrictive assumption that the electron trajectories are electrostatic field lines. In order to incorporate more microscopic features, an improved lucky electron modeling approach including ballistic and diffusive transport effects is proposed, which yields comparably favorable results if compared to Monte Carlo data.

Chemical energy release and dynamics of transitional, reactive shear flows

Results are reported from numerical studies of a compressible, subsonic reactive mixing layer, on the the effects of chemical-reaction exothermicity on the shear-layer development, and the dependence of these effects on initial conditions. The model solves the unsteady, conservation equations for mass, momentum, energy, and species concentrations. The convective transport equations are solved using the flux-corrected transport (FCT) algorithm and appropriate inflow and outflow boundary conditions. A one-step, irreversible, Arrhenius chemical reaction rate, and realistic (species- and temperature-dependent) modeling of diffusive transport are coupled with the convective transport using time-step splitting. The system studied consists of nonpremixed coflowing streams, where both the fuel (faster stream, hydrogen) and the oxidizer (slower stream, oxygen) are diluted in nitrogen. To facilitate the analysis of the results the flow is organized by low-level, single-frequency velocity perturbation at the inflow. The simulations show that energy release has the effect of reducing the shear layer growth and the amount of chemical product formed−relative to the corresponding cases for which exothermicity is not accounted for, in qualitative agreement with results from previous investigations. The relative mixing-layer growth reduction becomes more pronounced for larger energy release and lower Re, and is significant in terms of both, Reynolds stress, ρu′v′, and the velocity-fluctuation correlation u′v′. In spite of the relatively fast flows studied, for the regimes considered, the results on the initial mixing-layer growth are significantly sensitive to diffusive transport effects−more so in terms of Reynolds stress, than in terms of product formation. With larger energy release here associated to larger free-stream reactant molar fractions c0, the amount of chemical product in absolute terms is found to increase with energy release−but slower than c0, so that the product formation becomes effectively less efficient. The results of the present work highlight the difficulties involved in making general statements about the effects of exothermicity on the mixing-layer growth, indicating that a careful conceptualization of these properties in terms of initial conditions and other characteristic parameters of the reactive systems under study is required.