Simple Transport Model Research Articles

Reversible and Irreversible Adsorption Energetics of Poly(ethylene glycol) and Sorbitan Poly(ethoxylate) at a Water/Alkane Interface.

We simulate poly(ethylene glycol) (PEG) oligomers and model Tween 80 (polyoxyethylene sorbitan monooleate) molecules at water/alkane interfaces. Using the weighted histogram analysis method (WHAM), including an extension of WHAM to two reaction coordinates to remove hysteresis, we calculate interfacial potentials of mean force (PMFs) for PEG and Tween 80 using three force fields: the atomistic GROMOS 53a6OXY+D and two coarse-grained (CG) MARTINI force fields. Because the force fields have not yet been validated for PEO adsorption to hydrophobic interfaces, we calculate PMFs for alcohol ethoxylates C12E2 and C12E8 and find that they agree with semiempirical results of Mulqueen and Blankschtein [Langmuir 2002, 18 (2), 365-376] for the GROMOS 53a6OXY+D force field, whereas for both MARTINI force fields, PEO adsorbs too weakly to a clean hydrophobic interface. One MARTINI force field incorrectly shows depletion rather than adsorption to a clean hydrophobic interface. We find that the adsorption free energy for PEG oligomers at a clean, planar water/alkane interface is around 1.3 kBT per monomer for the atomistic force field but is less than half of this for the two CG force fields. With the newly validated GROMOS 53a6OXY+D force field, we bracket the dilute adsorption free energy for a model Tween 80 molecule at the clean water/squalane interface. We also calculate the pressure-area isotherm. We exploit these data with the Nikas-Mulqueen-Blankschtein (NMB) theory and a simple transport model to demonstrate a transition from irreversible to reversible adsorption with increasing surface coverage, consistent with experimental results of Reichert and Walker [Langmuir 2013, 29 (6), 1857-1867].

Applications of Bayesian temperature profile reconstruction to automated comparison with heat transport models and uncertainty quantification of current diffusion

In the context of present and future long pulse tokamak experiments yielding a growing size of measured data per pulse, automating data consistency analysis and comparisons of measurements with models is a critical matter. To address these issues, the present work describes an expert system that carries out in an integrated and fully automated way (i) a reconstruction of plasma profiles from the measurements, using Bayesian analysis (ii) a prediction of the reconstructed quantities, according to some models and (iii) a comparison of the first two steps. The first application shown is devoted to the development of an automated comparison method between the experimental plasma profiles reconstructed using Bayesian methods and time dependent solutions of the transport equations. The method was applied to model validation of a simple heat transport model with three radial shape options. It has been tested on a database of 21 Tore Supra and 14 JET shots. The second application aims at quantifying uncertainties due to the electron temperature profile in current diffusion simulations. A systematic reconstruction of the Ne, Te, Ti profiles was first carried out for all time slices of the pulse. The Bayesian 95% highest probability intervals on the Te profile reconstruction were then used for (i) data consistency check of the flux consumption and (ii) defining a confidence interval for the current profile simulation. The method has been applied to one Tore Supra pulse and one JET pulse.

A weakly coupled system of advection-reaction and diffusion equations in physiological gas transport

We deal with a simple model for oxygen transport in alveolar capillaries with exchange of oxygen between the capillaries and alveoli. This model is described by a weakly coupled three-component system of advection-reaction equations in capillaries and a linear diffusion equation in alveoli. We consider the equations in a bounded interval with appropriate boundary conditions. The goal of this article is to show that a steady state solution of the equations is asymptotically stable. To this end we first establish the existence of a unique solution for an initial-boundary value problem of the equations. Then we show the existence of a steady state solution. Finally we prove the main result on the asymptotic stability of the steady state with an exponential convergence rate. The proof can be done by using energy estimates for a large coupling constant.

Steady-state thermodynamics of non-interacting transport beyond weak coupling

We investigate the thermodynamics of simple (non-interacting) transport models beyond the scope of weak coupling. For a single fermionic or bosonic level —tunnel-coupled to two reservoirs—exact expressions for the stationary matter and energy current are derived from the solutions of the Heisenberg equations of motion. The positivity of the steady-state entropy production rate is demonstrated explicitly. Finally, for a configuration in which particles are pumped upwards in chemical potential by a downward temperature gradient, we demonstrate that the thermodynamic efficiency of this process decreases when the coupling strength between system and reservoirs is increased, as a direct consequence of the loss of a tight coupling between energy and matter currents.

Suspended single-walled carbon-nanotube field-effect transistor for gas sensing application

We investigate the pressure dependence of transfer characteristics of suspended single-walled carbon-nanotube field-effect transistors. We find that the gate bias around the charge neutral point with low drain current is appropriate for gas sensing application, while the high gate bias condition with high drain current that induces Joule heating in the suspended region for the desorption of the adsorbed molecules is preferable for the vacuum gauge application based on the heat exchange surrounding gas molecules, where the temperature at the suspended channel is investigated based on the simple one-dimensional heat transport model. We also revealed that the pressure dependence of the channel conductance at the gate bias around the charge neutral point can be explained by the Langmuir isotherm.

Theoretical analysis of hydrogen production by variable volume membrane batch reactors with direct liquid fuel injection

A direct liquid fuel injection/variable volume reactor integrated with a hydrogen selective membrane (CHAMP-DDIR) has been recently shown to be a promising new concept for hydrogen production in portable and distributed applications. The CHAMP-DDIR reactor performance has been analyzed using a simplified transport model with which conditions for maximum performance, e.g. highest volumetric power density, were identified. A prototype reactor demonstrated the ability to realize performance improvement, while also indicating a need for more a rigorous model for accurate exploration of the design and operation space. In this paper, we present a comprehensive reactor model which carefully considers the effects of heat and mass transfer, including rigorous treatment of multi-component species transport. The model is validated against experimental results through comparison of predicted and measured hydrogen production rate, reactor pressure, and temperature. The experimentally validated model is used to identify the relationship between CHAMP-DDIR design and operating parameters and the rate-limiting processes that govern reactor output. In addition, effects of heat and mass transfer limitations on CHAMP-DDIR performance are investigated by comparing the predictions among multiple cases with increasing level of complexity used for modeling the transport phenomena within the reactor.

Directed transport by surface chemical potential gradients for enhancing analyte collection in nanoscale sensors.

Nanoscale detectors hold great promise for single molecule detection and the analysis of small volumes of dilute samples. However, the probability of an analyte reaching the nanosensor in a dilute solution is extremely low due to the sensor's small size. Here, we examine the use of a chemical potential gradient along a surface to accelerate analyte capture by nanoscale sensors. Utilizing a simple model for transport induced by surface binding energy gradients, we study the effect of the gradient on the efficiency of collecting nanoparticles and single and double stranded DNA. The results indicate that chemical potential gradients along a surface can lead to an acceleration of analyte capture by several orders of magnitude compared to direct collection from the solution. The improvement in collection is limited to a relatively narrow window of gradient slopes, and its extent strongly depends on the size of the gradient patch. Our model allows the optimization of gradient layouts and sheds light on the fundamental characteristics of chemical potential gradient induced transport.

Optimization of Hybrid Silicon Lasers for High-Speed Direct Modulation

We report the optimization of hybrid silicon lasers for high-speed direct modulations by studying the small- and large-signal modulation responses based on the simple carrier transport model in this paper. The theoretical model matches well with published experimental data as the same structure parameters are used. To investigate the strong heating effect in hybrid lasers, we apply all of the carrier diffusion/capture and thermionic escape lifetimes to be temperature dependent in simulations. The frequency response of the small-signal analysis shows that the modulation bandwidth is much more sensitive to the temperature and the separate confinement heterostructure (SCH) thickness compared with the other limiting factors, such as the confinement factor, the interlayer thickness, etc. The largest modulation bandwidth decreases from 6.2 to 2.2 GHz under an injection current of 100 mA and an SCH thickness of 80 nm when the temperature rises from 300 to 350 K. The modulation speed will be greatly improved with a thinner SCH layer, particularly the p-type SCH layer, which helps shorten the diffusion lifetime. Furthermore, eye diagrams are also calculated under different bit rates, SCH thicknesses, and temperatures, respectively. With the increase of the temperature or SCH thickness, the quality of eye diagrams becomes worse. It shows that the modulation frequency can reach 10 Gb/s. We believe that this paper can serve as a guideline for the optimization of next-generation high-speed modulated hybrid silicon lasers.

Constraining groundwater flow in the glacial drift and saginaw aquifers in the Michigan Basin through helium concentrations and isotopic ratios

Abstract3He and 4He concentrations in excess of those in water in solubility equilibrium with the atmosphere by up to two and three orders of magnitude are observed in the shallow Glacial Drift and Saginaw aquifers in the Michigan Basin. A simplified He transport model shows that in situ production is negligible and that most He excesses have a source external to the aquifer. Simulated results show that 3He and 4He fluxes entering the bottom of the Saginaw aquifer are 7.5 × 10−14 and 6.1 × 10−7 cm3STPcm−2 yr−1, both of which are lower than fluxes entering the underlying Marshall aquifer, 1.0 × 10−13 and 1.6 × 10−6 cm3STPcm−2 yr−1 for 3He and 4He, respectively. In contrast, He fluxes entering the Saginaw aquifer are higher than fluxes entering the overlying Glacial Drift aquifer of 5.2 × 10−14 and 1.5 × 10−7 cm3STPcm−2 yr−1 for 3He and 4He, respectively. The unusually high He fluxes and their decreasing values from the lower Marshall to the upper Glacial Drift aquifer strongly suggest the presence of an upward cross‐formational flow, with increasing He dilution toward the surface by recharge water. These fluxes are either comparable to or far greater than He fluxes in deeper aquifers around the world. Model simulations also suggest an exponential decrease in the horizontal groundwater velocity with recharge distance. Horizontal velocities vary from 13 to 2 myr−1 for the Saginaw aquifer and from 18 to 6 myr−1 for the Marshall aquifer. The highly permeable Glacial Drift aquifer displays a greater velocity range, from 250 to 5 myr−1. While Saginaw 4He ages estimated based on the simulated velocity field display an overall agreement with 14C ages, 14C and 4He ages in the Glacial Drift and Marshall aquifers deviate significantly, possibly due to simplifications introduced in the He transport model leading to calculation of first‐order approximation He ages and high uncertainties in Glacial Drift 14C ages.

Dynamic connectivity in a fluvial network for identifying hotspots of geomorphic change

AbstractDynamical processes occurring on the hierarchical branching structure of a river network tend to heterogeneously distribute fluxes on the network, often concentrating them into “clusters,” i.e., places of excess flux accumulation. Here, we put forward the hypothesis that places in the network predisposed (due to process dynamics and network topology) to accumulate excess sediment over a considerable river reach and over a considerable period of time reflect locations where a local imbalance in sediment flux may occur thereby highlighting a susceptibility to potential fluvial geomorphic change. We develop a dynamic connectivity framework which uses the river network structure and a simplified Lagrangian transport model to trace fluxes through the network and integrate emergent “clusters” through a cluster persistence index (CPI). The framework was applied to sand transport in the Greater Blue Earth River Network in the Minnesota River Basin. Three hotspots of fluvial geomorphic change were defined as locations where high rates of channel migration were observed and places of high CPI coincided with two of these hotspots of possibly sediment‐driven change. The third hotspot was not identified by high CPI, but instead is believed to be a hotspot of streamflow‐driven change based on additional information and the fact that high bed shear stress coincided with this hotspot. The proposed network‐based dynamic connectivity framework has the potential to place dynamical processes occurring at small scales into a network context to understand how reach‐scale changes cascade into network‐scale effects, useful for informing the large‐scale consequences of local management actions.

The Predictive Analysis of the Energy Consumption and Environmental Emissions of Urban Passenger Transport of Beijing

A simple model of urban passenger transport was developed using computer based software called Long-range Energy Alternatives Planning System (LEAP), the model was used to carry out a research associated with urban passenger transport in Beijing to analyze the situation of energy consumption and environmental emissions ofCO2, CO, CH4, N2O, NOx, and SO2 for the base year 2012 and extrapolated till 2030 for the future predictions. The paper established four scenarios of business as usual, public transport, new energy vehicle, and P&N, and assessed the energy consumption and environmental emissions reduction potential of each scenarios in order to propose an optimal transport policy which have an excellent effect on reducing energy consumption and environmental emissions.

Modelling ELM heat flux deposition on the ITER main chamber wall

The interaction of ELM filaments with the ITER beryllium first wall panels (FWPs) is studied using a simple ad-hoc fluid model of the filament parallel transport, taking into account the full, three-dimensional structure of the FWPs, including magnetic shadowing effects. The calculated ELM surface heat loads are used as input to the RACLETTE heat transfer code to estimate the FWP surface temperature rise. The results indicate that controlled ELMs in ITER during burning plasma operation (ΔWELM≈0.6MJ) will not lead to melting or significant evaporation of the beryllium surfaces, even in the case of high ELM broadening and the minimum allowable distance between the primary and secondary separatrices. The ELM-averaged steady-state heat load also stays below the maximum power handling capability of the FWPs.

Effects of junctional correlations in the totally asymmetric simple exclusion process on random regular networks.

We investigate the totally asymmetric simple exclusion process on closed and directed random regular networks, which is a simple model of active transport in the one-dimensional segments coupled by junctions. By a pair mean-field theory and detailed numerical analyses, it is found that the correlations at junctions induce two notable deviations from the simple mean-field theory, which neglects these correlations: (1) the narrower range of particle density for phase coexistence and (2) the algebraic decay of density profile with exponent 1/2 even outside the maximal-current phase. We show that these anomalies are attributable to the effective slow bonds formed by the network junctions.

Transport of U(VI) through sediments amended with phosphate to induce in situ uranium immobilization

Phosphate amendments can be added to U(VI)-contaminated subsurface environments to promote in situ remediation. The primary objective of this study was to evaluate the impacts of phosphate addition on the transport of U(VI) through contaminated sediments. In batch experiments using sediments (<2 mm size fraction) from a site in Rifle, Colorado, U(VI) only weakly adsorbed due to the dominance of the aqueous speciation by Ca–U(VI)-carbonate complexes. Column experiments with these sediments were performed with flow rates that correspond to a groundwater velocity of 1.1 m/day. In the absence of phosphate, the sediments took up 1.68–1.98 μg U/g of sediments when the synthetic groundwater influent contained 4 μM U(VI). When U(VI)-free influents were then introduced with and without phosphate, substantially more uranium was retained within the column when phosphate was present in the influent. Sequential extractions of sediments from the columns revealed that uranium was uniformly distributed along the length of the columns and was primarily in forms that could be extracted by ion exchange and contact with a weak acid. Laser induced fluorescence spectroscopy (LIFS) analysis along with sequential extraction results suggest adsorption as the dominant uranium uptake mechanism. The response of dissolved uranium concentrations to stopped-flow events and the comparison of experimental data with simulations from a simple reactive transport model indicated that uranium adsorption to and desorption from the sediments was not always at local equilibrium.

On the response time of dye-sensitized solar cells to pulsed monochromatic illumination

It is common knowledge that the slow photocurrent response time of dye-sensitized solar cells (DSSCs) is due to trap-limited diffusion of photogenerated carriers in the mesoporous photoanode; therefore this quantity should depend on the carrier generation profile and on the carrier density, which are determined by the light wavelength and intensity, respectively. However, the dependence of the response time of DSSCs on these two parameters has scarcely been investigated. Here, we systematically measure the response time of two devices with different photoanode thickness to square-wave modulated illumination as a function of incident wavelength and of the intensity of the background bias light and we show that a simple transport model in the small-modulation approximation can account for the observed dependence. This information is relevant for the characterization of carrier transport in the device and for the design of correct photon conversion efficiency measurement setups when using lock-in techniques.

Numerical modeling of the effect of tidal stream turbines on the hydrodynamics and the sediment transport – Application to the Alderney Race (Raz Blanchard), France

A regional 2DH hydrodynamic model is used to estimate the tidal stream resource of a site located in a macrotidal environment with extreme tidal velocities. The study site is the Alderney Race (Raz Blanchard in French) which is a straight located in the English Channel between the Alderney Island and La Hague cape (France). The estimation of the resource is used to build two realistic tidal energy extraction scenarios consisting in placing a 290 MW tidal turbine array in two different areas. Then, we analyze the impact of turbines on the hydrodynamics and the sediment transport. The hydrodynamic perturbation is restricted to the vicinity of the array where the mean current velocity reduction reaches 0.3 m/s locally (corresponding to 15% of the baseline velocity). Focusing on the variable driving the bedload (the critical erosion threshold exceedance), we confirm that tidal energy extraction tends to reduce the bedload rate and to deflect the sediment fluxes. Our simulations with a simple model for the suspended sediment transport suggest that tidal energy extraction has a significant effect on the area of deposition of the particles transiting through the tidal farm. For the baseline, the sediment particles transiting through the tidal farm deposit mostly in the eastern part of the English Channel. Depending on the location of the tidal farm, the sediment mass balance between the eastern and the western part of the English Channel changes drastically.

Hydrate phase transition kinetics from Phase Field Theory with implicit hydrodynamics and heat transport

Most hydrate that forms or dissociates are in situations of constant non-equilibrium. This is due to the boundary conditions and Gibbs Phase rule. At a minimum this leaves a hydrate with two adsorbed phases in addition to hydrate and fluids. One adsorbed phase is governed by the mineral surfaces and the other by the hydrate surface. With pressure and temperature defined by local conditions, hydrate formation will never be able to reach any state of equilibrium. The kinetics of hydrate formation and dissociation are a complex function of competing phase transitions. This requires kinetic theories that include minimization of free energy under constraints of mass and energy transport. Since phase transitions also change density, further constraints are given by fluid dynamics. In this work, we describe a new approach for non-equilibrium theory of hydrates together with a Phase Field Theory for simulation of phase transition kinetics. We choose a three component system of water, methane and carbon dioxide for illustration. Conversion of methane hydrate into carbon dioxide hydrate is a win–win situation of energy production combined with safe long term storage of carbon dioxide. Carbon dioxide is able to induce and proceed with a solid-state exchange, but is slow due to mass transport limitations. A faster process is the formation of new hydrate from injected carbon dioxide and residual pore water. This formation releases substantial heat. This assists in dissociating in situ methane hydrate, making the conversion progress substantially faster, because heat transport is very rapid in these systems. But conversion of liquid water into carbon dioxide hydrate, in the vicinity of the hydrate core will increase temperatures to some portions of the surface. The dissociating regions of the methane hydrate core will show a local decrease in temperature, due to extraction of heat for methane hydrate dissociation from surroundings. Another reason for heat transport implementation is that regions of the system that contains non-polar gas phase will have low heat conductivity and low heat convection. At this stage we apply a simplified heat transport model in which “lumped” efficient heat conductivity is used. We illustrate the theory on the conversion of methane hydrate to mix methane–carbon dioxide hydrate using three initial hydrate sizes: 150Å×150Å, 500Å×500Å and 5000Å×5000Å. The hydrate cores used are spherical because it makes it easier to illustrate the impact of curvature. Symmetrical aspects simplifies the dependency to a two dimensional problem – although there are no such limitations in theory. The mineral surfaces are considered to be water wetting in these examples. It was observed that the smaller sizes convert to a more unstable mix hydrate for some periods of the simulation time, during which there were significant losses of the initial methane hydrate core. These instabilities are caused by local under saturated fluid phases around the hydrate core. Eventually a steady state progress was observed. The largest size system appeared to reach a steady state situation comparable faster than the two smaller systems.

Isotopic insights into microbial sulfur cycling in oil reservoirs.

Microbial sulfate reduction in oil reservoirs (biosouring) is often associated with secondary oil production where seawater containing high sulfate concentrations (~28 mM) is injected into a reservoir to maintain pressure and displace oil. The sulfide generated from biosouring can cause corrosion of infrastructure, health exposure risks, and higher production costs. Isotope monitoring is a promising approach for understanding microbial sulfur cycling in reservoirs, enabling early detection of biosouring, and understanding the impact of souring. Microbial sulfate reduction is known to result in large shifts in the sulfur and oxygen isotope compositions of the residual sulfate, which can be distinguished from other processes that may be occurring in oil reservoirs, such as precipitation of sulfate and sulfide minerals. Key to the success of this method is using the appropriate isotopic fractionation factors for the conditions and processes being monitored. For a set of batch incubation experiments using a mixed microbial culture with crude oil as the electron donor, we measured a sulfur fractionation factor for sulfate reduction of −30‰. We have incorporated this result into a simplified 1D reservoir reactive transport model to highlight how isotopes can help discriminate between biotic and abiotic processes affecting sulfate and sulfide concentrations. Modeling results suggest that monitoring sulfate isotopes can provide an early indication of souring for reservoirs with reactive iron minerals that can remove the produced sulfide, especially when sulfate reduction occurs in the mixing zone between formation waters (FW) containing elevated concentrations of volatile fatty acids (VFAs) and injection water (IW) containing elevated sulfate. In addition, we examine the role of reservoir thermal, geochemical, hydrological, operational and microbiological conditions in determining microbial souring dynamics and hence the anticipated isotopic signatures.

Scaling of olfactory antennae of the terrestrial hermit crabs Coenobita rugosus and Coenobita perlatus during ontogeny.

Although many lineages of terrestrial crustaceans have poor olfactory capabilities, crabs in the family Coenobitidae, including the terrestrial hermit crabs in the genus Coenobita, are able to locate food and water using olfactory antennae (antennules) to capture odors from the surrounding air. Terrestrial hermit crabs begin their lives as small marine larvae and must find a suitable place to undergo metamorphosis into a juvenile form, which initiates their transition to land. Juveniles increase in size by more than an order of magnitude to reach adult size. Since odor capture is a process heavily dependent on the size and speed of the antennules and physical properties of the fluid, both the transition from water to air and the large increase in size during ontogeny could impact odor capture. In this study, we examine two species of terrestrial hermit crabs, Coenobita perlatus H. Milne-Edwards and Coenobita rugosus H. Milne-Edwards, to determine how the antennule morphometrics and kinematics of flicking change in comparison to body size during ontogeny, and how this scaling relationship could impact odor capture by using a simple model of mass transport in flow. Many features of the antennules, including the chemosensory sensilla, scaled allometrically with carapace width and increased slower than expected by isometry, resulting in relatively larger antennules on juvenile animals. Flicking speed scaled as expected with isometry. Our mass-transport model showed that allometric scaling of antennule morphometrics and kinematics leads to thinner boundary layers of attached fluid around the antennule during flicking and higher odorant capture rates as compared to antennules which scaled isometrically. There were no significant differences in morphometric or kinematic measurements between the two species.

Experimental evaluation of a Markov multizone model of particulate contaminant transport.

The performance of a Markov chain model of the three-dimensional transport of particulates in indoor environments is evaluated against experimentally measured supermicrometer particle deposition. Previously, the model was found to replicate the predictions of relatively simple particle transport and fate models; and this work represents the next step in model evaluation. The experiments modeled were (i) the release of polydispersed particles inside a building lobby, and (ii) the release of monodispersed fluorescein-tagged particles inside an experimental chamber under natural and forced mixing. The Markov model was able to reproduce the spatial patterns of particle deposition in both experiments, though the model predictions were sensitive to the parameterization of the particle release mechanism in the second experiment. Overall, the results indicate that the Markov model is a plausible tool for modeling the fate and transport of supermicrometer particles.