Variation Of Transport Properties Research Articles

Temperature stability and enhanced transport properties by surface modifications of silica nanoparticle tracers for geo-reservoir exploration

Tracer tests are an important tool for characterizing and monitoring subsurface reservoir properties. However, they are limited both because of the tracer molecules constraining factors such as irreversible adsorption, retention, and degradations, i.e. interaction processes of fluorophore molecule with surrounding media resulting in a large variation in transport properties. Elaborate tests utilizing more than one tracer to distinguish time or location of injection are complex and interpretation is ambiguous because each tracer interacts differently. In this study, we present an approach to increase tracer stability and enhance the transport uniformity of different tracers, thus making tests utilizing multiple tracers simpler and more feasible. We present this concept of tracer multiplicity by encapsulating an anionic, cationic or amphoteric fluorophore inside mesoporous silica nanoparticle carriers coated with a protective titania layer. Upon encapsulation, increased thermal resistance and drastically lowered sorption affinity towards quartz sand was detected in batch and flow-through experiments. An additional advantage of the presented nanoparticle tracers over molecular tracers is their modularity, which is demonstrated by surface modifications and application of additives that greatly reduce sorption and increase recovery rates in the flow experiments. With the here presented concept of tracer multiplicity, we introduce a new approach for colloidal tracer design that has the potential to expand and enhance measurable parameters, measurement accuracy and simplicity of analysis.

Unmasking the reactants inhomogeneity in gas diffusion layer and the performances of PEMFC induced by assembly pressure

The gas diffusion layer (GDL), as the bridge to reactants and electrons in PEMFC, is a carbon-based porous component and would be deformed under compression to induce changes in the distributions of reactants and the corresponding performances of PEMFC; therefore, unmasking the impacts of assembly pressure on the distribution of the reactants in GDL is significant to improve the performance of PEMFC. In the present article, the structural response of GDL to assembly pressure was first studied; the variations in transport properties of GDL and the reactant distributions induced by assembly pressure were then discussed; the impacts on the dynamic performances of PEMFC were finally investigated. From the results, assembly pressure was found to have different effects on the regions of GDL under the rib and channel, significant gaps in GDL porosity and/or GDL permeability existed near the rib/channel transition region to worsen the inhomogeneity of reactants. Suffering assembly pressure, the distribution of current density became uneven, and the current density near the rib-channel border seriously rose to the aggravated risk of MEA thermal damage. Furthermore, the power density at specific efficiencies was raised under certain assembly pressures, which meant suitable assembly pressure(s) existed for better output performances of PEMFC.

Directional solidification induced compositional gradient for a quick evaluation of transport properties in p-PbTe thermoelectrics

Revelation of transport properties in a broad range of carrier concentration is indispensable for thermoelectrics to locate its performance optimum. This signifies inevitable efforts on the syntheses of many samples with various concentrations of dopants. Moreover, the realization of low carrier concentration is usually a great challenge for a precise control. Directional solidification can thermodynamically enable a spontaneous gradient in composition, therefore, naturally offers a convenient pathway to continuously manipulate the dopant concentration thus the carrier concentration by one synthesis. In this work, gradiently sodium-doped PbTe ingots with ∼10 mm in diameter are successfully synthesized by a vertical gradient solidification (VGS) technique. Such a composition gradient leads to an efficient manipulation of hole concentration in a broad range as well as a continuous variation in transport properties along the solidification direction. The comparable transport properties to the literatures demonstrate the strategy used here to be highly efficient for the evaluation of transport properties, which is believed to be equally applicable for efficient research of both existing and novel thermoelectric materials.

Nature of polaronic hopping conduction mechanism in polycrystalline and nanocrystalline Gd0.5Sr0.5MnO3 compounds

In the present study, we have investigated the structural, electrical and magneto-transport properties of polycrystalline and nanocrystalline Gd0.5Sr0.5MnO3 compounds. The variation of transport properties is modified by tuning the grain size of the material. In the high temperature semiconducting region, temperature dependent resistivity data can be well explained by non-adiabatic small polaron hopping (SPH) mechanism. In addition, the resistivity data for all compounds in the low-temperature paramagnetic region can also be well explained by the variable range hopping (VRH) model. The parameters obtained from SPH and VRH mechanisms are found to be reasonable. In the case of nanocrystalline compounds, there is an overlapping temperature range where both SPH and VRH models are valid simultaneously, and a new conduction mechanism - variable range hopping of small polaron s(VR-SPH) is satisfactorily valid for the whole temperature range of these compounds. However, for the polycrystalline compound, the overlapping temperature region between VRH and SPH models does not exist and the VR-SPH mechanism is not valid here. Thus, polarons play a leading role in selecting different conduction mechanisms in different temperature ranges.

Numerical Investigation of Irreversibility in Bioconvective Flow of Sisko Nanofluid with Arrhenius Energy

This paper investigates modeling and analysis of entropy generation in bioconvective non-radiative Sisko nanofluid flow by stretchable cylinder. Momentum relation is modeled in view of Darcy Forchheimer and porosity effects. Dissipation, Joule heating and heat generation impacts are accounted in energy relation. Mass concertation communication is constructed in manifestation of Arrhenius energy and chemical reaction. Brownian dispersion and thermophoretic effects of solid nanoparticles in Sisko liquid are stabilized by self-propelled gyrotactic microorganisms. The flow governing model is obtained utilizing boundary layer concepts. Fluid transport equations are made dimensionless via transformations and then tackled by NDSolve code in Mathematica package. Variation in transport properties versus effective parameters is examined via graphs and tables. It is perceived from obtained results that Sisko nanofluid velocity decays versus higher curvature parameter, Hartman number, porosity and Forchheimer variable. Further, it is observed that temperature distribution enhances for heat generation variable, Eckert number, Brownian movement variable, thermophoresis motion parameter and Prandtl number. Main observations are listed in the end.

A playground for compressible natural convection with a nearly uniform density

In the quest to understand the basic universal features of compressible convection, one would like to disentangle genuine consequences of compression from spatial variations of transport properties. For instance, one may choose to consider a fluid with uniform dynamic viscosity, but, then, compressible effects will generate a density gradient and consequently the kinematic viscosity will not be uniform. In the present work, we consider a very peculiar equation of state, whereby entropy is solely dependent on density, so that a nearly isentropic fluid domain is nearly isochoric. Within this class of equations of state, there is a thermal adiabatic gradient and a key property of compressible convection is still present, namely its capacity to viscously dissipate a large fraction of the thermal energy involved, of the order of the well-named dissipation number. In the anelastic approximation, under the assumption of an infinite Prandtl number, the number of governing parameters can be brought down to two, the Rayleigh number and the dissipation number. This framework is proposed as a playground for compressible convection, an opportunity to extend the vast corpus of theoretical analyses on the Oberbeck–Boussinesq equations regarding stability, bifurcations or the determination of upper bounds for the turbulent heat transfer. Here, in a two-dimensional geometry, we concentrate on the structure of numerical solutions. For all Rayleigh numbers, a change in the vertical temperature profile is observed in the range of dissipation number between $0$ and less than $0.4$ , associated with the weakening of ascending plumes. For larger dissipation numbers, the heat flux dependence on this number is found to be well predicted by Malkus's model of critical layers. For dissipation numbers of order unity, and large Rayleigh numbers, dissipation becomes related to the entropy heat flux at each depth, so that the vertical dissipation profile can be predicted, and so does the total ratio of dissipation to convective heat flux.

Heavy-fermion metallic state and Mott transition induced by Li-ion intercalation in LiV2O4 epitaxial films

The spinel-type ${\mathrm{LiV}}_{2}{\mathrm{O}}_{4}$ exhibits heavy-fermion metallic behaviors at low temperatures, but the effect of composition modulation on them is unknown. We realized the epitaxial growth of highly crystalline ${\mathrm{LiV}}_{2}{\mathrm{O}}_{4}$ films and conducted the electrochemical Li-ion intercalation (electron doping) to investigate systematic variation of transport properties in ${\mathrm{Li}}_{1+x}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$. We found that adjustment of Li content during pulsed-laser deposition was important to obtain films exhibiting the heavy-fermion metallic behavior comparable to that of a bulk single crystal. At low doping regime $(x\ensuremath{\le}0.5)$, resistivity increased with increasing electron filling. The slope of their linear ${T}^{2}$ dependence also increased in accordance of a Fermi-liquid model. At high doping regime $(x>0.5)$, a Mott transition was observed over a range of Li content where a new spinel phase and the original one coexisted. The results revealed that the electron doping induced enhancement of the electron correlation and the resultant Mott transition.

Enhancement in thermoelectric properties of n-type (La0.7Sr0.3MnO3)0.5.(NiO)0.5: composite and nano-structure effect

In the enhancement of thermoelectric properties of material for energy harvesting applications, the role of nano-composite made of two different materials plays a very crucial role. When materials with different crystal and electronic structure are synthesized in nano-crystalline phase to form a composite, they provide large power factor via energy filtering effect, whereas the lattice mismatching and grain boundaries of two phase scatter the phonons having wide range of frequencies, and effectively maintain the low thermal conductivity. In the present work, we have reported the structural and thermoelectric properties of (La0.7Sr0.3MnO3)0.5(NiO)0.5 composite synthesized by sol-gel method. Rietveld refinement of the XRD pattern confirms the presence of both phases. Thermoelectric properties are measured in 300–600 K temperature range. The magnitude of Seebeck coefficients, electrical resistivity, and thermal conductivity found to be monotonically increase with temperature upto the ∼450 K, which decrease gradually with further increase in temperature. Such variation in transport properties are resulted from the combine effect from the composite materials formed with semiconducting/metallic, and insulating electronic structure mixed in a 50:50 weight ratio. The calculated ‘figure-of-merit’ (ZT) shows an increasing trend with temperature. The value of ZT is found to be ∼0.05 at 600 K, which is nearly three times larger than that of La0.7Sr0.3MnO3 (ZT = 0.013) reported earlier. Our strategy of making nano-composite two materials with different electronic structures enhanced the ZT by ∼285%. The improvement in thermoelectric properties of n-type oxide materials can be useful to consider this material for applications in mid-temperature range.

Effect of free-stream inclination and buoyancy on flow past a square cylinder in large-scale heating regime

The combined effects of free-stream inclination (α) and heating [ϵ=(Tw−T∞)/T∞] on aerodynamic and heat transfer parameters are studied at fixed Reynolds number (Re = 100), Prandtl number (Pr = 0.71), cylinder inclination (ϕ=0°), Froude number (Fr = 1.0), and Mach number (M = 0.1) in the large-scale heating regime. For this purpose, a non-Oberbeck–Boussinesq compressible model for thermally perfect gases incorporating volumetric straining as well as transport property variations under large-scale heating is employed. The free-stream inclination (α) is varied in the range [0°, 90°] while the over-heat ratio (ϵ) is varied in the range of [0, 1]. It is found that at small free-stream inclinations (α≤45°), increase in heating causes a significant increase in mean drag coefficient, while at large α (α>45°), heating has little effect on mean drag coefficient. The mean lift coefficient (CL) increases by increasing ϵ for any value of α except α = 0. At a fixed heating level, the variation of mean CL is very non-monotonic with lower values at α=45° and 90°. It is found that the increase in flow inclination from 0° to 90° reduces the sensitivity of Strouhal number (St) to heating. For ϵ≤0.6, mean Nusselt number exhibits a non-monotonic trend with increase in α and attains a maximum value at α ≅ 40°. However, for ϵ>0.6, heat transfer decreases with increase in α and is maximum for aligned flow (α=0°).

Modeling long-term distribution of fast and thermal neutron fluence in degraded concrete biological shielding walls

Since neutron radiation can deteriorate mechanical properties of the concrete materials, it’s critical to obtain accurate neutron radiation levels in concrete structures during their service live. A two-group neutron diffusion model for fast and thermal neutron transport is developed in this study which considers the kinetic energy of the neutron more realistically. Based on the cross-property correlation theory, the variation of transport properties of concrete can be estimated using the degradation of mechanical properties of concrete, then, the thermal and fast neutron radiation fields with two representative energy levels as well as the thermal field in a concrete bioshield can be calculated. Coupled radio-thermo analysis of a section of simplified concrete bioshield was performed to 80 years as a case study, and the results were discussed and compared with the results from the one-speed model. The degradation of concrete induced by neutron radiation and elevated temperature can lead to considerable increases of thermal and fast neutron flux/fluence in the concrete. The profiles of thermal neutron flux/fluence are very different from the profiles of fast neutron flux/fluence. A proper treatment of the neutron energy dependence is needed to predict long-term neutron radiation levels in concrete biological shields accurately.

Probing thermoelectric properties of high potential Ca3PbO: An Ab Initio Study

The fascinating electron transport properties of anti-perovskites have gathered incredible research consideration. In the search for novel thermoelectric materials, a Density Functional Theory (DFT) based computational assessment of electron transport properties of Ca3PbO has been made based on the grounds of Boltzmann transport theory. The variation of transport properties specifically Seeback coefficient (S), electrical conductivity (σ), electronic thermal conductivity (κe) and power factor (PF) versus chemical potential (µ) have been plotted at 300 K, 600 K and 800 K. The present study proposes Ca3PbO to be an excellent n-type high temperature thermoelectric material. The optimized values of equilibrium structural parameters have been determined. The current analysis tends to stimulate experimental work towards designing and improving thermoelectric performance of Ca3PbO for realistic applications.

Adenine based molecular junction as biosensor for detection of toxic phosgene gas.

The possibility of adsorption of toxic phosgene gas (COCl2) molecule on one of the nucleobase of DNA-adenine-has been analyzed using the first principle calculations based on density function theory. In accordance with the geometry of the nucleobase, two possible positions have been considered for effective adsorption of gas molecule. The calculations performed on adsorption energies suggest that the gas molecule is able to physisorb at both the considered positions with negligibly small values of charge transfer. The in-depth analysis of electron charge densities depicts that there is no orbital overlapping between the gas molecule and adenine. We observe a significant variation of transport properties of adenine-based molecular junction on adsorption of phosgene molecule while calculation the transport parameters at both the equilibrium as well as non-equilibrium. Also, the variation of HOMO-LUMO gap of adenine molecule on adsorption of phosgene leads to alteration of current and voltage, thus implying that adenine-based sensor can be effectively utilized to sense the presence of phosgene gas in a given environment. Small adsorption energies and recovery time suggest that the rate of desorption of phosgene is very high; thus, the proposed adenine sensor can be effectively used as a highly stable and selective reusable sensor.

Micropolar fluid flow with temperature‐dependent transport properties

AbstractIn the present article, the heat transfer rate and the fluid flow of a micropolar fluid along with temperature‐dependent transport properties are scrutinized in the presence of heat generation. The variability in transport properties leads to a rise in the heat transfer and decreases the skin friction. Furthermore, Fourier's heat flux model is implemented in the analysis of heat transfer, employing a suitable transformation to convert the flow model into nonlinear ordinary differential equations. Numerical solutions are obtained by using the shooting method/bvp4c technique. Physical quantities of interest, such as local skin friction and Nusselt number, are discussed and computed. Skin friction decreases with the micropolar parameter but the Nusselt number shows the opposite behavior for the micropolar parameter.

Strong Thermopower Enhancement and Tunable Power Factor via Semimetal to Semiconductor Transition in a Transition-Metal Dichalcogenide.

Electronic band engineering is a promising approach to enhance the thermopower of thermoelectric materials. In transition-metal dichalcogenides (TMDCs), this has so far only been achieved using their inherent semiconducting nature. Here, we report the thickness-modulated band engineering of nanosheets based on semimetallic platinum diselenide (PtSe2) resulting in a thermopower enhancement of more than 50 times than that of the bulk. We obtained this by introducing a semimetal to semiconductor (SMSC) transition resulting in the formation of a bandgap. This approach based on semimetallic TMDCs provides potential advantages such as a large variation of transport properties, a decrease of the ambipolar transport effect, and a high carrier density dependence of the transport properties. Our observations suggest that the SMSC transition in TMDCs is a promising and straightforward strategy for the development of two-dimensional nanostructured thermoelectric materials.

Estimation of the radial distribution of axial velocities in fixed beds of spherical packing

The vessel wall of densely packed beds exert a strong influence on particle distribution that generates a radial porosity profile and, in turn, a radial profile of axial velocity. A direct consequence is the appearance of an uneven flow distribution on the bed cross-section and radial variations of transport properties. A widespread alternative for estimating such a velocity profile is to solve the so-called extended Brinkman equation, using an available radial porosity profile. To this end, it is employed a local version of an expression quantifying the drag force on the packing (usually the Ergun equation) and an effective viscosity to account for the radial transport of axial momentum. It is shown that the existing expressions to evaluate the effective viscosity are only suitable for a restricted range of Reynolds numbers. Supported on the expressions from averaging the microscopic conservation equations, it is reassessed the local evaluation of the drag force and the estimation of the effective viscosity. The use of the extended Brinkman equation with such changes allows a satisfactory description of the radial velocity profile for mono-sized spherical packing, as arisen from the comparison with available experimental results and with an important number of results from pore-scale simulation.

Vortex shedding suppression in mixed convective flow past a square cylinder subjected to large-scale heating using a non-Boussinesq model

In the present work, numerical simulations are performed to investigate the vortex shedding suppression phenomenon for mixed convective flows past a square cylinder in the large-scale heating regime. A full compressible flow model with variable transport and thermo-physical properties is employed to capture large-scale heating effects. The Reynolds number, Prandtl number, and Mach number are kept constant at Re = 100, Pr = 0.71, and M = 0.1, while the cylinder inclination (ϕ), free-stream inclination (α), and over-heat ratio (ϵ) are varied in the range [0, 45°], [0, 90°], and [0, 2], respectively. The governing equations are solved numerically using the particle velocity upwind (PVU-M+) scheme. The buoyancy parameter which governs the vortex shedding suppression process in the non-Boussinesq model is identified as RiNB=ϵFr222+ϵ, where Fr is the Froude number. Using the Stuart-Landau model, the neutral curves separating the steady and unsteady flow regimes are generated in the ϵ–ϕ and ϵ–α parametric spaces. The neutral curves show qualitatively similar characteristics as observed for Boussinesq models. The relative contribution of various large-scale heating effects in suppression of vortex shedding is also highlighted. This reveals that buoyancy effects followed by variations in transport properties play a major role in suppression of vortex shedding. The findings are also applicable to a range of low Re (O(100)) as supported by data obtained at Re = 130 for ϕ = 40°. The mechanism of vortex shedding suppression has been analyzed and extended for the large-scale heating scenarios.

Experimental study of molten tin sulfide thermal conduction

Molten thermoelectrics are proven candidates to harvest waste-heat from high-temperature industrial operations in the form of electrical power. However to date, the role of thermal conductivity remains to be evaluated in order to evaluate the overall efficiency. Herein, possible modes of heat transfer and their analysis for such molten semi-conductor are presented. Experimental estimates of thermal conductivity obtained on a power generation thermoelectric device operated with molten tin sulfide at temperature in excess of 900 °C are then presented. Results of the simulation of the temperature profile, the fluid mechanic model and electrical power performance show a key role of natural convection and a minor role of radiative heat transfer, and indicate a possible variation in transport properties of tin sulfides around 1000 °C.

First principles study of the structural, electronic, magnetic and thermoelectric properties of Zr2RhAl

The inverse Heusler alloy Zr2RhAl is investigated using first-principles calculations. The calculations are carried out using the full-potential linearized augmented plane wave plus local orbital (FP-LAPW + lo) method that is employed in the WIEN2k package. The structural, electronic, magnetic, and thermoelectric properties are studied. It is found that the ferromagnetic phase of Zr2RhAl is the most stable with lattice parameter 6.6328 Å. The calculations showed that there is an energy gap in the spin down channel whereas the spin up channel is metallic, resulting in a 100% spin polarization at Fermi level. This is an obvious characteristic of half-metallic materials. The calculated total magnetic moment of Zr2RhAl is found to be 2 μB which follows the generalized Slater-Pauling rule. The Curie temperature is estimated to be 607 K using the mean field approximation. Transport properties are studied using the Boltzmann theory with constant relaxation time approximation. The variation of transport properties with temperature is investigated using the two current model.

Long-term neutron radiation levels in distressed concrete biological shielding walls.

Neutron radiation can deteriorate mechanical properties of the concrete materials, and thus it is questionable that neutron transport properties of concrete can remain unchanged during the life span of biological shielding walls. one-speed neutron diffusion equation and heat conduction equation were used as governing equations for prediction of neutron radiation and thermal field in concrete, respectively. The potential variations of transport properties due to neutron radiation and elevated temperature were estimated. A simplified example of a typical concrete biological shielding wall was analyzed up to 80 years, and the results were discussed. The radiation damage and radiation heating lead to minor changes of the temperature profile in the concrete. However, neutron radiation and elevated temperature can result in considerable increases of neutron flux and fluence in the concrete. The damage of concrete induced by neutron radiation and elevated temperature can considerably accelerate the penetration of neutron radiation into the concrete. This work is the first attempt to deal with the degradation of neutron and heat transport properties of concrete and its effect on neutron fluence distribution in concrete, and provides a possible way to determine the long-term neutron and thermal fields in concrete biological shielding walls.

Turbulence modelling for flows with strong variations in thermo-physical properties

This paper presents a novel methodology for improving eddy viscosity models in predicting wall-bounded turbulent flows with strong gradients in the thermo-physical properties. Common turbulence models for solving the Reynolds-averaged Navier–Stokes equations do not correctly account for variations in transport properties, such as density and viscosity, which can cause substantial inaccuracies in predicting important quantities of interest, for example, heat transfer and drag. Based on the semi-locally scaled turbulent kinetic energy equation, introduced in [Pecnik and Patel, J. Fluid Mech. (2017), vol. 823, R1], we analytically derive a modification of the diffusion term of turbulent scalar equations. The modification has been applied to five common eddy viscosity turbulence models and tested for fully developed turbulent channels with isothermal walls that are volumetrically heated, either by a uniform heat source or viscous heating in supersonic flow conditions. The agreement with results obtained by direct numerical simulation shows that the modification significantly improves results of eddy viscosity models for fluids with variable transport properties.