Showing papers on "Transport phenomena published in 2013"
TL;DR: In this article, the authors characterized pool boiling on surfaces with wettabilities varied from superhydrophobic to super-hydrophilic, and provided nucleation measurements, and developed an analytical model that describes how biphilic surfaces effectively manage the vapor and liquid transport, delaying critical heat flux and maximizing the heat transfer coefficient.
428 citations
TL;DR: In this paper, a critical review of the current state of research in microchannels is presented with a focus on the future research needs, including single-phase gas flow, enhancement in singlephase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers.
Abstract: Heat transfer and fluid flow in microchannels have been topics of intense research in the past decade. A critical review of the current state of research is presented with a focus on the future research needs. After providing a brief introduction, the paper addresses six topics related to transport phenomena in microchannels: single-phase gas flow, enhancement in single-phase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers. After reviewing the current status, future research directions are suggested. Concerning gas phase convective heat transfer in microchannels, the antagonist role played by the slip velocity and the temperature jump that appear at the wall are now clearly understood and quantified. It has also been demonstrated that the shear work due to the slipping fluid increases the effect of viscous heating on heat transfer. On the other hand, very few experiments support the theoretical models and a significant effort should be made in this direction, especially for measurement of temperature fields within the gas in microchannels, implementing promising recent techniques such as molecular tagging thermometry (MTT). The single-phase liquid flow in microchannels has been established to behave similar to the macroscale flows. The current need is in the area of further enhancing the performance. Progress on implementation of flow boiling in microchannels is facing challenges due to its lower heat transfer coefficients and critical heat flux (CHF) limits. An immediate need for breakthrough research related to these two areas is identified. Discussion about passive and active methods to suppress flow boiling instabilities is presented. Future research focus on instability research is suggested on developing active closed loop feedback control methods, extending current models to better predict and enable superior control of flow instabilities. Innovative high-speed visualization and measurement techniques have led to microchannel condensation now being studied as a unique process with its own governing influences. Further work is required to develop widely applicable flow regime maps that can address many fluid types and geometries. With this, condensation heat transfer models can progress from primarily annular flow based models with some adjustments using dimensionless parameters to those that can directly account for transport in intermittent and other flows, and the varying influences of tube shape, surface tension and fluid property differences over much larger ranges than currently possible. Electronics cooling continues to be the main driver for improving thermal transport processes in microchannels, while efforts are warranted to develop high performance heat exchangers with microscale passages. Specific areas related to enhancement, novel configurations, nanostructures and practical implementation are expected to be the research focus in the coming years.
227 citations
TL;DR: In this paper, a review is provided for the determination of these effective transport properties through the various PEM fuel cell components, including the gas diffusion layer, microporous layer, catalyst layer and the electrolyte membrane layer.
170 citations
TL;DR: A pore-scale model based on the lattice Boltzmann (LB) method is developed that can capture coupled nonlinear multiple physicochemical processes including multiphase flow with phase separations, mass transport, chemical reactions, dissolution-precipitation processes, and dynamic evolution of the pore geometries.
Abstract: A pore-scale model based on the lattice Boltzmann (LB) method is developed for multiphase reactive transport with phase transitions and dissolution-precipitation processes. The model combines the single-component multiphase Shan-Chen LB model [X. Shan and H. Chen, Phys. Rev. E 47, 1815 (1993)], the mass transport LB model [S. P. Sullivan et al., Chem. Eng. Sci. 60, 3405 (2005)], and the dissolution-precipitation model [Q. Kang et al., J. Geophys. Res. 111, B05203 (2006)]. Care is taken to handle information on computational nodes undergoing solid-liquid or liquid-vapor phase changes to guarantee mass and momentum conservation. A general LB concentration boundary condition is proposed that can handle various concentration boundaries including reactive and moving boundaries with complex geometries. The pore-scale model can capture coupled nonlinear multiple physicochemical processes including multiphase flow with phase separations, mass transport, chemical reactions, dissolution-precipitation processes, and dynamic evolution of the pore geometries. The model is validated using several multiphase flow and reactive transport problems and then used to study the thermal migration of a brine inclusion in a salt crystal. Multiphase reactive transport phenomena with phase transitions between liquid-vapor phases and dissolution-precipitation processes of the salt in the closed inclusion are simulated and the effects of the initial inclusion size and temperature gradient on the thermal migration are investigated.
131 citations
TL;DR: This work provides a theoretical framework to address the role of polymer conformations in electronic transport through disordered semiconducting polymers and presents a simple model that reconciles observations at the local scale with device-scale measurements of charge mobility.
Abstract: Existing models for the electronic properties of conjugated polymers do not capture the spatial arrangement of the disordered macromolecular chains over which charge transport occurs. Here, we present an analytical and computational description in which the morphology of individual polymer chains is dictated by well-known statistical models and the electronic coupling between units is determined using Marcus theory. The multiscale transport of charges in these materials (high mobility at short length scales, low mobility at long length scales) is naturally described with our framework. Additionally, the dependence of mobility with electric field and temperature is explained in terms of conformational variability and spatial correlation. Our model offers a predictive approach to connecting processing conditions with transport behavior.
112 citations
TL;DR: In this paper, a multi-scale modeling framework combining finite volume method (FVM) and lattice Boltzmann method (LBM) was used to predict electrochemical transport reaction in proton exchange membrane fuel cell (PEMFC) cathode with a parallel gas channel (GC), a gas diffusion layer (GDL) with porous structures and a catalyst layer (CL) with idealized microstructures.
95 citations
TL;DR: In this article, a complete mathematical model of microwave ablation using the porous media approach is proposed, which uses transient momentum equations (Brinkman model extended Darcy model) and energy equation coupled with electromagnetic wave propagation equation to describe the specific absorption rate (SAR) profile, temperature profile and blood velocity profile within the porous liver.
93 citations
TL;DR: In this paper, a number of recent investigations have focused on the design, characterization, and fabrication of ultrathin vapor chambers for proximate heat spreading away from small hot spots, and the predominant transport mechanisms and operational limits have been found to be different under these conditions relative to conventional low-power heat pipes.
Abstract: Owing to their high reliability, simplicity of manufacture, passive operation, and effective heat transport, flat heat pipes and vapor chambers are used extensively in the thermal management of electronic devices. The need for concurrent size, weight, and performance improvements in high-performance electronics systems, without resort to active liquid-cooling strategies, demands passive heat-spreading technologies that can dissipate extremely high heat fluxes from small hot spots. In response to these daunting application-driven trends, a number of recent investigations have focused on the design, characterization, and fabrication of ultrathin vapor chambers for proximate heat spreading away from these hot spots. The predominant transport mechanisms and operational limits have been found to be different under these conditions relative to conventional low-power heat pipes. Noteworthy advances in the fundamental understanding of evaporation and boiling from porous microstructures fed by capillary action and improvements in vapor chamber characterization, modeling, design, and fabrication techniques are reviewed. Characterization of evaporation and boiling from idealized and realistic wick structures, observation of vapor formation regimes as a function of operating conditions, assessment of fluid dryout limitations, design of novel multiscale and nanostructured wicks for enhanced transport, and incorporation of these high-heat-flux transport phenomena into device-level models are discussed. These recent developments have successfully extended the maximum operating heat flux limits of vapor chambers.
91 citations
TL;DR: A comprehensive review of the state-of-the-art modeling studies regarding mass, heat and charge transport in a direct methanol fuel cells is provided in this article, where the authors discuss the basics of kinetics for electrochemical.
82 citations
TL;DR: In this paper, the authors investigated flow-field and heat transfer around a cylinder embedded in a layer of homogenous porous media and found that, in the presence of a porous layer around the cylinder, the wake length increases with decreasing the Darcy number while the critical radius of insulation increases.
74 citations
TL;DR: In this article, the authors proposed a methodology for the derivation of boundary conditions for both the velocity and the stress in a two-domain approach, where continuity of the velocity is postulated at the dividing surface and the jump coefficients are computed from the solution of an ancillary macroscopic closure problem.
TL;DR: In this paper, pore-scale analysis of thin-film evaporation through sintered copper wicks is performed using X-ray microtomography to generate geometrically faithful, feature-preserving meshes.
01 Jun 2013
TL;DR: In this article, the authors examined theoretically and computationally the transient magneto-hydrodynamic boundary layer flow and heat transfer in an incompressible rotating nanofluid over a stretching continuous s.
Abstract: This study examines theoretically and computationally the transient magneto-hydrodynamic boundary layer flow and heat transfer in an incompressible rotating nanofluid over a stretching continuous s...
TL;DR: A two-dimensional model for the mass transport of carbon dioxide from the gas mixture in nanoporous membrane contactors was developed in this paper, where a solution of 2-amino-2-methyl-1-propanol and piperazine (activator of absorption) was used as a chemical solvent for capture of CO 2.
01 Jul 2013
TL;DR: GOMA as discussed by the authors is a finite element program for modeling and analysis of manufacturing processes, particularly those involving free or moving interfaces, which is based on the premise that any boundary can be (1) moving or free, with an apriori unknown position dictated by the distinguishing physics, (2) fixed, according to a global analytical representation, or (3) moving in time and space under user-prescribed kinematics.
Abstract: GOMA is a two- and three-dimensional finite element program which excels in analyses of manufacturing processes, particularly those involving free or moving interfaces. Specifically, the full-Newton-coupled heat, mass, momentum, and pseudo-solid mesh motion algorithm makes GOMA ideally suited for simulating processes in which the bulk fluid transport is closely coupled to the interfacial physics. Examples include, but are not limited to, coating and polymer processing flows, soldering, crystal growth, and solid-network or solution film drying. The code is based on the premise that any boundary can be (1) moving or free, with an apriori unknown position dictated by the distinguishing physics, (2) fixed, according to a global analytical representation, or (3) moving in time and space under user-prescribed kinematics. The goal is to enable the user to predict boundary position or motion simultaneously with the physics of the problem being analyzed and to pursue geometrical design studies and fluid-structure interaction problems. The moving mesh algorithm treats the entire domain as a computational Lagrangian solid that deforms subject to the physical principles which dictate boundary position. As an added benefit, the same Lagrangian solid mechanics can be exploited to solve multi-field problems for which the solid motion and stresses interact with other transport phenomena, either within the same material phase (e.g. shrinking coating) or in neighboring material phases (e.g. flexible blade coating). Thus, analyses of many fluid-structure interaction problems and deformable porous media problems are accessible. This document serves as a user`s guide and reference for GOMA and provides a brief overview of GOMA`s capabilities, theoretical background, and classes of problems for which it is targeted.
01 Nov 2013
TL;DR: In this paper, the laminar boundary layer flow and heat transfer of non-Newtonian fluid from a permeable horizontal cylinder in the presence of thermal and hydrodynamic slip conditions is analyzed.
Abstract: The laminar boundary layer flow and heat transfer of Casson non-Newtonian fluid from a permeable horizontal cylinder in the presence of thermal and hydrodynamic slip conditions is analysed. The cyl...
TL;DR: In this article, the authors present a numerical model for the PEM fuel cell, which consists of non-linear, coupled partial differential equations representing the conservation of mass, momentum, species, charges and energy with electrochemical reactions.
Book•
30 Jan 2013
TL;DR: The aim of this book is to train students and non-specialists to use the spectral method for solving problems that model fluid flow in closed geometries with heat or mass transfer, and to bring a working knowledge of fluid mechanics and heat transfer.
Abstract: Transport phenomena problems that occur in engineering and physics are often multi-dimensional and multi-phase in character. When taking recourse to numerical methods the spectral method is particularly useful and efficient. The book is meant principally to train students and non-specialists to use the spectral method for solving problems that model fluid flow in closed geometries with heat or mass transfer. To this aim the reader should bring a working knowledge of fluid mechanics and heat transfer and should be readily conversant with simple concepts of linear algebra including spectral decomposition of matrices as well as solvability conditions for inhomogeneous problems. The book is neither meant to supply a ready-to-use program that is all-purpose nor to go through all manners of mathematical proofs. The focus in this tutorial is on the use of the spectral methods for space discretization, because this is where most of the difficulty lies. While time dependent problems are also of great interest, time marching procedures are dealt with by briefly introducing and providing a simple, direct, and efficient method. Many examples are provided in the text as well as numerous exercises for each chapter. Several of the examples are attended by subtle points which the reader will face while working them out. Some of these points are deliberated upon in endnotes to the various chapters, others are touched upon in the book itself.
TL;DR: In this paper, an electricallyconducting couple stress non-Newtonian model is employed to accurately simulate physiological fluids in peristaltic flow through a sinusoidally contracting channel of finite length.
Abstract: Magnetic fields are increasingly being utilized in endoscopy and gastric transport control. In this regard, the present study investigates the influence of a transverse magnetic field in the transient peristaltic rheological transport. An electrically-conducting couple stress non-Newtonian model is employed to accurately simulate physiological fluids in peristaltic flow through a sinusoidally contracting channel of finite length. This model is designed for computing the intra-bolus oesophageal and intestinal pressures during the movement of food bolus in the digestive system under magneto-hydro-dynamic effects. Long wavelength and low Reynolds number approximations have been employed to reduce the governing equations from nonlinear to linear form, this being a valid approach for creeping flows which characterizes physiological dynamics. Analytical approximate solutions for axial velocity, transverse velocity, pressure gradient, local wall shear stress and volumetric flow rate are obtained for the non-dimensional conservation equations subject to appropriate boundary conditions. The effects of couple stress parameter and transverse magnetic field on the velocity profile, pressure distribution, local wall shear stress and the averaged flow rate are discussed with the aid of computational results. The comparative study of non-integral and integral number of waves propagating along the finite length channel is also presented. Magnetic field and non-Newtonian properties are found to strongly influence peristaltic transport.
TL;DR: In this article, a model of the self-sustained thermally-induced oscillations of a two-phase system consisting of an isolated confined liquid-vapor meniscus (a single liquid plug adjoining a vapor bubble) inside a circular capillary tube, the tube length being exposed to a net temperature gradient, thereby creating a continuous cycle of evaporation and condensation, leading to thermally induced auto-oscillations.
TL;DR: A top-scale model to describe bulk flow of platelet suspensions is presented and dissipative particle dynamics are employed to model viscous flow dynamics and a novel and general no-slip boundary condition is presented that allows the description of three-dimensional viscous flows through complex geometries.
Abstract: Stresses on blood cellular constituents induced by blood flow can be represented by a continuum approach down to the μm level; however, the molecular mechanisms of thrombosis and platelet activation and aggregation are on the order of nm. The coupling of the disparate length and time scales between molecular and macroscopic transport phenomena represents a major computational challenge. In order to bridge the gap between macroscopic flow scales and the cellular scales with the goal of depicting and predicting flow induced thrombogenicity, multi-scale approaches based on particle methods are better suited. We present a top-scale model to describe bulk flow of platelet suspensions: we employ dissipative particle dynamics to model viscous flow dynamics and present a novel and general no-slip boundary condition that allows the description of three-dimensional viscous flows through complex geometries. Dissipative phenomena associated with boundary layers and recirculation zones are observed and favorably compared to benchmark viscous flow solutions (Poiseuille and Couette flows). Platelets in suspension, modeled as coarse-grained finite-sized ensembles of bound particles constituting an enclosed deformable membrane with flat ellipsoid shape, show self-orbiting motions in shear flows consistent with Jeffery’s orbits, and are transported with the flow, flipping and colliding with the walls and interacting with other platelets.
TL;DR: Although solute transport in biofilms is commonly thought to be diffusion dominated, this analysis shows that hydrodynamic dispersion effects may significantly contribute to transport.
Abstract: Many microorganisms live within surface-associated consortia, termed biofilms, that can form intricate porous structures interspersed with a network of fluid channels. In such systems, transport phenomena, including flow and advection, regulate various aspects of cell behavior by controlling nutrient supply, evacuation of waste products, and permeation of antimicrobial agents. This study presents multiscale analysis of solute transport in these porous biofilms. We start our analysis with a channel-scale description of mass transport and use the method of volume averaging to derive a set of homogenized equations at the biofilm-scale in the case where the width of the channels is significantly smaller than the thickness of the biofilm. We show that solute transport may be described via two coupled partial differential equations or telegrapher's equations for the averaged concentrations. These models are particularly relevant for chemicals, such as some antimicrobial agents, that penetrate cell clusters very slowly. In most cases, especially for nutrients, solute penetration is faster, and transport can be described via an advection-dispersion equation. In this simpler case, the effective diffusion is characterized by a second-order tensor whose components depend on (1) the topology of the channels' network; (2) the solute's diffusion coefficients in the fluid and the cell clusters; (3) hydrodynamic dispersion effects; and (4) an additional dispersion term intrinsic to the two-phase configuration. Although solute transport in biofilms is commonly thought to be diffusion dominated, this analysis shows that hydrodynamic dispersion effects may significantly contribute to transport.
TL;DR: An implementation, within a standard quantum chemistry package, of the NEGF formalism with an efficient approximation for the self-energy, which accounts both for absorbing boundary conditions and for exchange splitting of the energy bands in ferromagnetic electrodes is presented.
Abstract: In the field of molecular spintronics, experimental techniques have achieved a stage where it is feasible to explore the interplay between quantum electron transport and magnetism at the single molecule level. An example is a spin-polarized STM, which can probe local electrical currents through organic molecules deposited on magnetic surfaces. The atomistic complexity of nanoscale systems calls for a first-principles description of spin-dependent transport phenomena, e.g., based on the nonequilibrium Green’s function (NEGF) formalism merged with density functional theory (DFT). However, for the case of molecular junctions with transition metal electrodes, a computation of the underlying Kohn–Sham Hamiltonian can be a challenging problem: a simultaneous and accurate description of spin ordered magnetic surfaces together with the electronic structure of a molecule is required. In the present work, we provide a solution for this problem. We present an implementation, within a standard quantum chemistry packa...
TL;DR: It is shown that there exists some potential function directly defined on the space of the extensive and/or intensive variables for any transformation, and that meets the evolution criterion without any restriction on the chemical reaction kinetics.
TL;DR: In this paper, a three-dimensional computational fluid dynamics (CFD) model is developed for an anode-supported planar SOFC from the Chinese Academy of Science Ningbo Institute of Material Technology and Engineering (NIMTE).
Abstract: In this study, a three-dimensional computational fluid dynamics (CFD) model is developed for an anode-supported planar SOFC from the Chinese Academy of Science Ningbo Institute of Material Technology and Engineering (NIMTE). The simulation results of the developed model are in good agreement with the experimental data obtained under the same conditions. With the simulation results, the distribution of temperature, flow velocity and the gas concentrations through the cell components and gas channels is presented and discussed. Potential and current density distributions in the cell and overall fuel utilization are also presented. It is also found that the temperature gradients exist along the length of the cell, and the maximum value of the temperature for the cross-flow is at the outlet region of the cell. The distribution of the current density is uneven, and the maximum current density is located at the interfaces between the channels, ribs and the electrodes, the maximum current density result in a large over-potential and heat source in the electrodes, which is harmful to the overall performance and working lifespan of the fuel cells. A new type of flow structure should be developed to make the current flow be more evenly distributed and promote most of the TPB areas to take part in the electrochemical reactions.
TL;DR: A review of general approaches to modeling mass, heat, and momentum transfer through gaseous mixtures over the whole range of gas rarefaction can be found in this paper.
Abstract: In vacuum technology, one deals with gaseous mixtures more frequently than with a single gas, but the information about transport phenomena in mixtures published in the open literature is very poor. Moreover, methods to model mixture flows are more complicated than those for single gas. The aim of this work is to review general approaches to modeling mass, heat, and momentum transfer through gaseous mixtures over the whole range of gas rarefaction. This review is written in an easy, accessible manner avoiding hard mathematical derivations, though an extensive list of references is provided for readers wishing to find more details about the field. Results for some classical problems such as velocity slip and temperature jump coefficients, Poiseuille flow, Couette flow, and heat transfer for gaseous mixtures are presented in graphical form. A comparison of these results with those corresponding to a single gas is presented, which shows the peculiarities of the transport phenomena in mixtures and gives us an...
TL;DR: In this article, an electrical model of lithium-ion battery is presented based on a 1D analog representation of mass and charge transport phenomena, which can be directly implemented in standard simulation software used in electrical engineering (such as Saber, in our case).
TL;DR: In this paper, a flow separation technique was developed to experimentally demonstrate that the overall transport processes including pressure drop and heat transfer could be significantly improved by routing a portion of the incoming flow through a passive microjet.
TL;DR: In this article, the SU(2) Einstein-Yang-Mills model with backreaction was used to analyze the transport properties of p-wave superfluids within gauge/gravity duality, in particular the fluctuation modes of helicity zero in addition to helicity one and two modes studied earlier.
Abstract: We complete the analysis of transport phenomena in p-wave superfluids within gauge/gravity duality, using the SU(2) Einstein-Yang-Mills model with backreaction. In particular, we analyze the fluctuation modes of helicity zero in addition to the helicity one and two modes studied earlier. We compute a further transport coefficient, associated to the first normal stress difference, not previously considered in the holographic context. In the unbroken phase this is related to a minimally coupled scalar on the gravity side. Moreover we find transport phenomena related to the thermoelectric and piezoelectric effects, in particular in the direction of the condensate, as well as the flexoelectric effect. These are similar to phenomena observed in condensed matter systems.
TL;DR: In this article, a novel mathematical model is proposed, based on thermodynamics and transport phenomena fundamentals, that aims to capture the hydrogen pressure, temperature and molar volume evolution during a hydrogen vehicle's fill-up process.