Mass Transport In Systems Research Articles

A multiscale approach for stable relaxation parameter values in lattice Boltzmann simulations of heat and mass transport in porous media

Present work proposes a multiple time-scale lattice-Boltzmann method (LBM) for solving coupled convection-diffusion equations with large variation in diffusion coefficients. The key to the proposed approach is to vary the computational time-step over the solution domain according to the value of the diffusion coefficient. The proposed approach removes the limited applicability of classical LBM in simulating coupled transport phenomena with high diffusivity ratios. The proposed method is also found to be very efficient in solving heat and mass transfer problems in systems involving a combination of clear and porous media. The results indicate the efficacy of the proposed method in handling coupled flow, heat and mass transport in systems where diffusion coefficients values vary over several orders of magnitude.

MXE: A package for simulating long-term diffusive mass transport phenomena in nanoscale systems

We present a package to simulate long-term diffusive mass transport in systems with atomic scale resolution. The implemented framework is based on a non-equilibrium statistical thermo-chemo-mechanical formulation of atomic systems where effective transport rates are computed using a kinematic diffusion law. Our implementation is built as an add-on to the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code, it is compatible with other LAMMPS’ functionalities, and shows a good parallel scalability and efficiency. In applications involving diffusive mass transport, this framework is able to simulate problems of technological interest for exceedingly large time scales using an atomistic description, which are not reachable with the state-of-the-art molecular dynamics techniques. Several examples, involving complex diffusive behavior in materials, are investigated with the framework. We found good qualitative and quantitative comparison with known theories and models, with Monte Carlo methods, as well as with experimental results. Thus, our implementation can be used as a tool to understand diffusive behavior in materials where experimental characterization is difficult to perform. Program summaryProgram Title: MXE packageProgram Files doi:http://dx.doi.org/10.17632/s2mhjb8hyk.1Licensing provisions: GNU GPLv3Programming language: c++Supplementary material: The user manual and examples are provided along with the source code.External routines: MPI, LAMMPS, 12 December 2018 (http://lammps.sandia.gov/)Nature of problem: The simulation of diffusive mass transport in atomistic systems, involving vacancies, interstitials and solute atoms, is often challenging due to the exceedingly large time scale involved in these slow processes. Thus, molecular dynamics, a preferred technique to simulate atomistic systems, is not capable of accessing these long-term diffusive timescale, hindering its application to this kind of phenomena.Solution method: We present an implementation for simulating diffusive mass transport in atomic system. The methodology is based on two main pillars: (i) A non-equilibrium thermodynamic formulation of atomic system (Venturini et al., 2014); and (ii) Fokker–Planck master equation that encompasses the time evolution of the atomic molar fraction field in atomic systems (Ponga and Sun, 2018). The proposed implementation is built as a user package of the popular Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The implementation is flexible and robust, shows good parallel scalability and efficiency, and is compatible with all features available in LAMMPS.Additional comments including restrictions and unusual features: Unique features of the implementation involve the simulation of vacancy, interstitial, solutes and substitutional alloys for exceedingly large time scales.The current implementation in this package is limited to the Embedded Atom Model potentials.

A unified framework for heat and mass transport at the atomic scale

We present a unified framework to simulate heat and mass transport in systems of particles. The proposed framework is based on kinematic mean field theory and uses a phenomenological master equation to compute effective transport rates between particles without the need to evaluate operators. We exploit this advantage and apply the model to simulate transport phenomena at the nanoscale. We demonstrate that, when calibrated to experimentally-measured transport coefficients, the model can accurately predict transient and steady state temperature and concentration profiles even in scenarios where the length of the device is comparable to the mean free path of the carriers. Through several example applications, we demonstrate the validity of our model for all classes of materials, including ones that, until now, would have been outside the domain of computational feasibility.

Counting Single Redox Turnovers: Fluorogenic Antioxidant Conversion and Mass Transport Visualization via Single Molecule Spectroelectrochemistry

We report herein the use of single molecule spectroelectrochemistry (SMS-EC) to optically detect electrochemically activated individual molecules of the redox-active, tocopherol-like fluorogenic probe H2B-PMHC. Single molecule detection combined with a fluorescence burst frequency analysis is shown to provide a most sensitive method to monitor subnanomolar concentrations of the electrochemically activated probe. By measuring changes in activated probe concentration over time and distance from a planar electrode, mass transport characteristics of a prototype infinite planar electrode system are retrieved as a showcase study. We show that fluorescence activation may be achieved under both oxidative (direct oxidation of H2B-PMHC) and reductive (via O2•- production) conditions. Our results conducted in organic solvents broaden the scope of electrochemical reactions that can be studied by SMS-EC. The methodology we describe may potentially aid the study of mass transport in systems with complex electrode geome...

An implicit conservative scheme for coupled heat and mass transfer problems with multiple moving interfaces

A conservative scheme for phase transformations under mixed control of heat and mass transport has been deduced. Based on the conservative formulation of the Stefan condition for isothermal diffusion problems in two-phase systems by Illingworth and Golosnoy (Journal of Computational Physics 209 (2005) 207–225), a scheme for transformations under mixed control of heat and mass transport in systems containing multiple moving interfaces was developed. The scheme is illustrated for a phase bounded by two moving interfaces in cartesian, cylindrical or spherical 1D coordinates and for a fully implicit time discretization. In the limiting cases the scheme reduces to either an isothermal diffusion case or the thermally driven transformation of a pure component, for which good agreement with analytical solutions is obtained. The non-isothermal dissolution of a solid phase in a liquid phase illustrates the scheme under mixed control conditions.

Simulation of mass transport in systems with repulsive isotropic pair potential

The results of the numerical investigation of mass-transfer processes of extensive quasi-two-dimensional and three-dimensional nonideal dissipative systems are presented. Simulations were performed for different types of model pair potentials for inter-grain interaction that had various combinations of power-law and exponential functions. The parameters of calculations corresponded to the conditions of laboratory dusty plasma experiments. It was shown that the dynamics of grains in nonideal liquid-like systems on short observation times is close to the evolution of thermal oscillations in the crystal lattice.

A hybrid differencing scheme for mass transport in electrochemical systems

PurposeTo present a new hybrid differencing scheme for the numerical solution of an electromigration‐diffusion equation. The value of this work is evidenced by demonstrated improvement in the simulation of the Fu and Chan experiment when using the hybrid scheme.Design/methodology/approachA hybrid differencing scheme is developed which is based upon the solution of the pseudo‐steady state electromigration‐diffusion equation. In this scheme, a weighting parameter is calculated that varies the relative influence of the upwind node (relative to the direction of electromigration). This scheme significantly enhances the accuracy of electrochemical system mass transport models.FindingsThe hybrid scheme was compared to the upwind scheme. Use of the new hybrid scheme improved the accuracy of the model predictions by as much as 87 percent compared to the upwind scheme. However, use of the new scheme also increased the simulation time by between 6 and 43 percent. Deviations from electroneutrality and the presence of an activity coefficient gradient were detrimental to the stability of the hybrid scheme.Research limitations/implicationsThis scheme is presented in the paper as an one‐dimensional (1D) scheme. However, it could be extended to more than 1D but some artificial viscosity may result.Practical implicationsThe hybrid scheme developed and demonstrated herein is useful for researchers developing mass transport models of electrochemical systems. It has been proven capable of improving the accuracy of electrolyte mass transport models.Originality/valueThis is the first hybrid differencing scheme designed for the special characteristics of electrochemical mass transport systems. It greatly improves the accuracy of simulation results. This work is useful to those who mathematically model electrochemical systems.

Calcium mass transport and sandstone diagenesis during compaction-driven flow: Stevens Sandstone, San Joaquin basin, California

Sediment compaction provides a limited source of fluids for diagenesis and drives fluid flow at average rates of only ∼1 mm/yr. Nevertheless, many geochemical and petrographic studies of diagenesis provide evidence of significant mass transport in systems where fluid flow appears to be compaction driven. This apparent discrepancy could arise because diagenetic studies generally are concerned with relatively permeable petroleum reservoirs. Focused fluid flow through permeable zones could increase local flow rates and allow mixing of fluids from different sources. This study uses the Stevens Sandstone of the San Joaquin basin to explore the potential diagenetic effects of fluid focusing during compaction-driven flow over a 5 m.y. period. Reactive-transport simulations incorporate a new kinetic expression for plagioclase dissolution and suggest that rate-limited plagioclase dissolution drove calcium enrichment of pore fluids and caused precipitation of calcite, kaolinite, and albite. In spite of the importance of this rate-limited reaction, simulations show that influx of fluids from compacting shale could limit the distribution of calcite and kaolinite in adjacent sandstones. Although calcium mass transport is predicted on a scale of kilometers, upward flow of calcium-rich fluids from deep beds does not significantly increase calcite volume relative to closed-system predictions. Increased transverse dispersivity increases mixing, which further limits precipitation of calcite and kaolinite. Results are consistent with field observations of fluid chemistry, although simulations account for

Mass Transfer Between Phases in a Porous Medium: A Study of Equilibrium

Abstract To study mass transport in systems simulating oil recovery processes, different porous media were saturated with a mobile (carrier phase) and a stationary phase. Slugs of carrier phase containing a small amount of solute were displaced with pure carrier phase. By analogy to the chromatographic processes, the velocity of the solute can be predicted from a knowledge of the partition coefficient and the saturation provided that equilibrium between the two phases exists. Equilibrium was found to exist for different porous media, solutes and rates. The conditions were varied over the range normally encountered in the laboratory and in the field. The longitudinal dispersion of a solute undergoing interphase mass transfer was also investigated. Introduction The production of hydrocarbons by gas cycling, enriched gas drive and CO2 or alcohol displacement involves, among other factors, relative motion between two phases and compounds, hereafter called solute, which are soluble in both phases. The solute is carried forward by the faster flowing phase at a lower velocity than the average velocity of that phase. Retardation of the solute is caused by chromatographic absorption and desorption in the slower flowing phase and by the degree of departure from equilibrium. At equilibrium the concentration of solute in the two phases can be related by the equation* (1) where Csw and Cso are the concentration of solute in the aqueous and oleic phases respectively and K is the equilibrium ratio, or partition coefficient. Displacement theories must contain an explicit assumption with regard to equilibrium, i.e., whether the compositions can be related by Eq. 1. The existance of equilibrium depends, in general on the relative velocity between the phases. Unfortunately, other factors such as gravity segregation and viscous fingering, also depend on velocity. For this reason, whenever effects of rate on displacement were observed, it was practically impossible to discern what caused them - lack of equilibrium or the factors mentioned above. Equilibrium between phases has been the subject of extensive studies in fields such as extraction or chromatography. It has received only small attention in flow through the type of porous media encountered in oil production. For this reason a method was developed which makes it possible to study the movement of a solute as it is affected by rate, type of porous media, partition coefficient and carrier phase, but in the absence of segregation or fingering. The information obtained enables one to determine when the assumption of equilibrium can be made. Briefly, the method consists of (1) saturating the core with a mobile and an immobile phase, (2) injecting a slug made up of the same fluid as the mobile phase and a small concentration of mutually soluble solute, (3) measuring the lag and the peak height of the slug at arrival and (4) correlating these variables with fluid properties such as partition coefficient and mixing constants of the medium. PROPOSED MECHANISM The principles of chromatography are combined with the equation of longitudinal mixing to predict the velocity of a solute slug compared to the bulk velocity and the peak height of a slug. The equation so obtained is valid under equilibrium conditions only. Therefore, a comparison between experimental and predicted results will give a measure of departure from equilibrium. This work was done with either the oleic or the aqueous phase being immobile. For simplicity, the following development is based on the case where the oleic phase is immobile. However, the treatment is the same in either case. SPEC P. 51ˆ