Coupled Transport Processes Research Articles

Multiphysics and Multiscale Modeling of Coupled Transport of Chloride Ions in Concrete.

Chloride ions (Cl−)-induced corrosion is one of the main degradation mechanisms in reinforced concrete (RC) structures. In most situations, the degradation initiates with the transport of Cl− from the surface of the concrete towards the reinforcing steel. The accumulation of Cl− at the steel-concrete interface could initiate reinforcement corrosion once a threshold Cl− concentration is achieved. An accurate numerical model of the Cl− transport in concrete is required to predict the corrosion initiation in RC structures. However, existing numerical models lack a representation of the heterogenous concrete microstructure resulting from the varying environmental conditions and the indirect effect of time dependent temperature and relative humidity (RH) on the water adsorption and Cl− binding isotherms. In this study, a numerical model is developed to study the coupled transport of Cl− with heat, RH and oxygen (O2) into the concrete. The modeling of the concrete microstructure is performed using the Virtual Cement and Concrete Testing Laboratory (VCCTL) code developed by the U.S. National Institute of Standards and Technology (NIST). The concept of equivalent maturation time is utilized to eliminate the limitation of simulating concrete microstructure using VCCTL in specific environmental conditions such as adiabatic. Thus, a time-dependent concrete microstructure, which depends on the hydration reactions coupled with the temperature and RH of the environment, is achieved to study the Cl− transport. Additionally, Cl− binding isotherms, which are a function of the pH of the concrete pore solution, Cl− concentration, and weight fraction of mono-sulfate aluminate (AFm) and calcium-silicate-hydrate (C-S-H), obtained from an experimental study by the same authors are utilized to account for the Cl− binding of cement hydration products. The temperature dependent RH diffusion was considered to account for the transport of Cl− with moisture transport. The temperature and RH diffusion in the concrete domain, composite theory, and Cl− binding and water adsorption isotherms are used in combination, to estimate the ensuing Cl− diffusion field within the concrete. The coupled transport process of heat, RH, Cl−, and O2 is implemented in the Multiphysics Object-Oriented Simulation Environment (MOOSE) developed by the U.S. Idaho National Laboratory (INL). The model was verified and validated using data from multiple experimental studies with different concrete mixture proportions, curing durations, and environmental conditions. Additionally, a sensitivity analysis was performed to identify that the water-to-cement (w/c) ratio, the exposure duration, the boundary conditions: temperature, RH, surface Cl− concentration, Cl− diffusion coefficient in the capillary water, and the critical RH are the important parameters that govern the Cl− transport in RC structures. In a case study, the capabilities of the developed numerical model are demonstrated by studying the complex 2D diffusion of Cl− in a RC beam located in two different climatic regions: warm and humid weather in Galveston, Texas, and cold and dry weather in North Minnesota, Minnesota, subjected to time varying temperature, RH, and surface Cl− concentrations.

Faxén formulas for particles of arbitrary shape and material composition

Abstract

Natural and Anthropogenic Analogues for High-Level Nuclear Waste Disposal Repositories: A Review

ABSTRACTProjects involving deep geological disposal of nuclear waste are unique in that predictive models of long-term safety often involve geological timeframes. This manuscript provides a review of natural and anthropogenic analogues for high-level nuclear waste disposal in a deep geological repository. We also occasionally highlight analogues that have been used for low- and intermediate-level waste. Most studies define natural analogues as either naturally occurring or anthropogenic systems. In this paper, we distinguish between natural analogues and anthropogenic analogues because the latter generally provide non-technical (anecdotal) illustrations of concepts and processes for the safety case, whereas the former can provide technical and quantitative information. In addition, natural analogues can provide information over geological time scales (millions of years) and spatial scales (kilometers), whereas anthropogenic analogues provide information over a much more limited time scale (hundreds or thousands of years). Regardless of the definition, analogue studies provide one of the multiple lines of evidence intended to increase confidence in the safe geological disposal of high-level nuclear waste. They are deemed necessary because they complement the experiments that are carried out over a period of months or years. They also provide a way to validate numerical long-term safety assessment models with information and data covering geological time scales and spatial scales.The first part of this review describes the analogue concept. The second and third parts provide examples of natural and anthropogenic analogues for engineered barrier systems and natural barriers. Part four describes analogues for complex coupled transport processes. Finally, we discuss general and specific areas of future research.A concerted effort should be made to ensure that there is a transfer of data from the complex, natural analogue field studies to simplistic models which, by necessity, are used to evaluate the long-term safety of deep geological repositories. Field analogue studies should be planned to interface with laboratory experiments and, ultimately, with in situ field experiments, when the final repository site is selected. This will involve using natural analogue data in a quantitative way to support the deep geological repository safety case.

On Existence, Uniqueness and Two-Scale Convergence of a Model for Coupled Flows in Heterogeneous Media

This paper is concerned with the global existence, uniqueness and homogenization of degenerate partial differential equations with integral conditions arising from coupled transport processes and chemical reactions in three-dimensional highly heterogeneous porous media. Existence of global weak solutions of the microscale problem is proved by means of semidiscretization in time deriving a priori estimates for discrete approximations needed for proofs of existence and convergence theorems. It is further shown that the solution of the microscale problem is two-scale convergent to that of the upscaled problem as the scale parameter goes to zero. In particular, we focus our efforts on the contribution of the so-called first order correctors in periodic homogenization. Finally, under additional assumptions, we consider the problem of the uniqueness of the solution to the homogenized problem.

Fluid injection with supercritical reservoir conditions: Overview on morphology and mixing

The present work provides an overview on the possible phase transitions associated with supercritical fluid injection and a detailed evaluation of the mixing process between injectant fluid and quiescent ambience. The experiments cover superheated liquid disintegration, pseudo-boiling transition and single-phase jets under different nozzle pressure ratios. Pseudo-boiling effects emerge when rapid, subsequent changes in pressure/temperature interact with the non-linear behavior of thermodynamic response functions across the Widom line. The associated density fluctuations cause a significant increase in the scattering cross section and may lead to thermo-convective instabilities. Our analysis of the mixing process demonstrates the limited applicability of the adiabatic mixing model, which is often restricted to short residence times even in highly turbulent jets (Re=O(105)). Specifically, our findings show the importance of considering all coupled transport processes in the analysis of mixing problems at high pressures, in particular in presence of large mass concentration and temperature gradients.

Transport phenomena in fluid films with curvature elasticity

Abstract

Electrochemical pressure impedance spectroscopy applied to polymer electrolyte membrane fuel cells for investigation of transport phenomena

The performance of proton exchange membrane fuel cells (PEMFC) is partly governed by complex, coupled transport processes at high polarization which usually occur in the low frequency region. Electrochemical impedance spectroscopy (EIS) is a powerful tool to extract valuable information about ohmic, charge and mass transport processes but fails marginally below 1 Hz as limited information on mass transport can be extracted. In this work, electrochemical pressure impedance spectroscopy (EPIS) was performed by applying fluctuations of the gas pressure at the fuel cell cathode outlet in the frequency range 1 mHz to 1 Hz to peek into transport processes. An experimental bench was designed and setup for reliable EPIS measurements on a single 100 cm2 FC. Conditions of pressure modulation in the test bench were thoroughly examined, leading to a standard protocol of operation. Two cases were studied in an attempt to better understand and differentiate the various transport phenomena occurring in fuel cell operation. Sensitivity of impedance modulus to mass transport control by the presence of liquid water could be shown for frequency below 0.1 Hz. Moreover, EPIS spectra exhibit striking profile differences in both modules and phases, with variations of the oxygen mole fraction in the cathode gas fed, evidencing gas diffusion phenomena in particular over 0.1 Hz. EPIS has been shown here to become an attractive complementary technique to EIS, even if further work is required for full beneficiation of this promising tool.

Global weak solutions to degenerate coupled transport processes in partially saturated deformable elastic-inelastic porous media

In this work we prove the existence of global weak solutions to a degenerate and strongly coupled parabolic system arising from the transport processes through partially saturated deformable porous materials. The hygro-thermal model is coupled with quasi-static evolution equations modeling elastic and inelastic mechanical deformations. Physically relevant Newton boundary conditions are considered for water pressure and temperature of the porous system. The traction boundary condition is imposed on the deformable solid skeleton of the porous material. Degeneration occurs in both elliptic and parabolic part of the balance equation for mass of water. The coupling between water pressure, temperature, stress tensor and internal variables occurs in transport coefficients, constitutive functions and the decomposition of the total strain tensor into elastic and plastic parts due to mechanical effect and strain tensor due to thermal expansion.
 For more information see https://ejde.math.txstate.edu/Volumes/2020/63/abstr.html

Analysis of transport processes in a reacting flow of hybrid nanofluid around a bluff-body embedded in porous media using artificial neural network and particle swarm optimization

This paper investigates heat and mass transfer in a hybrid nanofluid flow impinging upon a cylindrical bluff-body embedded in porous media and featuring homogenous and heterogeneous chemical reactions. The analysis includes mixed convection and local thermal non-equilibrium in the porous medium as well as Soret and Dufour effects. Assuming single-phase mixture, a laminar flow of Al2O3-Cu-water (Aluminium oxide-Copper-water) hybrid nanofluid is considered and coupled transport processes are simulated computationally. Due to the significant complexity of this problem, containing a large number of variables, conventional approaches to parametric study struggle to provide meaningful outcomes. As a remedy, the simulation data are fed into an artificial neural network to estimate the target responses. This shows that the volume fraction of nanoparticles, interfacial area of the porous medium and mixed convection parameter, are of primary importance. It is also observed that small variation in the volume fraction of nanoparticles can considerably alter the response of thermal and solutal domains. Further, it is shown that the parameters affecting the thermal process can modify the problem chemically. In particular, raising the volume fraction of nanoparticles enhances the production of chemical species. Furthermore, particle swarm optimization is applied to predict correlations for Nusselt and Sherwood numbers through a systematic identification of the most influential parameters. The current study clearly demonstrates the advantages of using the estimator algorithms to understand and predict the behaviours of complex thermo-chemical and solutal systems.

Study on linear bio-structure print process based on alginate bio-ink in 3D bio-fabrication

Ink-jet printing is a non-impact printing technology in which drops jetted from an orifice onto a designated position. This technology can digitally transport fluids containing cells precisely onto desired substrates to construct three-dimensional organs. In order to obtain the stable uniform droplets, a stream is the key point of this technology. However, there are so many factors that affect the uniform droplet stream construction process: print parameters, material parameters, control method, etc. A good understanding of the various coupled transport processes that occur during bio-ink impact and spreading on bio-structure can improve the success of print-ability. This paper aims to obtain a good linear bio-structure with ink-jet printing technology. First, a typical droplet deposition process model is constructed; including droplet dynamics impact models and droplet diffusion cap models. Second, a model of successive droplet overlap, to form linear bio-structures, is constructed. Third, the finite element method is used to simulate the droplet impact, collision, and fusion process. Finally, the main influencing factors of the continuous injection printing process, namely the time interval between consecutive droplets and the droplet contact angles, are discussed. Sodium alginate is selected as bio-ink to verify the theory, and it is found that a good linear bio-structure could be obtained if the printing parameters are controlled optimally, i.e., if the initial contact angle is set as 60 degrees and the trigger frequency is set as 150 kHz. With a proper printing speed and gel coating, a good survival rate of printed cells could be obtained.

Homogenization of degenerate coupled transport processes in porous media with memory terms

In this paper, we establish a homogenization result for a doubly nonlinear parabolic system arising from the hygro‐thermo‐chemical processes in porous media taking into account memory phenomena. We present a mesoscale model of the composite (heterogeneous) material where each component is considered as a porous system and the voids of the skeleton are partially saturated with liquid water. It is shown that the solution of the mesoscale problem is two‐scale convergent to that of the upscaled problem as the spatial parameter goes to zero.

Precipitation patterns driven by gravity current.

A precipitation reaction can be driven by a gravity current that spreads on the bottom as a denser fluid is injected into an initially stagnant liquid. Supersaturation and nucleation are restricted to locations where the two liquids come into contact; hence, the flow pattern governs the spatial distribution of the final product. In this numerical study, we quantitatively characterize the flow associated with the gravity current prior to the onset of nucleation and distinguish three zones where the coupling of transport processes with the reaction can take place depending on their time scales. A scaling law associated with the region of Rayleigh-Taylor instability behind the tip of the gravity current is also determined.

The effect of w/c ratio and aggregate volume fraction on chloride penetration in non-saturated concrete

The diffusion of moisture and chloride in non-saturated concrete is considered as two coupled transport processes. Two coupling terms are added in the governing equations based on Fick’s first law to represent the interaction between the moisture and chloride during the transport processes. The first coupling term is for the effect of moisture movement on chloride transfer, and the other is for the effect of chloride transfer on moisture movement. In this paper, an experimental study was conducted to investigate the effect of the variation of the water-cement ratio and the aggregate volume fraction on the coupling parameter associated with the first coupling term (DCl-H). To this end, a chloride ponding test was designed and performed on two sets of fully and partially saturated concrete samples made with different water-cement ratios and aggregate volume fractions. The coupling parameter was determined based on the obtained test data. The results indicate that the coupling parameter DCl-H is strongly dependent on the concentration of chloride ions, water to cement ratio, and volume fraction of aggregate. A material model for the coupling parameter was developed using the experimental results.

Insights into the role of particle space charge effects in particle precipitation processes in electrostatic precipitator

Electrostatic precipitators (ESPs) are considered as cost-effective technology for separating the charged particle from flue gas. In this paper, a modified ESP model was developed to investigate the effects of charged particle on the electric field, ion density, particle charging and migration regarding various particle concentrations. Coupled processes of corona discharge, flow field, particle charging and transport were considered. Results showed that the particle space charge presented trajectory-dependent effects on the distributions of electric field and ion density in the gas flow direction. With the particle concentration increasing from 0 to 200 mg/m3, the electric field strength on the electrode surface was inhibited from 1.32 × 106 to 1.24 × 106 V/m, while the electric field strength on the plate surface was raised from 7.1 × 105 to 8.3 × 105 V/m. By contrast, the ion density within the whole domain decreased when particle space charge considered, and it decreased by more than 40% when the particle concentration was 50 mg/m3. The particle charging rate decreased due to particle space charge effects and consequently the overall particle charge decreased by 45.7% as particle concentration increased to 200 mg/m3. The correlation between collection efficiency and corona current was discussed, and it showed that the two didn't vary synchronously with the increasing concentration. A criterion for efficiency deterioration, under corona suppression conditions, was first proposed for instructing ESP design and operation.

Theory and Analytical Solutions to Coupled Processes of Transport and Deformation in Dual-Porosity Dual-Permeability Poro-Chemo-Electro-Elastic Media

The linear theory of dual-porosity and dual-permeability poro-chemo-electro-elasticity is presented. The theory outlines the dual-continuum formulation of multiple coupled processes involving solid deformation, pore fluid flow, and electrically charged species transport, within and in between two coexisting porosity systems of a fluid-saturated, poro-elastic medium. The described formulation is used to derive the analytical solutions to the inclined wellbore problem and axisymmetric Mandel-Type problem of dual-porosity, dual-permeability poro-chemo-electro-elasticity. The effects of chemical and electrical potentials on the distributions of stress and pore pressure are demonstrated by numerical examples pertaining to the considered problems. It is shown that the fully coupled nature of the solutions rigorously captures the seemingly anomalous time variations of the effective stress as driven by the pore fluid pressure disturbances, as well as the distribution and movement of anions/cations within the dual-porosity porous medium. The existing subset of published solutions on the subject is successfully reproduced as special cases of the solutions presented in this paper.

A colorimetric paper-based analytical device coupled with hollow fiber membrane liquid phase microextraction (HF-LPME) for highly sensitive detection of hexavalent chromium in water samples

A simple and highly sensitive procedure based on the combination of hollow fiber membrane liquid phase microextraction and a microfluidic paper-based analytical device (µPAD) was developed for pre-concentration and determination of hexavalent chromium in water samples. The hexavalent chromium was pre-concentrated using the HF-LPME technique via ion exchange or a coupled transport process through a supported ionic liquid (Aliquat 336) prior to colorimetric detection with diphenylcarbazide on the µPAD. The violet colour could be seen by the naked eye. Images from the µPADs were scanned using a commercial desktop scanner at 600 dpi resolution. ImageJ software was used for quantitative analysis by measuring the intensity values at green colour channel since it gave the best sensitivity among the RGB colour. Under optimal conditions, the calibration curve was linear in the range 10–90 µg L-1, with a limit of detection of 3 µg L-1. The developed method was successfully applied to determine the level of hexavalent chromium spiked into natural water samples at the parts-per-billion (ppb) level, and the results were in good agreement with those obtained using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The developed method was able to improve the detection limit of the conventional µPAD, and was expected to be used for the effective analysis of hexavalent chromium in natural water.

Quantification of solute penetration in an asymmetric fracture-matrix system

Solute and/or heat penetration into a fracture-matrix system is an important subject in subsurface transport. Solute and/or heat partitions among the fracture and its surrounding rock matrixes are based on coupled transport processes that are closely related to transport properties of the media. The penetration processes in an asymmetric fracture-matrix system are more complicated than that in a symmetric fracture-matrix system, due to different matrix properties. Accurate quantification of plume distribution in such a system is the basis of remediation design and risk assessment of a contaminated fracture-matrix system. Closed-form analytical and semi-analytical solutions (involving integrations) are obtained in this study to quantify the influences of asymmetric matrix properties on solute penetration processes in a fracture-matrix system. Advection, matrix diffusion, sorption, source decay and aqueous phase decay are considered. Numerical simulations are performed using HydroGeoSphere to test the solutions. Well matched curves of the numerical results and the closed-form solutions and semi-analytical solutions are obtained. The matrix porosity and retardation factor appear to significantly affect the spatiotemporal distribution of solute in a fracture-matrix system. Penetration depth in the matrix is a linear attenuation function of the horizontal distance from the source. And a new dimensionless number called the Z number (or the matrix penetration number) is defined to quantify the maximal penetration depth into the matrix. The solutions obtained can act as an effective tool for assessment of solute and/or heat transport in a fracture-matrix system. The solutions are also applicable to advection-dominated solute and/or heat transport in a thin aquifer bounded by two different aquitards.

Secondary-Phase Stochastics in Lithium-Ion Battery Electrodes.

Lithium-ion battery electrodes exhibit complex interplay among multiple electrochemically coupled transport processes, which rely on the underlying functionality and relative arrangement of different constituent phases. The electrochemically inactive solid phases (e.g., conductive additive and binder, referred to as the secondary phase), while beneficial for improved electronic conductivity and mechanical integrity, may partially block the electrochemically active sites and introduce additional transport resistances in the pore (electrolyte) phase. In this work, the role of mesoscale interactions and inherent stochasticity in porous electrodes is elucidated in the context of short-range (interface) and long-range (transport) characteristics. The electrode microstructure significantly affects kinetically and transport-limiting scenarios and thereby the cell performance. The secondary-phase morphology is also found to strongly influence the microstructure-transport-kinetics interactions. Apropos, strategies have been proposed for performance improvement via electrode microstructural modifications.

On degenerate coupled transport processes in porous media with memory phenomena

AbstractIn this paper we prove the global existence of weak solutions to degenerate parabolic systems coupled with an integral condition arising from the fully coupled moisture movement, transport of dissolved chemical species and heat transfer through porous materials. The problem under consideration covers a large range of problems including hygro‐thermo‐chemical modelling of concrete at early ages taking into account hydration (memory) phenomena. In the model, all changes of material properties are expressed as functions of state variables and the memory function (the so called hydration degree). Physically relevant mixed Dirichlet‐Neumann boundary conditions and initial conditions are considered. Existence of global weak solutions of the problem is proved by means of semidiscretization in time, proving necessary uniform estimates and by passing to the limit from discrete approximations. Degeneration occurs in the nonlinear transport coefficients which are not assumed to be bounded below and above by positive constants. Degeneracies in transport coefficients are overcome by proving suitable a‐priori ‐estimates based on De Giorgi and Moser iteration technique.

On the Limitations of Volume-Averaged Descriptions of Gas Diffusion Layers in the Modeling of Polymer Electrolyte Fuel Cells

Understanding of the coupled transport processes that occur in thin gas diffusion layers (GDLs) is necessary to develop improved designs. The traditional technique used to model GDLs is the volume-averaged approximation. However, the applicability of this approach has been long questioned, and the error in the results is unclear. In this work, the limitations of GDL volume-averaged models are examined under single-phase conditions. The lattice Boltzmann method is combined with tomography images of carbon-paper GDLs to assess the existence of a representative elementary volume (REV) in terms of various effective properties. Then, the predictions of GDL volume-averaged and pore-scale formulations are compared by using a CFD model of a differential cell. The results show that a REV cannot be clearly defined. This leads to inhomogeneities in the pore-scale model that the volume-averaged model is not able to capture despite the overall flux through the GDL is similar in both cases.