Transport properties of sorbing contaminants in a fractured granite under oxidizing conditions

Migration of some sorbing chemical species has been studied in a single rock fracture of 1 m scale in order to understand the transport behavior of contaminants at underground environments. For the tracers, tritium and anions were used as nonsorbing ones and some sorbing cations such as Sr, Co and Cs were used as well. The experimental study was focused on the identification of the retardation and matrix diffusion of the tracer in the fracture. The hydraulic conductivity in the fracture was determined from the pressure differentials between pairs of boreholes. The hydraulic data were used with a variable aperture channel model to characterize the aperture distribution in the fracture. A transport model has been developed to describe the migration of the solutes in the flow field by using a particle tracking method. Results were plotted in the form of elution curves and migration plumes in the fracture. The experimental elution curves have been explored with the transport model which takes into account sorption and diffusion into the rock matrix. This comparison may contribute to further understanding on the heterogeneous flow field and the interactions between rock and chemical species.     

Mass transport with varying diffusion- and solubility coefficient through a catalytic membrane layer

Mass transport, accompanied by chemical reaction through membrane reactor has been investigated in the case of varying diffusion coefficient and solubility coefficient. In reality, both parameters might depend on the concentration and/or on the inhomogeneity of the membrane layer. General mathematical models were developed to describe the mass transport, taking into account the external mass transfer resistances as well, when the solubility coefficient can vary, e.g. according to the Langmuir–Hinschelwood adsorption theory or when the value of diffusion coefficient depends on the concentration/anisotropy in the membrane. A general solution has been given that can be applied to most of the mathematical functions of the parameters mentioned. The concentration distribution and the mass transfer rate will be given in closed mathematical forms. The value of the mass transfer rates could be strongly altered by the varying diffusion- and/or solubility coefficient. The mathematical model and the effect of the varying parameters have been discussed in this paper.     

Effect of penetrant size and shape on its transport through a thermoset adhesive: I. n-alkanes

The diffusion behaviors of a series of n-alkanes, ranging from C₆ to C₁₇, through a polyamide-type polymeric matrix have been investigated by means of mass uptake measurements. Since n-alkanes are known to display negligible interactions with the polymer matrix, this study serves to isolate the effects of penetrant size and shape on the transport process without undue interference from polymer-penetrant interactions. It is established that the diffusion of the n-alkanes through the polymer matrix studied is Fickian and proceeds via a Henry's law-type mechanism. The diffusion coefficients, D, are evaluated based on a thin-film approximation of the Fickian equation. The activation energies of diffusion, E_d, are determined from the temperature dependence of D, using the Arrhenius equation. Correlations between the Arrhenius terms, E_d and D₀, are also established which enable the prediction of diffusion coefficients for similar polymer-penetrant systems. It is also demonstrated by means of activation energy calculations and molecular simulations that the n-alkanes assume a linear geometry within the polymer matrix and diffuse along their long axes.     

Measuring the voidage of a CFB through image analysis

The error associated with estimating the local concentration of particles observed in digital videos of the inner wall of a circulating fluidized bed (CFB) riser is evaluated. Understanding the dynamics of these rapidly moving particles will allow researchers to design cleaner and more efficient CFB facilities. However, the seemingly random motion exhibited by the particles in three dimensions, coupled with the varying image quality, make it difficult to extract the required information from the images. A method has been developed for automatically detecting particles from a video sequence. By exploiting the presence of specular reflections, individual particles are identified along the focal plane by an image filter specifically designed for this purpose. An evaluation of the proposed method indicates its potential to estimate particle count, location, and concentration in an efficient and reliable manner.     

Water transport through a PEM fuel cell: A one-dimensional model with heat transfer effects

Models play an important role in fuel cell design/development. The most critical problems to overcome in the proton exchange membrane (PEM) fuel cell technology are the water and thermal management. In this work, a steady-state, one-dimensional model accounting for coupled heat and mass transfer in a single PEM fuel cell is presented. Special attention is devoted to the water transport through the membrane which is assumed to be a combined effect of diffusion and electro-osmotic drag. The transport of heat through the gas diffusion layers is assumed to be a conduction-predominated process and heat generation or consumption is considered in the catalyst layers. The analytical solutions for concentration and net water transport coefficient are compared with recent published experimental data. The operating conditions considered are various cathode and anode relative humidity (RH) values at and 2 atm. The studied conditions correspond to relatively low values of RH, conditions of special interest, namely, in the automotive applications. Model predictions were successfully compared to experimental and theoretical I-V polarization curves presented by Hung et al. [2007. Operation-relevant modelling of an experimental proton exchange membrane fuel cell. Journal of Power Sources 171, 728-737] and Ju et al. [2005a. A single-phase, non-isothermal model for PEM fuel cells. International Journal of Heat and Mass Transfer 48, 1303-1315]. The developed easy to implement model using low CPU consumption predicts reasonably well the influence of current density and RH on the net water transport coefficient as well as the oxygen, hydrogen and water vapour concentrations at the anode and cathode. The model can provide suitable operating ranges adequate to different applications (namely low humidity operation) for variable MEA structures.     

Uniaxial tensile strength and fracture analysis of a hot-pressed boron carbide

Tensile specimens with three different gage dimensions were used to determine the tensile strength of a hot-pressed boron carbide. The strength-limiting flaw population and information on the fracture behavior under an applied tensile stress were also determined. The material follows traditional strength size-scaling for advanced ceramics, and flexure strength data for the same boron carbide was an excellent fit with the uniaxial tensile strength data. The strength-limiting flaw in all specimens was a carbon-based inclusion or agglomerate. The size of the fracture mirror around the origin and the fracture toughness value estimated from the measured strength-limiting flaw size are in excellent agreement with previously reported data. This information shows that standard four-point flexure strength tests can be used to confidently predict the uniaxial tensile strength of this hot-pressed boron carbide.     

A parametric study of axial segregation in granular systems

When a cylindrical container, partially filled with a binary granular mixture of particles that differ in size or density, is rotated around its axis, a spontaneous segregation of the two granular components may occur. In order to better understand this phenomenon, we have carried out an experimental study probing the effect of average particle size and relative size difference between particles on the onset of segregation. The experimental study is followed by a novel scaling analysis which relates the deterministic, convective driving force for particle segregation to the randomizing diffusional driving force present in these systems through the definition of an axial granular Péclet number. Values of this granular Péclet number are shown to successfully correlate with segregation behavior in the present study, as well as in comparable results in the literature.     

Modelling the transport of carbonic acid anions through anion-exchange membranes

Electrodiffusion of carbonate and bicarbonate anions through anion-exchange membranes (AEM) is described on the basis of the Nernst-Planck equations taking into account coupled hydrolysis reactions in the external diffusion boundary layers (DBLs) and internal pore solution. The model supposes local electroneutrality as well as chemical and thermodynamic equilibrium. The transport is considered in three layers being an anion exchange membrane and two adjoining diffusion layers. A mechanism of competitive transport of HCO₃⁻ and CO₃²⁻ anions through the membrane which takes into account Donnan exclusion of H⁺ ions is proposed. It is predicted that the pH of the depleting solution decreases and that of the concentrating solution increases during electrodialysis (ED). Eventual deviations from local electroneutrality and local chemical equilibrium are discussed.     

A fracture analysis of cathodic delamination at polyurea/steel interfaces

A fracture mechanics approach to accelerated life testing of cathodic delamination between steel and polyurea is presented. This required the hyperelastic behavior of the polyurea to be described by the Marlow model based on uniaxial tension and plane strain compression tests. Time-dependence was also considered but could be neglected if proper test protocols were followed in cathodic delamination tests using a strip blister specimen. The variation of J-integral with specimen geometry and loading parameter was obtained, which allowed the resistance to cathodic delamination to be expressed in terms the J-integral and the crack speeds obtained from the tests at several temperatures. The approach established that both temperature and stress can be used to accelerate the cathodic delamination, thereby providing a quantitative and rational basis for conducting accelerated testing. In addition, the discriminating nature of the approach for design purposes was exemplified by quantitatively establishing differences in the delamination resistance of three surface treatments.     

A transport phenomenon analysis criterion predicting pitting appearance during al anodization in sulphate electrolysis

The conditions of Al anodizing, bath temperatures, current densities, and concentrations of H 2 SO 4 and Al 2 (SO 4 ) 3 in pure H 2 SO 4 , and H 2 SO 4 +Al 2 (SO 4 ) 3 solutions where the growth of porous anodic alumina films is uniform/regular and nonuniform/abnormal with appearance of pitting, were experimentally determined. The equations describing the mass and charge transport phenomena and the composition of the solution at the pore bases during the regular oxide growth were solved. The current densities at which the H 2 SO 4 concentration becomes minimum at low film thicknesses were determined, which coincided with the experimental ones above which pitting appeared. They increased with the bath temperature and the H 2 SO 4 concentration and decreased with the Al 2 (SO 4 ) 3 concentration. A criterion for pitting apearance was thus formulated. It explained the promotion of pitting appearance by the low bath temperatures and H 2 SO 4 concentrations, high current densities, and sulphate additives, which is important for developing methods for producing regularly grown high-quality coatings.     

Numerical approximation of a flow and transport system in unsaturated-saturated porous media

In this paper we develop an approximation scheme for solving a coupled system of flow and contaminant transport with adsorption in unsaturated-saturated porous media. The flow model is based on nonlinear Richard's equation and is coupled with the transport equation through saturation and Darcy's velocity (discharge) terms. The approximation is based on time stepping, operator splitting and the method of characteristics (see Pironneau (Numer. Math. 38 (1982) 309), Douglas and Russel (SIAM J. Numer. Anal. 19(5) (1982) 871)). The nonlinear terms in Richard's equation are approximated by means of a relaxation scheme (see Jäger and Ka?ur (Math. Modelling Numer. Anal. 29 (1995) 605), Ka?ur (IMA J. Numer. Anal. 19 (1999b) 119), Ka?ur (SIAM J. Numer. Anal. 36(1) (1999a) 290)). The convergence of the approximation scheme is proved. Some numerical experiments in 1D are included.     

A model of non-Newtonian slurry flow in a fracture

An accurate calculation of a non-Newtonian slurry flow in a fracture is an important issue for fracture design (see for example, the book edited by Economides and Nolte [M.J. Economides, K.J. Nolte, Reservoir Stimulation, Third edition, Schlumberger, 2000.]). A model taking into account micro-level particle dynamics is developed here. The model shows that the slurry dynamics is governed to a significant extent by particle fluctuations about mean streamlines in a high-shear-rate flow. Particles migrate from zones of high shear rate at the fracture walls towards the center of the fracture where shear rates are lower. Thus, slurry flow in a fracture is characterized by non-uniform solids concentration across the fracture width. Low solids concentration near the walls leads to a reduction of slurry-wall friction as compared with that predicted by a model that does not take particle migration into account. Reduction in the friction at the wall leads to a reduction in the streamwise pressure gradient and hence in the net pressure.     

A numerical investigation of aerosol dynamics in a wall-less reactor

This paper describes a numerical investigation of aerosol formation during silane decomposition in a wall-less reactor. The wall-less reactor is amenable to numerical investigation because the homogeneous chemical reactions leading to the formation of solid particles are isolated from heterogeneous effects, such as occur at the walls of a laminar flow aerosol reactor. The flow/heat transfer and gas-phase chemical kinetics are simulated utilizing separate one-way coupled models. The aerosol dynamics model is based on a simplified sectional model originally developed by Okuyama et al. This model is modified to allow for the simulation of particle growth via condensation. Simulations have been performed which indicate that particle growth via condensation may be an important process. Additionally, the effects of total reactor pressure, temperature and inlet silane concentration on the dynamics of the aerosol population have been investigated. Conditions which result in the formation of larger and more numerous particles have been identified.     

Sensitivity analysis of HIV infection response to treatment via stochastic modeling

This work focuses on the analysis of HIV infection dynamics during the initial stages of infection when the viral load is low and random fluctuations may have a significant effect on the dynamics of the disease. Deterministic models that describe the expected progress of the infection cannot be employed to predict the probability of infection establishment at the primary stage. Consequently, stochastic simulations are used to determine the probability of successful infection in an average patient. A stochastic model based on Gillespie's algorithm is derived which includes mutant species and employed to determine the sensitivity of HIV infection to different treatment strategies such as single therapy and combination therapy with various efficacy values. The effect of treatment latency on virus clearance probability is also investigated.     

A stochastic analysis of the effect of hydrostatic pressure on the pit corrosion of Fe-20Cr alloy

The effect of hydrostatic pressure on the pit corrosion behavior of Fe-20Cr alloy was investigated in 3.5% NaCl solution by means of potentiodynamic polarization and potentiostatic technology, and the experiment data was analyzed based on stochastic theory. With the increase of hydrostatic pressure, the pit corrosion resistance of Fe-20Cr alloy was deteriorated, which was distinguished by the decrease of critical pit potential (E_cirt) and the increase of passive current density. The results also demonstrated that there exist two effects of hydrostatic pressure on the corrosion behavior of Fe-20Cr alloy: (1) the pit generation rate was evidently increased compared to that under lower hydrostatic pressure, and the metastable pits become faster and larger. However, it seemed that pit generation mechanism shows no hydrostatic pressure dependence; (2) the probability of pit growth increased with the increase of hydrostatic pressure, which implied that the metastable pit on Fe-20Cr alloy exhibited higher probability to become larger pit cavity during shorter time interval than that under lower hydrostatic pressure.     

A detailed analysis of the determination of fracture toughness by nanoindentation induced radial cracks

The correlation between the choice of reference material and model utilized in determining fracture toughness from indentation-induced radial cracks was critically investigated with six commonly used reference materials. Initially, the empirical constants in Anstis’s and Laugier’s equations were calculated, compared and analyzed. According to the values of the constants, the reference materials were categorized into three groups. This classification was further verified by evaluating the corresponding constants of 13 additional equations. To account for the classification, FIB technique was employed to examine the crack morphology in the reference materials – Si (100) features an almost Half-penny-shaped crack while the cracks in SiC (0001) display nearly as a rectangle, both far from the assumptions employed by the models. Subsequently, an improved and more reliable procedure to determine fracture toughness is proposed. Finally, with this procedure, the fracture toughness of a diamond/SiC composite film is determined to be 11.1 ± 1.1 MPa m^1/2.     

Influence of sample thickness on fracture behaviour of a polyketone and a polyketone-rubber blend

The influence of sample thickness on the fracture behaviour of an aliphatic polyketone and a blend of this polymer and 10 wt% core-shell rubber was studied. The sample thickness was varied from 0.1 to 8 mm. The skin morphology was studied by SEM. The fracture behaviour was studied on single edge notch specimen at a high strain rate (30 s⁻¹) in the temperature range of −40 to 120 °C. The fracture stress, fracture strain and fracture energies were determined. The temperature development in the notch area was followed with an Infra Red camera. The cavitation of the rubber particles was studied on tensile bars with a laser setup.With decreasing specimen thickness the fracture energies increased strongly and the brittle-ductile transition shifted to lower temperatures this both for the aliphatic polyketone and the polyketone-rubber blend. The deformation in these materials in accompanied with a strong temperature increase in the deformation zone. The addition of rubber particles decreases the sensitivity towards the thickness. However, in very thin samples the cavitation of the rubber particles is more difficult and the rubber toughening effect decreases. The strong thickness effects on the fracture toughness indicate for both the homo polymer and the blend indicate that data from a standard test with 4 mm thick samples are not representative for thin walled applications.     

A laser irradiation disc test for fracture testing of refractory fine ceramics

The critical stress intensity factor, tensile strength and crack stability were analysed for a zirconia refractory by parameter identification based on a laser irradiation test and a finite element simulation. Furthermore, the results for the specific fracture energy were determined for different assumed cohesive behaviours. The tests were carried out on notched discs that were irradiated at their centres. During the tests, temperatures are recorded by a thermo vision camera and the crack propagation by an acoustic emission recorder. Acoustic emission allowed for the determination of the onset of crack initiation (time t₁) and unstable crack propagation (time t₂). A finite element model representing the geometry of the sample was built to determine the fracture mechanical parameters from t₁ and t₂. This method is believed to be favourable for rather brittle refractory ceramics, whereas for less brittle materials, a wedge splitting procedure according to Tschegg is considered more favourable.