Search results for "Diffusion layer"

Mass Transport Analysis of Bicarbonate Buffer: Effect of the CO2–H2CO3 Hydration–Dehydration Kinetics in the Fluid Boundary Layer and the Apparent Ef…

2019

The main buffering system influencing ionizable drug dissolution in the human intestinal fluid is bicarbonate-based; however, it is rarely used in routine pharmaceutical practice due to the volatility of dissolved CO2. The typical pharmaceutical buffers used fail to capture the unique aspects of the hydration-dehydration kinetics of the bicarbonate-CO2 system. In particular, CO2 is involved in a reversible interconversion with carbonic acid (H2CO3), which is the actual conjugate acid of the system, as follows CO2 + H2O ⇌ H2CO3. In contrast to ionization reactions, this interconversion does not equilibrate very rapidly compared to the diffusional processes through a typical fluid diffusion b…

On the scavenging of SO2 by large and small rain drops: V. A wind tunnel and theoretical study of the desorption of SO2 from water drops containing S…

1993

An experimental and theoretical study has been carried out to investigate the rate of desorption of SO2 from water drops falling at terminal velocity in air. The experiments were carried out in the Mainz vertical wind tunnel in which water drops of various sizes containing S(IV) in various concentrations were freely suspended in the vertical airstream of the tunnel. The results of these experiments were compared with the predictions of three theoretical models, and with the experiments of Walceket al. This comparison shows that the predictions of the diffusion model of Kronig and Brink in the formulation given by Walcek and Pruppacher agree well with the experimental results for all relevan…

Diffusion, thermodiffusion, and thermal diffusion of polystyrene in solution

1962

Impact of cyclic mechanical stress on the electrical contact resistance between gas diffusion layer and bipolar plates in Polymer Electrolyte Membran…

2018

WHEC 2018, 22nd edition of the WORLD HYDROGEN ENERGY CONFERENCE, Rio de Janeiro, BRESIL, 17-/06/2018 - 22/06/2018; and collisions) and the mechanical clamping stress applied during the stack assembly. The internal stresses come from the shrinking and the swelling of the membrane caused by water content and the difference between the thermal expansion coefficients of the cell components. These mechanical stresses have an impact on the global fuel cell's performance and one of the most sensitive cell's components to this pressure is the Gas Diffusion Layer (GDL). This layer, assembled with the membrane electrode assembly, provides the reactant gases, evacuates the produced heat and …

Space charge effect on competitive ion transport through ion-exchange membranes

2002

A mathematical model of the competitive electro-transport of two counter-ions through an ion exchange membrane based on the Nernst-Planck and Poisson equations is developed. A three-layer system is considered: the membrane and two adjacent diffusion layers. Concentration profiles in the three layers, effective transport numbers as functions of the current and current-voltage characteristics are calculated. Deviation from the local electroneutrality in space charge region near the depleted solution/membrane interface is taken into account. It is shown that the space charge region grows with the voltage applied. However the fluxes of the competitive counter-ions at over-limiting currents are …

Numerical analysis of the influence of ultrasonic vibration on crystallization processes

2011

The challenge in the future fabrication of semiconductor bulk crystals is the improvement of the crystal quality with a simultaneous increase of the yield. For that, a proper control of mass transfer within the fluid phase is required. Besides the damping of violent convective fluctuations, the thickness of the diffusion boundary layer, causing morphological instability, has to be decreased. The influence of ultrasound in molten Germanium was analyzed by numerical simulations. The simulations were provided by applying commercial software packages ANSYS ® and FLUENT ® . ANSYS ® was used to model the ultrasonic wave propagation in the whole growth system consisting of melt and crystal, crucib…

Characterization process to measure the electrical contact resistance of Gas Diffusion Layers under mechanical static compressive loads

2016

AEM2016. International conference on Advanced Energy Materials, University of Surrey, Guildford, ROYAUME-UNI, 12-/09/2016 - 14/09/2016; Recent research has identified the mechanical properties of the fuel cell internal components (in particular, the Gas Diffusion Layers - GDLs) as key-parameters to obtain high final performances of the generator. The mechanical compression modulus of these components, the stability of their mechanical properties with respect to temperature and humidity, and their ability to interact with water have an impact on the electrical contact resistances in the stack and, by consequence, on the overall performance of the electric generator. Reducing the losses by co…

Multiphysical modeling of dissimilar welding via interlayer

2011

Abstract A multiphysical finite element modeling of dissimilar welding via interlayer material was proposed. A 2D model including heat transfer, fluid flow and level set problems allowed to simulate the morphology and the composition of melted zone in horizontal plane. The calculated thickness of melted interlayer was used as a main criterion for the choice of optimal welding conditions, when the chemical interaction between the joined materials must be avoided. A 1D diffusion model at the limit of melted zone allowed estimating the length and the composition of diffusion layer between one of the materials and the interlayer basing on previously calculated local temperature gradient. The si…

How Electrical Heterogeneity Parameters of Ion-Exchange Membrane Surface Affect the Mass Transfer and Water Splitting Rate in Electrodialysis

2020

Electrodialysis (ED) has been demonstrated as an effective membrane method for desalination, concentration, and separation. Electroconvection (EC) is a phenomenon which can essentially increase the mass transfer rate and reduce the undesirable water splitting effect. Efforts by a number of researchers are ongoing to create conditions for developing EC, in particular, through the formation of electrical heterogeneity on the membrane surface. We attempt, for the first time, to optimize the parameters of surface electrical heterogeneity for ion-exchange membranes used in a laboratory ED cell. Thirteen different patterns on the surface of two Neosepta anion-exchange membranes, AMX and AMX-Sb, w…

Mechanistic analysis and experimental verification of bicarbonate-controlled enteric coat dissolution: Potential in vivo implications

2019

Enteric coatings have shown in vivo dissolution rates that are poorly predicted by traditional in vitro tests, with the in vivo dissolution being considerably slower than in vitro. To provide a more mechanistic understanding of this, the dependence of the release properties of various enteric-coated (EC) products on bulk pH and bicarbonate molarity was investigated. It was found that, at presumably in vivo-relevant values, the bicarbonate molarity is a more significant determinant of the dissolution profile than the bulk pH. The findings also indicate that this steep relationship between the dissolution of enteric coatings and bicarbonate molarity limits those coatings' performance in vivo.…