Laminar fluid flow and mass transfer in a standard field and laboratory emission cell

The field and laboratory emission cell (FLEC) is becoming a standard method of characterizing pollutant emissions from building materials. Based on this method, the material and the inner surface of the FLEC cap form a cone-shaped cavity. The airflow is distributed radially inward over the test surface through a slit in a circular-shaped channel at the perimeter of the chamber. After mass transfer, the air is exhausted through an outlet in the center. Usually, emission rate profiles are obtained using such cells. However, the local convective mass transfer coefficients are now needed. In this study, laminar fluid flow and mass transfer in a standard FLEC are investigated. The velocity field and moisture profiles are obtained by solving Navier-Stokes equations numerically. The whole geometry, including the air inlet and outlet, channel, air slit, and emission space, are included in the numerical modeling domain. The mean convective mass transfer coefficients are calculated and compared with the experimental data. In the test, distilled water is used in the FLEC lower chamber to substitute the emission surface. Mass transfer data are obtained by calculating humidity differences between the inlet and outlet of a gas stream flowing through the FLEC. The study concentrates on assessing the variations of velocity and humidity profiles, as well as convective mass transfer coefficients, in the cell.     

Simultaneous heat and moisture transfer through a composite supported liquid membrane

Composite supported liquid membranes (SLM) are an efficient transfer media to recover heat and moisture from exhaust air due to the high moisture diffusivity in the liquid layer. However, heat transfer has adverse effects on moisture transfer since the water concentration in the LiCl solution decreases at higher temperatures. This study gives a detailed quantitative analysis of these effects. More specifically, simultaneous heat and moisture transfer through a composite supported liquid membrane is modeled. The SLM involved comprises three layers: two hydrophobic porous skin layers and a hydrophilic porous support layer where a layer of LiCl liquid solution is immobilized in the macro and micro pores as the permselective substance. The equations governing the heat mass transport in the microstructures, as well as the transfer of heat and moisture in the air streams adjacent to the membrane, are solved numerically in a coupled way. An experiment has been built to validate the model. The results found that though heat transfer has adverse effects on moisture transfer, in general, the effects on moisture effectiveness are quite limited. The high moisture permeation rates of SLM can be sustained when there is concomitant simultaneous heat transfer.     

Mass transfer of volatile organic compounds from painting material in a standard field and laboratory emission cell

The field and laboratory emission cell (FLEC) is becoming a standard method of characterizing pollutant emissions from building materials. It is significant to use the emission profiles from FLEC to scale the emissions of building materials in real buildings. The dynamics of mass transfer in such an FLEC are the key to perform this task. In this study, the mass transfer mechanisms of the total volatile organic compounds from a wet painting in an FLEC are experimentally and numerically investigated. A three-dimensional mass transfer model, which takes into account the convective mass transfer between the material and the air, the diffusion in the paint film and in the substrate, is developed. The emissions from a water-based emulsion paint are quantified to assess the model. The concentration fields in the film and substrate are calculated to demonstrate the processes of internal volatile organic compounds diffusion.     

The effect of baffle shape on the performance of a polymer electrolyte membrane fuel cell with a biometric flow field

Flow field design on the cathode side, inspired by leaf shapes, leads to a high performance, as it achieves a good distribution of reactants. Furthermore, the addition of baffles to the cathode channel also increases the supply of reactants in the cathode catalyst. However, research on the addition of baffles to the cathode channel has still been limited to straight channels and conventional flow fields. Therefore, in this work, a numerical study was conducted to investigate the effect of baffles on the leaf flow field on the performance of a polymer electrolyte membrane fuel cell. The generated 3D model is composed of nine layers with a 25-cm² active area. The beam and chevron shapes of the baffles, which were inserted into the mother channel, were compared. The simulation results revealed that the addition of beam-shaped baffles that are close to each other can increase the current and power densities by up to 18% due to the more uniform distribution of the oxygen mass fraction.     

Mass transfer characteristics of the hydrophilic membrane with the isolation of heat transfer effect by isothermal bath

The efficiency and lifetime of a proton exchange membrane fuel cell (PEMFC) system is critically affected by the humidity of incoming gas which should be maintained properly for normal operating conditions. But the experimental characteristics of the humidifier are rarely reported. Water transport through the hydrophilic membrane is a coupled phenomenon of heat and mass transport. In this study, a laboratory scale test bench is designed to investigate the characteristics of water transport through the hydrophilic membrane. The mass transfer capability of the hydrophilic membrane is evaluated over various flow rates, temperature, pressure, and flow arrangements. In the experiment, the test bench is submerged in a constant temperature bath in order to isolate the effect of temperature variation between dry air and humid air. The results show the water transport of the hydrophilic membrane is significantly affected by operating temperature and operating pressure. Additionally, the flow arrangement demonstrates a minor effect but it should be considered along with the heat transfer effect.     

Oxygen permeation through Nafion 117 membrane and its impact on efficiency of polymer membrane ethanol fuel cell

We investigate oxygen permeation through Nafion 117 membrane in a direct ethanol fuel cell and elucidate how it affects the fuel cell efficiency. An obvious symptom of oxygen permeation is the presence of significant amounts of acetaldehyde and acetic acid in the mixture leaving anode when no current was drawn from the fuel cell (i.e. under the open circuit conditions). This parasitic process severely lowers efficiency of the fuel cell because ethanol is found to be directly oxidized on the surface of catalyst by oxygen coming through membrane from cathode in the absence of electric current flowing in the external circuit. Three commonly used carbon-supported anode catalysts are investigated, Pt, Pt/Ru and Pt/Sn. Products of ethanol oxidation are determined qualitatively and quantitatively at open circuit as a function of temperature and pressure, and we aim at determining whether the oxygen permeation or the catalyst's activity limits the parasitic ethanol oxidation. Our results strongly imply the need to develop more selective membranes that would be less oxygen permeable.     

Heat,air and moisture transfer through hollow porous blocks

The combined heat, air and moisture transfer in building hollow elements is of paramount importance in the construction area for accurate energy consumption prediction, thermal comfort evaluation, moisture growth risk assessment and material deterioration analysis. In this way, a mathematical model considering the combined two-dimensional heat, air and moisture transport through unsaturated building hollow bricks is presented. In the brick porous domain, the differential governing equations are based on driving potentials of temperature, moist air pressure and water vapor pressure gradients, while, in the air domain, a lumped approach is considered for modeling the heat and mass transfer through the brick cavity. The discretized algebraic equations are solved using the MTDMA (MultiTriDiagonal-Matrix Algorithm) for the three driving potentials. Comparisons in terms of heat and vapor fluxes at the internal boundary are presented for hollow, massive and insulating brick blocks. Despite most of building energy simulation codes disregard the moisture effect and the transport multidimensional nature, results show those hypotheses may cause great discrepancy on the prediction of hygrothermal building performance.     

Development of a miniaturized polymer electrolyte membrane fuel cell with silicon separators

The fabrication process and electrochemical characterization of a miniaturized PEM fuel cell with silicon separators were investigated. Silicon separators were fabricated with silicon fabrication technologies such as by photolithography, anisotropic wet etching, anodic bonding and physical vapor deposition (PVD). A 400 μm × 230 μm flow channel was made with KOH wet etching on the front side of a silicon separator, and then a 550 nm gold current collector and 350 nm TiN_x thin film heater were respectively formed on the front side and the opposite side by PVD. Two separators were assembled with the membrane electrode assembly (MEA) having a 4 cm² active area for the single cell. With pure hydrogen and oxygen under atmospheric pressure without humidification, the performance of the single fuel cell was measured. A single cell operation led to generation of 203 mW cm⁻² at 0.6 V at room temperature, which corresponded to 360 mW cm⁻³ in terms of volumetric fuel cell power density, with 20 ccm of gas flow rate of hydrogen and oxygen at the inlet.     

Modeling heat and moisture transfer through fibrous insulation with phase change and mobile condensates

This paper reports on a transient model of coupled heat and moisture transfer through fibrous insulation, which for the first time takes into account of evaporation and mobile condensates. The model successfully explained the experimental observations of Farnworth [Tex. Res. J. 56 (1986) 653], and the numerical results of the model were found to be in good agreement with the experimental results of a drying test. Based on this model, numerical simulation was carried out to better understand the effect of various material and environmental parameters on the heat and moisture transfer. It was found that the initial water content and thickness of the fibrous insulation together with the environmental temperature are the three most important factors influencing the heat flux.     

Effects of hydrophilic/hydrophobic properties of gas flow channels on liquid water transport in a serpentine polymer electrolyte membrane fuel cell

The droplet dynamics in the serpentine flow channel of a hydrogen fuel cell has been numerically investigated to obtain ideas for designing a serpentine channel with the aim of effectively preventing flooding. Three-dimensional two-phase flow simulations employing the volume of fluid (VOF) method have been performed. Liquid droplets emerging from four adjacent pores at the hydrophobic bottom wall are subjected to airflow in the bulk of the serpentine flow channel. The effects of contact angle variation of the channel walls on liquid water removal have been tested in terms of liquid water saturation and coverage of liquid water on the gas diffusion layer (GDL) surface. The numerical results show that the hybrid case, which consists of hydrophilic channel walls at the straight part and hydrophobic walls at the turning part of the serpentine flow channels, enhances water removal compared with two other cases in which the channel wall is homogeneously hydrophilic or hydrophobic. The three-dimensional visualization of liquid water droplets reveals that the hydrophobic wall at the turning part reduces the water saturation in the channel and the hydrophilic wall at the straight part prevents the liquid water from covering the GDL surface.     

Investigation of moisture transfer mechanism in porous media during a rapid drying process

In this work, the moisture transfer mechanism in wet porous media during a rapid drying process was investigated experimentally and analytically. By means of scanning electron microscopy, the rapid drying processes for potato, carrot, and radish species were observed and recorded. A new displacement model using the pressure gradient in a porous material during rapid drying was suggested. To analyze this displacement flow in a porous material, the variables of this flow in a single capillary tube, such as velocity, flow rate, as well as the displacement time of internal moisture, were calculated. © 2000 Scripta Technica, Heat Trans Asian Res, 30(1): 22–27, 2001     

Performance of laboratory polymer electrolyte membrane hydrogen generator with sputtered iridium oxide anode

The continuous improvement of the anode materials constitutes a major challenge for the future commercial use of polymer electrolyte membranes (PEM) electrolyzers for hydrogen production. In accordance to this direction, iridium/titanium films deposited directly on carbon substrates via magnetron sputtering are operated as electrodes for the oxygen evolution reaction interfaced with Nafion 115 electrolyte in a laboratory single cell PEM hydrogen generator. The anode with 0.2 mg cm⁻² Ir catalyst loading was electrochemically activated by cycling its potential value between 0 and 1.2 V (vs. RHE). The water electrolysis cell was operated at 90 °C with current density 1 A cm⁻² at 1.51 V without the ohmic contribution. The corresponding current density per mgr of Ir catalyst is 5 A mg⁻¹. The achieved high efficiency is combined with sufficient electrode stability since the oxidation of the carbon substrate during the anodic polarization is almost negligible.     

Development of flow field design of polymer electrolyte membrane fuel cell using in-situ impedance spectroscopy

The design of a flow field channel of a polymer electrolyte membrane fuel cell (PEMFC) was investigated by computational fluid dynamic (CFD) simulation and in-situ three-channel impedance spectroscopy. To investigate the efficiency of the in-situ three-channel impedance spectroscopy method, it was adopted with a heterogeneous stack, which was composed according to three different types of flow field design. The in-situ three-channel method proved its validity by showing corresponding result with that obtained from the experiments and CFD simulation at the same experimental condition. This study demonstrates that a heterogeneous stack and in-situ three-channel impedance spectroscopy are powerful tools for predicting and analyzing the performance of a fuel cell stack.     

Free vibration analysis of a polymer electrolyte membrane fuel cell

A free vibration analysis of a polymer electrolyte membrane fuel cell (PEMFC) is performed by modelling the PEMFC as a 20 cm × 20 cm composite plate structure. The membrane, gas diffusion electrodes, and bi-polar plates are modelled as composite material plies. Energy equations are derived based on Mindlin's plate theory, and natural frequencies and mode shapes of the PEMFC are calculated using finite element modelling. A parametric study is conducted to investigate how the natural frequency varies as a function of thickness, Young's modulus, and density for each component layer. It is observed that increasing the thickness of the bi-polar plates has the most significant effect on the lowest natural frequency, with a 25% increase in thickness resulting in a 17% increase in the natural frequency. The mode shapes of the PEMFC provide insight into the maximum displacement exhibited as well as the stresses experienced by the single cell under vibration conditions that should be considered for transportation and stationary applications. This work provides insight into how the natural frequencies of the PEMFC should be tuned to avoid high amplitude oscillations by modifying the material and geometric properties of individual components.     

Performance equations for a polymer electrolyte membrane fuel cell with unsaturated cathode feed

A mathematical formulation for the cathode of a membrane electrode assembly of a polymer electrolyte membrane fuel cell is proposed, in which the effect of unsaturated vapor feed in the cathode is considered. This mechanistic model formulates the water saturation front within the gas diffusion layer with an explicit analytical expression as a function of operating conditions. The multi-phase flows of gaseous species and liquid water are correlated with the established capillary pressure equilibrium in the medium. In addition, less than fully hydrated water contents in the polymer electrolyte and catalyst layers are considered, and are integrated with the relevant liquid and vapor transfers in the gas diffusion layer. The developed performance equations take into account the influences of all pertinent material properties on cell performance using first principles. The mathematical approach is logical and concise in terms of revealing the underlying physical significance in comparison with many other empirical data fitting models.     

A method for predicting heat and moisture transfer through multilayered walls based on temperature and moisture content gradients

A mathematical formulation applied to a numerically robust solver is presented, showing that moisture content gradients can be used as driving forces for heat and moisture transport calculation through the interface between porous materials with different pore size distribution functions. For comparison purposes, several boundary conditions are tested—in order to gradually increase the discontinuity effects—and a detailed analysis is undertaken for the temperature and moisture content distributions and sensible and latent heat fluxes, when the discontinuity on the moisture content profile is taken or not into account.     

Apparent contact angles of liquid water droplet breaking through a gas diffusion layer of polymer electrolyte membrane fuel cell

The lattice Boltzmann method is used to simulate the three-dimensional dynamic process of liquid water breaking through the gas diffusion layer (GDL) in the polymer electrolyte membrane fuel cell. An accurate method is introduced to analyze asymmetric droplet shape. Ten micro-structures of Toray GDL were built based on a stochastic geometry model. It was found that asymmetric droplets are produced on the GDL surfaces. Their local apparent contact angles vary with different view angles and geometries. They are different to the idealized contact angles by symmetric simplification. It was concluded that the apparent contact angles are influenced by GDL structures and view angles. This information can help to bridge the gap between mesoscale and cell-scale simulations in the field of fuel cell simulation.     

Network based optimization model for pin-type flow field of polymer electrolyte membrane fuel cell

The flow field of a bipolar plate distributes reactants for polymer electrolyte membrane (PEM) fuel cells and removes the produced water from the fuel cells. It greatly influences the performance of fuel cells, especially the concentration losses. Pin-type design, as one of the widely utilized flow field configurations, has the advantage of low pressure drop but the disadvantage of uneven flow distribution and stagnant areas. In this paper, a network-based optimization model was developed to optimize the pin-type flow field configuration. Two optimization cases, uniform flow distribution with and without considering reactant consumption along flow channels, were investigated and the corresponding optimized designs were developed. Three-dimensional numerical simulations of PEM fuel cell were conducted to validate the proposed optimization model and also to compare the performance of the fuel cells using the optimized designs and the regular pin-type design. The numerical results verified the designs generated with the present optimization model. Both the optimized designs exhibited substantially higher fuel cell performance compared to the regular pin-type configuration. Moreover, the optimized design with considering reactant consumption performed even better than the one without considering the consumption, emphasizing the significance of reactant consumption in the flow field optimization model.     

Review of hydrogen crossover through the polymer electrolyte membrane

Proton exchange membrane fuel cell (PEMFC) is considered to be a promising, clean, and efficient energy conversion device. At present, the main challenges faced by the application of PEMFC in the automotive are cost and durability. Hydrogen from anode to the cathode through polymer electrolyte membrane (i.e. crossover hydrogen) affects the durability of fuel cells. In this paper, the existing literature on hydrogen crossover is reviewed and summarized from consequences, causes, mitigation measures, and detection methods. The influences of hydrogen crossover on the components and performance are discussed. The causes are analyzed from structural permeation and membrane degradation. The methods of alleviating the degradation of the membrane are summarized. The electrochemical and non-electrochemical monitoring methods are described, and the segmented current method is explained separately. The existing problems and research prospects are put forward, which lays a foundation for further research on hydrogen crossover and improvement fuel cell durability.