Numerical analysis of the influence of the channel cross-section aspect ratio on the performance of a PEM fuel cell with serpentine flow field design

This work numerically investigates the influence of the channel cross-section aspect ratio (defined as the ratio height/width) on the performance of a PEM fuel cell with serpentine flow field (SFF) design. The local current densities, velocity distributions, liquid water concentration in the membrane, hydrogen and oxygen concentrations and temperature were analyzed in the PEM fuel cell for 10 different aspect ratios, varying between 0.07 and 15, to understand the channel cross-section aspect ratio effect. The area of the channel cross section (1.06 mm²) and the total effective reactive area of the PEM fuel cell (256 mm²) were maintained constant in all cases. The obtained results show that at low operating voltages the cell performance is independent of the channel cross-section aspect ratio. At high operating voltages, where the influence of mass transporting velocity is predominant, as the channel cross-section aspect ratio increases the cell performance is improved. The models with high aspect ratio show, in general, more uniform current distributions, with the higher maximum and minimum intensity values, temperature distributions with smaller gradients and a superior contain of water in the membrane, which allows to obtain a higher performance. From these models the 10/06 and 12/05 aspect ratio present the best combination of variables, as shown by their polarization curves.     

Measures of specificity of fuzzy sets under T-indistinguishabilities

The concept of measure of specificity of fuzzy sets is extended in this paper to measure of specificity under the knowledge of T-indistinguishabilities. Four axioms of measure of specificity under T-indistinguishabilities are given. An algorithm to compute the inference independent set /spl Tfr/ from a fuzzy set and a T-indistinguishability is used to measure the specificity of fuzzy sets under T-indistinguishabilities satisfying the axioms.     

New redox deposition-precipitation method for preparation of supported manganese oxide catalysts

A new preparation method has been developed to produce homogeneous coatings of manganese compounds on alumina-coated monoliths by redox deposition-precipitation using acetone as solvent. The deposition is produced by reduction of manganese permanganate with ethanol. The Lewis acid sites on the alumina catalyze this reaction. Thus, the precipitation is produced preferentially on the surface of the monolith and not in the bulk of the solution. Monoliths prepared by this method are very active for the complete oxidation of oxygenated volatile organic compounds (VOCs). These monoliths consist of a mixture of complex phases including aluminum, sulfated alumina, potassium sulfate and manganese oxide.     

The effects of an electrostatic field and air stream on water jet break-up length

An experimental study of the break-up length of a circular section liquid jet subject to the combined action of a coaxial airstream and an electrostatic field is reported. A high speed photographic technique has been employed and the results have been correlated empirically by the following dimensionless equation: where “b” and “k” are functions of Reynolds number.     

Fracture toughness of epoxy resins modified with polyethersulfone: Influence of stoichiometry on the morphology of the mixtures

Toughened mixtures containing 15 wt % polyethersulfone were made with diglycicdyl ether of bisphenol-A resin and 4,4′-diaminodiphenylmethane curing agent, with amine/epoxy group stoichiometric ratios varying from 0.6 to 1.5. Fracture behavior of the modified mixtures has been investigated as a function of the stoichiometry in the matrix. Morphology has been analyzed by transmission and scanning electron microscopy. The increase of amine content in the matrix results in a further increased fracture toughness. This behavior has been related to the changes on the ductility of the matrix upon stoichiometric ratio, but also to the changes on microstructural features of the modified mixtures as stoichiometric amine/epoxy group ratio increased. These morphological changes have been interpreted in terms of spinodal decomposition during curing of the epoxy matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 183–191, 1998     

Conductive additive content balance in Li-ion battery cathodes: Commercial carbon blacks vs. in situ carbon from LiFePO4/C composites

Two samples of commercial conducting carbon black and the carbon generated in situ during LiFePO₄/C composite synthesis from citric acid are studied, with the aim of finding out whether carbon from the composite can fulfil the same function as carbon black in the electrode blend for a Li-ion battery. For this purpose, the carbon samples are analyzed by several techniques, such as X-ray diffraction, Raman spectroscopy, transmission electron microscopy, granulometry, BET specific area and conductivity measurements. Different cathode compositions and component proportions are tested for pellet and cast electrodes. Electrochemical results show that a moderate reduction of commercial carbon black content in both kinds of cathodes, by adding more LiFePO₄/C composite, enhanced the electrochemical behaviour by around 10%. In situ generated carbon can partially replace commercial conducting carbon black because its high specific surface probably enhances electrolyte penetration into the cathode, but it is always necessary to maintain a minimum amount of carbon black that provides better conductivity in order to obtain a good electrochemical response.