439 research outputs found
A review of performance degradation and failure modes for hydrogen-fuelled polymer electrolyte fuel cells
A qualitative account of the causes and effects of performance degradation and failure
in hydrogen-fuelled polymer electrolyte fuel cells (PEFCs) is given in the present review. The purpose
of the review is to establish a backbone understanding of the phenomenological processes
that occur within the PEFC, how they interact, how they are influenced through elements of
design, manufacturing and operation, and ultimately how they result in performance degradation
and cell failure. In the current work, 22 common faults are identified which are induced by 48
frequent causes. The major PEFC components considered here that are susceptible to faults are
the polymer electrolyte-based membrane, the anode and cathode catalyst layers, gas diffusion
and microporous layers, seals and the bipolar plate. Faults pertaining to these components can
cause irreversible increases in activation, mass transportation, ohmic and fuel efficiency losses,
or indeed cause catastrophic cell failure
Onset of cellular instabilities in spherically propagating hydrogen-air premixed laminar flames
Using high-speed Schlieren and Shadow photography, the instabilities of outwardly
propagating spherical hydrogen-air flames have been studied in a constant volume
combustion bomb. Combustion under different equivalence ratios (0.2 w 1.0), temperatures
(298 K w 423 K) and pressures (1.0 bar w 10.0 bar) is visualized. The results show that
flames experience both unequal diffusion and/or hydrodynamic instabilities at different
stages of propagation. The critical flame radius for such instabilities is measured and
correlated to the variations of equivalence ratio, temperature and pressure. Analysis
revealed that equivalence ratio affects unequal diffusion instability via varying the Lewis
number, Le; increased temperature can delay both types of instabilities in the majority of
tests by promoting combustion rate and changing density ratio; pressure variation
has minor effect on unequal diffusion instability but is responsible for enhancing
hydrodynamic instability, particularly for stoichiometric and near-stoichiometric flames
A proposed agglomerate model for oxygen reduction in the catalyst layer of proton exchange membrane fuel cells
Oxygen diffusion and reduction in the catalyst layer of PEM fuel cell is an important process in fuel cell modelling, but models able to link the reduction rate to catalyst-layer structure are lack; this paper makes such an effort. We first link the average reduction rate over the agglomerate within a catalyst layer to a probability that an oxygen molecule, which is initially on the agglomerate surface, will enter and remain in the agglomerate at any time in the absence of any electrochemical reaction. We then propose a method to directly calculate distribution function of this probability and apply it to two catalyst layers with contrasting structures. A formula is proposed to describe these calculated distribution functions, from which the agglomerate model is derived. The model has two parameters and both can be independently calculated from catalyst layer structures. We verify the model by first showing that it is an improvement and able to reproduce what the spherical model describes, and then testing it against the average oxygen reductions directly calculated from pore-scale simulations of oxygen diffusion and reaction in the two catalyst layers. The proposed model is simple, but significant as it links the average oxygen reduction to catalyst layer structures, and its two parameters can be directly calculated rather than by calibration
Jablotron 100-Based Corporation Access System
Bakalářská práce se zabĂ˝vá problematikou návrhu a realizace pĹ™ĂstupovĂ©ho systĂ©mu podniku s elektronickĂ˝m zabezpeÄŤovacĂm systĂ©mem Jablotron 100 a jeho propojenĂm s jinĂ˝mi systĂ©my. Byl navrĹľen a naprogramován mĹŻstek s API rozhranĂm pro ovládánĂ systĂ©mu Jablotron 100. Tento mĹŻstek byl pouĹľit pro realizaci webovĂ©ho uĹľivatelskĂ©ho rozhranĂ k ovládánĂ pĹ™ĂstupovĂ©ho systĂ©mu. Dále byl vytvoĹ™en koncept zĂskávánĂ dat docházky ze systĂ©mu Jablotron 100.This bachelor thesis deals with the design and implementation of an enterprise access control system based on the electronic security system Jablotron 100 and its connection with external systems. The bridge with an API interface for controlling the Jablotron 100-based corporation access system has been designed and programmed. This bridge was used to implement the web user interface to control the access system. Furthermore, the concept of collecting attendance data from the Jablotron 100-based system was created.
Trends of ductility at 5°C.
In recent years, diatomite has been successfully adopted in asphalt modification to overcome the problems of polymer modified asphalt, because of the advantages in wide sources, low price, and worthy technical characteristics. Although the improvement of the high-temperature performance of the modified diatomite asphalt has been verified in previous studies, the diatomite will bring negative impact on the low-temperature resistance. Hence, the objective of this study is to seek a new channel to improve the comprehensive performance of the diatomite modified asphalt binder. Considering the advantage of the SBR in improving the low-temperature performance of asphalt binder, the diatomite/SBR composite modified asphalt binder (DSA) and the corresponding preparation technology are developed to obtain an improved comprehensive performance via the orthogonal experiment method in this study. Moreover, the modification mechanisms of the DSA are revealed using fluorescence microscopy (FM) tests, Gel permeation chromatography (GPC) tests, and Fourier transform infrared spectroscopy (FTIR) tests.</div
Descriptive statistics of main variables.
Share repurchase is not only an important financial policy of the company, but also a financial policy with a very wide range of influence. A scientific and reasonable share repurchase policy has a significant financial effect in terms of an equity structure optimization, capital structure adjustment, cash holding control, and even management compensation system design. The long-standing share repurchase in the U.K. and U.S. companies and the recent practice of share repurchase by Chinese listed companies imply an important financial proposition: share repurchase is a high-quality financial policy that, when properly applied, will help reduce the cost of capital. The data in this study show that share repurchases can reduce the level of firms’ cost of capital and play an important role in enhancing their competitiveness. This study also finds that Chinese listed companies, when formulating and implementing share repurchase programs, do not subjectively view share repurchases as a policy tool to adjust cash holdings and capital structure. In addition, they do not view share repurchases as an alternative to cash dividends but use equity incentives for management as the main intention for share repurchases. This decision-making concept is not conducive to the rational use of share repurchase policies in the long run and should be a cause for concern in the business community. Finally, the leverage and the cash holding path are the main motives affecting share repurchases and are the important fundamental mechanisms in this study, while the mediating mechanism test finds that share repurchases mainly reduce the cost of capital through two information perspectives of analyst following and stock liquidity. The heterogeneity analysis shows that the role of share repurchases in reducing the cost of capital is more obvious in enterprises with fierce product market competition, high institutional investor shareholding.</div
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