40 research outputs found
Transient non-isothermal model of a polymer electrolyte fuel cell
In this paper we present a one-dimensional transient model for the membrane electrode assembly of a polymer-electrolyte fuel cell. In earlier work we established a framework to describe the water balance in a steady-state, non-isothermal cathode model that explicitly included an agglomerate catalyst layer component. This paper extends that work in several directions, explicitly incorporating components of the anode, including a micro-porous layer, and accounting for electronic potential variations, gas convection and time dependence. The inclusion of temperature effects, which are vital to the correct description of condensation and evaporation, is new to transient modelling. Several examples of the modelling results are given in the form of potentiostatic sweeps and compared to experimental results. Excellent qualitative agreement is demonstrated, particularly in regard to the phenomenon of hysteresis, a manifestation of the sensitive response of the system to the presence of water. Results pertaining to pore size, contact angle and the presence of a micro-porous layer are presented and future work is discussed
A transient PEMFC model with CO poisoning and mitigation by O2 bleeding and Ru-containing catalyst
In this paper we present a transient, fully two-phase, non-isothermal model of carbon monoxide poisoning and oxygen bleeding in the membraneelectrode assembly of a polymer electrolyte fuel cell. The model includes a detailed description of mass, heat and charge transport, chemisorption,electrochemical oxidation and heterogeneous catalysis (when oxygen is introduced). Example simulation results demonstrate the ability of themodel to qualitatively capture the fundamental features of the poisoning process and the extent of poisoning with respect to channel temperatureand concentration. Further examples show how the multi-step kinetics can interact with other physical phenomena such as liquid-water flooding,particularly in the anode. Carbon monoxide pulsing is simulated to demonstrate that the complicated reaction kinetics of oxygen bleeding canbe captured and even predicted. It is shown that variations in the channel temperature have a convoluted effect on bleeding, and that trends inperformance on relatively short time scales can be the precise opposite of the trends observed at steady state. We incorporate a bi-functionalmechanism for carbon monoxide oxidation on platinumâruthenium catalysts, demonstrating the marked reduction in the extent of poisoning, theeffect of variations in the platinumâruthenium ratio and the influence of temperature. Finally, we discuss the implications of the results, extensionsto the model and possible avenues for experimental work
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
Imaging Observations of Quasi-Periodic Pulsatory Non-Thermal Emission in Ribbon Solar Flares
Using RHESSI and some auxiliary observations we examine possible connections
between spatial and temporal morphology of the sources of non-thermal hard
X-ray (HXR) emission which revealed minute quasi-periodic pulsations (QPPs)
during the two-ribbon flares on 2003 May 29 and 2005 January 19. Microwave
emission also reveals the same quasi-periodicity. The sources of non-thermal
HXR emission are situated mainly inside the footpoints of the flare arcade
loops observed by the TRACE and SOHO instruments. At least one of the sources
moves systematically both during the QPP-phase and after it in each flare that
allows to examine the sources velocities and the energy release rate via the
process of magnetic reconnection. The sources move predominantly parallel to
the magnetic inversion line or the appropriate flare ribbon during the
QPP-phase whereas the movement slightly changes to more perpendicular regime
after the QPPs. Each QPP is emitted from its own position. It is also seen that
the velocity and the energy release rate don't correlate well with the flux of
the HXR emission calculated from the sources. The sources of microwaves and
thermal HXRs are situated near the apex of the loop arcade and are not
stationary either. Almost all QPPs and some spikes of HXR emission during the
post-QPP-phase reveal the soft-hard-soft spectral behavior indicating separate
acts of electrons acceleration and injection, rather than modulation of
emission flux by some kinds of magnetohydrodynamic (MHD) oscillations of
coronal loops. In all likelihood, the flare scenarios based on the successively
firing arcade loops are more preferable to interpret the observations, although
we can not conclude exactly what mechanism forces these loops to flare up.Comment: 22 pages, 10 figure
An Observational Overview of Solar Flares
We present an overview of solar flares and associated phenomena, drawing upon
a wide range of observational data primarily from the RHESSI era. Following an
introductory discussion and overview of the status of observational
capabilities, the article is split into topical sections which deal with
different areas of flare phenomena (footpoints and ribbons, coronal sources,
relationship to coronal mass ejections) and their interconnections. We also
discuss flare soft X-ray spectroscopy and the energetics of the process. The
emphasis is to describe the observations from multiple points of view, while
bearing in mind the models that link them to each other and to theory. The
present theoretical and observational understanding of solar flares is far from
complete, so we conclude with a brief discussion of models, and a list of
missing but important observations.Comment: This is an article for a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
Extratores quĂmicos empregados na determinação de P como indicadores ambientais em solos tratados com lodo de esgoto
Reduced-dimensional models for straight-channel proton exchange membrane fuel cells
A comprehensive description of proton exchange membrane fuel cell (PEMFC) performance includes the transport phenomena, phase change and electrochemical reaction inside the several components, which possess disparate characteristics and together form a complex three-dimensional geometry. Much of the modelling work in this area has, therefore, relied on the techniques of computational fluid dynamics (CFD). The comprehensive three-dimensional (3D) approach can, however, be prohibitively time consuming. Consequently, it is not the ideal basis for a rapid screening tool that operates under a wide range of design options and operating conditions. Mathematical models and solution procedures using simplified models with reduced dimensions have been proposed to address this issue. Such approaches are computationally efficient, but no systematic study has been conducted to qualitatively or quantitatively assess the impact of the neglected dimensionality on the accuracy of the resulting model. In this paper, we compare results from a hierarchy of reduced-dimensional models to the results from a comprehensive 3D CFD model for a single, straight-channel unit cell. The quality of the simulation results from reduced-dimensional models, including the cell voltage and the distributions of current density and relative humidity, are assessed. We demonstrate that the 2 + 1D approach, which includes mass transport in the 2D cross-section of the channel and membrane electrode assembly and integrates along the flow channel, is optimal in terms of both efficiency and accuracy. It provides a sound basis for a simulation tool that can be used in the early stages of a unit-cell design cycl
Effect of Pt nano-particle size on the microstructure of PEM fuel cell catalyst layers: Insights from molecular dynamics simulations
The effect of Pt nano-particles size on the microstructure of catalyst layers in a Polymer Electrolyte Fuel Cell is investigated by means of molecular dynamics simulations. The catalyst layer model includes carbon-supported platinum, perfluorosulfonate ionomer (PFSI), hydronium ions and water molecules. Three different Pt nano-particle sizes, i.e. 1, 2 and 3 nm, are studied, and simulations provide visualization of the distinct micro-morphologies of the CL corresponding to each nano-particle size. The results are analyzed using pair correlation functions, showing that different microstructures are obtained for different Pt nano-particle sizes, and also that inclusion of PFSI in the simulations impacts significantly the final configuration of Pt nano-particles. Water molecules are found to distribute near the side chains of PFSI and surface of Pt nano-particles, but far from the graphite surface. Side chains form clusters and exhibit different dispersion toward the Pt surface. The orientations of the side chains in the vicinity of the Pt surface are analyzed in detail. The dispersion of perfluorosulfonate ionomer is found to strongly influence the merging of Pt nano-particles and, consequently, the CL microstructure formation.Peer reviewed: YesNRC publication: Ye