The effect of Nafion film on the cathode catalyst layer performance in a low-Pt PEM fuel cell

A single--pore model for performance of the cathode catalyst layer (CCL) in a PEM fuel cell is developed. The model takes into account oxygen transport though the CCL depth and through the thin Nafion film, separating the pore from Pt/C species. Analytical solution to model equations reveals the limiting current density $j_N^{\rm lim}$ due to oxygen transport through the Nafion film. Further, $j_N^{\rm lim}$ linearly depends of the CCL thickness, i.e., the thinner the CCL, the lower $j_N^{\rm lim}$. This result may explain unexpected lowering of low--Pt loaded catalyst layers performance, which has been widely discussing in literature.Comment: 11 page

Potentials near the curved anode edge in a PEM fuel cell: Analytical solution for placing a reference electrode

We consider a PEM fuel cell with concentric circular electrodes: the small anode and the large cathode. A model for in-plane distributions of the cathode overpotential ηc and the membrane potential Φ in the anode-free region of the cell is developed. Mathematically, the problem reduces to the axially symmetric Poisson–Boltzmann equation for ηc. An approximate analytical solution shows that |ηc| exhibits rapid decay to zero with the radius, while |Φ| grows to the value of |η0c|, the cathode overpotential in the working domain of the cell. For typical η0c, the radial shape of ηc far from the anode edge only weakly depends on η0c; this effect is analogous to Debye screening in plasmas. The smaller the anode radius, the faster Φ approaches η0c with the distance from the anode. It follows, that a reference electrode for measuring the cathode overpotential in the working area can be placed at a small distance from the curved anode edge

Performance of a PEM fuel cell with oscillating air flow velocity: A modeling study based on cell impedance

A model of PEM fuel cell impedance is developed taking into account imposed harmonic perturbation of the air flow velocity in the cathode channel. The flow velocity modulation with the amplitude proportional to AC amplitude of the cell potential lowers the resistivity $R_h$ due to oxygen transport in channel. When relative amplitudes of velocity and potential oscillations are equal, a complete compensation of $R_h$ occurs. This effect explains experimental findings of Kim et al. (doi:10.1016/j.jpowsour.2008.06.069) and Hwang et al.(doi:10.1016/j.ijhydene.2010.01.064), who demonstrated significant improvement of PEM fuel cell performance under oscillating air flow velocity.Comment: 6 pages, 4 figure

Performance of a PEM fuel cell cathode catalyst layer under oscillating potential and oxygen supply

A model for impedance of a PEM fuel cell cathode catalyst layer under simultaneous application of potential and oxygen concentration perturbations is developed and solved. The resulting expression demonstrates dramatic lowering of the layer impedance under increase in the amplitude of the oxygen concentration perturbation. In--phase oscillations of the overpotential and oxygen concentration lead to formation of a fully transparent to oxygen sub--layer. This sub--layer works as an ideal non polarizable electrode, which strongly reduces the system impedance.Comment: 4 pages, 3 figure

Comparison of two physical models for fitting PEM fuel cell impedance spectra measured at a low air flow stoichiometry

Local impedance spectra of a segmented PEM fuel cell operated at an air flow stoichiometry of λ = 2 are measured. The local spectra are fitted with the recent 1D and quasi–2D (q2D) physical models for PEMFC impedance. The q2D model takes into account oxygen transport in the gas channel, while the 1D model ignores this transport assuming infinite stoichiometry of the air flow. Analysis of the q2D expression for the GDL impedance Z∞gdl at λ → ∞ shows that the contribution of Z∞gdl to the total cell impedance rapidly decays with the frequency growth. We derive an equation for the boundary frequency flim, above which this contribution is small. We show that the 1D model can be fitted to the high–frequency part (f > flim) of a spectrum acquired at λ ≃ 2, ignoring the low–frequency arc due to the oxygen transport in the channel. Comparison of fitting parameters resulted from the 1D and q2D models confirms this idea

A new numerical strategy with space-time adaptivity and error control for multi-scale streamer discharge simulations

This paper presents a new resolution strategy for multi-scale streamer discharge simulations based on a second order time adaptive integration and space adaptive multiresolution. A classical fluid model is used to describe plasma discharges, considering drift-diffusion equations and the computation of electric field. The proposed numerical method provides a time-space accuracy control of the solution, and thus, an effective accurate resolution independent of the fastest physical time scale. An important improvement of the computational efficiency is achieved whenever the required time steps go beyond standard stability constraints associated with mesh size or source time scales for the resolution of the drift-diffusion equations, whereas the stability constraint related to the dielectric relaxation time scale is respected but with a second order precision. Numerical illustrations show that the strategy can be efficiently applied to simulate the propagation of highly nonlinear ionizing waves as streamer discharges, as well as highly multi-scale nanosecond repetitively pulsed discharges, describing consistently a broad spectrum of space and time scales as well as different physical scenarios for consecutive discharge/post-discharge phases, out of reach of standard non-adaptive methods.Comment: Support of Ecole Centrale Paris is gratefully acknowledged for several month stay of Z. Bonaventura at Laboratory EM2C as visiting Professor. Authors express special thanks to Christian Tenaud (LIMSI-CNRS) for providing the basis of the multiresolution kernel of MR CHORUS, code developed for compressible Navier-Stokes equations (D\'eclaration d'Invention DI 03760-01). Accepted for publication; Journal of Computational Physics (2011) 1-2

ПОСТРОЕНИЕ ПРОТОТИПА АВТОМАТИЗИРОВАННОЙ БЕСПРОВОДНОЙ СИСТЕМЫ МОНИТОРИНГА ЗАПЫЛЕННОСТИ ВОЗДУХА НА ПРОИЗВОДСТВЕННЫХ ПЛОЩАДКАХ

Various studies over the past decades have shown that fine dust particles can pose serious health hazards, contributing to the development of respiratory and cardiovascular diseases. Potential links have been identified between the likelihood of occurrence of diseases such as chronic obstructive pulmonary disease (COPD), asthma, lung cancer, with the concentration of dust in the air. There are correlation data showing that an increase in the MPC of dust in the air by only 50 μg / m3 leads to an increase in mortality by 1-8%. Dust particles have a significant impact on the life of plants and animals. Many industrial, experimental and medical processes can be safely carried out only at certain values ​​of the concentration of dust particles in the air: for example, dust consisting of solid particles less than 850 microns in size, suspended or settled in a gas environment, is capable of self-combustion and explosion in the air. Such dust is classified as combustible dust and, at certain concentrations, can cause industrial accidents associated with explosions of dust / air mixtures. The creation of an automated system capable of controlling the dustiness of the air in various production facilities is an urgent task. This work is devoted to the creation of such a system that combines the required number of dust sensors connected to each other in a wireless network, which makes it possible to control the measurement process and record the measurement results of each specific sensor.Разнообразные исследования последних десятилетий показали, что мелкие пылевые частицы могут представлять серьезную опасностью для здоровья, способствуя развитию  респираторных и сердечно-cосудистых заболеваний. Выявлены потенциальные связи между вероятностью возникновения таких заболеваний, как хроническая обструктивная болезнь легких (ХОБЛ), астма, рак легких, с концентрацией пыли в воздухе. Есть корреляционные данные, свидетельствующие о том, что при увеличении ПДК пыли в воздухе всего лишь на 50 мкг / м3, приводит к увеличению смертности на 1-8 %. Твердые пылевые частицы оказывают существенное влияние на жизнь растений и животных. Многие промышленные, экспериментальные и медицинские процессы могут быть безопасно осуществлены только при определённых значениях концентрации пылевых частиц в воздухе: например, пыль, состоящая из твердых частиц  размером менее 850 мкм, находящихся во взвешенном или осевшем состоянии в газовой среде, способна к самостоятельному горению и взрыву в воздухе. Такая пыль классифицируется как горючая пыль и, при определенных концентрациях, может стать причиной несчастных случаев на производствах, связанных со взрывами пылевоздушных смесей. Создание автоматизированной системы, способной контролировать запылённость воздуха в различных производственных помещениях является актуальной задачей. Данная работа посвящена созданию подобной системы, объединяющей в себя необходимое количество датчиков пыли, связанных между собой в беспроводную сеть, позволяющую управлять процессом измерения и фиксированием результатов измерения  каждого конкретного датчика

An adaptive grid refinement strategy for the simulation of negative streamers

The evolution of negative streamers during electric breakdown of a non-attaching gas can be described by a two-fluid model for electrons and positive ions. It consists of continuity equations for the charged particles including drift, diffusion and reaction in the local electric field, coupled to the Poisson equation for the electric potential. The model generates field enhancement and steep propagating ionization fronts at the tip of growing ionized filaments. An adaptive grid refinement method for the simulation of these structures is presented. It uses finite volume spatial discretizations and explicit time stepping, which allows the decoupling of the grids for the continuity equations from those for the Poisson equation. Standard refinement methods in which the refinement criterion is based on local error monitors fail due to the pulled character of the streamer front that propagates into a linearly unstable state. We present a refinement method which deals with all these features. Tests on one-dimensional streamer fronts as well as on three-dimensional streamers with cylindrical symmetry (hence effectively 2D for numerical purposes) are carried out successfully. Results on fine grids are presented, they show that such an adaptive grid method is needed to capture the streamer characteristics well. This refinement strategy enables us to adequately compute negative streamers in pure gases in the parameter regime where a physical instability appears: branching streamers.Comment: 46 pages, 19 figures, to appear in J. Comp. Phy

Spontaneous Branching of Anode-Directed Streamers between Planar Electrodes

Non-ionized media subject to strong fields can become locally ionized by penetration of finger-shaped streamers. We study negative streamers between planar electrodes in a simple deterministic continuum approximation. We observe that for sufficiently large fields, the streamer tip can split. This happens close to Firsov's limit of `ideal conductivity'. Qualitatively the tip splitting is due to a Laplacian instability quite like in viscous fingering. For future quantitative analytical progress, our stability analysis of planar fronts identifies the screening length as a regularization mechanism.Comment: 4 pages, 6 figures, submitted to PRL on Nov. 16, 2001, revised version of March 10, 200

Gamma-ray halo around 3C 279: looking through the Sun on the 8th of October

We discuss how the solar occultations of bright sources of energetic gamma rays can be used to extract non-trivial physical and astrophysical information, including the angular size of the image when it is significantly smaller than the experiment's angular resolution. We analyze the EGRET data and discuss prospects for other instruments. The Fermi Gamma Ray Space Telescope will be able to constrain the size of a possible halo around 3C~279 from observations it makes on the 8th of October each year.Comment: 6 pages, 4 eps figures, mn2e.cls style; v.2: numerous but minor changes to match the published versio