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Proton exchange membrane fuel cells (PEMFCs) are considered one of the viable solutions to the decarbonization of the transport sector. However, their performance and durability are yet to be competitive with internal engine vehicles due to the complex interaction of electrochemical and physical phenomena in PEMFCs. The electrochemical and physical phenomena that occurred in PEMFCs, including the polarization curves, profiles of the reactant and product species, velocities of species, as well as temperature distribution, could be described by coupling the reaction kinetics with the transport processes of mass, momentum, energy, and charge. In this chapter, the fundamentals and operating principles of PEMFCs are explained along with the governing equations that describe various electrochemical and multi-physics transport processes in PEMFCs.

Nonequilibrium Thermodynamics: Transport and Rate Processes in Physical, Chemical and Biological Systems, Fourth Edition emphasizes the unifying role of thermodynamics in analyzing natural phenomena. This updated edition expands on the third edition by focusing on the general balance equations for coupled processes of physical, chemical and biological systems. Updates include stochastic approaches, self-organization criticality, ecosystems, mesoscopic thermodynamics, constructual law, quantum thermodynamics, fluctuation theory, information theory, and modeling the coupled biochemical systems. The book also emphasizes nonequilibrium thermodynamics tools, such as fluctuation theories, mesoscopic thermodynamic analysis, information theories, and quantum thermodynamics in describing and designing small scale systems.

This article presents both an experimental as well as a theoretical study on the effect of MethylHy droxyEthylCellulose (MHEC) on drying in porous materials using Nuclear Magnetic Resonance Imaging (NMR). MHEC, a water soluble polymer, is normally added to glue mortars as a water retention agent in order to improve the drying by increasing Accepted the open time. However, the exact processes that determine the drying rate of a glue mortar are unknown. In this study, we therefore focus on investigating the drying of a Fired day Brick (FCB) saturated with an aqueous MHEC solution. By using a FCB as a model system, the influence of hydration and changing of pore sizes due to hydration can be removed. The performed NMR experiments show a transition from homogeneous drying for a water saturated FCB toward an inhomogeneous (front receding) drying behavior for FCB saturated with increasing MHEC concentration. The capillary number (Ca) indicates a homogeneous drying for water saturated brick (Ca«i) and an inhomogeneous drying (Ca»1) in case of saturated brick with more than 1.5 wt% MHEC. Analysis of capillary number shows that the main parameter determining the capillary numbers are viscosity, surface tension, contact angle and evaporation rate. Among this viscosity change match with change in capillary number and therefore viscosity has a major influence on the drying behavior and rate. Based on the measured profiles, the moisture diffusivity is calculated. Using an empirical equation, the moisture diffusivity and NMR profiles were fitted to obtain the key parameters. From these analyses we conclude that 1) the moismre diffusivity scales with viscosity, which is the main parameter in shift in drying behavior. 2) Surprisingly, the presence of MHEC reduced the evaporation, which is unexpected since evaporation of MHEC solutions, for instance present in capillaries do not exhibit this behavior. 3) A minimum moisture diffusivity is found which shows to correlate to the evaporation process, which indicates that this might be the result of a vapor dominated diffusion flux.

International audience; In most modelling works on bioreactors, the substrate assimilation is computed from the volume average concentration. The possible occurrence of a competition between the transport of substrate towards the cell and the assimilation at the cell level is generally overlooked. In order to examine the consequences of such a competition, a diffusion equation for the substrate is coupled with a specific boundary condition defining the uptake rate at the cell-liquid interface. Two assimilation laws are investigated, whereas the concentration far from the cell is varied in order to mimic concentration fluctuations. Both steady and unsteady conditions are investigated. The actual uptake rate computed from the interfacial concentration is compared to the time-averaged uptake rate based on the mean far-field concentration. Whatever the assimilation law, it is found that the uptake rate can be correlated to the mean far-field concentration, but the actual values of the parameters are affected in case of transport limitation. Moreover, the structure of the far-field signal influences the substrate assimilation by the microorganism, and the mean interfacial uptake rate depends on the ratio between the characteristic time of the signal and the diffusional time scale, as well as on the amplitude of the fluctuations around the mean far-field concentration in substrate. The present work enlightens some experimental results and helps in understanding the differences between the concentration measured and that present in the microenvironment of the cells. (C) 2012 Elsevier Ltd. All rights reserved.

A simplified model for gas–solid reactions in fluidised bed (FB) is proposed. Such models already exist for catalytic gas–solid reactions (CGSRs), providing general description of the system in terms of main governing parameters. Expansion of this approach to non-catalytic gas–solid reactions (NCGSRs) is difficult, because the solid reactant takes part in the reaction. Therefore, FB reactor models for NCGSR are usually devised only for specific cases, and a general analysis has not been presented up to date. The present model allows analysis of different types of NCGSR in a generalised way, handling catalytic reactions as a particular, simpler, case. It is shown that the reactor behaviour can be described by three governing dimensionless parameters. Two additional parameters, quantifying the importance of diffusion effects in single particles are also identified, and their impact on reactor behaviour is analysed. Possible simplifications are explored. Model limitations, that is, assumption of isothermal bed and particle and the occurrence of only one reaction, are discussed. Examples are outlined to show the applicability of the method.

22 pages, 18 figures, 4 tables, A historical data set is used to describe the coastal transition zone off Northwest Africa during spring 1973 and fall 1975, from 17° to 26°N, with special emphasis on the interaction between subtropical (North Atlantic Central Waters) and tropical (South Atlantic Central Waters) gyres. The near-surface geostrophic circulation, relative to 300 m, is quite complex. Major features are a large cyclonic pattern north of Cape Blanc (21°N) and offshore flow at the Cape Verde front. The large cyclone occurs in the region of most intense winds, and resembles a large meander of the baroclinic southward upwelling jet. The Cape Verde frontal system displays substantial interleaving that may partly originate as mesoscale features at the coastal upwelling front. Property–property diagrams show that the front is an effective barrier to all properties except temperature. The analysis of the Turner angle suggests that the frontal system is characterized by large heat horizontal diffusion as a result of intense double diffusion, which results in the smoothing of the temperature horizontal gradients. Nine cross-shore sections are used to calculate along-shore geostrophic water-mass and nutrient transports and to infer exchanges between the coastal transition zone and the deep ocean (import: deep ocean to transition zone; export: transition zone to deep ocean). These exchanges compare well with mean wind-induced transports and actual geostrophic cross-shore transport estimates. The region is divided into three areas: southern (18–21°N), central (21–23.5°N), and northern (23.5–26°N). In the northern area geostrophic import is roughly compensated with wind-induced export during both seasons. In the central area geostrophic import is greater than wind-induced export during spring, resulting in net import of both water (0.8 Sv) and nitrate (14 kmol s−1), but during fall both factors again roughly cancel. In the southern area geostrophy and wind join to export water and nutrients during both seasons, they increase from 0.6 Sv and 3 kmol s−1 during fall to 2.9 Sv and 53 kmol s−1 during spring, This research has been supported by the Spanish Ministerio de Educación y Ciencia, through project CANOA (Ref. no. CTM2005-00444/MAR). MVP would like to acknowledge Consejo Superior de Investigaciones Científicas for funding through an I3P grant

A simplified model for gas–solid reactions in fluidised bed (FB) is proposed. Such models already exist for catalytic gas–solid reactions (CGSRs), providing general description of the system in terms of main governing parameters. Expansion of this approach to non-catalytic gas–solid reactions (NCGSRs) is difficult, because the solid reactant takes part in the reaction. Therefore, FB reactor models for NCGSR are usually devised only for specific cases, and a general analysis has not been presented up to date. The present model allows analysis of different types of NCGSR in a generalised way, handling catalytic reactions as a particular, simpler, case. It is shown that the reactor behaviour can be described by three governing dimensionless parameters. Two additional parameters, quantifying the importance of diffusion effects in single particles are also identified, and their impact on reactor behaviour is analysed. Possible simplifications are explored. Model limitations, that is, assumption of isothermal bed and particle and the occurrence of only one reaction, are discussed. Examples are outlined to show the applicability of the method.