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A three dimensional, steady state, non-isothermal, single phase model was developed and simulations were carried out in order to find the effect of cathode channel dimensions (width, depth and height) on the performance of an air-breathing fuel cell. The model was solved using commercial CFD package Fluent (version 6.3). Separate user defined functions were written to solve the electrochemical equations and the water transport through the membrane along with the other governing equations. Analyses were carried out for three different channel widths (2, 4 and 6�mm), for three different channel depths (2, 6 and 10�mm) and for three different cell heights (15, 30 and 45�mm). Cell characteristics like current distribution, species distribution, oxygen mass transfer coefficient, cell temperature, cathode channel velocities and net water transport coefficients are reported. The results show that the cell performance improves with increase in cathode channel width, channel depth and with decrease in cell height. Maximum power density obtained was 240�mW/cm2 for a channel width of 4�mm and channel depth of 6�mm. When the channel depth was 2�mm the performance was limited mainly due to the resistance offered by the channel for the buoyancy induced flow. For channel depths higher than 2�mm, the diffusion resistance of the porous GDL also contributed significantly to limit the performance to low current densities. At low current densities the fuel cell is prone to flooding whereas at high current densities ohmic overpotential due to dehydration of the membrane significantly contributes to the overall voltage loss. � 2009 Elsevier Masson SAS. All rights reserved.

Experimental surface-average heat transfer coefficients of a vertical tube located at various positions along the axis of a Circulating Fluidized Bed riser (CFB) are presented. Experiments were carried out on the tube in a 100 mm ? 100 mm cross-section and 5.5 m tall CFB cold unit using silica sand of mean particle size 363 ?m as the bed material. The copper tube was 9.6 mm OD and 0.6 m high placed in the core of the riser with hot water flowing though it. The tube was located axially at distances of 0.97 m, 1.62 m, 3.0 m and 4.0 m from the distributor plate. The fluidizing air velocity and solid circulation flux were varied in the range of 4.5-7.3 m�s-1 and 21-72 kg�m-2�s-1 respectively. The measured average heat transfer coefficient varied in the range of 58-101 W�m-2�K-1 and showed a decreasing trend from the riser bottom to the riser exit. At a given location, increasing the fluidizing velocity resulted in reduced heat transfer coefficient while increasing the solid circulation flux enhanced it. At a given average suspension density, the local dynamics of gas and solids adjacent to the tube surface affected the heat transfer. Gas convective contribution was found to be substantial at high gas velocities especially at higher tube locations. Using the experimental data for the axial tube, two empirical heat transfer correlations were developed in terms of the important design and operating parameters including a non-dimensional height parameter. Comparisons were made with literature available under similar but not identical operating parameters. ? 2002 �ditions scientifiques et m�dicales Elsevier SAS. All rights reserved.