| The traditional proton exchange membrane fuel cells (PEMFC) is generally low-temperature fuel cells which requires strict gas humidity and cooling system, while operation of PEM fuel cells at high-temperature (>80℃) is considered an effective way to improve performance in terms of reaction kinetics, catalyst tolerance, heat rejection and water management. Therefore, high-temperature PEM fuel cells have received wide attention in recent years.At first, the basic theory of high-temperature proton exchange membrane fuel cell is described and the mathematical models of computer simulation such as the control equations of computational fluid dynamics (CFD), electrochemical reaction equations and the gas diffusion in the fuel cells are introduced.Three single cell base-cases were obtained by theory calculation of their pressure losses in this investigation which based on a high-temperature PEM fuel cells, including parallel straight flow field, multi-serpentine flow field I and multi-serpentine flow field II. Based on the FLUENT simulation, the parallel straight flow field operated the best performance. Then three optimal programs were developed to analyze the performance based on the simulated results, which took into account different parameters of the flow structure. There were three aspects for the analysis such as membrane water content, molar concentration of oxygen distribution and current density distribution between catalyst layer and diffusion layer interface. The results showed that it was the best for 95℃PEM fuel cells to adopt parallel flow field. A set of optimum values were found for the parallel flow field, in which both channel widths and rib widths were lmm, and the best channel height was 0.4mm according to the numerical simulation.The analysis on the condition of different cathodal relative humidity is considered, focusing on three characteristics of the current density. And, the relative air admission humidity mainly manifests to the battery performance's influence in the air admission oxygen differential pressure difference, the different oxygen differential pressure causes the battery internal pressure drop change to be big. According to the Nernst formula, the oxygen differential pressure the influence manifests to the battery in the oxygen density to the fuel cell electrode dynamics. In concentration polarization scope (when 3.0 A/cm) in membrane water content and in MEA oxygen molar density being smaller than ohmic polarization scope (when 1.2A/cm2) target, which is the primary reason for the performance drop.Based on the fore-mentioned computation and the analysis result, a new honeycomb flow field is simulated and makes the contrast with the traditional parallel flow field cell performance. When the percentage open area is 50%, a hexagon unit honeycomb flow field and the circular unit honeycomb flow field displays the best battery performance. Regarding the new honeycomb flow field, comes up the analysis from the theory, the fractal layer are more, the cell performance is better. But in the actual production, regarding the different size request's honeycomb flow field, some suits own best lamination number, but by no means fractal layer better. Under the same operating condition and the boundary condition, two structure flow field's battery performance surpasses traditional obviously the parallel flow field. In two circular unit honeycomb flow field flow of gas process has not presented the local flow dead angle and the burble phenomenon, displays the fuel cell performance slightly is also strong in the hexagon shape unit honeycomb flow field. |
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