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Unformatted text preview: Rigorous 3-D Mathematical Modeling of PEM Fuel Cells II. Model Predictions with Liquid Water Transport Sandip Mazumder* ,z and James Vernon Cole* CFD Research Corporation, Huntsville, Alabama 35805, USA In this part of the paper, we present a model to treat formation and transport of liquid water in proton exchange membrane ~ PEM ! fuel cells ~ FCs ! in three-dimensional ~ 3-D ! geometry. The performance of modern-day PEM FCs at high current density are largely dictated by the effective management of liquid water. In the first part of this paper, a rigorous model was presented to model PEM FCs using a computational fluid dynamic technique. It was found that under the assumption of no liquid water formation, the model consistently overpredicted measured polarization behavior. In the model presented here, the phase change process is modeled as an equilibrium process, while the transport of liquid water is governed by pressure, surface tension, gravity and electro-osmotic drag. Results show that the inclusion of liquid water transport greatly enhances the predictive capability of the model and is necessary to match experimental data at high current density. © 2003 The Electrochemical Society. @ DOI: 10.1149/1.1615609 # All rights reserved. Manuscript submitted July 17, 2002; revised manuscript received May 1, 2003. Available electronically September 22, 2003. Despite several studies on water management in proton exchange membrane ~ PEM ! fuel cells ~ FCs ! within the past decade, effective water management has remained elusive. This is partly due to the fact that once liquid water is formed, it is transported within the membrane-electrode-assembly ~ MEA ! by several co-existing and comparable forces, and therefore, it is extremely difficult to control its motion. The other complication results from the conflict that while liquid water is necessary to maintain high electrical conduc- tivity of the membrane, excessive water can result in clogging of the electrodes. In particular, in a hydrogen-air PEM FC operating at high loads, water is generated at the cathode due to electrochemical reactions, and significant condensation and clogging can occur within the cathode if the liquid water is not managed properly. Thus, it is neither desirable to remove water completely, nor is it desirable to have it in excessive amounts, and this poses a design challenge. The best scenario is to have liquid water in optimum amounts in certain regions of the MEA only, and this is almost impossible to achieve. One can only hope to alleviate the problem by understand- ing how much water is produced, where it finally resides within the cell, and what effects it has on the performance of the cell. Part of the role of modeling and simulation is to answer these questions....
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