Numerical Study of Liquid Water Formation and Transport in Porous Media

Abstract

Fundamental understanding of liquid water formation and transport in fuel cells is of great importance for achieving high durability, reliability, and maximizing cell performance. For low temperature fuel cells, water may exist as vapor or liquid phase and is involved in various electrochemical reactions and transport mechanisms. The condensation process occurs when local water vapor pressure exceeds the saturation pressure. In this work, we first investigate the permeation of liquid water in the gas diffusion layer (GDL). To this aim, a two-dimensional isothermal model considering reconstructed GDL microstructure in conjunction with VOF model is implemented. The effects of mixed wettability of materials, operating temperature, and Polytetrafluoroethylene (PTFE) loadings on liquid water transport in the porous media are investigated numerically. The liquid water breakthrough and the capillary fingering regime are captured and compared. The results show that liquid water saturation in the GDL increases with decreasing PTFE loadings. Moreover, the results demonstrated that the local microstructure in the GDL has a strong effect and can dominate the overall water distribution. To further study water condensation, the isothermal model is expanded to non-isothermal for including physics of phase change heat transfer and species transport. In addition, anisotropic thermal conductivity is incorporated to study its effect on temperature distribution and liquid water distribution. The results show that initially the liquid water is formed as micro droplets and then these micro droplets are combined to form large droplets. Moreover, the impacts of water generation rate, temperature gradient, operating pressure, and contact angle on condensation have been investigated. The findings of this work are expected to provide insights into designing more reliable GDL in fuel cells. Figure 1