Factors Impacting Liquid Droplet Instability in a PEFC Flow Channel

To achieve optimal performance with minimal parasitic losses and degradation, the relationship between water removal parameters such as flow rate and the diffusion media (DM) surface properties must be clearly identified. An extensive experimental study of the influence of controllable engineering parameters, including surface PTFE (Teflon™) coverage (ranging from 5% to 20% of wt.) and operational air flow rate, on liquid droplet deformation at the interface of the DM and the gas flow channel was performed. A new visualization technique was developed to better understand the droplet mechanisms with enhanced optical access of both side and top views of the flow channel of a simulated H₂ PEFC. A telecentric lens and 5 mm by 5 mm prisms embedded in the flow channel side walls were used for the first time to measure droplet receding and advancing surface angles in an enclosed flow channel. The influence of channel air flow rate and emerging droplet size on droplet characteristics with varying PTFE content in the DM was investigated to identify the conditions under which the droplet tends toward an unstable state. The results indicate that operational conditions, droplet height, chord length, and level of surface hydrophobicity of the DM directly affect the droplet instability. At high flow rates, the surface hydrophobicity of the DM enhances the efficacy of droplet removal, and helps to avoid local channel flooding, however at low flow rates, regardless of the amount of PTFE content, droplet instability (and removal) is unaffected by the DM surface PTFE content.