Numerical Investigation Of The Two Phase Flow In The Gas Flow Channel Of PEMFC

Proton exchange membrane fuel cell (PEMFC) is a clean and efficient energyconversion device. Water management is critical to the performance, stability andreliability of PEMFC. Effective water transport and removal in the flow channel of aPEMFC is significantly important to the critical water management in PEMFCs.Investigation of the air-water two phase flow, especially the liquid water transport andremoval process, in the cathode flow channel of PEMFC not only helps reveal thewater transport and removal mechanisms in the micro-scale channels including thePEMFC flow channels, but also benefits the flow channel design for better watermanagement, hence better performance of PEMFC.In this study, a dynamic wettability model (including the sliding angle and thedynamic contact angle effect) is developed for the two phase flow in the flow channelof PEMFC, on the basis of an existing static wettability model (only the static contactangle effect considered). Based on the dynamic wettability model, numericalsimulations are carried out focusing on the two phase flow in the flow channel ofPEMFC. The contents of this thesis are as follows:1. Under the assumption of the elliptic water spreading shape on the solidsurface and the linear dynamic contact angle distributions, a theoretical correlation isderived between the sliding angle and the dynamic contact angles, the two importantparameters representing the surface dynamic wettability. The correlation describes theinteraction between the sliding angle and dynamic contact angles. Then, the slidingangle and dynamic contact angles determined by this correlation are incorporated inan existing numerical model (the static wettability model), forming the dynamicwettability model. Using the dynamic wettability model, the effect of the dynamicwettability on the water transport in the flow channel of PEMFC is investigated. Theresult shows that for the same static contact angle, as the sliding angle is decreased,the contact angle hysteresis is decreased, the water droplet height is increased, and thewater droplet is moving faster on the gas diffusion layer (GDL) surface. Therefore,the GDL surface having a smaller sliding angle benefits the water transport andremoval in the flow channel of PEMFC.2. The water emerging location at the GDL surface can influence the watertransport and removal in the flow channel significantly. Thus, water transport anddynamics in the flow channel with various water emerging locations are investigated.It reveals that the water droplet can oscillate on the GDL surface along the channelwidth direction and the oscillation amplitude increases as the water emerging locationdeviates more from the GDL surface center. It is found that the water droplet can bedriven to the channel sidewall by the aerodynamics when the water emerging location deviates from the GDL surface center to a certain amount.When the contact angle ofthe channel surface is sufficiently small, the water droplet can be removed from theGDL surface by the channel surface capillary wicking effect. This shows theimportance of the channel surface contact angle on the water removal from the GDLsurface.3. Two novel channels are designed to facilitate the water removal from theGDL surface, i.e. the channel with a hydrophilic needle and the channel with ahydrophilic plate. Water transport and dynamics, especially the water removal processfrom the GDL surface, in the two novel channels are investigated numerically. It isfound that once the water droplet touches the needle or the plate, it will be removedfrom the GDL surface by the needle or the plate surface capillary wicking effect. Itindicates that the pressure drop in the two novel channels is small compared to that ina serpentine flow channel, making the present approaches viable for use in theconventional parallel flow channels for PEMFCs. It is also found that compared to thechannel with a hydrophilic needle, the channel with a hydrophilic plate shows moreefficient water removal from the GDL surface and smaller pressure drop in the flowchannel.