Investigations On Water Dynamics Of Proton Exchange Membrane Fuel Cells At The Fiber Scale And Channel Scale

As the devices to convert the chemical energy of hydrogen and oxygen into electric energy,proton exchange membrane fuel cells(PEMFC)have been widely used in automobiles,space flights and mobile power stations.PEMFC is considered to be the one of the most promising clean energy conversion devices since its final product is water.The accumulation of liquid water in PEMFC would result in the performance degradation of PEMFCs.Hence the water management has been a key research topic of PEMFCs.The dimensions of PEMFC channels are always in millimeter scale.The main force applied on the liquid water is the adhesion force from the wetted wall and the drag force from the air flow.The mechanical model of the droplet is the foundation of the investigation of the droplet dynamics in channels.In this thesis,the adhesion force of the droplet from gas diffusion layer(GDL)is modeled at the fiber scale and apparent contact angle scale considering contact angle hysteresis.The force balance model and contact angle hysteresis model are established in the VOF(Volume of Fluid)framework.The investigations of the water droplet dynamics are conducted at the fiber scale and apparent contact angle scale respectively,which can guide the optimization of PEMFCs.Besides that,as a part of works in the water management of PEMFCs,the effects of the accumulated liquid water in GDLs and channels on the performance of PEMFCs have been investigated based on the full scale PEMFC model.The main content s and results of this paper can be summarized as:(1)GDLs consist of randomly arranged fibers.As a droplet moves on GDLs,th e interaction of water and fibers is the source of the adhesion force from GDLs.The GDLs are simplified as the fiber layers with ordered fibers to investigate the adhesion forces applied on droplets.The mechanical model is modeled in the VOF framework with the forces from fiber layers being modeled by the surface tension force and pressure force.The surface tension force and pressure force is computed by a secondary development program.The variations of the forces from a single fiber and fiber layers during the droplet dynamic process are modeled based on the force model.The effects of fiber spaces,fiber layer numbers,fiber orientations and the contact angle s of fibers on the droplet dynamics are investigated.Besides that,a comparison of the fiber s cale model and the apparent contact angle scale model is conducted(2)The droplet dynamics on GDLs are investigated at the apparent contact angle scale ignoring the interaction of droplet and a single fiber.The contact line of a droplet is simplified based on the double ellipse model and the contact angle hysteresis model is established in the VOF framework with the contact angles change constantly along the contact line.The comparison of the contact angle hysteresis model and the experimental data in the references indicates that the model can be used to predict the adhesion force and model the droplet dynamics on smooth surfaces,rough surfaces and GDLs.(3)The drag force from air flow is computed as the pressure force caused by the inertia change of air flow and the viscous force due to the gradient of tangential velocity at the droplet surface.The droplet detachment velocities are used as evaluations of droplet mobility and the effects of the contact angle,contact angle hysteresis,channel dimension and channel geometry on the droplet dynamics are investigated.(4)Considering the interaction of droplets and sidewalls of PEMFC channels,the droplet dynamics and the drag forces exerted on the film flow,slug flow,corner flow and trapped flow at the turning corner are investigated.The difference of the values of the drag force and the difference of force components including the pressure force and viscous force would result in the different droplet dynamic behaviors.The pressure force,viscous force,droplet detachment velocity and the pressure drop of channel for the film flow,slug flow,corner flow and trapped flow are investigated and evaluated systematically.(5)The three dimensional physical model for the full scale PEMFCs considering the heat and mass transfer,phase change of water and chemical reaction is established.The effects of liquid water in GDL on the performance of the PEMFC is investigated based on the evaluation of the mass transfer limitation.The liquid water patterns in channels including the film flow and slug flow are also considered in the full scale PEMFC model and the effects of the liquid water in channel s on the performance of PEMFCs are investigated.