Numerical Simulation Of Cathode Flow And GDL Diffusion Law In Proton Exchange Membrane Fuel Cells(PEMFCs)

Effective water management shows an important impact on the performance of proton exchange membrane fuel cells(PEMFCs).The liquid droplet movement mechanism in the cathode channel of the fuel cell and the formation of gas-liquid two-phase flow mode are important to water management in PEMFCs,which has attracted a great deal of research attention in the past decades.This thesis employed an advanced computational fluid dynamic method with the volume of fluid(VOF)method to simulate effects of various operating parameters on the liquid water flow mechanism and the gas-liquid two-phase flow mode in the cathode channel of a fuel cell.In addition,porous medium model combined with the VOF model was used to understand the effects of inside and outside pressure of gas diffusion layers(GDLs)and gas flow rate on liquid water through the GDL into the cathode channel.The simulation results show that liquid water passes through the GDL micropores and enters the cathode channel to form droplets and grows continuously.Due to the drag forces from the convective air flow,the droplet size and shape will undergo a series of changes.Coalescence between droplets reduces the liquid water coverage of the GDL surface.A critical pore distance of 0.6 mm is observed before the droplet coalescence.The shape of the micropores is found to show a significant effect on the flow pattern of the two-phase flow in the channel.The occurrence of the bridge flow accelerates removal of liquid water from the flow channel,but leads to a large pressure drop and pressure fluctuations.The gas flow rate affects the droplet morphology,while the effects on the droplet motion period is negligible.Effect of liquid phase flow rate is opposite.It is also found that as the gas-liquid phase flow rate increases,the average pressure drop of the channel will also increase,but its increase with the liquid phase flow rate is small.The standard deviation of the inlet and outlet pressure drop will increase significantly with the increase of the gas-liquid phase flow rate,which will result in unstable output voltage.Combined with the porous medium model simulation,it is found that with an increase in the pressure difference across the GDL and the liquid water velocity increases the volume and the number of droplets entering the gas flow channel through GDL while the volume of a single droplet and the water content in the GDL decrease.The increase in the gas phase flow rate of the channel reduces the pressure difference across the GDL,thereby affecting the flow of the liquid water.