Numerical Simulation Of Cross Flow In Microstructure Of Gas Diffusion Layer In Proton Exchange Membrane Fuel Cell

Effective control of the dynamic balance between cathode liquid water removal and membrane hydration is of great importance in improving the performance of proton exchange membrane fuel cells(PEMFCs). As a typical flow pattern in gas diffusion layer(GDL), cross-flow favors to the effective removal of liquid water in the cathode, which means enhanced mass transport to and from the reaction sites at the catalyst layer. On the other hand, the contact angle of GDL surface usually displays non-uniform characteristics with a differential distribution due to the hydrophobic treatment. Up to date, there are few studies to investigate the effects of cross flow intrusion and non-uniform contact angle on the properties of water transport in GDL microstructure.In order to investigate the process of water intrusion in GDL, a three-dimensional two-phase unsteady model of the microstructure of GDL has been developed in this study based on volume-of-fluid(VOF) method. To simulate the realistic situation of water transport with cross flow in GDL, contact angle gradient in GDL is established. Correspondingly, the effects of pressure difference and contact angle gradient on the process of water intrusion are investigated numerically.The results show that during a two-phase transport process along the in-plane direction in GDL, the liquid water first moves through some of the pores that are easy to penetrate, forming a “fingering transport” mode; and after that, with more liquid water accumulated, the rest of the pores can also be filled, forming a “steady transport” mode. It is also suggested that the effect of differential pressure on the water transport is more significant than of contact angle. Increasing the differential pressure or decreasing the contact angle of GDL accelerates the liquid water intrusion, and this effect is weakened at higher differential pressures and contact angles. For a GDL with variable contact angle, the water transport characteristics in different cross sections normal to the through-plane direction are similar to the corresponding fixed contact angle cases in these cross sections, and the overall process of water intrusion with variable contact angle is similar to its corresponding average fixed contact angle case. At the end of the water intrusion processes, quasi steady state can be reached, and the relationship among the pressure drop(capillary pressure), steady-state liquid water volume fraction and steady-state liquid water mass flux can also be obtained based on this model. The time needed to reach the steady state becomes shorter in the cases of higher differential pressure and lower contact angle. With the increment of the quasi steady-state liquid water volume fraction, the capillary pressure(pressure drop) increases correspondingly, and the increment becomes more significant at higher liquid water volume fractions. The liquid water mass flux through GDL in quasi steady state is proportional to the pressure drop, and with a lower contact angle, liquid water can enter the GDL more easily(higher mass flux). Overall, the water transport behavior in the case of variable contact angle resembles the case of corresponding average fixed contact angle.