Numerical Simulation Of Droplet Dynamic Behaviors On Modified Gdl Surface Of PEM Fuel Cell

Proton exchange membrane fuel cell(PEMFC),as the fuel cell with the highest technology maturity and the most extensive application at present,is one of the green new energy technologies with the most development potential,and is also considered to be the core driving force in the third energy revolution of mankind.The problem of cathode water management in PEMFC has always been the focus of research by scholars.The study of cathode electrode water management can not only effectively avoid the occurrence of electrode "water flooding" phenomenon,but also promote the gas mass transfer of reactants and improve the performance of PEMFC.To investigate reasonable water management strategies,in this work,droplet dynamic behaviors in a gas flow channel(GFC)of proton exchange membrane fuel cell(PEMFC)with gas diffusion layer(GDL)of various wettabilities are observed by using the volume of fluid(VOF)method in this work.Droplet deformation,detachment,breakup and multiple droplets coalescence are investigated via an analysis of forces over the droplet.Results show that droplets tend to detach from a hydrophobic GDL surface at a high airflow velocity;while exhibiting significant deformation even breakup on a hydrophilic GDL surface.Besides,the interaction among the pressure force,shear force,wall adhesion force(arising from surface tension)and lift force over the droplet is examined to qualitatively elucidate the droplet dynamic behaviors.It is found that a hydrophobic GDL with a moderate airflow velocity can facilitate water removal from the GFC and maintain a relatively small spreading area;however,too high or too low an airflow velocity can cause a breakup or a slow drainage of the droplet,especially on a hydrophilic GDL surface.Thus,the design and operation of the PEMFC should consider both the wettability of GDL surface and the airflow velocity to achieve effective water management.In order to further optimize the water management of PEMFC and improve the drainage performance of GDL,this study modified the traditional GDL and proposed the mixed wettability GDL and the gradient pore size distribution design GDL.The simulation results show that the reasonable size of the mixed wettability GDL interval and the design of the hydrophobic span can accelerate the droplet discharge and effectively promote the fuel mass transfer compared with the traditional GDL.The change of the wettability of GDL usually results in the change of the surface aperture size,thus,droplet dynamic behaviors on GDL of different pore sizes and gradient pore size distribution designs are numerically simulated by using the VOF method.Results indicate that droplet has a relatively shorter detachment time and a smaller detachment radius on both super hydrophobic and hydrophilic GDL comparing with that on a moderate hydrophobic GDL regardless of pore size.Moreover,large pore can facilitate droplet detachment,but it increases droplet detachment radius and pressure drop.In comparison of uniform pore size distribution,for transverse gradient pore size distributions,a small pore distance at a relatively low airflow velocity or a large pore distance at a high airflow velocity can reduce droplet detachment time and pressure drop.For the longitudinal pore size distribution,a large pore in the upstream and small pore in the downstream with a large pore distance benefits the decrease of droplet detachment time and pressure drop.These findings can guideline design of a GDL with a proper gradient pore size distribution,which can improve water removal performance of fuel cell and maintain a relatively low pressure drop,at the same time,it is of guiding significance for GDL design of gradient pore size distribution.