Numerical Investigation Of Two-phase Flow In PEMFC Porous Media Via Lattice Boltzmann Method

Proton exchange membrane fuel cells(PEMFCs)are a high efficiency energy conversion device that produces no emission and they are an alternative power source to replace the traditional internal combustion engine for automotive vehicles.However,some key technical issues such as water management have impacts on cell performance and durability,thus hampering the full commercialization of fuel cell vehicles.The transport of water within the fuel cell significantly affects the performance of fuel cell,therefore,proper water management,especially for the porous media of the cathode,is crucial to improve the performance of PEMFC.The porous model of PEMFC catalytic layer(CL)was reconstructed based on the Quartet Structure Generation Set(QSGS)method first in the present study.Multirelaxation time Lattice Boltzmann method was then employed to simulate the effect of solid surface properties,pore structure,dynamic viscosity ratio and surface tension on the two-phase transport process in the reconstructed catalytic layer.The simulation results show that when the solid surface is hydrophilic,liquid water will quickly immerse and fill the pore space,blocking the transmission channel of the reactants and resulting in "flooding" phenomenon.Under a certain pressure gradient,the water immersion rate gradually decreases with the increase of contact angle,and the volume of water infiltrated in the catalytic layer decreases.When the contact angle increases to 130°,the infiltration of water is significantly decreased,and the degree of immersion is less than 10%.Implementing a suitable material contact angle,therefore,helps in the drainage of water within the CL.The surface properties significantly affect the transport pattern of water in the porous media.The pressure gradient,viscosity ratio and liquid surface tension have little effect on the transport.In a model with a hydrophilic surface,water would uniformly penetrate into the pores of various sizes in a "stable displacement mode".In the case of high hydrophobic surface,water would preferentially form multiple protruding head filling some pores of the porous structure in a "capillary finger mode",and merge and separate continuously in the immersion process.Increasing pressure gradient would facilitate the transport of water within the CL.With the increase of working temperature in PEMFC,the immersion rate and amount of water increase.Secondly,the effect of the crack on the two-phase transport in the CL-MPL(micro porous layer)was simulated in the present study.The simulation results show that the larger the cross-sectional area of the crack,the more preferentially the water enters into the crack and the easier water penetrates the model through the computation domain,which indicates that water would be more easily discharged from the catalytic layer into the gas diffusion layer(GDL).Furthermore,water would continue to penetrate into the larger surrounding spaces during the transfer of water within the crack.The longer the crack,the faster the water travels and the less time it would take to reach the breakthrough point.The presence of cracks reduces the water content in the catalytic layer around the crack and can thus effectively avoid the occurrence of flooding.Cracks with a certain degree of curvature would also reduce the rate of water transfer.Finally,the porous structure model of a GDL was reconstructed based on the morphological structure method.A compression model was obtained by finite element module in ANSYS,and the transport of two-phase flow in the GDL before and after compression was investigated.The results show that as the amount of compression is increased,it is more difficult for water to immerse into the GDL either below the rib or below the channel area.The effect of compression on water transport under the rib is particularly pronounced,and water is more likely to accumulate under the rib,thereby causing flooding in the fuel cell.Subtle differences in the transmission of water in two compression models are observed.The micro finite element compression model can more accurately reflect the deformation of carbon fiber in GDL,which can help us to gain understanding on the impact of compression on water transport.