Two-phase Flow Simulation And Flow Field Optimization Of Proton Exchange Membrane Fuel Cell

The proton exchange membrane fuel cell(PEMFC)is an efficient and clean energy conversion device that can directly convert the chemical energy stored in the fuel into electrical energy.PEMFC has the advantages of simple structure,high energy density,rapid start and stop,quiet and reliable operation,and environmental friendliness.It is considered to be one of the most promising fuel cell types.However,due to a series of technical issues such as life,cost,water and thermal management,PEMFC is still difficult to achieve large-scale commercial applications.The flow field of PEMFC plays the role of reactant supply and water removal,it can be said that its structure has a great influence on cell performance.A good flow field structure can ensure the reactant to reach the reaction interface uniformly under the premise of low pressure drop,and the water generated by the reaction can be removed in time.Meanwhile,in order to be more valuable and cost-effective,the flow field structure should also be easy to manufacture.It is also considered that using a suitable and comprehensive mathematical model to simulate the flow field in the early stage is helpful to reduce the research and development(R&D)cost and shorten the R & D cycle.Therefore,this paper studies the flow field of PEMFC by numerical simulation,in order to provide some guidance for the structure design and optimization of PEMFC flow field.Firstly,based on the theory of heat transfer,mass transfer and electro-chemistry,a threedimensional non-isothermal two-phase flow model of a PEMFC with single straight channel is developed.The model includes a catalyst layer agglomeration sub-model,and considers the influence of the cooling channel.The grid independence of the model is studied by comparing multiple cases with different mesh densities,and the validity of the model is verified based on the experimental data under different cathode and anode platinum loadings.Then,based on the proposed PEMFC mathematical model,combined with the Nelder-Mead simplex optimization method,the channel cross-section structure is optimized to improve the overall performance of PEMFC.A trapezoidal cross-section is obtained by optimization.The results show that compared with the rectangular cross-section,the trapezoidal cross-section can improve the transfer of oxygen and is more efficient to remove the liquid water in the porous media.In terms of polarization performance,The power density of PEMFC with the trapezoidal cross-section has increased by about 5% at high current density.This paper also proposes a novel flow channel with circular chamfer cross-section based on the optimized trapezoidal cross-section.The study show that the cross-section with circular chamfer has significant advantages in liquid water removal.The maximum liquid water saturation in the cathode gas diffusion layer is reduced by more than 20% compared with the rectangular cross-section;After optimizing the fillet radius,the polarization performance can be improved while keeping the pressure drop in the flow channel almost unchanged.Finally,based on the three-dimensional structure of the porous metal material,geometric reconstruction of the porous metal flow field is carried out.Then,combined with the flow analysis and the three-dimensional two-phase non-isothermal model,the performance of the porous metal flow field are studied.Firstly,it is found that the fluid flow in the porous metal flow field is more gentle and uniform,but the pressure drop in the flow field is larger,about twice that of the parallel flow field.Then,through structural analysis,it is found that the contact area between the direct flow channel and the gas diffusion layer is significantly larger than the porous metal flow channel,accounting for 50% of the flow field area,while that of the porous metal flow channel is only15.67%.Compared to the straight channel,it is also found that the polarization performance of the porous metal flow field is greatly improved in the high current density area,and the oxygen supply inside the cell can be greatly improved,and it also has a greater advantage in the drainage capacity.In addition,when the operating pressure is 1.7 atm,the performance difference between the two different flow channels in the area dominated by ohmic polarization is much smaller than when the operating pressure is 1 atm.