Study On The Water Management Of Main Flow Fields Based On Corn Vein Structure For PEMFC Bipolar Plates

Proton exchange membrane fuel cell（PEMFC）has the advantages of high energy conversion rate,environmental friendliness,rapid start-up and low operating temperature,holding a bright future for various applications.It is also considered to be one of the solutions to tackle two major problems of modern society,i.e.environmental pollution and energy crisis.Bipolar plate is a major component of PEMFC,affecting the power density,weight and cost of the whole PEMFC.The flow field of bipolar plate directly determines the distribution of reactants and products in fuel cells,and plays a key role in water management.Its main function is to provide and distribute the reactive gas,reduce the polarization loss and discharge the water generated by the reaction.Based on the mathematical model of the macro-structure of corn vein and the methodology of bionics,bionic main flow fields have been designed for PEMFC bipolar plate in this work,which is aimed at improving the water management performance of PEMFC.Then simulation investigation is carried out by using Fluent（Software）.Finally,the parameters for laser processing on graphite plate have been optimized in order to realize the low-cost and high-efficiency fabrication of flow channel.The main contents in this thesis are as follows:First,this thesis has demonstrated the feasibility of applying corn-vein-inspired structure to the flow field of PEMFC bipolar plate.In order to obtain the mass transfer characteristics of corn vein system,corn leaves representing different growth stages（developmental period and maturity period）have been collected,from which the macro-structure and size parameters have been measured and calculated.A series of related size ratios between different vein diameters and intervals as well as opening ratio have been acquired.Then the theoretical basis for the design of bionic flow field is founded based on these extracted ratios and the self-resemblance of corn vein system.Second,several key governing conservation equations have been profiled for the PEMFC model,including four fundamental conservation equations,electrochemical equations and water transport equations.The numerical simulation process is introduced.Meanwhile the basic assumptions and boundary conditions of the model have also been set.Third,a design principle has been proposed for the bionic main flow field of PEMFC bipolar plate based on the mathematical model extracted from the macro-structure of corn vein.Then,five bionic flow fields with distinct structures have been set up,and investigations have been carried out on the characteristics of polarization curves and power density curves,pressure drop,reactant and current distribution,and water management performance under high and low current density by the simulation with Fluent.Parallel-II flow field with the main channel width of0.5 mm demonstrates the best battery performance,and the maximum power density is 0.67 w/cm²,which is 4.69%and 23.35%higher than those of Parallel-I and Parallel-IV flow fields,respectively.It is probably because when the transverse pressure drop acts on the parallel II flow field with narrow main channel,forced convection is formed in the flow field,which improves the distribution of components in the cell.Parallel II has also shown the best water management capability.Under the condition of high current density,with the water generated by electrochemical reaction increasing,the bionic flow field can quickly discharge the water under the synergistic effect of the main channel and sub-channels,resulting in good drainage capacity.Under the condition of low current density,the water mass fraction distribution is more uniform in the bionic flow fields,whereas there is no local water shortage phenomenon.Last,the parameters for laser processing on graphite plate have been optimized,based on the theory of laser processing.A"two-step"processing technology has been designed.In the first step,high-power and low-speed sintering is used to achieve the desirable depth of flow channel on graphite plate.In the second step,low-power and high-speed sintering is utilized to clean the debris on the channel surface to improve the finishing degree of flow channel.A comprehensive orthogonal test with three factors and three levels has been conducted to optimize the processing parameters of the"second step"laser marking.The effects of different processing parameters on the processing performance of flow channel have been preliminarily determined through the analysis of each examined factor.Then a comprehensive scoring method has been employed to convert the assessments into a single comprehensive index for analysis.The optimal parameter combination has been determined as V₃P₁A₃,which is channel#21 fabricated with a laser processing speed of 2000 mm/s,a processing power of20%,and a filling angle of 60°.The results of single index analysis are basically consistent with the optimal parameter combination obtained by the multi-index comprehensive scoring method.