Study And Optimization Of Bionic Flow Field In Proton Exchange Membrane Fuel Cell

Hydrogen fuel cell is a power generation device that uses hydrogen as fuel and converts the chemical energy in the fuel directly into electrical energy through an electrochemical reaction.Among the different hydrogen fuel cell classifications,the proton exchange membrane fuel cell（PEMFC）uses a polymeric membrane that conducts ions as the electrolyte,and has the advantages of fast start-up,high energy conversion efficiency,and low operating temperature,making it popular in military,aerospace,and automotive fields,and is recognized as a major contender for future transportation power systems.As one of the core components of PEMFC,the structure and form of the internal flow field of the bipolar plate play an important role in the fluid transport and distribution.In recent years,inspired by natural or biological factors,researchers have proposed a class of biomimetic flow field structures.Compared with the conventional flow field,this structure can achieve better fluid distribution,mass and heat transfer,and thus effectively improve the output performance of fuel cells,which has high research potential.In this thesis,the structural characteristics of two types of bionic flow fields are investigated separately.Firstly,based on the leaf-vein type bionic flow field,the influence of different angles between the main and branch channels on the performance of the PEMFC cathode is modeled and analyzed by the computational fluid dynamics（CFD）method.The results show that the uniformity of oxygen distribution in the cathode flow field increases with the increase of the included Angle.In addition,the increase of the angle is also conducive to the removal of the product water attached to the flow channel to avoid its accumulation and blockage of the flow channel.However,when the included angle increases above 30°,the oxygen content in the branched flow channel region decreases sharply and the water content increases sharply.At this situation,Changing the included Angle will no longer be beneficial to the improvement of the performance on the fuel cell cathode side.Secondly,in addition to the numerical analytical model for simulation,a performance prediction model for multivariable complex conditions is established with the help of neural network to save computational time and resources,and the refined optimization search for the included angle between the main and branch channels of the leaf-vein type bionic flow field is completed based on this model.The prediction results show that the output parameter of the model peaks at an angle of 23° under appropriate input conditions.The numerical analysis of the model with the same input conditions confirms that the combined performance of the flow field in terms of fluid distribution,voltage and power output is better than other solutions.Finally,based on the structural characteristics of the existing bionic flow field,this thesis proposes a new bionic flow field structure--the spide-web type flow field,and uses the polygonal structure and the number of spiral flow channel layers of this flow field as the key variables to establish a numerical analytical model,and solves to compare the differences between different schemes in terms of oxygen distribution on the cathode side,water removal performance and cell output performance.The results show that the complex polygonal structure is not conducive to the adequate flow of the reaction gas in the second half of the flow field,resulting in the weakened electrochemical reaction here,and the generated water will also collect here due to the lack of gas purging,blocking the flow channel and eventually causing the degradation of the fuel cell performance.In contrast,increasing the number of spiral flow channel layers under the same polygonal structure can effectively enhance the convection under the ribs,which is a reliable way to improve the uniformity of oxygen distribution in the flow field and increase the efficiency of water removal.In addition,increasing the number of polygonal sides or spiral flow channel layers can increase the effective contact area between the reaction gas and the MEA,which is more conducive to the full electrochemical reaction,and thus the output voltage and power density of the fuel cell will gradually increase.However,the increase in output voltage and power caused by the increase in the number of polygonal sides or spiral flow channel layers is limited,and the difference in output performance between adjacent schemes with higher polygonal sides or spiral flow channel layers is small and does not meet expectations.Finally,the spide-web type flow field scheme with good overall performance is compared with the straight flow field and the serpentine flow field with the same effective area.The results show that the spider-web type flow field has better fluid distribution performance than the above two types of flow fields.Although the output voltage and power are slightly lower than those of the serpentine flow field,the pressure stability in the spide-web type flow field is much better than that of the serpentine flow field,which further confirms the desirability of this structure in improving the performance of PEMFC.