| As an effective and clean energy device,the proton exchange membrane fuel cell(PEMFC)has received much attention in the field of new power resources and is promising to be widely used in aeronautics,astronautics,automobiles and portable equipment.It is not surprising that the dynamic condition,clamping method and structural design have great effects on the structural stress,performance and reliability of the stack during the serving time.Taking the PEMFC stack as the studied objective and the numerical simulation as the main method,the structural performance analysis and optimization design of the stack are carried out in this dissertation.The current research status of the fuel cells is first introduced in Chapter 1,and then the key problems in the stack design,i.e.dynamic load,clamping force and reliability,are also elaborated.The main research contents are divided into the following four chapters:In Chapter 2,the dynamic responses and fatigue damage of PEMFC stacks under vibration and impacting load are analyzed.As there is still lack of the dynamics simulation of the large stack,this chapter starts from a mode analysis based on finite element method(FEM).Using this method,we can distinguish the local vibration from the stack global modes,predict the vibration responses,such as deformed shape and direction,and discuss the effects of the clamping configuration and the clamping force magnitude on vibration modes.As the structure design for the stack has a great effect on the component fatigue life,the dynamic stress of the stack under random vibration is analyzed afterwards.Based on the Miner fatigue damage theory,the fatigue lives of the fuel cell components are assessed.The component fatigue life contours illustrate that the fatigue lives of bolt regions close to nuts and the outside regions of sealant and PEM are relatively short.As a result,the torque release,leak rate increase,sbhrinkage of electrochemical reaction region and degradation of stack performance may occur during the service time.In the analy-sis of mechanical response of a large stack clamped by belts under impact load,a homogenization FEM model is established.The numerical results indicate that the unit cells close to the two endplates have a worse anti-impact performance than those in the middle of the stack.When subjected to a large impact in the direction parallel with the cells,the stack may give rise to interface slippage between cells.showing a downward bowing phenomenon.These FEM analysis results may provide referential information for the detailed design of membrane electrode assembly(MEA),sealant,endplate,and etc.In Chapter 3,the endplate structure clamped by steel belts is optimized.According to the FEM analysis results in the previous chapter,optimization objectives are determined as a maximum rigidity of endplate and a most uniform distribution of contact pressure.Steel-belt clamping method is a new clamping method used for large stack during recent years.Compared with bolt-clamped stack,the steel belt-clamped stack shows advantages of compact structure,elegant appearance,andetc.However,when the 3D topology optimization method available for bolt-clamped stack is employed directly,the complex stress state of the belt-clamped stack can readily lead to a "blur" or "tiny" structure problem.Therefore,a multi-objecti,ve stepwise optimization on the endplate clamped by steel belts is proposed.We first divide the endplate cross section(2D)optimization problem into a two-step optimization,i.e.shape optimization and topology optimization,to improve the numerical convergence and stability.The simulated annealing method is introduced to promote the numerical efficiency and precision for the nonlinear contact optimization problem.Then the grid parameterization method is used,resulting in a smooth boundary topology optimization.Finally,based on the 2D cross section shape optimization and the improved topology optimization,a 3D optimized topology is obtained using a geometry parameterization method.The finally optimized endplate not only gives a small mass but also improves greatly the uniformity of the contact pressure distribution and increases the fatigue life of the fuel cell components.In Chapter 4,an equivalent stiffness-mass model for performance analysis and structural parameter design of stack is set up.The analysis of structural stress and stack performance are multi-scale and multi-factor coupled problems.For the FEM analysis,a huge number of elements will be needed to get an accurate solution.Thus a large computational resource will be taken for the fine numerical analysis.Therefore,a reasonable and simplified numerical analysis method is desired for improving the efficiency of PEMFC stack.In this dissertation,an equivalent stiffness-mass model is proposed.The model consists of a number of springs and point masses in order to get the parameterized structural characteristics.Based on this model,the effects of the structural parameters,clamping force,temperature and dynamic load on the structural stress of components can be efficiently analyzed.This chapter provides a theoretical basis for the study of the stack reliability in the next chapter.In Chapter 5,the structure parameter design and performance analysis of stacks using the equivalent stiffness-mass model are introduced.The proper range of clamping force for typical stacks is obtained according to the structural strength,gas tightness,and non-slippage condition between the contact components.Based on this,a method to analyze the strength and fatigue reliability is also introduced in the stack structure design.The reliability of stack depends on the stress status of each MEA and sealant,and the stress status is related to clamping force and thicknesses of the two components.According to the calculation results based on the equivalent stiffness-mass model,the allowable value of these key design parameters can be effectively obtained in the initial stage of product design.This can improve the reliability and increase the power density of stacks during the serving time. |
