A Thermo-fluid Electrochemical And Mechanical Coupled Numerical Simulation For Functionally Graded SOFCs With Stereo Electrodes

SOFC has been a priority research focus in the new power area due to its advantage of zero pollution,solid structure,high energy transfer efficiency,widely suitable to fuels, multiple scales of power supply and flexible installation,etc.During the transferring process of the energy in the SOFC system,the heat and mass transfer and electrochemical reactions are complex.Therefore,the output power and mechanical performance of SOFCs influence its energy transfer efficiency and service life. The energy transfer efficiency and mechanical performance are influenced by the ionic conductivity of the components,the resistance of the interface between the anode/cathode and electrolyte and the activate areas of the reactions.It is difficult to validate the influence of each parameter on the SOFC performance due to the relating influence. Hence,the geometry,material design and working conditions of SOFCs are choose and optimized to improve the comprehensive performance of SOFCs based on the numerical simulation.On the basis of the SOFC principle,the thermo-fluid model was constructed to describe the multiphase and heat and mass transfer in the system.The dust-gas model was selected to describe the transfer of multi-component gas in the porous media and channel. The BFD(Brinkman-Forschheimer-Darcy) was used to describe the seep of gas in the porous electrodes and research the mass sources,momentum sources and energy sources in the process of the transfer of multi-phase and multi-component gas.Then the finite software and external routine were coupled to solve these models.The intelligent electrochemical model was constructed to simulate the couple relation of the reaction and thermo-fluid model during the process of the energy transfer in the SOFC system.The ionic transfer,current,voltage and the output power in the corrugated and multi-plane SOFCs were studied.The software design,intelligent pre-processing and secondary exploitation of FLUENT software were presented.To avoid the stripping and gas leakage caused by the thermal expansion coefficients difference or circle operating under the high temperature and improve the contact of the interface between the electrodes and electrolyte influenced the output power of SOFC,the functionally graded layers were designed between the electrodes and electrolyte to make the physical parameters and porosity of the graded layers change continuously.By this means,the best geometry design was expected to improve the high power density and mechanical performance of SOFC. SOFC operates in the high temperature conditions,so the contact of interface between the cathode/anode and the electrolyte influence severely on the output power of SOFC.In particular,the stripping and gas leakage was caused by the difference of the components material of SOFC and the cyclical high temperature operating conditions.In order to release the deformation and crack induced by the difference of the components material of SOFC,in the thesis,the idea of the functionally graded material(FGM) is introduced into the design of SOFC.Between the electrodes,including cathode and anode,and electrolyte, the FGM layers with continuously graded physical prosperity and porosity are prepared. With the same volume of SOFC unit,the comprehensive performances of functionally graded SOFC were compared with those of SOFC without graded layers to seek the optimal design of SOFC with high power density and ideal mechanical performance.In order to enlarge the adaptability of SOFC to fuels,the multi-specious mixture generated by decomposing universal hydrocarbon was used as the fuel gas.The influences of reforming reaction of methane,water shift reaction and electrochemical reaction on the output power of SOFC were researched.The results show that the hydrogen mass fraction is increase along the fuel flow direction in the small type SOFC using the hydrocarbon fuel,hence,the hydrogen and hydrocarbon fuels can be supplied circularly.The industrial exhaust gas can be used as the fuel of SOFC,which is validated in the theory.Based on the temperature distribution calculated from the actual operation process, thermal stress of complex structure SOFCs(corrugated and octagon type) were first analyzed.A novel octagon typed SOFC was presented after the comparisons of the electrochemical performance of different structures,which could obtain the desired effect of both the mechanics performance and the improvement of cell energy density.The complex impedance method was employed to test the electrical conductivity of functionally graded SOFCs.It was experimentally validated that functionally graded layers can effectively improve the interface contact condition between cathode or anode and electrolyte,thus the decrease of the interface resistance.Moreover,the relationships were also investigated among ionic conducting properties,operation temperature,structure design and materials.