Study On Internal Temperature Distribution And Heat And Mass Transfer Characteristics In PEMFC

Proton exchange membrane fuel cell(PEMFC)is a device that can directly convert chemical energy stored in fuel(hydrogen and oxygen)into electrical energy,and the operation process does not involve combustion.It has the characteristics of high energy conversion efficiency,clean and pollution-free,and has become a very promising new energy source to replace fossil fuels today.In the electrochemical process of PEMFC,a lot of heat will be released and product water will be generated.If the heat is not discharged and drained in time,local hot spots and flow channel blockage will occur in the cell,which will cause irreversible damage to the proton exchange membrane and gas diffusion is blocked,and ultimately reduce the output performance and service life of the cell.In this paper,the temperature distribution and heat and mass transfer characteristics of PEMFC stack are studied by combining experimental and numerical analysis.The effects of coolant temperature and oxygen stoichiometry on the temperature distribution and heat and mass transfer inside the cell are analyzed,and the parametric correlation between them and the output performance of PEMFC is established,which ultimately provides a reference for the water and heat management strategy of PEMFC.The specific research content is as follows:(1)Established an experimental testing plan for PEMFC stack performance testing and thermocouple temperature monitoring.The test stack consists of fifteen single cells with a classical parallel gas-liquid direct current channel structure.In order to study the temperature distribution and heat and mass transfer laws inside the stack,the commercial test platform was used in the experimental scheme designed in this paper to monitor the inlet and outlet parameters and output power of the stack and the internal single cell.Thermocouple temperature sensors are embedded on the cathode plate of the single cell at different positions in the stack to monitor the temperature data of the stack in real time,and analyze the temperature distribution characteristics and output performance of the stack and the internal single cell.Then,numerical simulations were conducted on the single cell.Based on the experimental monitoring of the reaction gas and coolant inlet and outlet parameters,external wall temperature,and current voltage of the single cell in the center of the PEMFC stack,numerical simulations were conducted on the single cell,which were close to the actual operating environment inside the stack,to analyze the internal heat and mass transfer characteristics of the single cell in the PEMFC stack.(2)The influence of coolant temperature on temperature distribution and heat and mass transfer in PEMFC stack was analyzed.The temperature distribution and heat and mass transfer characteristics in the stack are analyzed when the coolant temperature ranges from 50℃ to 70℃.The results show that the average temperature and temperature difference of a single cell inside the stack exhibit a "parabolic" distribution,and the average temperature and temperature difference of a single cell in the center of the stack are the highest,with the highest output performance.As the coolant temperature increases,the temperature of the stack increases,and the thermal balance coefficient inside the stack decreases.The effect of water phase change heat release inside the stack on the thermal balance of the stack decreases.At the same time,the temperature on each single cell board in the stack gradually increases along the coolant flow direction,and the temperature gradient shows a linear increasing trend with the increase of current density.With the increase of coolant temperature,the smaller the slope of the temperature gradient linear curve,the more uniform the temperature distribution inside the stack,and the higher the output performance of the stack.Based on the numerical simulation results,it can be seen that under various operating conditions,at the interface between the cathode and anode gas diffusion layers and the catalytic layer,due to the high reaction consumption and pressure drop of hydrogen,there is a reflux at the outlet,and a trend of first decreasing and then increasing along the flow direction occurs.Oxygen is gradually consumed and reduced along the air flow direction;With the increase of coolant temperature,the uniformity of reaction gas and water distribution inside the cell is improved,which improves the output performance of the cell.(3)The influence of oxygen stoichiometry on temperature distribution and heat and mass transfer in PEMFC stack was analyzed.The temperature distribution and heat and mass transfer characteristics in the stack are studied when the hydrogen stoichiometry is 1.5 and the oxygen stoichiometry is 1.6-2.6.The results of experiments and numerical simulations indicate that as the oxygen stoichiometric number increases,the average temperature and temperature difference of the stack gradually decrease,the thermal equilibrium coefficient decreases,and the effect of water phase change heat release on the internal thermal equilibrium of the stack decreases.Sufficient oxygen allows hydrogen to participate more fully in the reaction,and the output power of the stack is highest when the oxygen stoichiometric number reaches 2.2.An excessively high oxygen stoichiometry will increase the thermal equilibrium coefficient of the stack,and the impact of water phase change on the thermal equilibrium of the stack will increase.The slope of the temperature gradient curve of the single cell in the stack also gradually decreases first and then increases with the increase of the oxygen stoichiometry,which is the lowest when the oxygen stoichiometry is 2.2,indicating that when the oxygen stoichiometry is 2.2,the water phase change has the smallest impact on the thermal balance of the stack,the temperature distribution uniformity in the stack is the best,and the output performance of the stack is the highest.Based on numerical simulation,it can be seen that as the stoichiometric number of oxygen increases,the air flow rate increases,which enhances the water and gas transmission capacity inside the cell channel,improves the uniformity of gas and water diffusion distribution,and reduces the difference in water and gas mass fraction and temperature difference between the area below the channel and the area below the rib.As a result,the output performance of the cell gradually increases.But after the oxygen stoichiometry exceeds 2.2,the excessively high air flow rate leads to a decrease in the uniformity of water heat distribution on the membrane,resulting in a decrease in cell performance.Therefore,when the stoichiometric number of oxygen is 2.2,the diffusion distribution of hydrogen,oxygen,and water is the most uniform at the interface between the diffusion layer and the catalytic layer.The temperature and water distribution along the flow direction on the central section of the proton exchange membrane are the best,and the temperature difference between the area below the flow channel and the area below the rib is the smallest,resulting in the best output performance.