| Proton exchange membrane fuel cell?PEMFC?is a high efficient device,which directly transfers chemical energy stored in the fuel into electric energy.With the advantages not only including light weight,high energy density,no moving parts wear and low noise,but also including zero emission,environment friendly and low infrared radiation,PEMFC has been considered as a reliable source of power,widely used in civilian and military fields.Water,thermal and gas managements seriously affect the performance of fuel cell,thus structural optimization,enhancement of mass transfer,and improvement of the management level of gas,water and heat are the key issues for the development of fuel cells.The dissertation focuses on the relationship between the management level of water,heat and gas and its performance in the proton exchange membrane fuel cell,and develops the enhancement theory and method of the water/heat/gas in the fuel cell,and carries out numerical simulation and experimental verification.Improvement of the management level of gas and enhancement of mass transfer of the reactants could improve the performance of the fuel cell.In this dissertation,the effect of adding blockages in the flow channel was studied both by numerical simulation and experiment.Numerical results were successfully validated by the experimental data,confirming the performance improvement that occurred with blockages in the flow channel,particularly in the shape of rectangle and at the cathode side.The results also indicated that closer arrangement of the blockages corresponded to better performance.The analysis of the angle between the velocity vector and the concentration gradient in the convective mass transfer of the reactants showed that,the addition of the blockages in the flow channel could effectively reduce the angle.Combined with the field synergy principle,it was proved that the synergy between the velocity vector and the concentration gradient could be enhanced under the turbulence of the blockages,and effective mass transfer coefficient was proposed to measure the mass transfer capability of the fuel cell.Thermal management was another important issue limiting the development of the fuel cell.A three-dimensional model with coolant channels was developed to investigate the temperature distribution of PEMFCs under the impacts of different operating conditions,including Reynolds numbers of the coolant water,current densities,and relative humidity levels of reactants at operating temperature of 353K.And a novel thermal resistance model was proposed to validate the rules governing temperature distribution in fuel cells.Numerical simulation results revealed that relative humidity had a significant effect either on cell performance or on temperature distribution,and the temperature of the interface between the catalyst layer?CL?and gas diffusion layer?GDL?in the cathode was the highest of any part in the fuel cell.Limited by the temperature of the coolant water,small rise of reactant temperature would have little influence on the overall temperature distribution.However,the local temperature was too high at current density of 2 A·cm-2 with fully humidification,and the temperature should be kept in a reasonable range by lowering the coolant inlet temperature.Meanwhile,the phase change and its effect on the heat balance in a PEMFC stack was investigated both theoretically and experimentally.The effect of the phase change on heat balance must be accounted for to yield reliable results.During the heating process,convective heat transferred by the coolant water could be enhanced by elevating the flow rate of the coolant.During the cooling process,the thermal balance coefficient can reach 1.0.However,because of the large temperature difference between the MEA and the coolant water at high operating current density,a bipolar plate with high heat conductivity coefficient was required to improve temperature uniformity in the stack.Water and heat managements were inseparable in the fuel cell due to the gas-liquid phase transition process during the operation.There were three simplified configurations proposed to investigate the water transport characteristics in a PEMFC flow channel.In open system,liquid water in the cathode could be blown away with the exhaust gas.Simulation results showed that,higher inlet flow velocity accelerated water drainage.And hydrophobic surface was considered to be a preferable design for water removal.In closed system,water droplet could flow out of the flow channel without external force,just under an appropriate initial velocity with varied and decreasing surface contact angle of hydrophobic surface.Compared with the conventional flow fields,the removal time of water droplet in hydrophobic channel with variable contact angle could be greatly shortened by up to 44.7%,which was reduced from 17.79 ms and 11.33 ms to 9.84 ms.Considering the phase transition during the movement,the liquid water moved faster in the hydrophilic channel with decreasing effective diffusion area.Meanwhile,a 3D transient model,with user-defined boundary,has been proposed to study the effect of gas purging.Gas purging could effectively reduce the accumulation of water in the inlet region of flow channel,and it was positive on performance improvement.And the output voltage and the water distribution in the cathode channel responded immediately to the gas purging,while the water distribution in other regions was delayed.Combined with the enhancement of mass transfer and the optimization of water transport performance in the flow channel,a new optimized 3D flow field was proposed to investigate gas flow characteristics,water transport characteristics,and operation characteristics.The new type of flow field could effectively separate liquid water from gas flow,that the water could be partially removed through the flow gap.And the 3D flow field could enhance the mass transfer in the flow channel,and improve the performance of fuel cell.In addition,the inclination of the channel had little effect on the performance,while a larger inclination would result in a larger pressure drop.However,3D flow field caused considerable waste of the reactants,and the exhaust gas recycling was very necessary.Voltage abnormality of the fuel cell may occur,with flooding or starvation in the channel due to the poor water management and mass transfer,which could accelerate the rate of carbon corrosion and cause the performance degradation of the fuel cell.A mathematical model was established to study the potential characteristic of the solid phase potential and the membrane phase potential under different operating conditions,the extent of corrosion was qualitatively investigated and guidance for inhibiting carbon corrosion was provided.Based on thermodynamics,the effect of different operating conditions on the carbon corrosion rate of was investigated according to chemical equilibrium constant.It was found that,the rate of carbon corrosion could be slowed down by reducing the operating temperature,increasing the system pressure,reducing the stoichiometry and lowering the relative humidity of the reactants.The optimization of flow field and the managements of water and heat of fuel cell were investigated both theoretically and experimentally in this dissertation,based on mass transfer enhancement.The results had some guidance on the structure design and system optimization of fuel cells. |
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