Study On Numerical Simulation Of Mass And Heat Transfer And Cooling Mode In PEMFC

As an energy conversion device,the proton exchange membrane fuel cell(PEMFC)converts chemical energy into electrical energy with a high energy conversion rate and produces water with little pollution to the environment.In the era of advocating green development,PEMFC has attracted widespread attention.During the operation of PEMFC,there is a complex mass transfer and heat transfer process inside,and its water,heat and reaction gas concentration distribution is closely related to its performance.However,its microstructure brings a large cost to the actual monitoring of the correlation distribution,and numerical simulation is an effective and economic means.In order to optimize the mass transfer and heat transfer process of PEMFC and improve the performance of the cell,the numerical simulations of the mass transfer and heat transfer process of PEMFC are carried out in this paper and the main research work is as follows:Firstly,the steady-state,two-phase flow,non-isothermal model of the serpentine PEMFC is established to study the influence mechanism of external operating conditions and internal physical parameters on its steady-state performance.The results shows that the performance of the cell can be improved by changing the appropriate operating conditions and physical parameters: the water content of the PEMFC membrane can be increased by appropriately lowering the inlet temperature;the reaction gas concentration can be improved by increasing the operating pressure;In the region of medium and high current density,the wetting effect of the membrane can be enhanced by increasing the inlet humidity and the water content of the membrane can be increased.In the region of high current density,the accumulation of liquid water and liquid water saturation can be reduced by decreasing the inlet humidity.The diffusion efficiency of reaction gas can be improved by increasing the porosity of GDL.the ability of the cell to exclude liquid water can be enhanced by increasing the permeability.the hydrophobicity can be increased and the water saturation can be deceased by improve the contact angle.Secondly,based on the steady-state model,the transient model of PEMFC under multiple time scales is established to study the transient response speed and correlation of the key variables in PEMFC when the load changes.The results show that the transient responses of the membrane average temperature and current density are highly consistent,which reach steady state the fastest among all variables.The liquid water saturation reach steady state the most slowly.The average molar concentration of oxygen and hydrogen is affected by the accumulation of liquid water in the transient state to different degrees.The hydrogen reaches steady state the more slowly than the oxygen.The transient response of the membrane average water content and proton conductivity is highly consistent.And there is an overshoot in the variation trend,which is influenced by the wetting effect of liquid water.In the subsequent time,the variation trend is similar to that of liquid water saturation,the time to reach steady state is basically the same.It can be seen that the variation trend of above variables is correlated with the liquid water saturation.At the same time,a steady-state and non-isothermal model of water-cooled PEMFC stack is established.From the perspective of the relationship between the cooling water flow direction and the gas inlet direction,a variety of cooling modes are designed and their cooling effects are compared.The results show that the membrane temperature equilibrium between the adjacent cells and under the different flow channels in a cell can be improved by keeping the flow direction of cooling water adjacent consistent.Under co-current gas flow patterns,the best cooling effect can be obtained by keeping the flow direction of all cooling water consistent,and consistent with the inlet direction.Under counter-current gas flow patterns,the best cooling effect can be obtained by keeping the flow direction of all cooling water consistent with the direction of cathodic inlet direction.Finally,combined with the experimental platform and electrochemical impedance analyzer,a 5k W water-cooled stack is used to test the influence of cooling water inlet temperature and verify the validity of the model.The results show that 55℃ is the best inlet temperature of cooling water.Under the optimum temperature,the experimental results are in good agreement with the simulation results,and the model is accurate.The simulation results have certain application value.