- 學位論文
PEM燃料電池-非等溫兩相流模擬分析
A non-isothermal two-phase flow at cathode of PEM fuel cell
摘要
In the present study, fuel cell performance on the PEMFC cathode is investigated numerically. The computational domain includes the gas diffusion layer, the catalyst layer and the membrane. The modeling framework is assuming that the transport process is diffusion controlled and the convection transport is neglected. Both the single phase and two-phase flows are studied. The predicted results are validated with the experimental results of Liu et al.. Based on the analytic solution of Gural et al., an analytic temperature solution at the cathode of the PEMFC is presented. To allow for the analytic solution, the heat sources considered are reaction and ohmic heating in the catalyst layer, ohmic heating in the membrane and no heat effect in the gas diffuser layer. In deriving the analytic temperature solution, the temperature in the reaction term is held constant at the inlet temperature. This may affect the accuracy of the solution. To further examine the effect of the constant temperature assumption, the simulations are applied to different inlet temperature conditions. However, the difference between the analytic solution and the non- isothermal simulation remains approximately constant over a wide range on inlet temperature. This indicates that the error of the analytic solution is at most 10% compared with the numerical simulations. Based on the analytic solution, the influences of various operational parameters on the fuel cell performance and temperature distributions are also examined. The single phase, non-isothermal model is addressed next. However, the predicted limiting current is too high compared with the measurements by Liu et al.. In the practical application of the PEM fuel cell, condensed water would exist at high current density. The effect of liquid water is to reduce the diffusivity of the gas phase species. If the pores in a porous media are occupied by liquid water, the gas phase species can not reach the catalyst layer.In our model, we use ε^(eff) = ε(1-s^b)to simulate this effect. In the past studies, the effective surface area for reactions is modified using the correction factor (1-s), i.e. b = 1.However, in the present study, the porosity is modified using the correction factor (1-s^(2/3)), i.e. b is b = 2/3. The rationale is that because s is defined as the ratio of the volume of liquid water to the volume of void space. When the liquid water is generated by the chemical reaction, it blocks the void cross-sectional area of the porous diffuser, i.e. s^(2/3). Thus, when s increases, the porosity decreases. The predicted IV curves using the adopted porosity modification compare favorably with the measurements. The performances of the non-isothermal cases exceed those under the isothermal conditions.
並列摘要
In the present study, fuel cell performance on the PEMFC cathode is investigated numerically. The computational domain includes the gas diffusion layer, the catalyst layer and the membrane. The modeling framework is assuming that the transport process is diffusion controlled and the convection transport is neglected. Both the single phase and two-phase flows are studied. The predicted results are validated with the experimental results of Liu et al.. Based on the analytic solution of Gural et al., an analytic temperature solution at the cathode of the PEMFC is presented. To allow for the analytic solution, the heat sources considered are reaction and ohmic heating in the catalyst layer, ohmic heating in the membrane and no heat effect in the gas diffuser layer. In deriving the analytic temperature solution, the temperature in the reaction term is held constant at the inlet temperature. This may affect the accuracy of the solution. To further examine the effect of the constant temperature assumption, the simulations are applied to different inlet temperature conditions. However, the difference between the analytic solution and the non- isothermal simulation remains approximately constant over a wide range on inlet temperature. This indicates that the error of the analytic solution is at most 10% compared with the numerical simulations. Based on the analytic solution, the influences of various operational parameters on the fuel cell performance and temperature distributions are also examined. The single phase, non-isothermal model is addressed next. However, the predicted limiting current is too high compared with the measurements by Liu et al.. In the practical application of the PEM fuel cell, condensed water would exist at high current density. The effect of liquid water is to reduce the diffusivity of the gas phase species. If the pores in a porous media are occupied by liquid water, the gas phase species can not reach the catalyst layer.In our model, we use ε^(eff) = ε(1-s^b)to simulate this effect. In the past studies, the effective surface area for reactions is modified using the correction factor (1-s), i.e. b = 1.However, in the present study, the porosity is modified using the correction factor (1-s^(2/3)), i.e. b is b = 2/3. The rationale is that because s is defined as the ratio of the volume of liquid water to the volume of void space. When the liquid water is generated by the chemical reaction, it blocks the void cross-sectional area of the porous diffuser, i.e. s^(2/3). Thus, when s increases, the porosity decreases. The predicted IV curves using the adopted porosity modification compare favorably with the measurements. The performances of the non-isothermal cases exceed those under the isothermal conditions.