Pore And Single-cell Scales Modeling Multiphase Transport Phenomena In The Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell(PEMFC)has been one of promising alternative power sources in the automobile industry due to its outstanding merits such as high efficiency,low noise and zero emissions.One of the main challenges for the development of high-performance PEMFC is the water management in cell components with different scale such as flow channel(～1 mm)and porous gas diffusion layer(GDL,10～20μm).In the operation of PEMFC,the excessive product water can block reactants transport and reaction,especially in the high current density condition.Hence,fully understanding the multiphase flow mechanisms in flow channel and gas diffusion layer is crucial for high-performance PEMFC design and optimization.However,the unique liquid water behaviors in flow channel are not known under the high current density condition because of complex turbulent flow.The pore-scale liquid water dynamics in GDL are also poorly understood.The effects of GDL microstructures on PEMFC’s performance are unclear.This dissertation addresses these knowledge gaps through pore and single-cell scales modeling techniques:direct numerical simulations(DNS)of two-phase turbulent flow,pore-scale simulations of two-phase flow and mass transfer and full-cell simulation of three-dimensional(3D)PEMFC’s performance.A DNS model of the two-phase turbulent flow in fuel cell channel is developed with volume of fluid(VOF)approach for tracking the air/water interface.By resolving the whole range of spatial and temporal scales of turbulence,the results of the two-phase DNS model show that the deformation of the water droplet is asymmetric and broken into small pieces/films,and is significantly different from the laminar and the corresponding k-εmodels.It is suggested that the effect of turbulence on the two-phase transport in fuel cell flow channel is significant and needs to be considered for water management by using the DNS model.A pore-scale two-phase GDL model is developed and validated to investigate the two-phase flow and oxygen transport in the microstructures of GDL.In this model,GDL microstructures are reconstructed via a stochastic method.The local liquid water saturation distributions along the through-plane(TP)direction and in-plane(IP)direction are studied.The model is subsequently employed to investigate the effect of mix-wettability caused by different Polytetrafluoroethylene(PTFE)spatial distribution on the liquid water dynamics and correlation between capillary pressure P_l and liquid water saturation s in the GDL.It is found the mixed-wettability GDL shows a P_c-s curve more close to experimental data,indicating that the mixed wettability in the PTFE treated GDL needs to be taken into account to accurately predict two-phase behaviors.An oxygen diffusion model is further developed in conjunction with two-phase GDL model to study the effective oxygen diffusivity in the dry or partially-saturated GDL.The predicted effective diffusivity in dry GDLs is compared with various diffusivity models from literature.Reasonable agreements with other models were obtained.The effects of different local water profiles and porosity distribution on the effective oxygen diffusivity in both the TP and IP directions are investigated and compared with a lattice Boltzmann model(LBM)and experimental data.The present results are in good agreement with other studies.It is found that the local water profile has significant impacts on the effective diffusivity in partially-saturated GDLs and the diffusivity in the TP direction is more sensitive to the water distribution than the IP direction.The two-phase GDL and oxygen diffusion model are also used to optimize the perforationparameters,which can reduce the liquid water level in the GDL.Different perforation depths and diameters are investigated,in comparison with the GDL without perforation.It is found that perforation can considerably reduce the liquid water level inside a GDL.The perforation diameter(D=100μm)and the depth(H=100μm)show pronounced effect.Also,two different perforation locations,i.e.the GDL center and the liquid water break-through point,are investigated.Results show that the latter perforation location works more efficiently.Moreover,the perforation perimeter wettability is studied,and it is found that a hydrophilic region around the perforation further reduces the water saturation.Finally,the oxygen transport in the partially-saturated GDL is studied using an oxygen diffusion model.Results indicate that perforation reduces the oxygen diffusion resistance in GDLs and improves the oxygen concentration at the GDL bottom up to 101%(D=100μm and H=100μm).A single-cell 3D PEMFC multiphase model is developed and validated in open-source platform Open FOAM.The detailed PEMFC modeling aspects in Open FOAM are elucidated comprehensively.The model is used to study the effects of different relative humidity at the anode/cathode inlets.It is found that the different relative humidity at the anode inlet impacts the cell performance significantly,while the relative humidity has minor effects on the cell performance.The effect of intrinsic homogeneous transport properties is demonstrated by improving macro-scale 3D PEMFC model to pore-scale 3D PEMFC model where the microstructures of a GDL are used.The results suggest that the volume-average PEMFC model predicts higher performance than the pore-scale PEMFC model because of different estimations of the effective electronic conductivity in these two models.This work has significantly contributed to the understanding of two-phase turbulent flow in the flow channel,pore-sale liquid water dynamics and oxygen diffusion in the microstructures of the GDL.The work is also the first one to deliver detailed PEMFC modeling aspects for low-temperature PEMFC in Open FOAM,which greatly contributes to the high-efficiency and multi-functions PEMFC solver development.