【作者】 郝亮；

【导师】 郑平；

【作者基本信息】 上海交通大学 ， 动力工程及工程热物理， 2010， 博士

【摘要】 随着清洁替代能源研究的发展,质子交换膜燃料电池已经成为氢能源利用的理想能源转换装置。在质子交换膜燃料电池所发生的各种传递现象中,燃料组分的输运和水管理始终是关系着电池性能和及其可靠性的关键问题,然而,以前的宏观数值模拟因其局限性,不能精确的描述这些传输现象。本研究论文主要针对这些问题,在孔隙尺度上对气体扩散层及气体微通道内的单相和两相传输现象进行了研究。本研究论文以揭示与碳纸扩散层多孔介质有关的流动和传输机理为目标,以介观尺度的格子Boltzmann模型为理论研究方法,辅以必要的实验测量,探讨气体扩散层的微观结构以及润湿特性及其分布对气液传输的影响,以弥补宏观模型和实验观测在微观定量研究上的不足和缺陷。主要的研究内容包括了碳纸气体扩散层内的单相流动、碳纸扩散层内液体水的动态传输特性以及相对渗透系数和毛细压力-饱和度关系、气体微通道中液体水在扩散层表面上的动态传输特性。具体的研究内容和结论如下:1.碳纸扩散层各向异性渗透系数研究。数值构建了碳纸多孔材料的三维孔隙结构,并采用单相多松弛格子Boltzmann方法获得了压缩以及PTFE处理后的碳纸扩散层在孔隙尺度上的详细流动信息,结果表明:碳纸扩散层的渗透系数和弯曲度表现出明显的各向异性特性,其中in-plane方向的渗透系数大于through-plane方向渗透系数,而弯曲度正好相反;压缩引起的碳纸扩散层渗透系数的变化依然能用Kozeny-Carman半经验关联式和∧-base理论公式进行合理的预测,但是由于PTFE处理后的碳纸不具有单一的纤维结构,导致这些公式在这种情况下不再适用;Bruggeman方程计算的碳纸弯曲度正好处于in-plane和through-plane弯曲度之间,表明此方程仅仅考虑了多孔介质弯曲度的各向同性的宏观平均效果;分形理论模型预测得到的碳纸扩散层的through-plane渗透系数与Kozeny-Carman半经验关联式接近,但却不适用于in-plane方向和PTFE处理后的碳纸渗透系数的预测。2.碳纸扩散层内气液两相传输特性研究。采用多相自由能格子Boltzmann方法,对碳纸扩散层中液体水的传输机理以及扩散层孔隙结构和润湿特性对水传输的影响进行了研究;从孔隙模拟的角度,获得了碳纸扩散层内两相传输的相对渗透系数和毛细压力与水饱和度的关系曲线,并将所获得孔隙模拟结果与经验公式以及实验测量进行了比较。研究结果表明了碳纸扩散层的润湿特性在液体水的动态传输过程中所扮演着重要角色。在均匀润湿的情况下,碳纸扩散层的疏水性越强,水的传输越明显的表现为毛细指进的特征;而在疏水性不强时,水的传输表现为稳态驱替的特征,这种情况容易发生扩散层的水淹。在非均匀润湿的情况下,液体水选择亲水的路径进行传输,保留疏水孔用于气相传输,这种布置能更加有效的排出催化层的生成水,从而最大程度的利用更多的有效孔隙传输气体反应物,降低燃料电池的质量传输损失。此外,本文通过孔隙模拟获得的PTFE疏水处理的碳纸扩散层的Drainage和Imbibition毛细压力-饱和度关系曲线也较好的吻合了实验测量值,并充分体现了碳纸扩散层所具有的亲水和疏水孔隙共存的特征。根据研究结果拟合的碳纸扩散层毛细压力-饱和度关联式,相对于标准Leverett函数,能更加有效的对碳纸扩散层的毛细压力做出预测。3.疏水碳纸扩散层表面上液滴的动态特性研究。研究了液体水通过碳纸扩散层中的微孔隙,在扩散层表面上浮现、生长和最后脱离的动态过程,探讨了微通道中的气体流速以及扩散层表面的疏水程度对液滴脱离方式和液滴断裂大小的影响。研究结果证实了液滴与扩散层内部液体水传输孔隙间连接的表面张力是影响液滴脱离扩散层表面的主要因素,且液滴的脱离方式与通道内的气体流动剪切力有关。此外,本文所提出的基于液滴宏观力平衡的分析模型,对液滴在气体微通道内形成的大小以及断裂后的液滴在疏水扩散层表面上的运动速度也都给出了合理的预测结果。本文的研究在微观孔隙尺度上揭示流体在质子交换膜燃料电池气体扩散层内以及扩散层和气体通道界面上的流动和传质机理,为优化质子交换膜燃料电池的水管理和燃料组分传输策略提供帮助,同时也为质子交换膜燃料电池系统模型的开发提供重要的理论依据。更多还原

【Abstract】 With rapid progress of the researches on alternative and clean energy sources, proton exchange membrane fuel cells (PEMFCs) have become a promising energy conversion device for hydrogen applications. Among various and complicated transport phenomena occur in PEMFCs, reactant gas diffusion and water management are two very important issues which are greatly related to the performance and durability of the PEMFCs in order to become a commercial reality. Macroscopic models, however, are deficient to study these transport phenomena in PEMFCs. In this thesis, the single and two phase transport in the gas diffusion layer (GDL) and gas flow channel of PEMFCs are investigated using pore-scale simulations, thus providing more fundamental understandings of these transport problems.Due to the limitations of macroscopic models and existing experimental methods at the present time, the lattice Boltzmann method (LBM), which is based on mesoscale kinetic theory, has been used in present work to perform quantitative investigations of gas and water transport within the very thin carbon paper GDL with actual pore structure and wettability distribution taken into consideration. The major contents in this thesis include the investigations of single and two phase transport processes in carbon paper GDL, relative permeability and capillary pressure-saturation relationship of carbon paper GDL and water droplet dynamics on the surface of carbon paper GDL in a gas channel. The details are described as follows:1. Anisotropic permeability of carbon paper GDL. The multiple-relaxation-time (MRT) LBM is used to study the anisotropic permeabilities of carbon paper GDLs. The porous carbon paper is numerically reconstructed using the stochastic method by taking into consideration of various porosities and microstructures to imitate different GDL compression ratios and PTFE contents. The detail flow field of single phase fluid in the GDL microstructure is presented and the resulting permeability and tortuosity are calculated, which show anisotropic characteristics of the reconstructed carbon paper with in-plane permeability higher than through-plane and in-plane tortuosity lower than through-plane. The simulated and measured permeabilities are compared with the Kozeny-Carman and∧-base relations, which predict permeabilities as a function of porosity accurately for GDL with different compression ratios, but inaccurately for GDL with different PTFE contents. The Bruggeman equation is faulty to predict the anisotropic tortuosities. The fitted relations of tortuosity and porosity are obtained from simulation results and are used in a fractal model, which indicates that the fractal model only provides good predictions on through-plane permeabilities of carbon paper GDL.2. Two-phase transport in carbon paper GDL. Water transport dynamic behaviors and water distribution in the microstructure of carbon paper GDL is investigated using the multiphase free-energy LBM. The relative permeabilities and capillary pressure-saturation relations of carbon paper with different wettability distributions and PTFE contents are obtained from pore-scale simulations, and are compared with empirical relationships and experimental measurements. The results show that the wettability plays a significant role on water saturation distribution in water transport in GDL. For highly hydrophobicity, the water transport falls in the regime of capillary fingering, while for neutral wettability, water transport exhibits the characteristic of stable displacement, although both processes are capillary force dominated flow with same capillary numbers. In addition, the introduction of hydrophilic paths in the GDL leads the water to flow through the hydrophilic pores preferentially, which would facilitate the removal of liquid water more effectively, thus alleviating flooding in catalyst layer (CL) and GDL. In addition, the pore-scale simulated capillary pressure-saturation relationships of carbon paper with different PTFE contents are in good agreement with experimental results, indicating the coexistence of both hydrophilic and hydrophobic properties in the PTFE treated GDL. The fitted capillary pressure curves in present works could provide more accurate predictions of the effects of two-phase flow in PEMFC models than using the standard Leverett-Udell empirical relationship for sand.3. Water droplet dynamic behavior on the hydrophobic GDL surface. The dynamic behaviors of water droplet emergence, growth, detachment and subsequent movement on the GDL surface are presented. The detached mode or detached size of the droplet under the influence of gas flow velocity and GDL surface wettability is investigated. The results confirm that the emerging water droplet holding on the GDL surface is attributed to the surface tension force from the connecting of the water in the GDL pore and the emerged part on the GDL surface. It shows that liquid water removal is facilitated by a high gas flow velocity on a more hydrophobic GDL surface. Furthermore, a simplified analytical model based on force balance is presented to predict the droplet detachment size and moving velocity, and the predicted results are in good agreement with the mesoscopic simulation results.The results of the present work provide physical insight for understanding the reactant gas and product water transport in the porous GDL and water droplet dynamics on the interface of GDL and gas flow channel. The results are helpful for design of optimal reactant diffusion and water management strategies for PEMFCs, and also provide theoretical basis for the development of system models of PEMFCs.更多还原