PEM燃料电池气体扩散层微观结构中交叉流的数值模拟研究

发布时间：2018-08-13 19:40

【摘要】：如何有效地利用阴极反应生成的水和加湿的反应气体对膜进行适当的水合作用,并及时地排出阴极液态水,是提高质子交换膜燃料电池性能的关键。作为气体扩散层中一种典型的流动形式,交叉流能够有效地排出阴极液态水。同时,由于气体扩散层的疏水处理工艺,会导致其表面的接触角分布不均匀。交叉流以及非均匀接触角对气体扩散层微观结构中液态水的传输过程均有影响,目前,与此相关的研究还很少。本研究基于已经开发的气体扩散层真实微观结构的重建模型和VOF方法,建立了一个三维、两相流瞬态模型,研究接触角(固定和渐变)、压力差(毛细压力)对交叉流在气体扩散层微观结构中传输特性的影响。研究结果表明,交叉流在气体扩散层的传输过程中,会先通过一些容易渗透的孔隙,形成“毛细指进模式”;然后越来越多的液态水侵入到气体扩散层中,填满其余的大部分孔隙,形成“稳定传输模式”。研究证实,对液态水的传输模式来说,压力差比接触角具有更显著的影响。较高的压力差和较小的接触角能够加快液态水的侵入过程,使交叉流动过程达到稳定状态所需的时间缩短;对于渐变接触角工况,在垂直于流道方向的不同横截面上,气体扩散层表面的接触角也不相同。在这些截面上,交叉流的传输特性与其在相对应的固定接触角工况下具有相同的变化规律;对于交叉流在气体扩散层中的整个传输过程,渐变接触角工况与其相对应的固定接触角中间值工况下的变化规律相似。当交叉流的传输过程接近结束时,达到规定的稳定状态。在稳定状态下,毛细压力(压力差)随液态水体积分数的增加而增大,这个趋势在液态水体积分数较大的时候更加明显;在稳定状态下,气体扩散层中液态水的质量流量与压力差呈正比例,且当接触角较小时,气体扩散层中液态水的质量流量更高。总体来说,对于渐变接触角及其相对应的固定接触角中间值的工况,压力差对交叉流稳态传输特性的影响规律相似。
[Abstract]:The key to improve the performance of proton exchange membrane fuel cells is how to effectively use the water generated by the cathodic reaction and the humidified reaction gas for proper water cooperation and timely discharge of cathode liquid water. As a typical flow form in gas diffusion layer, cross flow can effectively discharge cathode liquid water. At the same time, due to the hydrophobic treatment of the gas diffusion layer, the surface contact angle distribution is not uniform. Cross-flow and non-uniform contact angle affect the transport process of liquid water in the microstructure of gas diffusion layer. Based on the developed reconstruction model of the true microstructure of the gas diffusion layer and the VOF method, a three-dimensional and two-phase flow transient model is established. The effects of contact angle (fixed and gradual) and pressure difference (capillary pressure) on the transport characteristics of cross flow in the microstructure of gas diffusion layer are studied. The results show that the cross flow in the gas diffusion layer will first pass through some easily permeable pores to form a "capillary pointing mode", and then more and more liquid water will invade into the gas diffusion layer. Fill most of the remaining pores to form a "stable transport mode". It is confirmed that the pressure difference is more significant than the contact angle in the transport mode of liquid water. Higher pressure difference and smaller contact angle can speed up the invasion process of liquid water and shorten the time required for the cross flow process to reach a stable state. For the gradual contact angle condition, at different cross sections perpendicular to the direction of the flow channel, The contact angle of the gas diffusion layer surface is also different. On these sections, the transport characteristics of cross flow are the same as those under the corresponding fixed contact angle, and for the whole transmission process of cross flow in gas diffusion layer, The variation law of the gradual contact angle is similar to that of the fixed contact angle. When the transmission process of the cross-flow is near the end, the specified stable state is reached. Under steady state, capillary pressure (pressure difference) increases with the increase of liquid water volume fraction, which is more obvious when the volume fraction of liquid water is larger; in stable state, the capillary pressure (pressure difference) increases with the increase of liquid water volume fraction. The mass flow of liquid water in the gas diffusion layer is proportional to the pressure difference, and when the contact angle is small, the mass flow rate of liquid water in the gas diffusion layer is higher. As a whole, the influence of pressure difference on the steady state transmission characteristics of cross flow is similar for the condition of gradual contact angle and its corresponding intermediate value of fixed contact angle.
【学位授予单位】：天津大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TM911.4

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