【作者】 丁玉栋；

【导师】 廖强；

【作者基本信息】 重庆大学 ， 工程热物理， 2010， 博士

【摘要】 便携式电子设备和无线通讯技术的发展对电源的要求越来越高,传统电池已很难满足要求。而且,当今能源危机和环境污染日益严峻,促使人们寻求一种高效、清洁、廉价方便、续航能力强的新能源动力技术。而直接甲醇燃料电池（DMFC）可以使用液态甲醇溶液作为燃料,与传统动力装置相比具有其独特的优点,如能量密度高、启动响应快速、零排放、低温运行、可利用现有的能量供应系统等,是最有希望尽快商业化的下一代移动式电源。因此,国内外众多研究单位开展了关于DMFC的研究。至今,国内外对DMFC的研究主要集中在化工、材料和电池性能的整体优化方面,对于电池中关键的传输现象和传输机理等热物理问题的研究多为数值模拟,系统的实验数据较缺乏。因此,本文从工程热物理学科角度出发,研究液相进料DMFC内两相流动及其传输特性。主要内容包括:液相进料DMFC的设计与制造;实验系统的建立及DMFC性能研究;DMFC阳极流道CO₂气泡动态特性研究;DMFC阳极流场两相流动特性及其对电池性能的影响;液相进料DMFC阳极流动阻力特性;DMFC阴极流动阻力特性及水淹过程; DMFC多孔扩散层气液两相逆流传输模拟实验研究。主要研究成果如下:（1）在流道内扩散层表面与流道侧壁面的角区和碳布纤维束之间的交叉空隙处优先生成CO₂气泡; CO₂气泡生长和脱离主要受浮力、曳力、剪切升力和表面张力的控制;气泡生长速率随电流密度和接触环直径的增大而增大;甲醇浓度、甲醇溶液流速和温度增加,气泡脱离直径变小;扩散层表面润湿性越好,气泡的脱离直径越小。（2）甲醇溶液流量增加,流道内单个气泡的体积减小,气泡从流道排出的速度加快,电池性能提高;甲醇溶液温度变化对流道内气泡的生成及聚合过程影响很小;甲醇浓度、电池阴阳极压差增大,流道内的CO₂气泡增多,体积增大;电池采用下进料、上排气方式有利于CO₂气泡的排出,电池的性能较好;（3）相同操作条件下,蛇形流场内产生的压降大于平行流场;放电电流密度、甲醇溶液流量和温度增加,流场进出口压降增加;甲醇浓度提高,进出口压降均略有减小;流场进出口压降随流道宽度的变小而增大;在相同流道宽度情况下,流场进出口压降随流道深度的变小而增大;流场进出口压降与流道当量直径和流场的开孔率均有关系。（4）建立DMFC阳极蛇形流场进出口压降均相模型,计算结果表明计算数据与实验数据变化趋势一致,但相对误差较大;用无因次准则整理文献中微小槽道内气液两相流流型实验数据,绘制微小槽道内水平流动气液两相流流型图,建立基于流型模型的DMFC阳极蛇形流场进出口压降模型,计算结果优于均相模型但与实验数据间的相对误差仍然偏大;将附着在扩散层表面未脱离的CO₂气泡视为连续重复的粗糙单元,并引入系数k表征放电电流密度对气泡分布均匀性的影响,提出计算DMFC阳极蛇形流场进出口压降的改进方法,计算结果表明:采用Beattie & Whalley关系式确定两相均质粘度,模型计算数据与实验数据间相对误差小,模型预测效果较好。（5）恒电流放电时,平行流场中首个液滴大多首先在流场右上区域冒出;流场中新液滴的出现具有瞬间涌出特性而且总是优先出现在流场板和扩散层交界的夹角处及扩散层表面碳纤维束交叉处;液滴的生长过程具有非连续性,与流道边壁相接触的液滴和液柱的生长速度均大于流道中间位置及靠近流道中间位置的未与流道边壁接触的液滴的生长速度,而且液柱有逆气流方向反向生长现象。电池恒电流放电时,流场内液态水的存在对流场压降的影响较大;随着氧气流量增大,阴极扩散层表面首个可见液滴的生成时间随之增加,阴极流道内的液态水逐渐减少,而且流场中大液滴数量和形成液柱的长度也减少,电池性能得到提高,但流场进出口压降却增加;进气温度升高,阴极扩散层表面首个可见液滴的生成时间同样随之增加,而且阴极流道内的液态水减少,流场中大液滴数量和形成液柱的长度也减少,流场进出口压降降低,电池性能得到提高。（6）利用多孔网络实验模拟研究DMFC多孔扩散层气液两相逆流传输特性,结果表明:可渗透壁面上多孔成泡时常伴随气泡回缩现象,气泡进入快速生长期后,多孔网络内出现明显的水侵渗现象;喉道内水侵渗机制呈“活塞状”,孔内侵渗机制包含三种:（1）孔初始状态有相邻两喉道被气相占据,其余两相邻喉道被液相占据,呈“弯月形”侵渗;（2）孔初始状态有三个相邻喉道被液相占据,剩余一个喉道被气相占据,呈“球缺形”侵渗;（3）孔初始状态有两个相对的喉道被液相占据,另外两个相对的喉道被气相占据,以几何截面突变处的“夹断”气柱方式侵渗;多孔网络内空气驱替水的机制包含三步:（1）喉道内呈“活塞状”驱替液相;（2）气相由喉道进入孔,呈“球冠形”驱替;（3）气相占据整个孔;空气流量小,气驱液在多孔网络内呈现稳定驱替（stable displacement）现象,随着空气流量的增加,逐渐向“树冠形”分布转变;多孔网络内气相驱动压力随气相流量和水流量的增加而增加;水流量增加,多孔网络内水最大侵渗高度增加,气相饱和度略有减小。更多还原

【Abstract】 The increased functionality demands for present and future create a critical need for smaller, less-costly, environmentally safe, highly efficient and long-lasting power in portable energy applications. However, the performance capability of conventional electrochemical battery is unlikely to keep pace with the expanded power requirements. Direct methanol fuel cells （DMFCs） has inherent advantages over other battery system in particular for portable electronics application for the view point of using aqueous methanol solution as fuel, high energy density, rapid start-up and response, low operating temperature, zero emission and easy-to-recharge character. It has been widely recognized as the most promising power sources for the next generation portable electronic equipment. Consequently, a large number of research works focus on the development of DMFCs.Based on the literature survey, it is easy to find that many researches focused on the chemicals, materials and optimization of system performance. However, systematic experiments on the transport mechanisms inside it is still lack. Therefore, a homemade transparent DMFC was developed to visualize the two-phase flow and transport in the anode and cathode flow field. The main contents of this thesis include: The design and manufacture of a liquid feed DMFC; Test bench setup and experimental study of the cell performance; Two-phase flow and transport characteristics in the anode flow field; Visualization of water flooding and characterization of flow resistance in the cathode flow field; Building a pressure drop model for two-phase flow in the single serpentine anode flow field; Experimental study on the gas-liquid countercurrent flow in porous diffusion layer of DMFCs. The main results are summarized as follow:（1） For the anode using a parallel flow field, it is observed that the pores around the corner of the channel ribs and the intersection of the carbon cloth fibres were favorable sites for the emergence of CO₂ gas bubbles under constant current mode. When the gas slugs are removed, the fraction of gas coverage reduced gradually. The growth and departure of CO₂ bubbles are mainly controlled by the balance between buoyant, drag, shear lift and surface tension force in the horizontal flow channels of the vertical DMFC. Both a higher current density and a bigger diameter of contact ring induce the growth of CO₂ bubbles. The bubble departure diameter decreases with the increasing in methanol flow rate, methanol concentration and operating temperature. A more hydrophilic gas diffusion layer usually lead to smaller bubble departure diameters.（2） Increasing the flow rate of the methanol solution, the individual CO₂ bubbles emerging into the channels become smaller, and the coalescent gas slugs become shorter and less as well. This results in an improved methanol supply from the flow field to the catalyst layer, and consequently the cell performance. However, further increase in the methanol flow rate over a particular value does not result in the improvement of the cell performance. The temperature change of the methanol solution shows no influence on the formation and coalescence of CO₂ gas bubbles. There are more CO₂ bubbles and larger gas slugs can be found in the channels when the methanol concentration and the pressure difference between the anode and the cathode increases. Feeding methanol from lower and exhausting CO₂ gas from upper enhanced the removal of CO₂ bubbles, and hence the cell performance.（3） It is also found that the pressure drop between inlet and outlet is larger in serpentine flow field than that in parallel flow field under the same operational parameters. With the increase of current density, methanol solution flow rate and temperature, the pressure drop of anode flow field increases. The pressure drops in serpentine and parallel flow field decrease slightly with the increasing of methanol concentration when the DMFCs are operated under the same current density. The result from the parallel flow plates with the same channel depth and different channel widths shows that the pressure drop increases with the decrease of channel width. On the other hand, it is found that the pressure drop increases with the decrease of channel depth when the same channel width and different channel depths are used. It is clear from the experimental data that both the channel equivalent diameter and the open ratio plays an important role on the pressure drop.（4） A homogenous model is developed for the first time for the calculation of the two-phase flow pressure drop in the serpentine flow field of a DMFC. The results show that the relationships between pressure drop and methanol solution flow rate and current density agree with those of experiences, but the relative error is not neglectable. The separated model based flow pattern in mini-channels is also built for the improvement of the predictive validity. Although the results also agreed with the experimental results and the relative error less than homogenous model, satisfied accuracy are still not achieved. The dynamics of CO₂ bubbles has a great influence on the frictional pressure drop of the two-phase flow in flow channels. Therefore, the attached CO₂ bubbles can be treated as continued repeating rough units, and using a factor k to characterize the effect of discharge current density on the distribution of bubbles, an improved method for calculating the frictional pressure drop of the two-phase flow is proposed. The results show that the relative error is smaller than the homogenous model and the separated model, the predictive validity is good when using the relation of Beattie & Whalley to calculating two-phase homogeneous viscosity.（5） It was observed that the first droplet usually emerged in the up-right region of the parallel flow field and most liquid droplets usually appear in the downstream of channels. The growth of new droplets has the instant effusing character and they usually appear from the pores around the corner of the channel ribs and the intersection of the carbon cloth fibres. The droplets has the character of pulsed growth, the land-touching droplets developing on each side of the channel and water columns contacting the channel wall grow faster than those far from the wall. In addition, it is interesting to note that some water columns grow in a direction opposite to the oxygen flow. Moreover, it is shown that with a higher oxygen flow rate and a higher inlet oxygen temperature usually lead to a much more forming time in for a water droplet on the surface of gas diffusion layer, whereas droplets can be more easily removed from the wall. These enhance the mass transfer of the oxygen, and thus improve the cell performance. The pressure drop significantly affects the liquid water in the flow field. Under the constant current discharge operating mode, the pressure drop increases with the increase in the flow rate of the oxygen or at a lower temperature of the cell. Increasing the temperature of the oxygen leads to the decrease of the pressure drop.（6） From results of two-phase counter flow in the porous layer of DMFCs, it is found that the formation of the multiple bubbles usually accompanies with the bubble retrieve phenomenon. Water invasion process can be observed when the bubble is at the rapid growth stage. During the water invasion process, snap-off occurs at the junction between the pore and throat due to the velocity difference between liquid and gas phases. Water invasion in the network is rather complex. In throats, displacement is piston-like. Three types of invasion can be observed in the pores: （1） two neighboring throats are filled by gas and the rest two are filled by liquid, forming meniscus-like invasion （2） crown-like invasion under the condition that three throats are filled by liquid and the last one is gas-filled.; （3） snap-off invasion under the situation that the opposite two throat are liquid-filled while the other two are filled by gas. There are three steps for the gas invasion in the network: （1） piston-like displacement in the throats due to the pressure effects; （2） crown-like invasion in the pores; （3） pore filling process. The stable displacement occurs at a smaller air flow rate. With the increase in the air flow rate, air distribution in the network can be transformed to dendrite. With increasing the gas and liquid flow rate, gas pressure increases in the network. The largest invasion height of the liquid increases with the increasing in liquid flow rate. The gas saturation in the network slightly reduces with the increase in the liquid flow rate.更多还原