【作者】 陈晨；

【导师】 庄林；

【作者基本信息】 武汉大学 ， 物理化学， 2011， 博士

【摘要】 人类文明的进步与化石能源(特别是“黑金”石油)的利用密不可分,但化石能源的过度开采和利用引发了严重的能源和环境危机。从短期来看,缓解石油危机须有效地提高石油采收率,高效利用石油资源,并治理石油工业的环境污染。从长远的角度来看,彻底解决能源枯竭和环境污染这两大棘手的问题的出路在于发展清洁的新能源。氢能是一种可再生的清洁能源,其产生、储运以及燃料电池应用是目前能源领域的研究热点。本论文工作以三次采油和燃料电池为背景,运用分子模拟方法对若干重要过程和关键材料进行微观和动态的模拟研究。本工作包含两个部分,第一部分与石油开采和地下水污染治理有关。我们以毛细管限域流体为计算模型,运用多体耗散粒子动力学(MDPD)方法对毛细管中多种流体行为进行了多方面的研究,主要进展如下：1.毛细管中液体自发吸入和自发退出过程的MDPD模拟。建立了液体在毛细管中所形成的静态接触角与固-液相互作用力参数之间的关系,为调控液体润湿性提供依据。研究了自发吸入与自发退出过程的发生条件,并对不同浸润性的液体的动态过程进行了系统的模拟。在经过两次修正的(以动态接触角代替静态接触角、考虑惯性阻力)Lucas-Washburn方程中引入滑动距离b,并推导相应的数学公式,成功地将其应用范围拓展至不完全浸润管壁液体的自发吸入和自发退出过程。2.受外力驱动的毛细管中液-液驱替过程的MDPD模拟。建立油/水界面张力γE与油-水相互作用参数AE的对应关系,为界面张力的调控提供依据。设计受外力驱动的毛细管中液-液驱替过程的计算模型,可定量调节驱替外力f、水/毛细管界面张力γw、油/毛细管界面张力γo和油/水界面张力yE。当固定水-毛细管相互作用参数Aw、油-毛细管相互作用参数Ao和油-水相互作用参数AE只改变驱替外力f时,我们发现f必须大于临界值,驱替才能发生,并且驱替时液体流量与外力大小成正比。依据此规律,获得了模拟中启动压力fs的计算公式,讨论了参数Aw和AE对启动压力的影响；给出残油量ROC的计算方法,并讨论了参数AW、AE和f对毛细管残油量的影响。这些结果产生了一组外力驱替参数的优选原则,为实践提供有价值的参考。3.毛细管中液-液自发驱替过程的MDPD模拟。设计了合理的油/水/毛细管三相界面静态接触角计算模型,提出弯液面位置的新定义。设计毛细管中液-液自发驱替过程的计算模型,模拟了各种不同条件下的自发驱替过程。针对油/水界面不分离的情况推导了描述毛细管中液-液自发驱替过程的数学表达式,以模拟所得动态接触角代入该公式,得到了与模拟计算相吻合的预测曲线。我们进一步在该数学式中采用MKT理论描述动态接触角与弯液面运动速度之间关系的,使公式的应用不再依赖模拟所获得的动态接触角。本论文第二部分工作采用分子动力学(MD)模拟研究了燃料电池的一种新型聚电解质膜—碱性聚合物电解质(APE)膜的结构和动态性质,主要进展如下：1.将文献报道的Nafion膜模拟经验移植到对碱性聚电解质(APEs)的模拟中,研究了侧链长度不同的三种APE膜在不同含水量下的结构和动态性质。2.对特定类型的APE膜,随含水量的增加,膜的密度先增大后减小。根据该规律我们提出膜中空体积的概念,用以解释水分子进入APE膜时的填充行为,促进对聚合物膜溶胀规律的理解。3.分析了膜内多种原子(水分子中氧原子OW、氢氧根离子中氧原子Oh、季铵根离子中氮原子Nq)之间的径向分布函数(RDF),发现APE膜中季铵官能团均匀分布,氢氧根离子在含水的膜中发生电离并聚集于含水区域：APE膜存在小范围的亲水/憎水相分离结构,有别于Nafion膜的较大面积相分离结构。4.通过均方位移(MSD)的计算,分析了膜中水分子与氢氧根离子不同条件下的自扩散系数,发现两者相比Nafion膜均偏低,但两类粒子在APE膜中扩散活化能较大,提升工作温度可大幅提高APE膜离子电导。更多还原

【Abstract】 Modern civilization is established on the utilization of fossil fuels, especially the "black gold" petroleum; but now has to face severe energy and environmental crises due to the overexploitation. As a short-term resolution to the crises, the efficiency of oil recovery and utilization must be improved while effectively reducing environmental pollutions. As a long-term resolution, new clean energy sources are necessary, such as hydrogen, whose production, storage, distribution, and fuel-cell application are very hot research subjects nowadays.This thesis is about the molecular simulations of key processes and materials in oil recovery and fuel cells. The contents include two parts. The first one is related to oil recovery and groundwater remediation, which share common features of capillary-confined fluids. Many-body dissipative particle dynamics (MDPD) methods are employed in this work, resulting in the following main results:1. MDPD study of spontaneous capillary imbibition (SCI) and drainage (SCD).The relationship between static contact angles and solid-liquid interaction parameters is established, which enables the definition of fluid wettability. Startup conditions for SCI and SCD are studied and a series of such processes are simulated for fluids with distinct wettability. We have refined the modified Lucas-Washburn equation (with the static contact angle replaced by the dynamic contact angle, and the inertial resistance included) to incorporate the slip length b, such that SCI and SCD processes with partly-wetting fluids can also be well described.2. MDPD study of forced capillary displacement (FCDis).The relationship between the oil/water interfacial tensionγE and the oil-water interaction parameter AE is established, which enables the control of the interfacial tension between oil and water. A simulation model for FCDis is designed, in which external force f,water/capillary interfacial tension yw, oil/water interfacial tension yo, and oil/water interfacial tensionγE can be quantitatively regulated. With fixed water-capillary interaction parameter AW, oil/capillary interaction parameter AO and oil/water interaction parameter AE, it is found that FCDis can only take place when f exceeds a critical value and the flow rate is proportional to f.Accordingly, a parameter named starting force fs is derived and the influences of AW and AE are studied. Meanwhile, the residual oil content (ROC) could be extracted from the simulation, which is influenced by AW, AE, and f. Systematic investigations have produced a set of optimized conditions for FCDis, which may instruct realistic applications.3. MDPD study of spontaneous capillary displacement (SCDis).An appropriate oil/water/capillary 3-phase static contact angle model is designed, and a new definition for meniscus position is proposed. A simulation model for SCDis is designed, and a series of simulations under various conditions are conducted. A differential equation is derived to make use of the simulated dynamic contact anglesθd, which can describe the SCDis process very well. Further, after plugging in the relationship betweenθd and the meniscus velocity obtained from the MKT theory, the above differential equation is refined to be independent of the simulatedθd.The second part of the thesis is about molecular dynamics (MD) simulations on the static and dynamic properties of alkaline polymer electrolyte (APE) membrane, a novel material for fuel cells. The main results are summarized as follows:1. The static and dynamic properties of three types of APE membranes with different side chains are simulated using the force field for Nafion simulations reported in the literature.2. It is found that the density of hydrated APE membranes peaks at certain water content, according to which the concept of vacuum volume is proposed and demonstrated to be useful in explaining the filling behavior of water, and helpful in understanding the swelling nature of APE membranes.3. Radial distribution functions (RDFs) are calculated between typical atoms, including the water oxygen Ow, the hydroxide ion oxygen Oh, and the quaternary ammonium (QA) ion nitrogen Nq, which provide a variety of structural information of APE membranes. The QA functional groups are found to distribute uniformly in the APE membrane, while the hydroxide ions can be dissociated and assemble in the aqueous domains. There exist hydrophilic/hydrophobic phase separations in small range in APE membranes, differing from the wide-range phase separation observed in Nafion.4. Self-diffusion coefficients of water molecules and hydroxide ions are obtained by calculating their mean squared displacement (MSD) in the APE membranes. Although both coefficients are smaller than those in Nafion, the corresponding particle diffusion activation energies are larger in APE membranes, indicating greater conductivity can be obtained at elevated temperatures.更多还原