电场下液滴界面输运与传热特性的分子动力学研究

发布时间：2018-05-11 20:17

本文选题：液滴 + 界面输运　；参考：《华北电力大学(北京)》2016年博士论文

【摘要】：电场作用下液滴界面输运和传热特性在电雾化、静电纺丝、电喷干燥、电喷离子化以及原油电脱水等高新技术中扮演重要角色。在这些技术应用过程中,电场下液滴的蒸发、分裂、合并以及液滴的界面传热是发生的最基本物理过程。电场下多组分液滴中不同粒子(水分子、离子和高分子)间复杂相互作用,特殊团簇结构的形成,以及带电粒子的定向运动,导致电场下液滴界面输运和传热过程中出现诸多新问题和新现象。目前,实验和理论研究很难从分子角度揭示液滴中粒子间复杂相互作用,导致新问题和新现象出现的机理尚不明晰,这成为高新技术进一步优化和发展的瓶颈。本文采用分子动力学方法模拟电场下液滴的界面输运与传热行为,研究内容包括三部分：(1)电场下单个导电液滴(溶有盐的水液滴)的蒸发和分裂特性；(2)两个导电液滴的电合并行为；(3)高温带电壁面上水液滴的强化换热。本文研究目的在于揭示电场下液滴界面动力学过程中蕴含的新机理和新规律,同时提出调控液滴界面输运和传热行为的有效方法,为高新技术的优化和发展提供坚实的理论基础。电场下液滴的蒸发和分裂模拟结果表明,电场作用下液滴中加入盐,一方面可以强化液滴的蒸发和分裂特性,另一方面能够优化液滴中分子链形貌。此外,调节液滴中盐浓度,能够有效调控液滴的蒸发和分裂行为。观察液滴中粒子运动轨迹发现,液滴中存在可以自由移动和形成“离子对”两种状态的离子。液滴的蒸发和分裂行为取决于液滴中自由离子数目。自由离子在高压电场下高速运动会导致自由离子逃离液滴,发生子液滴喷射行为,从而强化液滴蒸发和分裂特性。此外,离子与分子链的特殊基团会形成配位结构,离子运动会带动分子链运动,起到优化分子链形貌的目的。因此,在静电纺丝、电喷干燥、电喷离子化等高新技术应用中,向液滴中加入盐可以调控制备产物的质量和性能。电场下邻近导电液滴会发生液滴的合并与不合并行为。当施加电场低于临界电场强度时,两个液滴接触后完全合并；然而,当电场强度高于临界值时,导电液滴接触后发生液滴的部分合并或反弹行为。本部分工作首次从分子角度观察到液滴接触后离子的传递过程,并揭示电荷的传递是导致液滴不合并行为的微观机理。模拟结果还发现,尺寸越大的导电液滴合并与不合并行为的临界电场值越小。施加脉冲电场,可以提高液滴发生合并与不合并的临界电场强度,同时提高液滴电合并效率,更有利于原油的电脱水。当壁面温度远高于液体饱和温度时,壁面上液滴会出现液滴的莱登弗罗斯特现象。即液滴与壁面间出现蒸汽层,严重阻碍液滴与壁面间换热。高温壁面上水液滴的传热行为模拟结果表明,湿润性越强的壁面与液滴间换热量越高,壁面上越容易发生液滴的莱登佛罗斯特现象。当高温壁面带有足够多电荷时,带电壁面会形成高强电场,可以显著强化液滴与壁面间相互吸引,增加液滴固-液界面处水分子数密度。壁面带电能够有效抑制液滴莱登佛罗斯特现象的出现,同时强化液滴与高温壁面间换热。
[Abstract]:The transport and heat transfer characteristics of droplet interface under the action of electric field play an important role in high and new technologies such as electric atomization, electrospun, electrospray drying, electrospray ionization and electric dehydration of crude oil. In these applications, the evaporation, splitting, merging and interface heat transfer of droplets under electric field are the most basic physical processes. The complex interaction of different particles (water molecules, ions and polymers) in multi component droplets, the formation of special cluster structure and the directional motion of charged particles lead to many new problems and new phenomena in the process of transport and heat transfer in the droplet interface under the electric field. The complex interaction between the new problems and the new phenomena is not clear, which is the bottleneck of the further optimization and development of high and new technology. In this paper, the molecular dynamics method is used to simulate the interfacial transport and heat transfer behavior of droplets under the electric field. The content of the study includes three parts: (1) a single conductive liquid drop (solution with salt water droplets) under the electric field The characteristics of evaporation and splitting; (2) the electric merging behavior of two conductive droplets; (3) the enhanced heat transfer of water droplets on the high temperature charged wall surface. The purpose of this study is to reveal the new mechanism and new laws contained in the dynamic process of the droplet interface under the electric field. At the same time, an effective method to regulate the transport and heat transfer behavior of the droplet interface is proposed, which is a new and high technology. The optimization and development of the operation provide a solid theoretical basis. The simulation results of evaporation and splitting of droplets under electric field show that adding salt in the droplet under the action of electric field can strengthen the evaporation and splitting characteristics of the droplets on the one hand, and on the other hand, it can optimize the molecular chain morphology in the droplet. In addition, the concentration of salt in the liquid droplet can effectively control the evaporation of the droplets. The movement of particles in the droplets shows that there are two ions in the droplet that can freely move and form the "ion pair" state. The evaporation and splitting behavior of the droplets depends on the number of free ions in the droplets. The free ions escape the droplets and the droplets of the generation of the free ions in the high pressure electric field. In addition, the special groups of ions and molecular chains will form the coordination structure, the ion movement will drive the molecular chain movement and optimize the molecular chain morphology. Therefore, adding salt to the droplets can be controlled in the high and new technology applications such as electrospun, electrospray drying, and electrospray ionization. The quality and performance of the prepared products. The combination and non merger behavior of droplets will occur near the electric field near the electric field. When the electric field is lower than the critical electric field strength, the two droplets are completely merged; however, when the electric field strength is higher than the critical value, the partial merge or rebound of the droplets after the contact of the conductive liquid drops. The transfer process of ion after droplet contact is observed from the molecular point of view for the first time, and it is revealed that the transfer of charge is the micro mechanism that leads to the unmerged behavior of the droplets. The simulation results also show that the smaller the critical electric field of the larger size of the droplets is combined with the unmerged behavior, the imposing the pulse electric field can improve the combination and non merger of the droplets. The critical electric field strength, at the same time increasing the combined efficiency of liquid drop, is more conducive to the electric dehydration of crude oil. When the wall temperature is far higher than the liquid saturation temperature, the droplet on the wall appears Lydon Frost phenomenon. That is, the droplets appear between the droplets and the wall surface, which seriously impede the heat transfer between the droplets and the wall. The simulation results show that the higher the heat transfer between the wall and the droplet is higher, the more easily the Lydon Frost phenomenon occurs on the wall surface. When the high temperature wall has enough charge, the charged wall will form a high-strength electric field, which can significantly strengthen the mutual attraction between the droplets and the wall, and increase the number of water molecules at the liquid droplet solid liquid interface. Wall electrification can effectively inhibit the appearance of Lydon Frost phenomenon of droplets and enhance heat transfer between droplets and high temperature walls.

【学位授予单位】：华北电力大学(北京)
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O35

【相似文献】