【作者】 王遵敬；

【导师】 过增元；

【作者基本信息】 清华大学 ， 工程热物理， 2002， 博士

【摘要】 蒸发与凝结是一类在工程与技术领域有着广泛而重要应用的相变现象，但由于其复杂性，现有理论仍难以准确预测蒸发与凝结流率。本文采用分子动力学模拟、理论分析和实验研究相结合的方法，对蒸发与凝结过程进行了从微观机理到宏观规律的系统研究，以求为工程应用中更好地预测蒸发/凝结流率提供改进的方法和建议。主要研究成果归纳如下：对汽液界面进行了分子动力学研究，揭示出宏观尺度的平界面在分子尺度上是随时间起伏涨落的曲分界面，分界面的涨落区域就是汽液界面的厚度，相应的激光散斑实验也定性地证明了界面涨落区的存在；研究还发现在汽液平衡体系中，界面附近存在局域热非平衡区域，并指出了其可能的原因及影响。对蒸发与凝结过程进行了分子动力学研究，提出了统计凝结系数的新方法--特征时间法，该方法通过统计获得有效区分反射过程和凝结再蒸发过程的特征时间，从而使求得的凝结系数近似程度更好。模拟获得的Lennard-Jones流体和水在平衡条件下的凝结系数表明：水和氩的凝结系数为温度的减函数；水的凝结系数略高于氩的凝结系数；虽然在水的模拟中考虑了分子的极性、旋转等因素，但模拟获得的水与氩的凝结系数之间的差距并不显著，具体原因仍有待于进一步的探讨。论文还采用瞬态法研究了非平衡条件下氩的蒸发系数，模拟研究的初步结果显示，非平衡分子动力学方法获得的蒸发系数与平衡分子动力学方法获得的凝结系数之间差距并不显著，由于可比数据少，上述结果还有待于更多的模拟验证。考虑界面的涨落结构和相间输运过程的动力学特征，将分子运动方向的影响引入过渡态分子的自由容积计算中，修正了平衡条件下蒸发/凝结系数的过渡态理论统计表达式。对于氩的凝结系数，该修正式获得的凝结系数能与本文及其他作者的模拟结果相吻合，计算准确度可望优于现有的其它凝结系数统计理论计算方法。采用准稳态方法对水的蒸发过程进行了实验研究。研究表明，使用界面温度获得的蒸发系数实验结果与分子模拟获得的凝结系数差距不大，但采用主流区温度获得的蒸发系数与模拟计算结果相差可达4~5个量级。由于界面处存在明显的温度跳跃，界面温度的准确实验测量难度较大，这很可能是文献报道的蒸发/凝结系数的实验研究结果数据分散程度较大的主要原因。本文则提供了一种能获得准稳态蒸发/凝结过程的界面温度分布的热分析方法。更多还原

【Abstract】 Evaporation and condensation phenomenon has been studied actively for decades because of its extensive and significant applications in various fields of technology and engineering. However, because of the complexity of the liquid-vapor phase transition, until now, we still have difficulty in accurately predicting the evaporation or condensation flux. The present dissertation mainly focuses on MD study as well as statistical theoretical and experimental investigations of evaporation and condensation phenomenon. According to these studies, some suggestions and method for the calculation of the evaporation or condensation flux are provided. The results can be summarized into the following aspects:Molecular dynamics study of liquid-vapor interface shows that the planar liquid-vapor interface at macroscopic level is in fact a wavy surface fluctuating with time, and the length scale of the fluctuating region of the wavy surface is the thickness of the liquid-vapor interface. With speckle laser visualized experiment, the fluctuation of the interface can be verified qualitatively. Moreover, MD simulations indicate that in the liquid-vapor equilibrium system, there exists a local non-equilibrium region near the interface. We analyzed the reason of the local non-equilibrium region existence and its possible effect on the calculation of the liquid-vapor phase change flux.With MD method, we studied evaporation and condensation process. By statistically analyzing the behavior of the colliding molecules with the interface, we presented a novel method, namely, the characteristic time method, to calculate the evaporation/condensation coefficient. In this method, the condensed then re-evaporated process is considered. The condensed then re-evaporated molecules are successfully distinguished from the reflected molecules with their different characteristic time, which make this method more reasonable to calculate condensation coefficient than the previous MD statistical methods. With the characteristic time method, we also studied the condensation coefficients of water and argon in liquid-vapor equilibrium system. The simulated condensation coefficient decreases with the increase of temperature for both argon and water, and the condensation coefficient of water is larger than that of argon. Though the polarity and the rotation are considered in the simulation of water, the difference between the condensation coefficients of water and argon is not remarkable. Further study is in <WP=6>demand. Moreover, with a transient simulation, we calculated evaporation coefficients of argon under non-equilibrium conditions. The rudimental MD results indicate that there is no notable difference between the evaporation coefficients from the non-equilibrium simulation and the condensation coefficients from the equilibrium simulation.Based on the fluctuation of the liquid-vapor interface and the thermodynamic characteristic of the liquid-vapor interphase, we modified the transition state theoretical calculation of the condensation coefficient by taking into account the effect of molecular moving orientation in the free volume calculation for the activated complex. The calculated condensation coefficients of argon from the modified theoretical formula agree well with the MD simulation results from different authors.Quasi-stable evaporation process of water is experimentally investigated. The experimental results of the condensation coefficient obtained from the liquid and vapor temperatures near the interface and the results from MD simulations are in the same order, but those obtained from the bulk liquid and bulk vapor temperatures are four to five orders lower than the results from MD simulations. From the vapor phase to the liquid phase, the temperature jumps near the interface. Therefore, it is difficult to accurately measure the temperatures near the interface. This may be the main reason of the large difference between the evaporation/condensation coefficients obtained from different experiments. A method for calculating the temperature更多还原