Molecular Dynamics Research And Experiment Of Evaporation And Condensation

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 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...