Molecular Dynamics Simulation Of Vapor Condensation

In the processes of condensation, boiling, crystallization, catalysis, and other engineering problems, the nucleation phenomenon plays a critical role. Nucleation is a complicated physical process which contains the microcosmic and macroscopic process, the micro mechanism of nucleation is not totally understood. This thesis is focuses on the nucleation phenomenon in the process of condensation. Based on the statistic thermodynamics theories, we adopt molecule dynamics simulation (MDS) and Lennard-Jones (L-J) potential function to investigate the nucleation process.Firstly, the characteristic of fractional dimension number of condensation nucleus under the cooling velocity of 0.0002 was analyzed theoretically based on the fractal theory, and it was found that the fractional dimension number of condensation nucleus was 1.593. The nucleation process was analyzed by the cluster theory, and the results show that the process of vapor condensation is divided into latent heat releasing and sensible heat releasing processes. A model of force function was built up and simulation of non-homogeneous nucleation process was carried out. By comparing the homogeneous nucleation process with the non-homogeneous nucleation process under the same cooling velocity of 0.0002, it was found that the nucleus of condensation of homogeneous nucleation is random, but the nucleus of condensation of non-homogeneous nucleation is fixed, it just is the position allocated.Secondly, the homogenous nucleation of argon vapor was studied by molecular dynamic simulation, and the nucleation rate was calculated by three methods, CNT theory, MFPT and Cluster theory. It was indicated that under the same simulation condition the nucleation rates calculated by MFPT and Cluster theory belonged to the same order of magnitude, which were three orders of magnitude bigger than that of CNT theory. The difference among them can be explained by the equilibrium thermodynamics assumption, when the nucleation rate was calculated by CNT theory. However, for three methods the critical Cluster number of homogenous nucleation was very closely to 20. Additionally, by the Cluster theory analysis, it turned out that the nucleation rate was bigger as the cooling rate increasing, which was close to practical homogeneous nucleation.Thirdly, the bubble nucleation in superheated argon liquid was studied by molecular dynamics simulation. The simulation results show that the process of bubble nucleation is divided into three stages. The first stage is the waiting process, in which pressure of system is almost constant. In the second stage, numerous small bubble nucleuses appear and pressure of system increases rapidly. The third stage is the aggregation process of small bubble nucleuses, in this stage the pressure of system increases slowly to a stable value. By analyzing simulation data, the bubble nucleation rate is got, which is eight orders of magnitudes bigger than the result of classic nucleation theory. The bubble nucleation rate increases along with the increasing of density, which agrees well with classic nucleation theory.At last, the anode material Ni/YSZ of solid-oxide fuel cell (SOFC) was firstly analyzed by molecular dynamics simulation, and the model of Ni/YSZ was discussed mainly.