Molecular Dynamical Study On Some Thermal Phenomena

It is difficult to investigate some heat transfer phenomena such as microscale heat transfer, ultra-speed heating and phase change heat transfer by traditional methods and tools. So molecular dynamics simulation(MDS) probably becomes an effective tool to discuss these phenomena. In this dissertation, MDS has been used to predict the thermodynamic properties and transport coefficients of an argon system with Lennard-Jones potential to reveal the propagation mechanism of a disturbance in the medium, to discuss the interface characteristics under saturated condiction.Numerical results of the energy, the velocity distribution, the mean free path, the mean collision time, the heat capacity and the self-diffusion coefficient agree well with the theoretical/experimental data.In order to possibly reduce the computative time, the relationship of thermodynamic and transport properties of different system dimensions was discussed and developed. One dimensional molecular dynamical simulation(1DMDS) was used and the predicted 3D results agree well with the experimental data. Since 1DMDS can reduce computer time and reduce parameter fluctuation, it can simulate a system of large number particles to obtain bulk parameters. A two-molecule-model was used to predict the conduction coefficient of gas Argon, which qualitively agree with the experimetal data.Results of MDS show that the pressure disturbance propagates by waves, while the temperature disturbance travels still in diffusive way, even though the initial temperature pulse duration is as short as only 5 times the relaxation time of the liquid argon. However, the very fast, large temperature disturbance propagates in part by diffusion and in part by waves. Both analyses and numerical results indicate that such heat/temperature waves resulting from the dissipations of the pressure waves are not the heat waves inthe sense of the C-V model or second sound.The characteristics of interface between the liquid and vapor phases under saturation condition are presented in terms of MD simulation. The thickness of the interface increases remarkably as the temperature rise close to the critical temperature. The temperature in the interface is not equal to the saturation temperature, but exhibits a nonmonotonous variation. Furthermore, some thermodynamic states correspond to the states in the unstable region of Van der Waals loop. This implies that the continuous phase transition appears between liquid and vapor phases without bubbles and droplets.Experiments of Freon(R13) in a vessel under slow heating has been carried out. The interface thickness is very thin and not avaible to be measured as the temperature is far lower than the critical temperature. The measured data by laser speckle technique show that the interface may be so thick, even up to the order of milimeter to centimeter as the temperature is close to the critical temperature.