Molecular Dynamics Simulation Of The Effect Of Transcritical Effect On The Physical Properties Of Liquid Nitrogen

The environment of high power fuel injection engine combustion chamber is often in a supercritical environment.The fluid will present a different state from the subcritical fluid in the normal environment when it is in supercritical state.In the state,the surface tension of the fluid disappears and its viscosity is equivalent to that of the air.Thus,the gas-liquid interface is not clear.The trans-critical change in physical properties of the fluid fuel could make an important effect on the engine spray combustion process.Although the physical property parameters of subcritical fluids and supercritical fluids can be obtained from experiments or empirical formulas already.The trans-critical transition law of fluid properties has not been well understood because of the unsteady transition characteristics of the trans-critical change process of fluid properties,especially for the non-equilibrium transition process of single droplet from subcritical state to supercritical state.In this thesis,a single liquid nitrogen droplet is taken as the research object.The surface tension and viscosity of the droplet in heated condition has been studied by molecular dynamics(MD)method,in which the transform path of the droplet is subcritical state,partial supercritical state and complete supercritical state.First,the surface tension distribution and viscosity of liquid nitrogen in a subcritical environment have been studied by molecular dynamics(MD)method.And the simulation data of density,temperature,pressure and viscosity are obtained.The reliability of the selected parameters and the correctness of the programming are verified by comparing the data with the experimental values.The simulation results show that as the simulated temperature increases,the thickness of the gas-liquid interface layer increases,and the difference of gas-liquid density decreases when the liquid nitrogen under the subcritical environment.The change of environment temperature and pressure will have an important influence on the interface behavior of liquid nitrogen.Because of the increase of temperature,the molecules in the liquid phase move to the gas phase,which leads to the decrease of pressure difference and surface tension between the gas phase and the liquid phase.Then,the temperature of nitrogen is introduced into liquid nitrogen by the heat bath method,so that the temperature of liquid nitrogen reaches the critical temperature or even exceeds the critical temperature,which is the transforms of liquid nitrogen from subcritical to supercritical.The simulation results show that when the environment changes from subcritical to supercritical,the density difference between gas and liquid phase decreases,and the interface thickness increases.When the liquid nitrogen transform into supercritical state,the density is almost straight line,the gasliquid interface area disappears,the interface thickness disappears,the surface tension disappears,and the internal & external pressure difference is almost zero,which is consistent with the phenomenon that the surface tension disappears in supercritical environment.