Molecular Dynamics Simulation Of Liquid-Gas Interface In Sub/supercritical Surroundings

The liquid-gas interface refers to the dividing surface between two different phases, which has a certain thickness and volume. It is highly significant to comprehensive understand the liquid-gas interfacial behavior for the development of energy-efficient and low-emission internal combustion engine. However, the characteristics of micro-scale and instability make it difficult to realize the goal. Therefore, by using Molecular Dynamics, this dissertation is intended to reveal the microscopic mechanism of liquid-gas interfacial behavior while the ambient condition changes from vacuum to supercritical one from two aspects:single-droplet evaporation and nanojet injection.In the aspect of single-droplet evaporation, the liquid-vapor interfacial properties of n-heptane in sub/supercritical surroundings were investigated using molecular dynamics method and based on OPLS-AA (optimized potentials for liquid simulations all-atom potential function). For vacuum surroundings, the interfacial properties at 293K-533K and critical temperature of n-heptane were obtained to verify the accuracy of OPLS-AA potential function in this simulation. For sub/supercritical surroundings, the temperature distribution and interfacial properties are investigated while the ambient condition changes from vacuum to supercritical one. It is found that the calculated critical temperature is slightly lower than the real value, and calculated values of interfacial properties are in a good agreement with the experimental data at temperatures lower than 433K, while discrepancy increases at higher temperatures. The density difference between the liquid and gas phases as well as the surface tension decrease when the ambient conditions transition from subcritical to supercritical, however, the fluids could not become supercritical in lower supercritical surroundings, only in sufficiently high supercritical ones could fluids transition to supercritical state.For the non-equilibrium molecular dynamics simulation of nanojet ejection, the same single-droplet evaporation model was used to investigate the accuracy of united-atom potential function, then the effect rules of nozzle orifice diameter、push speed to nanojet were explored by using this potential function; particularly, we focused on the ejection and liquid-vapor interfacial behavior in supercritical nanojet. The results prove that only when nozzle orifice diameter is bigger than 7～8 molecular lengths of heptane can the nanojet become stable, the appropriate push speed for heptane is 200～300 m/s; besides, low supercritical ambient conditions make the density difference between the liquid and gas phases as well as the surface tension decrease but not disappear, only a sufficiently high supercritical ambient environment would make fluids transition to the supercritical state.