Molecular dynamics investigation of the vaporization characteristics of n-alkane blended fuels under different ambient conditions

Abstract

Molecular dynamics (MD) simulation is a powerful tool to reveal the microscopic characteristics of supercritical transitions. However, the accuracy of MD depends strongly on the potential model that describes the interaction forces between atoms. In this study, four commonly used potential models for long-chain n-alkanes in MD simulations are evaluated, and a hybrid model is introduced. The vaporization and phase-transition characteristics of n-alkane blended fuels with different mole fractions are then explored under a wide variety of ambient conditions by using the hybrid model. Compared to the commonly used potentials, the hybrid model shows higher accuracy for predicting the thermodynamic and transport properties. In subcritical environments, vaporization belongs to typical two-phase evaporation with a sharp gas-liquid interface. The preferential evaporation of the light-end component is obvious, and the evaporation rate of the heavy-end component is maximized after the light-end component is consumed. Under supercritical conditions, the interface dissolves rapidly, the evaporation rates for both the light-and heavy-end components increase simultaneously, and both components coexist throughout the evaporation process. Based on the maximum potential energy and evaporation rate, a new criterion for the supercritical transition is proposed. The dimensionless transition time, which reflects the proportion of the sub/supercritical stage within the lifetime, is nearly independent of the ambient temperature and fuel composition; instead, it mainly depends on the ambient pressure. Finally, an empirical formula is obtained by curve-fitting to describe the variation in the dimensionless transition time with ambient pressure.