A molecular dynamics investigation of n-alkanes vaporizing into nitrogen: transition from subcritical to supercritical

The injection, evaporation and mixing processes of hydrocarbon fuels into a supercritical environment are not yet well understood. The present paper investigated evaporation of three n-alkane fuels into nitrogen under various temperatures and pressures by molecular dynamics simulations. The emphasis was to understand at what conditions, when and how the transition from classical two-phase evaporation to one phase diffusion-controlled mixing takes place. The reduced ambient temperature and pressure range from 0.8 to 2.4 and 0.55 to 14.3, respectively. Scaling law was explored with hopes to extend the conclusions to macroscopic systems. The dimensionless transition time from subcritical to supercritical (with respect to the liquid lifetime) was found to be independent of liquid film thickness, but it has strong dependence on ambient temperature and pressure. With higher ambient temperature and pressure, the transition occurs earlier in the liquid's lifetime. Correlations for dimensionless transition time were proposed to describe such dependence. Additionally, a threshold dimensionless transition time of 0.35 may be used to separate the subcritical-dominated regime and the supercritical-dominated regime on the P-T diagram. Lastly, the normalized liquid lifetime (by a reference lifetime and the film thickness) depends only on the ambient temperature and pressure. As pressure increases, it decreases for subcritical-dominated cases mainly because the enthalpy of vaporization reduces with increasing pressure. The trend, however, is reversed for supercritical-dominated cases where the liquid lifetime increases slightly with increasing pressure. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.