Evaporation of liquid fuel in a high-pressure air is a process that critically influences fuel–air mixing in advanced propulsion systems. In this work, we use molecular dynamics (MD) simulations to study the validity and accuracy of the Schrage equation in quantifying the evaporation and condensation rates of n-dodecane (a diesel surrogate) in air (approximated as N2 gas) with gas pressure varying from 0 atm to above the critical pressure of n-dodecane. The MD simulation results show that the evaporation coefficient (αe) is higher than the condensation coefficient (αc) at the evaporating n-dodecane surface and is lower than αc at the condensing n-dodecane surface due to the nonzero macroscopic velocity in the vapor. The difference between αe and αc decreases with decreasing evaporation/condensation flux. Even in cases with low evaporation flux, our modeling results show that ignoring the small difference between αe and αc could result in significant errors (60%) in the prediction of the evaporation flux from the Schrage equation. We also find that high N2 gas pressure (∼20 atm) could increase the saturated n-dodecane vapor pressure/density by over 15% due to the Poynting effect. The enhanced saturated vapor density is mainly caused by the high molar volume of liquid n-dodecane, which considerably increases the Gibbs free energy of liquid n-dodecane under a high-pressure N2 gas. Ignoring the enhancement of the saturated vapor density under high gas pressure could make the Schrage equation invalid in the prediction of evaporation and condensation rates of n-dodecane. When inequality between αe and αc and the Poynting effect on saturated vapor density are both taken into account, the Schrage equation gives accurate predictions of evaporation and condensation rates of n-dodecane in a high-pressure N2 gas.
Tausif et al. (Fri,) studied this question.