Phase transitions of multi-component fuel droplets under sub- and supercritical conditions

摘要

For a multi-component hydrocarbon mixture under supercritical conditions, the mechanism and criterion for the transition of the dominant mixing mode from evaporation to diffusion are not well established. In this paper, phase transition processes of three-component hydrocarbon fuel (5.3 wt% isooctane, 25.8 wt% n-dodecane and 68.9 wt% n-hexadecane) droplets in sub- and supercritical nitrogen environments were studied using molecular dynamics, in comparison with those of single-component n-hexadecane droplets. The initial diameters of the droplets were 25.5 nm (three-component) and 26.5 nm (single-component), respectively. Based on the quantitative Voronoi tessellation, a new criterion, which was a combination of two dimensionless critical values of H-c = 0.85 and W-c = 0.35, was proposed to determine the transition of the dominant mixing mode from evaporation to diffusion during fuel-ambient gas mixing. Results indicated that when the ambient pressure ranged from 2 MPa to 10 MPa and the ambient temperature ranged from 750 K to 1200 K, the density difference between the vapor phase and the liquid phase decreased gradually with increasing ambient pressure or decreasing ambient temperature. And the dominant mixing mode gradually transitioned from evaporation to diffusion. Increasing the ambient pressure did not necessarily promote the occurrence of phase transition, while increasing ambient temperature accelerated the phase transition monotonically. Light fuel components increased the minimum pressure of the diffusive mixing zone. A major finding was that under a certain ambient pressure, the supercritical transition temperature had not only a minimum but also a maximum.