Liquid vaporization under thermodynamic phase non-equilibrium condition at the gas-liquid interface

Liquid vaporization under thermodynamic phase non-equilibrium condition at the gas-liquid interface is investigated over a wide range of fluid state typical of many liquid-fueled energy conversion systems. The validity of the phase-equilibrium assumption commonly used in the existing study of liquid vaporization is examined using molecular dynamics theories. The interfacial mass flow rates on both sides of the liquid surface are compared to the net vaporization rate through an order-of-magnitude analysis Results indicated that the phase-equilibrium assumption holds valid at relatively high pressures and low temperatures, and for droplets with relatively large initial diameters (for example, larger than 10 µm for vaporizing oxygen droplets in gaseous hydrogen in the pressure range from 10 atm to the oxygen critical state). Droplet vaporization under superheated conditions is also explored using classical binary homogeneous nucleation theory, in conjunction with a real-fluid equation of state. It is found that the bubble nucleation rate is very sensitive to changes in saturation ratio and pressure; it increases by several orders of magnitude when either the saturation ratio or the pressure is slightly increased. The kinetic limit of saturation ratio decreases with increasing pressure, leading to reduced difference between saturation and superheat conditions. As a result, the influence of non-equilibrium conditions on droplet vaporization is lower at a higher pressure.