Liquid–vapor equilibrium in a gravitational field

Abstract

The effect of a constant gravitational field on the existence and position of the liquid–vapor boundary of a pure classical fluid is quantitatively discussed on the basis of two simple models: (a) a perfect gas and incompressible liquid model, and (b) a van der Waals fluid model. The van der Waals equation is used as a function of reduced parameters, and it is shown that two dimensionless parameters suffice to describe the vertical concentration profile and the liquid–vapor boundary position. With this model, it is shown how the gravitational field induces a vertical phase separation, an observation so common that it is usually taken for granted but not modeled. The conditions for a gravitational field to produce gas condensation are also discussed.