Three boundary features are available for vapor transport; the Vapor Inflow, the
Vapor–Liquid Interface, and the
Vapor–Liquid-Mixture Interface feature. All three features prescribe the equilibrium vapor pressure for one or several vapor species, either at an inlet connected to an upstream system involving liquids, or at a vapor–liquid interface. Evaporation or condensation occurs when the partial pressure in the adjacent gas domain differs from the vapor pressure at the interface. Below the conditions prescribed at the interface and the resulting heat of evaporation is defined.
The saturated water pressure psat,w (Pa) at a liquid water surface is defined as a function of temperature
T using the correlation:
where R is the universal gas constant,
Mw is the molar mass of water, and
ρ the vapor density.
Here fiL is the fugacity in the liquid phase, and
fiV the fugacity in the vapor phase for component
i. The fugacity in each phase depends on the temperature and pressure, as well as on its mass fraction in the liquid (
ωl,i) and vapor phase (
ωv,i), respectively. Using the Thermochemistry coupling, functions for the fugacity
fi =
fiL =
fiV at the interface are created automatically.
where ΔHvap, w denotes the heat of vaporization of water,
jw is the water mass flux at the interface,
us is the Stefan velocity, and
n a normal unit vector.
where ΔHvap,i is heat of vaporization of species
i. The evaporative heat flux is defined and announced by the vapor–liquid interface feature for use when modeling heat transfer. It can for example be applied using a Boundary Heat Source feature in Heat Transfer in Fluids Interface.