Moist Surface
This node should be used to model evaporation from and condensation to a solid surface partially covered with liquid water. It adds the evaporation flux to the boundary conditions of Equation 7-4 of Moist Air (MT Interface) node, and computes the corresponding latent heat source to be added in the heat transfer equation. In addition, it computes the liquid water concentration accumulated on the surface, during evaporation and condensation processes.
It can be applied on the exterior boundaries of a domain where the Moist Air (MT Interface) node is active.
In time-dependent studies, the evaporation flux gevap (SI unit: kg/(m2s)) is defined as:
where Mv is the molar mass of water vapor (SI unit: kg/mol), K is the evaporation rate factor (SI unit: m/s), csat is the saturation concentration of vapor (SI unit: mol/m3), cv is the vapor concentration (SI unit: mol/m3), and cl is the liquid water concentration on surface (SI unit: mol/m2).
The liquid water concentration on surface is computed by solving the following equation:
with the initial condition:
In stationary studies, the initial liquid concentration is used in the definition of the evaporation flux:
The latent heat source qevap (SI unit: W/m2) is obtained by multiplying the evaporation flux by the latent heat of evaporation Lv (SI unit: J/kg):
With these definitions, gevap is negative when condensation occurs, and positive during evaporation. Whereas the condensation process does not depend on the liquid concentration on the surface, the evaporation process only happens when the (initial or time-dependent) liquid concentration on surface is positive.
Moist Surface Settings
Enter values or expressions for the Evaporation rate factor K and the Initial liquid water concentration on moist surface cl,init.
See Wet Surface node for the modeling of evaporation and condensation on a solid surface completely covered with liquid water.
When a Heat and Moisture multiphysics coupling node is active, the resulting latent heat source is automatically added as a boundary heat source into the heat transfer equation.
Location in User Interface
Context menus
Moisture Transport in Air>Moist Surface
Moisture Transport in Building Materials>Moist Surface
Ribbon
Physics Tab with interface as Moisture Transport in Air or Moisture Transport in Building Materials selected:
Boundaries>Moist Surface