Moist Air
Use this node to model moisture transfer in moist air through vapor diffusion and convection. The moisture content variation is expressed through the transport of vapor concentration, which itself can be expressed as the product of molar mass of water, relative humidity, and vapor saturation concentration:
(7-3)
(7-4)
with the following material properties, fields, and source:
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Μv (SI unit: kg/mol) is the molar mass of water vapor.
•
 φ (dimensionless) is the relative humidity.
•
csat (SI unit: mol/m3) is the vapor saturation concentration.
•
D (SI unit: m2/s) is the vapor diffusion coefficient in air.
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u (SI unit: m/s) is the air velocity field.
•
G (SI unit: kg/(m3⋅s)) is the moisture source (or sink). See Moisture Source node.
For a steady-state problem, the relative humidity does not change with time and the first term disappears.
Model Inputs
This section has fields and values that are inputs to expressions used in Equation 7-3 and Equation 7-4. The temperature is used for the definition of the saturation conditions, whereas the velocity field is used in the convection term.
The default Temperature Τ and Velocity field u are User defined. When additional physics interfaces are added to the model, the temperature and velocity field variables defined by these physics interfaces can also be selected from the list. For example, if a Heat Transfer in Building Materials interface is added, you can select Temperature (ht) from the list. If a Laminar Flow interface is added, you can select Velocity field (spf) from the list.
If the node was added automatically after selecting the Heat and Moisture Transport predefined multiphysics interface, the temperature of the Heat and Moisture multiphysics node is used by default and the input field is not editable. To edit the Temperature field, click Make All Model Inputs Editable ().
The Moisture Transport in Building Materials Interface
The Heat and Moisture Transport Interfaces
Heat and Moisture
See Turbulent Mixing to account for eddy diffusivity in moisture convection by turbulent flows.
For a detailed overview of the functionality available in each product, visit http://www.comsol.com/products/specifications/
Location in User Interface
Context menus
 Moisture Transport in Air>Moist Air
Moisture Transport in Building Materials>Moist Air
Ribbon
Physics Tab with interface as Moisture Transport in Air or Moisture Transport in Building Materials selected:
Domains>Moist Air