Moist Air (MT Interface)
Use this node to model moisture transfer in moist air through vapor diffusion and convection.
For small vapor concentration conditions, the density of moist air is supposed not to change with variations of moisture content, and a diluted species formulation is used. In this case, the moisture content variation is expressed through the transport of vapor concentration cv:
(7-3)
with the following material properties, fields, and source:
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Μv (SI unit: kg/mol) is the molar mass of water vapor.
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  (dimensionless) is the relative humidity.
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csat (SI unit: mol/m3) is the vapor saturation concentration.
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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.
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G (SI unit: kg/(m3⋅s)) is the moisture source (or sink). See Moisture Source node.
For larger vapor concentration conditions, the variation of moist air density are modeled, and a concentrated species formulation is used. In this case, the moisture content variation is expressed through the transport of vapor mass fraction ωv:
(7-4)
where ρ is the moist air density (SI unit: kg/m3).
For a steady-state problem, the relative humidity does not change with time and the first term in Equation 7-3 and Equation 7-4 disappears.
Model Input
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 ().
Diffusion
Set a value for the Diffusion coefficient, D. The default value 2.6e-5 m²/s is valid at T=298 K and pA=1 atm. You may set a temperature or pressure-dependent diffusion coefficient through the use of the variables mt.T and mt.pA.
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/
Condensation Detection in an Electronic Device with Transport and Diffusion: Application Library path Heat_Transfer_Module/Power_Electronics_and_Electronic_Cooling/condensation_electronic_device_transport_diffusion
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