COMSOL 6.3 - Open Boundary

Open Boundary

Use this condition to define a boundary as the limit between a moist air (or hygroscopic porous medium) domain of the geometry and the rest of the same moist air (or hygroscopic porous medium) domain that is not represented in the geometry.

At the open boundary both inflow and outflow conditions are supported. Use the Inflow and Outflow features on boundaries where only inflow or outflow conditions are expected.

For an outgoing moist airflow across the boundary, it applies a zero diffusive flux condition:

(7-7)

For an incoming flow of velocity field u across the boundary, it accounts by default for the moisture flux induced by the flow rate. This is done through a Danckwerts condition on the vapor concentration:

Note that on the exterior boundaries of a porous medium, the moisture flux contains both the vapor diffusive flux and the liquid capillary flux:

Alternatively the open boundary condition can set a constraint on the vapor concentration:

(7-8)

Both types of conditions are based on the knowledge of the upstream moisture content (from the vapor concentration, the vapor mass fraction, or the relative humidity and the temperature) at the outer boundary of the virtual domain, but the concentration and mass fraction ones should be preferred to account for the feedback of the model moisture sources and moisture constraints on the inlet moisture profile.

Both the upstream vapor concentration custr, and vapor mass fraction ωustr are defined on this boundary. Their expressions depend on the chosen inputs.

This node can be applied on the exterior boundaries of a computational domain in which either a Moist Air (Moisture Transport Interface) node, a Hygroscopic Porous Medium node with set to , or a Moist Air (Hygroscopic Porous Medium) subnode when it belongs to a Hygroscopic Porous Medium node with set to is active. In the latter case, this node only constrains the vapor flux.

Upstream Properties

When is set to in the list, set both the Tustr and the ϕw,ustr from which the upstream vapor concentration and the vapor mass fraction can be defined:

For , enter values or expressions. Else, select any available input (like defined in an node under ).

Alternatively, choose in the list, and set directly a value or expression for the custr. The vapor mass fraction is then defined as

Finally, choose in the list to set a value or expression for the ωustr. The vapor concentration is then defined as

Inflow Condition

Unfold this section to change the inflow condition:

•

The condition (default) prescribes the flux defined from the upstream vapor concentration and the fluid velocity at the boundary. Using this condition, the moisture condition is not strictly enforced at the inlet. Instead a flux is set on this boundary. The higher the flow rate, the smallest difference between the inlet and upstream vapor concentration.

•

The option enforces the upstream vapor concentration on the boundary.

This section is not available when this node is added to the Moist Air (Hygroscopic Porous Medium) subnode. The condition always applies.


	Evaporative Cooling of Water: Application Library path Heat_Transfer_Module/Phase_Change/evaporative_cooling 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

Ribbon

Physics tab with interface as , or any version of the Moisture Transport interface selected:

Physics tab with > selected in the model tree: