Setting Boundary Conditions and Values

The thermal insulator consists of a closed heated box with two internal solid glass panels surrounded by air. The boundary specifications required for this analysis are wall thermal conditions and radiation properties at the boundaries.

A fixed heat flux of 1000 W/m^2 needs to be applied to the base of the box to provide the heat source. The top of the box is given a fixed temperature of 300 K which corresponds to the temperature of the environment. The vertical sides of the solid panels which are in contact with the environment need to be set to a fixed heat flux of -62.5 W/m^2. All other wall boundary conditions will be considered to be adiabatic, which is the default condition. Contact-type conducting boundaries will also be used at solid-fluid interfaces.

Optical radiation properties emissivity, reflectivity, transmissivity, and radiation temperature must also be set at all boundaries. Kirchoff's Law (emissivity = absorptivity) is assumed by the solver and you should also check that, for each boundary or interface, emissivity + reflectivity + transmissivity = 1.

We will start with the wall boundary definitions for the fluid region.

The heat flux value for this boundary can now be set.

STAR-CCM+ adds the decimal point and units automatically, so by simply entering 1000 you get an entry of 1000.0 W/m^2. A positive value for heat flux indicates that heat is entering the domain while a negative value would indicate that heat is leaving the domain.

The conditions for the side-1, side-2 and side-3 wall boundaries (side walls of the fluid region) will be left at the default adiabatic setting but the optical properties at these boundaries must be set. We will set the side-1 wall boundary next.

The conditions for the side-2 and side-3 wall boundaries are identical to those for the side-1 boundary and may be set up in the same way manually or copied and pasted from the side-1 boundary.

The conditions for the top wall boundary are set next.

Now the boundary conditions must be set for the solid-fluid interfaces. The emissivity and reflectivity of the solid-fluid interfaces must be specified at each Interface [In-place #] boundary, not the Interface boundary, as the latter became defunct when the interface was created (see the previous section Creating Interfaces). However, transmissivities for interface boundaries are set under the Interfaces node rather than at the boundaries within the Regions node.

Note that it is not necessary to set the emissivity and reflectivity for interface boundaries in the solid region. We can now set the solid wall boundary conditions and values in a similar way to the fluid ones.

The heat flux value for this boundary can now be set.

The conditions for wall boundary side-solid2 are the same as for side-solid1 and may be copied and pasted from the side-solid1 boundary.

Finally, we must now specify the transmissivities of the interface boundaries: