Default interior and default wall
 

While applying boundary conditions in FLUENT it makes two new zones on its own. One is interior and other is wall. For interior by default we cannot give any boundary condition. On what basis these two zones are created?

In my case I have three plates in an enclosure. In meshing I created named selection in these two ways-
1. plate faces -(i selected all faces of all the three faces)
2. plate metal -(i selected bodies of the three plates
3. enclosure - (i selected body of the enclosure)

By body i mean the total volume.

FLUENT created the plate interior and plate and solid wall as zones

Second case
1. plate faces - (all faces of plate)
2. plate volume- (all three bodies of plates)
3. enclosure faces-(all outer faces of enclosure)
4. enclosure volume- (whole body of enclosure)

This time FLUENT created interior enclosure volume and interior enclosure wall as two zones.

These two zones are important regarding the application of boundary condition.
Please help me understand about the concept behind this?

In Fluent, "interior domain" represents the fluid field which is situated in the interior of your walls, in in this area we can not apply the BCs because it is the field of calculation.

Walls that created by default are the walls of your geometry that you have not defined during the creation of geometry so Fluent defined it by default, (yes you can apply the BCs in walls)

Okay, this is really helpful.

How does FLUENT decide which region to create as interior? Suppose, I want to know the temperature distribution inside the enclosure i.e. around the plate. Then the field of calculation must be enclosure volume. Correct?
In my first post, I have mentioned two different named selections. Different interior zones were created in those two named selections.

How should I ensure that FLUENT creates the interior in that particular region where I need to know the value of some parameter (e.g. temp. distribution in the enclosure volume)?

If I understand your question, to know the temperature distribution in any area of your 3D computational domain, you must define "iso-surfaces" then view your temperature fields in any surfaces of your domain (To create at Surface > iso-surface.)

I will try about creation of iso-surfaces. But I did a little bit of experimenting. As mentioned in my earlier post I had two different walls in as zones. I applied my boundary conditions to the named selection created by me i.e. enclosure boundary. The next time I applied it to the wall-enclosure region created by FLUENT. The results in the latter case are wrong. I think I should conclude that whatever may be the case never apply the boundary conditions to the regions created by the regions created by FLUENT i.e. default interior and default wall... Please correct me if I am wrong...

Normally you should apply the boundary conditions on what you have created (named selection)..
When you apply the boundary conditions on the block "wall" created by default in Fluent, it sends error because Fluent he gather all the walls that you have not defined in the "named selections" so it can not apply BCs on a block! so you must define your BC in "named selection"