what does it stand for in semi cross-section of tubes in boundary condition
 

Hi all,

I am working on 3D tube banks heat exchanger.

I have encountered a problem that is relevant '' boundary condition ''.

I have added a photo and the question is what boundary condition I am going to define in cross-section of tubes?

External flow domain that are between in tubes have been defined by Symmetry

But I really not sure that whether or not it is symmetry boundary condition in cross-section of tubes.Because if I define a wall thickness ( shell conduction zone ) , maybe some problem/problems will appear.

Can anybody help me with this situation ?