internal boundary conditions
 

My aim is to simulate a sidepod with a radiator inside and calculate mass flow rate through it. I read Fluent guide and all about the porous jump and radiator BCs but I couldn't find anything about the definition of internal boundaries. In 2D I was able to model the radiator by projecting a line body. In 3D I tried creating a surface body but the solver won't recognize it as a boundary. Then how do I define an internal boundary? I know it may be a stupid qeustion but I'm a newbie of CFD and couldn't find the answer. Thanks in advance.

p.s. In case you need to know I'm using the default ANSYS mesher.