[Helium]

hi guys,
i am using heat exchanger model and I need to understand what happens to working fluid. If, for example, I am using a liquid, once I establish the direction (let's say from top to bottom of a cube) some fluid "originates" from the top face of my cube, then flows through the internal domain to reach the bottom face. Once the fluid reaches the bottom face, where does it go?
I know this is a silly question, but in my case I have this cubic exchanger inside a bigger chamber. To save computational time (because the exchanger is just a small part of the system) I want to avoid to create other channels to mesh because I am only interested in knowing what happens at the bottom face, I do not really need to inquire what happens inside the tubes. So basically, as this liquid "magically" appears on the top face, can I suppose that after the outlet it does not inundate the bigger chamber?
Or do I need to create some tubes to direct the liquid after the bottom face/outlet zone to avoid the flooding?