Hi Mr Rob
As always, thank you very much for your help. The ANSYS forum staff will definitely make an appearence in the acknowledgement section of my thesis.
Just to make sure I understood your suggestion: do you mean a geometry like the one below?
If that is indeed the case, how do I tell Fluent it should use Layering in the tube layers next to the inner zone? As of now, it doesn't let me choose an adjacent zone to the Inner zone, so I cannnot even set the ideal height for the adjacent layer.