[Linse]

Sounds like a nice - but possible! - challenge you are looking at!

Maybe there have been major changes, but to my knowledge you have to define the different regions via another dictionary file, not via blockMesh.
The blockMeshDict is there to define the mesh.
Another file is there for setting up the different regions.

If you look at the tutorial case for chtMultiRegionFoam (for example $FOAM_TUTORIALS/heatTransfer/chtMultiRegionFoam/multiRegionHeater ) you will find that there is a file called makeCellSets.setSet. This is the file where the different regions are defined.
For a basic understanding you might look into https://blinseis.web.cern.ch/blinsei...RegionFoam.pdf . I really have to review this howTo, but it describes a bit of setting up a basic case for chtMultiRegionFoam, including few explanations concerning the *.setSet file.

Unfortunately I do not know anything about round structures in setSet. So it would be great if you could share your experiences with that afterwards! ;-)

Hope this helps a bit...