Correct, merging the solid into one or other fluid zones will have the same effect. Just be aware that the surface area may differ from reality: consider a pipe the inner surface area is different to the outer surface area. In your case that shouldn't matter if the pipe wall is thin relative to the diameter but it is something to remember for the report.https://forum.ansys.com/discussion/comment/113066#Comment_113066

Ok, understood. Thx very much for your helpn