Normally we use open inner region to define a new fluid area, hence the new area is also given the name fluid. In a conjugate heat transfer case I have a body of water with empty channels inside which are areas to be defined as immersion heating elements. If I create these regions using open inner region and then define the regions as solids, will the regions really act as solids or will the solver still model them as having convection like a fluid?
It doesn’t really matter. The CAD part would be created as fluid region. But then you can also define them to be solid part - it’s what we specify in the material definition stage.