Pipe in cylinder
 

Hi.
I want simulate fluid flow in vessel. Collectors and distributers of the vessel are pipes with some holes. I want to simulate both flow inside the pipes and the cylinder. How should I model the geometry? The pipes should be solid or surface?

You should draw the liquid volume. So, if you want to include the volume inside the pipe as well, than this liquid should be connected to the liquid in the vessel through the holes in the pipe.

The pipe can be solid that is cut out of the liquid. Conceptually, that is most easy.
If you want to model it as a thin-sheet/surface, than the strategy depends on the mesher and solver, so difficult to tell beforehand

I am using Icem or ansys meshing as a mesher, and Fluent as a solver. I think meshing with surface pipe would be easier. Although, subtracting surface pipe from fluid domain is not possible, as far as I know. Is it?

Subracting a Solid volume (pipe) from the liquid is very easy in Spaceclaim. Just draw both and subtract them from each other. In DesignModeler, it should be the same, I think, but have no experience with it. It is old and a crappy piece of software. What ANSYS version do you use?

If you model the pipe as a thin surface, then you cannot subtract them from each other since the thin surface has no volume. In ICEM you should treat this thin surface as Internal or Split Wall (Mesh > Part Mesh Setup). Then ICEM sees it as an object to mesh. Otherwise it will just ignore it.

An Internal wall has the same mesh on both sides of the thin surface. A Split wall has different mesh on both sides.
The internal wall has the advantage that the mesh is conformal on the surface which has advantages in e.g. heat transfer through the wall and adds flexibility in the setup in Fluent since you can switch between wall and interface. The latter one allows you to vanish the pipe wihout making a new geometry and mesh.
The advantage of a Split wall is that you can have diferent mesh settings on both sides which gives more flexibility in refining the mesh on the inside, if you think this is necessary.