Little Problem with FluegasDuct Mixer
 

I want to simulate the Mixing of 2 gases in the gas channel and want to compare which kind of Mixer is better - the Problem is i have hardly any experience in Gambit, if i substract the volume of the Mixer from the duct it doesn´t work, if i only create faces it also doesn´t work - in the tutorials i can´t find any help, and i have no access to the ansys site because i have only a student version - please Help me !!:confused:

Sounds you are getting close to the geometry set-up, what specifically is the problem when you subtract the Mixer?

Then i get an error, because of a negative volume
 

and i can´t solve it in Fluent, and the Gas is also going through my mixer !:(

So, it is a meshing problem that you have. What scheme are using for your volume mesh? My experience is that T-grid is quite robust in giving a mesh that will at least work in Fluent. You have probably already seen that it is one of the options in Gambit. You can convert T-grid tetrahedral meshes to polyhedrals in Fluent, which are more satisfying.

Failing that, you may have to simplify your geometry to make a consistent mesh.

You'll need to be careful if the mixer is part of the domain that the faces between the fluid volume and mixer volume are connected to allow fluid through.

Sometimes it is better just to start with very simple geometry and get the steps down and then move up to your actual case. All of the boundary and internal conditions can be saved so it is not too time consuming to do this. Good Luck.