Hi Kai,
Its a really cool model, I think the issue here is that because it is full geometry the computer cant automatically detect where the flow should be. This can be remedied in two ways, create a parametric mesh, and explicitly define the material point within the flow region, or go back to onshape and create a solid that represents just the fluid you wish to simulate and mesh it as automatic internal.
If you get stuck with either of these methods I am more than happy to help,
And if you are sucessful it would be cool if you could write your resolution maybe with the resulting mesh for future reference 
Cheers,
Darren