It is probably better to convert the STL file to a surface/solid body before going for the CFD mesh. In form, the STL file appears as a mesh (because, well, it is based on triangulation). But the file mainly stores the coordinates of the vertices of the triangles along with the coordinate of the normal to its exterior. And, the triangles generated for the STL file often do not satisfy the shape requirements for downstream FEM/CFD applications because the "STL mesh" is generally less watertight. I think SpaceClaim can do the heavy job of converting the STL file to a usable format for CFD. You can use MeshMixer (Autodesk) if you need to repair the STL file before converting to surface/solid body in SpaceClaim.
If you are using openFoam with snappyHexMesh you must make sure, that your meshes are air or fluid tight. The stl must be point or vertex identical with the base mesh that you export out of your CAD system. The point identical meshes for the in and outlets can be generated with blender.
I've created a tutorial on YouTube where you can see the workflow.
https://youtu.be/2cywdPboXyw
More information you also can find on our website engineerdo.com
The issue is that my geometry is complicated. When I tried to convert it into solid, SpaceClaim gave me a warning that the size and the processing will be one or two order of magnitude bigger.