In CFD, the domain we model is the fluid region, not the solid region. The normals must point towards the fluid region (i.e. out of the solid). For internal flows where there are inlets and outlets, the stl file will be an open shell describing the interface (wall) between solid and fluid regions.
Note that the upcoming 19.1 release will have a option to display the stl normal vectors and flip them if needed, to make it easier to spot and fix this common error.
