Imported Geometry Shows up as a Hollow Block??

Hi all, I am a newbie in Comsol Multiphysics so please forgive my silly question.

I am trying to model fluid flow inside a pipe. I made the model using SolidWorks then I realised Comsol doesn't recognise the domain inside the pipe, so I made another SolidWorks part that is the inner part of the pipe just like a cylindrical block. (I realised there is a Cap Faces function just after)

But when I import it into Comsol, it seems that the cylindrical block becomes only just a void enclosed by surfaces. When I use the hide function in setting up inlet conditions, I could hide the outer surface and even see the inner wall of the cylindrical block. And my solution never converged, don't know if there's anything to do with this. I try importing the geometry using .sldprt and .x_b but the problem still exists. The part I imported into Comsol is definitely showing up as a cylindrical block in SolidWorks, I used the Join function in SolidWorks to transform the pipe assembly into a part, don't know if that's relevant.

Thanks!