Assigning material properties to an interior volume

I want to assign the empty volume within a closed container to be a fluid and assign material propertied to this fluid.

I imported a closed 3D model that is made up of a number of solid parts contained within a closed chamber made up of a tube with end flanges. Each part comes in as a solid domain that I can assign material properties to. What I want to do is to create a domain from the empty volume within the closed container and assign material propertied to this domain so I can model heat transfer through the fluid in the container. I am not sure how to create a domain out of the empty space within a closed volume.
Is there a way to do this within COMSOL or within Solidworks?

Thank you