Defining free space as domain

Hi all,

i have an issue with Comsol. I have a CAD Model of an electrical box with some power components. Defining the materials and heat losses is not the problem as i can select the components without problem. To know how much the interiour heats up and how much the influence is to other components i want to simulate the heat transfer in fluid (air) inside the box, but i cant figure out how to define the free space. I already read the following topics here:

www.comsol.com/community/forums/general/thread/46155/
www.comsol.com/community/forums/general/thread/32396/
www.comsol.com/community/forums/general/thread/35578/
www.comsol.com/community/forums/general/thread/36376/
www.comsol.com/community/forums/general/thread/32396/


None of them appear to help me as i dont have a tubular object to use the "cap faces" nor a surface to use the conversation to solid function. Is there a way i can do it in comsol o is it better i reacte a negative volume in the CAD application and define it as my air domain?

To simulate the walls of the box is not of my interest as it is quite isolating and closed, so i only use it for visualization purposes to create plots that are nearer to the physical model on my desk.


Thanks,
Adrian