Filling an "empty" space

Hello, I'm new to Comsol and a novice at SW2012

It seems in order to model fluid flow through a heat exchanger, the volume filled with fluid (helium in this case) must be included in the model as a "solid", so that the inlet and outlet boundary conditions can be defined. Is this the case, and is there an easy way to create a part that "fills" the void? I have an existing SW2012 model of the HX.

Thanks,
Geoff