I would like to run CFD on a plenum/volute solid model.  Since I have the plenum, I need to model the flow path (the empty space).  I have used the "Combine" feature before with varying levels of success.  If I enclose the plenum with a large square block, then use the "combine" feature to remove the original model, I am left with the model I want, enclosed in a block and I can not figure out how to quickly get rid of the block.

What I have done in the past is use a horrendous amount of extruded-cuts to remove just the excess block area.  I am hoping there is a simpler way to achieve this.

The part is one "solid", so if there is a feature that allows me to split a solid into multiple solids, that may be what I want.  Also, if anyone has other suggestions on how to do this easier please contribute.

Thanks