set up the control volume
 

in my case, i have the compartment which is already shell; have inlet and outlet hole, have floor to cover the lower part (cover the shell), have fuel cell engine (rectangular box), 1st fan (circle surface) at the front of the fuel cell as intake fan, 2nd fan (circle surface) at the back of fuel cell engine as exhaust fan. i have done 'fill' in design modeler to get the air domain and i did it. what should i do to do the next step? is it the boolean (substract) or surpress?