How do dual fan power supplies work does there need to be a whole or vent on the top of the case for it to work or can you it just be in a case without out one?

I have a dual fan PSU it has a fan at the front drawing in warm air from the machine and an exhaust fan at the rear

There was a top fan and a rear fan.

Are you sure that it was not rear and bottom?

In any case the rear fan should exhaust and any other fan intake so to answer your original question there would not be any opening on the top of the case.

What is the make and model of this PSU?