If all it took to describe a pipe were a length and a diameter, we wouldn't need engineers. Thankfully for us, pipe flow can be very complex whether or not the cross section is circular.
Fill a tank with gas. It is a bit boring. Fill it completely with liquid, and that is also boring.
Why are you asking "How hot does it get?" when you really need to ask "How big should this be to keep it from getting too hot?"