To all budding thermodynamics engineers here:
Experiment all you like with different fans, reversed-flow fans etc. but you had better have replacement parts / assemblies on hand to replace the stuff you fry in the process. There are specific reasons for every single little hole and vent cut into the chassis and the direction and flow of every fan. Some of the best-paid brightest engineers in the world work for Apple and you think not one of them thought about reversing a PSU fan to make a "push-pull" setup? Are you kidding?
If you stop and think for a sec, you'll quickly realize that reversing one of the PSU fans will simply cause the airflow to go around and around in a circle through the fans, never reaching the rear of the unit (where most of the hot stuff is) and the PSU will overheat faster than you can say "BTU".
In addition to the four obvious openings and the bottom slot in the front of the MDD, there are lots of little air holes punched into the sides that allow the PSU fans to draw cool air from the bottom between the steel inner frame and the plastic outer covers. Cool air enters the bottom, flows up through the space between the covers and chassis, is drawn into the PSU fans then is (and this is important) forced through the length of the PSU, exhausting hot air out the back of the computer along with the hot CPU air from the main fan.
>>>>>> Cool air IN bottom front……hot air OUT top rear. <<<<<<<
All of this is covered up by the motherboard, the drive cages and the PSU itself, but strip an MDD down and you quickly see there's a method to the madness, with carefully laid-out air passages feeding the fans with the intakes and outflows as far apart as possible to keep the inflow and outflow separated to discourage warm exhaust from finding its way back to the intake. Even the little fan in the door has a specific role, drawing air up through the side to cool the optical drive(s) compartment.
With the access door closed, the main fan has its intake well isolated from the interior and draws almost 100% outside air blowing directly across the CPU heat sink and out the back. Virtually ALL of the CPU heat is exhausted right out that way. Additional heat from additional PCI cards does accumulate inside the upper region but realistically, except for some video cards, most PCI accessory cards are not notorious heat generators. If you're concerned about the interior overall temp of the MDD, an effective solution is one of those little mounts-in-a-PCI-space fans. They're perfect for this job in an MDD, providing a little more general exhaust flow out the very top rear of the computer.
Lastly, let me remind all that aiming an IR temp reader at the PSU tells you almost nothing. It's NOT the outside that matters, it's the temp of the switching transistors themselves and there's no way to measure that during operation with an IR from the outside.
