[sfn42]

I wonder if you could just put a conventional heatsink in there to cool the air inside the box?

You would have a liquid block on the CPU but you'd also have a heat sink on top that transfers heat from the air to the coolant block, working in reverse compared to normal air cooling heatsinks. The temperature difference would cause passive air circulation and the liquid cooling would now cool both the CPU and the air in the box, without fans.

Seems like something someone would have thought about and tested already though.

[matt-p]

Not really practical it wouldn't transfer much energy at all. Let's say that your coolant comes in at 30 degrees c, well if your air is 40° and you've got no fans you can do the maths but it may as well be 0.

[sfn42]

I was imagining the coolant comes in at a lower temp like 20 and maybe keeps the air from going above 40.

It doesn't have to do that much, but maybe you're right. I'm sure they'd be doing this if it was practical, being able to onit thousands of fans would probably save a pretty penny both on hardware and electricity.