But not convect. Hence why it’s much, much harder than removing heat on Earth.
Almost. It’s proportional [0] to T_hot^4 - T_cold^4. For a 100C surface with emissivity 1, that’s about 1kW/m2 if there is no radiation coming back, which really isn’t very high. You cannot cheat this with fancy folded-up radiating surfaces (it’s thermodynamically impossible, and the actual mechanism that kills it is one fin of the heatsink radiating right at the next one).
So cooling in space is hard. You’re not getting GPU-like power densities without a physically immense radiating surface extending way past those GPUs.
[0] Caveat: emissivity can depend on wavelength, and the law holds independently at each wavelength. So this can introduce interesting effects, which is how all the fancy prototype roof-cooling materials work, and it’s also related to how “spectrally selective” windows and window films work.
I've seen tests done on heavy PC loops (ie multi-GPU) both high-flow and low-flow, as well as on car engines, in different coolant flow configurations. The results from all of those are that the water doesn't rise meaningfully in temperature between components.
Unless I did my back-of-the-napkin math wrong, this seems reasonable. If you have a single 10mm ID pipe going through a 1U server and up to the next, then for a full 42U rack you have about 1.7kg of water going through the servers. If the flow rate is about 1s per server (so 42 seconds for the full rack) and each 1U server dumps 500W of energy into the water, there should be just a 3 degree C difference in the water temperature between the first and the last server.
> Delta T between two sockets is 5 degrees C
And secondly, ~5-10 degrees is what I see on my dual-socket workstation, and have been wondering about this delta ever since the first day I started monitoring the temperatures. At first, I thought that the heat sink wasn't installed properly but after reinstalling it the delta remained. Since I didn't notice any CPU throttling or whatsoever I figured it's "normal" and ignored it.
I mean, water travels from one socket to another, so one processor adds heat equal to 5 degrees C under nominal load. The second socket doesn’t complain much, but this is enormous amounts of heat transferred in a such quick pace.
You need two circuits, and a CDU between them. Coolants needs maintaining. You add antifreeze, biocides, etc.
Air is brute force. It cools everything it touches. Liquid cooling is serialized in a node. Two sockets? Second will be hotter. HBA not making good contact? It’ll overheat.
You add extensive leak detection subsystems, the amount of coolant moving in your primary circuit becomes massive.
Currently you can remove 97% of the heat via liquid (including the PSUs), and it’s cheaper to do so than air, but it’s not “rails, screws, cables, power on”. Air cooled systems can be turned on in a week. Liquid cooled ones take a month.
However, using liquid is mandatory after some point. You can’t cool systems that dense and under that load with air. They’ll melt.