this post was submitted on 24 Mar 2025
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Noctua's pumpless 'thermosiphon' liquid cooling unit is expected to be released in 2026 and has already given me a free lesson in basic thermodynamics (www.pcgamer.com)
submitted 1 week ago by [email protected] to c/[email protected]
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Interesting to see a major brand trying to do this. I wonder Icegiant will respond.

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[–] [email protected] 11 points 1 week ago (4 children)

So it's a big heatpipe?

[–] [email protected] 10 points 1 week ago (1 children)

"Yes" with the asterisk that there is no phase change, and the flow paths are segregated.

In a heat pipe, water is installed such that it is kept near it's liquid-gas phase change point on the pressure-temperature curve. When heated, it turns to "steam", travels thru the center of the pipe, condenses back to liquid on the cold/fins side (giving off all it's heat), then returns via capillary action on the metal foam walls of the pipe.

In a thermosiphon, the water never leaves the liquid phase. It simply relies on the density change based on temperature (hot water becomes less dense, and will rise to the top of a column) to force some circulation to occur. The hot fluid rises out of the heatsink and displaces the cooled water in the radiator, which then flows down the other side to return to the heatsink.

Very old cars (<1920) used to rely entirely upon the thermosiphon effect, rather than a pump.
It's not terribly efficient, especially at higher dissipated power densities. They are also very prone to being overloaded with heat, if the overall loop temperature gets too high and/or the radiator loses some efficiency (e.g clogged with dust), the water can start to boil on the hot plate side and you'll lose basically all cooling effect when your siphon is blocked with steam.

[–] [email protected] 9 points 1 week ago* (last edited 1 week ago)

That's not how the article describes it, at least:

Essentially it creates a vapour chamber-like effect by using heat emitted from the CPU to evaporate a refrigerant, which then moves up a vapour tube into a fan-cooled condenser, where it cools off, condenses back to a liquid state, and makes its way back to the CPU to be heated again—no pump required.

Which sounds exactly like a heat pipe.

Edit: I guess the difference is that heat pipes use wicks and capillary action to return the liquid phase, where the thermosiphon instead uses gravity, which makes it a little easier to produce and higher capacity, but vulnerable to changes in orientation.

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