Colder water better thermal conductor?

[Pengwuino]

Is colder water a better thermal conductor then warmer water?

Say for example you nee dsomething to be cooled at 70C and you run water to it in a system. Which would cool this "something" quicker, 10C water or 20C water?

 

[russ_watters]

10C water will. But its not a question of thermal conductivity, but rather approach temperature. Since heat transfer rate is proportional to temperature difference, the greater temperature difference (approach temperature), the greater the heat transfer. So 70-20=50 and 70-10=60. 60 is 20% bigger than 50, so 10C water has the potential to cool something 20% better than 20C water. I say "potential" because flow rate makes a big difference.

 

[Pengwuino]

Ok check this out. I am on a forum with a lot of people who like to water-cool theri computer systems. Its not... literal water cooling lol, but a series of tubing going to the main components into water blocks and make use of a radiator and pump. This one guy proposed that:

Well, if you have time, check it out http://www.overclock.net/showthread.php?t=20358

Guess who i am on that forum :D

 

[russ_watters]

Ehh, this is a really tough issue because the he source and heat sink have similar, but opposing constraints: for the heat source (the cpu) you want the coldest water possible and the highest flowrate possible, but for the heat sink (the radiator), you want the warmest water possible (but again, highest flowrate). Increasing the flowrate will bring the two temperatures closer together.

Your idea that if you double the flowrate, you simply cut the delta-T in half and the overall heat transfer stays the same is close but not quite right. Because of the approach temperature issue I mentioned above, it isn't a linear relationship: you will get more heat transfer by increasing the flow rate. Its easiest to see this by looking at it the other way: what if you decrease the flow rate to the point where the water ends up at 70C? It can't get any warmer, so that's a limit to your delta-T.

In HVAC, you match your equipment to each other to maximize performace and efficiency, looking at performance curves and such. The way most overclockers seem to do watercooling, though, they have an absurdly large radiator - and that's a good thing. The best cooling comes from the lowest possible temperature and the highest possible flowrate entering the CPU block.

 

[Pengwuino]

Lol some of these guys have radiators out of Buicks and Pontiacs :P