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If you have a liquid-based cooling system, like the loop in the picture below, does the fluid speed actually matter?

I can see how cycling the liquid too slowly would be bad, since the fluid would be interfacing with the block for longer and what you want is to move the heat (and subsequently the fluid) away from the block. What I fail to see is how moving too quickly could also impair cooling performance as stated in a lot of online forums. One argument I clearly remember from reading about this a while back was:

You shouldn't crank the pump speed too fast or the water won't have time to pick up the heat from the waterblock as well.

That, however, makes no sense to me. It's not like the fluid and the block are interfacing in a discrete frame-like manner where if you speed it up it'd be somehow 'waiting' for the next water frame to come around, there's a continuous flow so I don't see how that would be an issue.

How does fluid speed influence the cooling efficiency in a closed loop?

Typical loop

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What I fail to see is how moving too quickly could also impair cooling performance as stated in a lot of online forums. One argument I clearly remember from reading about this a while back was:

and:

You shouldn't crank the pump speed too fast or the water won't have time to pick up the heat from the waterblock as well.

The latter statement you quoted is frankly speaking pseudo-scientific poppycock.

Maintaining high rates of cooling water always promotes cooling: heat energy carried off per unit of time is increased.

High cooling water flow rates increase heat transfer coefficients (by promoting turbulence), as well as keeping the cooling water at lower temperature, which further promotes cooling, as the heat carried off per unit of time is directly proportional to the temperature difference between the cooling water and the object to be cooled (see Newton's law of cooling).

Of course there may be other, practical limitations to cooling water rates, such as pump rating and pressure build up. But within these margins, the faster the flow of cooling water, the better the cooling.

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    $\begingroup$ I can also imagine a case where cavitation is a problem. I kind of like the idea of cpu coolers where this sort of thing is an issue. $\endgroup$
    – user107153
    Jul 25, 2016 at 21:16
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    $\begingroup$ I can only see flowing too fast being an issue if you are excessive turbulence, friction and heating from pumping can warm the heat transfer fluid and reduce the cooling efficiency. There will be some optimum flow rate and it is possible that for all practical pumping speeds faster might still be better for most systems. $\endgroup$
    – s0rce
    Jul 26, 2016 at 0:18
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    $\begingroup$ @tfb You can sidestep that issue by simply increasing the pressure in the loop. The next big hurdle I can think of, is when you bring the fluid velocity up to the speed of sound in the fluid. Apparently fluid moving that fast tends to "start ignoring the turns in your pipes". $\endgroup$
    – Aron
    Jul 26, 2016 at 8:50
  • $\begingroup$ @Aron "Simply" increasing the pressure in the loop might be anything but simple. It would require stronger pipes, for one, and promote leaking. It's a trade-off. $\endgroup$
    – Luaan
    Jul 26, 2016 at 12:48
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    $\begingroup$ @Luaan when the rest of my post is talking about supersonic fluid flow...it is simple. $\endgroup$
    – Aron
    Jul 26, 2016 at 13:02

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