[Alchemy]

Originally posted by myv65
In the literal sense, anywhere. There's no such thing as a perfect energy barrier. In a more practical sense, the CPU and pump are responsible for virtually all energy that you have to get out of the fluid eventually. What's comical is seeing some folks use pumps that are rated as high or higher than the energy used by a CPU. At some point people need to realize that the closer you get a fluid to ambient, the more it takes to close the gap further still. It's also possible to go over the top where a pump adds so much energy that your final CPU temperatures begin to climb. Yeah, we're talking tenths of a degree here, but when the differences are that small, what is the point of using an ever larger pump?

Well, if you draw a boundry condition around the entire piping system, the only sources of energy entering the system would be the pump and the chips being cooled. Every point in the system will be at a higher temperature than ambient, so there would be no heat transfer into the system at all.

Actually, now that I think about it, any parts inside the case might possibly be cooler than the air inside the case. So I guess you're right after all.

Regarding pump sizing, I've been trying to understand how the so-called "sweet spot" is possible for cooling systems. Convective heat transfer resistance *must* reach the minimum limit as the flow rate through the system is brought to infinity.

My best guess of why maximum heat tranfer does not coincide with minimum CPU temperatures is because at very high flow rates (and correspondingly high TDH) the pump duty becomes large enough to overtake the benefits of improved heat transfer.

If you pick the most important part of the system and consider the change in fluid temperature across the radiator as Q=UA(deltaT), UA is increasing very slightly as flow rate increases, but if you have to increase Q more than that to match it, you're going to end up with an even larger temperature gradient. Not good.

What do you think?

Us engineering geeks are the only ones really concerned about the distinctions between potential energy, kinetic energy, work, etc., etc.

I'm still a bit wary of calling pressure change in the system "potential energy" because it really isn't, but I'm going with it here b/c I think it's too pedantic and too complicated to call it anything else, or explain why it can be expressed in terms of height.

I assume from your posts you're a mechanical engineer?

I am a little confused by your final comments about friction/turbulence in water. I guess I want to separate your comment from what happens within a pump. In a pump there is a lot of wasted energy because of secondary flows that don't contribute to system flow. The best of centrifugal pumps max out well less than 100% efficient with many "pond pumps" running under 50% efficient. The excess energy put into the impeller that doesn't generate flow generates heat. This heat isn't heat transfer but rather due to non-useful "churning of the water". So in this respect, it really is the internal friction of the water. It is not, however, quite the same as the turbulence in as established flow stream. I think this is what you were getting at in your post.

Well, perhaps I should have said "due *entirely* to its own turbulence," that is, not turbulence created in the pump.

If you think about it, when you dead-head a centrifugal pump so that the impeller does no shaft work, you're still doing work on the fluid. It's essentially the same setup Joule used to prove the first law of thermodynamics.

On that note:

Originally posted by bigben2k
So now you've got me wondering: if "the friction in water due to its own turbulence is basically nil unless you have supersonic flow", then where does the heat come from? Friction of water to inner tube/channel surface?

Yep. Exactly. And since most plastic tubing is very smooth, the vast majority of the heat is being created at the fittings.

Alchemy