First of all, my apologies. Much of what follows is sure to upset some folks that have posted in this thread. I intend no disrespect and have no desire to upset anyone, but I find it hard to read the mix of fact and fiction found here. For the most part, people stating claims have been correct. Unfortunately, there are interspersed errors and misinterpretations mixed in with the rest.
gmat,
You've got a pretty good grip on most of this, but some of your statements make me wonder. You have obviously studied the topic of heat transfer, and most of what you've said is true. Some of it, though, is nonsense. To your credit, you're only referencing someone else's flawed results.
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Theres a point in the circuit where pressure switches from positive to negative. (not: thats *always* the case even with our poor centrifugal pumps) you'll want that point to be *before* your rad and *after* your waterblock(s).
For more info on that there's a very good article at overclockers.com
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Someone posted about this on AMDMB a couple of weeks ago. My reply: "Absolute garbage".
Heat transfer from a surface to a liquid or gas depends on many factors including surface shape/roughness, delta-T, and myriad fluid properties (most of which are temperature dependent). For incompressible liquids, pressure has no bearing on heat transfer. Sure, if you wanna get technical, water
is compressible, but its bulk modulus is ~200,000 psi. The pressures in our systems pale in comparison.
It's also an error to state that pressure always changes from positive to negative. This too depends on a couple of things. First, if your system has an open air reservoir, then the only regions of below-atmospheric pressure reside in the pump casing. Second, if your system is truly sealed, the suction line
may be at less than atmospheric pressure. Usually it will be, but if your system was filled with relatively cool water and heated, the expansion of the water will create a static pressure higher than atmospheric. Whether or not you get a vacuum at the suction then depends on elasticity of the tubing and the overall temperature rise of the fluid as well as flow rate and velocity in the lines.
Anyway, getting back to the overclockers thing, I'm specifically referring to a claim that relocating the block and radiator lowered this guy's temperatures on the order of 3°C. This is absolute crap. A system dumping 100 watts into the fluid (75 W CPU and 25 W pump) needs only about 32 gph to keep the peak water differential to 1°C. Claiming a 3°C change only tells me that the guy doesn't know what he's doing.
Sorry if I sound like I am going off on you, but just want folks to fully understand that pressure has no bearing on heat transfer coefficient of an incompressible fluid (aside from determing flow rates).
NoSoupForYou,
You said something about delta-Ts and radiators that didn't sit right. Perhaps I've misinterpreted your meaning.
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What increased flow does is lower the delta-T between the coolant and the rad required for the heat flow. This decreases the overall coolant temp. It does not increase dissipation.
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Not really. You are forgetting the other side of the equation, er, radiator. The heat transfer from the fluid to the tubing in the radiator is generally not the limiting factor in radiator performance. What
is usually limiting is the air flow across the radiator. A higher fluid flow rate will reduce the (already really, really low) temperature differential between fluid and tubing walls. This is normally smaller than the delta-T between air and tubing, especially at the fluid inlet to the radiator.
This next bit also applies to the argument of "parallel" radiators or flow rate through radiators. It should be completely obvious that a radiator will deliver its "best" fluid outlet temperature if the fluid stays in for a very, very long time. Given enough time, the fluid will eventually reach the ambient air temperature, for all practical purposes. On the opposite extreme, very high fluid velocity
may leave insufficient time for the heat to escape. This should also be obvious. Imagine a radiator with a fluid flow path one inch long. No way you could get the water to cool off because you'd have insufficient time and area to get the job done.
Everyone isn't using huge radiators simply because:
1) There's only so much room.
2) You can get all the performance you need with a reasonable sized radiator. Beyond a certain point, you have rapidly diminishing returns.
To all,
There is a disconnect for some understanding the relationship of pressure, flow rate, flow velocity, and heat transfer. Pressure along with flow resistance determine flow rate. Flow rate divided by cross sectional area determine flow velocity. Flow velocity, and not pressure or flow rate, has the largest impact on heat transfer coefficient.
What a
good block does is transform upstream pressure into high velocity. It does this by reducing flow area. The pressure drop that occurs over the block is much like the spray nozzle we put on our garden hoses. It you run a hose with no nozzle, the water comes out with good flow rate, but not impressive speed. A spray nozzle drops the overall flow rate (like a water block), but results in much higher velocity in the flow that remains. This is the key to getting good heat transfer in the block, where all energy must pass through a very small area.
Velocity is also the key in the radiator, but it's the air velocity that is key. Increasing air velocity allows decreasing radiator size and dwell time.
Again, sorry if any feelings got hurt and sorry for such a long post, but hopefully it helps clarify some things.