myv65 mentioned something a while back that eventually sunk in to the point I could make use of it.
Quote:
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Flow rate (L^3/T) multiplied by pressure drop (F/L^2) = units of F*L/T (power). It's as simple as that. Measure this stuff in metric and the unit conversions are a little easier, but it's not so bad in imperial units.
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In other words:
If you take a flowrate (say liters per minute) through a waterblock and convert it into cubic meters per second...
m^3 / s
...and you take the pressure drop (say meters of H2O) across the waterblock at that flowrate, and convert it into Pascals (or equivalently Newtons per square meter)...
N / m^2
...and you multiply that flowrate by that pressure drop...
( m^3 / s ) * ( N / m^2 ) = N * m / s
...you get Watts.
1 Watt = 1 N * m / s
For lack of a better term, I'm calling this power dissipated in water blocks 'Hydro Power'.
I was curious as to what the thermal resistance of waterblocks would look like graphed vs 'Hydro Power'. BillA was kind enough to let me have the dataset which three of the graphs in his article
Waterblock Bench Testing Results are based on. From this data I produced the following graph:
About the graph:
It's important to note that the horizontal axis of the graph is on a log scale. Each major vertical line represents a factor of 10 increase in 'Hydro Power' over the vertical line to the left. (The chief benefit of using a log scale is that it makes the differences in the curves of each WB standout more clearly.)
Also important to note, is that the Watts referred to in "C/W" is completely separate from Watts of 'Hydro Power'. The former refers to heat transferred from the CPU to the water. The latter refers to power which is supplied by a pump, and is dissipated in the WB.
The 'Hydro Power' really is dissipated in the WB and results in additional heat in the coolant which the radiator must remove. Just as a certain amount of electrical power dissipation can be the result of, a 'high' voltage and 'low' current, or a 'low' voltage and 'high' current; a certain amount of 'Hydro Power' dissipation can be associated with different ratios of flowrate and pressure drop. The relevant flowrate to pressure drop ratio for a particular block cannot be determined from this graph. (Refer to BillA's graphs for that information.)
So what?
That's what I'm hoping to find out from you guys. Everything I know about fluid dynamics I learned from studying sailing, model rocket design, or watercooling on the forums. No formal training whatsoever. I'm curious about what explains the differences in these curves, but all I can do is speculate.
Particular things I'm curious about
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The WB75 block has a vastly steeper slope than the other WB's suggesting that at about 10 Watts of 'Hydro Power' its cooling performance will exceed that of the TC-4. 10 Watts of 'Hydro Power' would take (by overclockers standards) a monster pump to produce, and the total heat put into the system by such a pump would most likely outweigh the benefit of the WB thermal resistance achieved. However, maybe understanding this big difference in slope would enable someone to design a WB with the same slope but shifted substantially to the left.
Why do the Swiftech blocks show so much curvature on this graph compared to the others? My speculation is that: At low 'Hydro Power', the flow of water through the block has little effect on the water near the baseplate - it just loops from inlet to outlet. At medium 'Hydro Power', the inlet flow is impinging on the base and the cooling is therefore substantially improved. At high 'Hydro Power', a lot of the applied 'Hydro Power' is being used in maintaining eddies as shown below in red.
The outgoing water has to fight it's way past the eddies and the jet and a lot of the applied 'Hydro Power' is wasted in generating turbulence that is of little benefit for cooling. Also, warmed water from the eddies gets mixed into the jet on the way down.
My speculations on the Swiftech curves are very much uneducated guesses, so please feel free to point out their faults or suggest alternative explanations.
I've got some other thoughts on this stuff, but it's after midnight and this post is long enough already.
Let me know what you think.