Quote:
Originally posted by airspirit
I think Ben's first post was on the mark 100%. This is NOT measuring the cooling power at all.
An easy way to describe this would be to take two blocks, one of high resistance, and one of low resistance. Looking at the original equations, you would raise W by increasing the resistance of the block. Raising the resistance of the block would lower the flow rate. Lowering the flow rate decreases the efficiency of the block, and you get higher temps.
Conversely, with a low resistance block, you get more flow, and more efficiency, and while W goes down, temps lower as well.
The perfect waterblock would draw heat from the die in the most efficient manner possible while providing the least amount of resistance to flow. Increasing the resistance of a block and thereby increasing the amount of time a slug of coolant remains in it will NOT lead to better temps. While I commend you on the effort, I think you are measuring a different type of force, not the cooling potential of the block.
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If I thought that just dissipating power in a waterblock would cool my CPU, I'd simply set up my watercooling system out in the garage and wouldn't bother mounting the waterblock on the CPU.
You think I'm suggesting that increasing the power dissipation in a water block is good, in and of itself? Not even close.
The fact of the matter is, for any given block, there is a relationship between the hydraulic power applied to the block, and the thermal resistance of the block. Within limits, the thermal resistance of the block decreases as more hydraulic power is applied. Because a waterblock is part of a system including (usually) a centrifigual pump, considering issues related to transferring power from the pump to the block is important. (It's not necessary to look at the system in terms of power transfer to determine how well it will cool, there may not even be any particular benefit to doing so, it's just a viewpoint that comes 'naturally' to me, and I find it interesting to consider watercooling systems in these terms.)
Because I'm an electrical engineer, I tend to think about a lot of this stuff in electrical analogies. One tool from electrical engineering that appears somewhat relevant is
The Maximum Power Transfer Theorem. In short, it says that if I have a voltage source with a resistor permanently attached to one of its output leads, then to get the maximum power into the load, I need to connect a load with the same resistance as the permanently attached resistor.
This would be more directly applicable to watercooling systems if flow resistances behaved akin to Ohm's Law or:
dP = Rf * Q
Where:
dP is pressure drop
Rf is flow resistance
Q is flow rate
Instead, flow resistances generally behave as:
dP = Rf * Q^2
Because centrifugal pumps behave somewhat like ideal pressure sources in series with an inherent flow resistance, a modified version of the Maximum Power Transfer Theorem may be applicable enough to be useful. From the very limited number crunching I've done so far, it appears that the maximum power will be transferred to the load, when the load resistance is twice the inherent resistance of the (idealized) pump. I'll let someone who's had a math class much more recently than I, attempt to prove or disprove this.
The following graph shows the PQ curve for an Eheim 1048 pump as well as the following equation as some indication of how accurate a simple simulation of a pump might be.
dP = 1.5 - 0.015 * Q^2 (1.5 is the max head, and 0.015 is the inherent flow resistance)
It's getting too late at night to go on with this now. Some examples of looking at watercooling systems in terms of power transfer tomorrow night.