I'm confused, Radiator Flow Rate, More not always better?

[Martinm210]

I think I figured out the problem with my particular setup when trying to see flow rate effects:

A larger delta across the inlet and outlet of the radiator will net a lower "Water Out" temperature with lower flow rates. This is simply a function of the water's specific heat value and how lower flow rates does indeed mean a lower "Water Out" temperature, but if you were measuring the average loop temperature, THAT would be a different story.

What this means to me is that using "Water Out" temperature only in radiator testing while letting flow rate vary is BAD!!!

More appropriately, we should be measuring "Water Out" and "Water in" and averaging those two temperatures. This way the Delta across the radiator which is VERY dependent on flow rate is canceled out in testing.

That or you always need to test at exactly the same flow rate. That's not necessarily taking any pressure drop differences or flow rate performance differences into account, but it's at least canceling out the In vs Out delta dependency on flow rate.

Bottom line, measuring radiator performance must either fix the flow rate, or preferably measure and average the temperature of the inlet and out.


This same issue goes for CPU testing. Using the water out only is again going to create some skewing of results if flow rates vary and you compare water to core. If ambients were held exactly the same, the blocks that are the most restrictive will produce water temperatures from the radiator that are slightly lower, this is not so much due to lower pump heat dump, it's mostly because of the delta across the radiator.

Anyhow, I think I finally understand and learned some important test procedure things:

1) You should not test radiators with "Water Out" only and allow flow rate to vary. This will make the more restrictive radiators appear to perform better, when it's really just a specific heat and flow rate variable that's making the water out temperatures lower.
2) Radiator testing or comparing of water temperature should compare the water loop temperature average "Water In + Water Out/2". This will cancel out this delta T problem that's extremely sensitive to flow rate.
3) Radiator testing using "Water Out" should ONLY be used if you fix the flow rate constant for all tests. This still does not test the real world where the flow rate is dependent on the loop, and affected by the radiator pressure drop, but it's still at least eliminating the radiator delta problem.
4) CPU block testing - Again this "Water Out" comparison is not good with varied flow rates. Ideally you need to fix ambient and compare core to ambient only. If you fix the ambient temperature and compare core temps to water out temps, you'll artificially be making the lower restriction blocks look better because by nature of the higher flow rate the radiator water out temperature is higher. You will indeed get lower water out temperatures with the more restrictive blocks simply because of the lower flow rate across the radiator and larger delta across the radiator. To me this just means using "Water/Core" as a means to compare blocks is not an apples to apples comparison. Apples to apples is a direct comparison to ambient, and that will mean two slightly different water temperatures running across the block.

Anyhow, my testing setup is not working for flow rate testing. I either need to fix flow rates at 1.5 GPM, or if I want to see flow rate effects...I need to measure both inlet AND outlet water temps.

Regardless, I think the Coolingmasters test was good, so there is still some interesting thing happening...just not to the extent that my first test was showing.

I thought something wasn't right.....

I also realize that my early CPU block testing and comparison to water out temperature was WRONG! The rad testing is still ok though as I think fixing the flow rate at 1.5 GPM saves that one.