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

[Martinm210]

Been playing around with some flow specific radiator testing and getting wierd results. After referring to other tests, I noticed the old coolingmasters review measured something similar. On their review they measured that with lower RPM fans, many of the rads begain showing worse performance with higher flow rates:



Any theories as to why?

[millertime359]

maybe because the fluid is moving too faster for the slower fans to keep up with cooling the radiators? Or I just completely missing it?

[NaeKuh]

maybe because the fluid is moving too faster for the slower fans to keep up with cooling the radiators? Or I just completely missing it?

your completely missing it..

im gonna blame the eternal badboy AMBIENTS martin.

[flak-spammer]

I would imagine that the heat transfer between the water and the fins is quite high but the forced air the air cannot remove the heat from the fins fast enough. That's just my guess.

[Martinm210]

I don't know.

I know the net amount of time the water is in the radiator is the same regardless of flow rate when you consider the recirculation part of the loop. Double the flow rate, and it's only in there half the time, but twice as often, etc.

But the water itself and turbulence I figured would always be better with higher flow rates and increased velocity.

I can't make sense of it..?

[Sadasius]

How about liquid friction?

[Martinm210]

How about liquid friction?

That could amount to some, but at most I think it could only be a few watts. If the pump only consumes 15-18 watts at 1.5GPM and you're only loosing a very small portion of that (Pressure/Flow energy) due to pressure drop, I would think pressure drop produced heat could only be a few watts at most.

They are showing that some of these radiators are loosing nearly 20 watts worth of dissipation between 1 LPM(.2GPM) and 11 LPM(2.2GPM).

That's more loss than the pump would consume. Also all the radiators seem to have a similar loss. Since the GTS is serveral times more restrictive, if the loss was all pressure drop induced, I would have expected the GTS to show this stronger.??

I'm sure that's some sort of contribution though. Energy has to go somewhere and pressure drop converts to heat.

[millertime359]

I know the net amount of time the water is in the radiator is the same regardless of flow rate when you consider the recirculation part of the loop. Double the flow rate, and it's only in there half the time, but twice as often, etc.

O, ok I understand this then, but if you have less air moving over the fins, less heat is dissipated, even though water spends the same time in the rad no mater what, the quality of the time it spends there is worse as it moves faster?

So seeing that a slower fan should perform worse?

[nikhsub1]

There is no mystery here. Liquid flow rate and air flow rate are conjoined twins. This has been well documented years ago by Bill Adams and his extensive radiator testing.

[skinnee]

I am curious on this as well...the first couple radiator test runs I did I forgot to control the flow rate and saw some odd numbers. I don't have the thermal engineering background to give a concrete reason as to why.

so, on this one... :popcorn:

[silverphoenix]

maybe because the fluid is moving too faster for the slower fans to keep up with cooling the radiators? Or I just completely missing it?

I don't know.

I know the net amount of time the water is in the radiator is the same regardless of flow rate when you consider the recirculation part of the loop. Double the flow rate, and it's only in there half the time, but twice as often, etc.

But the water itself and turbulence I figured would always be better with higher flow rates and increased velocity.

I can't make sense of it..?

I think millertime has part of it right, in our normal loops it might not be but these guys tested with a water chiller and not a closed recirculating loop. There is probably a non linear curve to how much heat the radiator dissipates depending on flow, and a non linear curve on how well the air absorbs heat depending on cfm. There is I'm guessing a turning point where curves go down when the flow is increased, and this means there is a perfect equilibrium point where there is a certain amount of flow that you can reach that will give you the best temps at that certain CFM.

[Martinm210]

There is no mystery here. Liquid flow rate and air flow rate are conjoined twins. This has been well documented years ago by Bill Adams and his extensive radiator testing.

Do you recall any specifics about what's going on it detail?

I don't understand it,but it's very interesting especially when there's becoming an increasing number of users that prefer very very slow speed fans....

It does me there are circumstances where you may want to PURPOSELY run radiators in parallel or series. You might even consider some sort of bypass plumbing setup to tune the radiator where it peaks in heat dissipation.

[Bond Number]

In their test how do they vary their flowrate?
needle valve, change in voltage

[MomijiTMO]

Excuse me Martin but how slow is slow? Are we talking about sub 1000rpm?

[Boogerlad]

heat dump from fans? Unlikely though.

[MpG]

If the flow through the radiator is higher, the pressure drop must be higher too. It is possible that the pressure is altering the radiator physically, albeit very subtly? Maybe making the tubes flex outwards a little, which would make the tubes' inner surface/CSA ratio worse?

You couldn't have waited another week or two to post this, Martin? I'm halfway through building a cabinet to house three triple rads, three pumps, and I was planning on running sub-600rpm fans. This is throwing a monkey wrench into my plans, darn it.

[Martinm210]

The cooling masters guys and in my test, I'm pulling with the fans, so it can't be heat dump from the fans. Also in the coolingmasters test, they show this affect occurring to a very minor degree with the nexus fans at 12V, it's not really flat past 1GPM until they tested with the Panaflos at 7V.

And we're not talking huge differences, but something and perhaps more with certain radiators than others.

I just see this as another possible optimization. The radiator manufacturers could either build in their own bypasses to emphasize performance in one area, or the user could do it with plumbing/valves.

I'm certainly seeing this in my closed loop testing now and I'm repeating my tests, so it's checking out.

[Martinm210]

If the flow through the radiator is higher, the pressure drop must be higher too. It is possible that the pressure is altering the radiator physically, albeit very subtly? Maybe making the tubes flex outwards a little, which would make the tubes' inner surface/CSA ratio worse?

You couldn't have waited another week or two to post this, Martin? I'm halfway through building a cabinet to house three triple rads, three pumps, and I was planning on running sub-600rpm fans. This is throwing a monkey wrench into my plans, darn it.

I think your on to something there.

I believe that does happen with some. I know in my 480GTX pressure drop test, that the curve was very linear compared to most pressure drop curves. Those tubes are very thin for good heat transfer, it probably doesn't take alot of pressure to start flexing them round..

If this is the case, you might consider running those rads in parallel...

[MpG]

I just see this as another possible optimization. The radiator manufacturers could either build in their own bypasses to emphasize performance in one area, or the user could do it with plumbing/valves

Maybe this is the reason for the gap inside the TC rads?

Now that I think about it some more, any physical deformation that interferes with low-speed fan cooling, should only be magnified with the high-speed fans. Which doesn't seem to be happening.

[Martinm210]

Maybe this is the reason for the gap inside the TC rads?

Now that I think about it some more, any physical deformation that interferes with low-speed fan cooling, should only be magnified with the high-speed fans. Which doesn't seem to be happening.

Could be, although I would have expected a cutout hole in the dividing plate if it was intentional.

What I find interesting is how low a flow rate we're talking about. 1LPM is about .2GPM, that's hardly any flow rate at all

I'm seeing up to 20% performance differences depending on flow rate so far and possibly more. That's pretty significant especially if you consider doing some flow bypassing also has the benefit of reducing overall system pressure drop and increasing flow rate for your blocks where more flow rate has been shown to make improvements...win win..

[Erasmus354]

Maybe this is the reason for the gap inside the TC rads?

Now that I think about it some more, any physical deformation that interferes with low-speed fan cooling, should only be magnified with the high-speed fans. Which doesn't seem to be happening.

It wouldn't be magnified with high speed fans because the supposed deformation is caused by the water and not the fans. The tubes expanding would create a smaller space for air, which makes the radiator more restrictive for air passing through. Therefore it is less efficient on lower rpm fans while the higher rpm fans can power through it (like a thicker vs thinner rad or a high fpi vs low fpi).

[hellcamino]

maybe because the fluid is moving too faster for the slower fans to keep up with cooling the radiators? Or I just completely missing it?

You are closer to the truth than NaeKuh thinks! I wasted my breath on this one a long time ago, blocks may like as much flow as you can feed them but radiators certainly do not unless you ramp up the amount of air that you are horsing through the core.

@Martin, if you have any super finger choppers (220cfm+ delta's), try those on the same radiators and the cooling performance will improve. Please wear gloves and ear plugs though!

[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.

[NaeKuh]

heres comes back martin setting another higher standard for standarized testing.

[millertime359]

Very interesting Martian. I learned something today for sure. The problem was it wasn't what I should have been learning in class