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

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Originally Posted by relttem

That is exactly true, but not all radiators can provide that necessary delta, therfore they are limited. All radiators have a specified heat load that they max out at..

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Can you explain the "Max Out" part?

I haven't seen that in any testing yet, they all seem to progress with more and more heat dissipation as the delta increases. Obviously if we approached boiling in the the fluid, that would be a limit, but I just havn't even began to see any sort of limit. I've tested and measured up to 20C deltas which is approaching my own personal comfort level and I know my temp probes also have their limits, but I just havn't really seen any sort of limit in testing.:shrug:

I can see a limit when you say something like what is the limit for a "10C water/air delta", but that's just a point of reference. I know there are some small physical changes to the specific heat/thermal conductivity values, but I just haven't seen any sort of limit yet other than my own self imposed one.

I tend to stay away from the thought of a radiator "Limit". I hear that alot in the forums with folks thinking that if they have a 1000 watt radiator, there's no performance loss to be had by adding another 100 watts to the loop, etc. That's obviously an incorrect way in thinking because every single watt of heat adds to the water temperature and your receiving core temperatures.

From what I gathered, you could potentially dissipate 1000 watts from a single 80mm radiator if you had a high enough water/air delta and enough air flow.

Honestly, when I was worked with Modine the radiators were sized way above anything that you would typically see. But, you can max out a radiator if, for example, you find a tiny radiator online that isn't really made for cooling a CPU, find one that is cooling something small. Or, make a simple radiator..just flow your liquid thru a short copper tube and blow air over it. Then, slowly add heat. The tube-radiator will peak out at a certain load. I have done it with refrigeration systems in that the refrigerant mass flow rate wasn't able to handle the amount of heat, then the temperatures start to run away until something breaks.

Again, the statement "if you had high enough water/air delta and enough air flow" has a limit. That is why a radiator will peak out. I think another thread on here talked about blowing thru a straw. If you get a martini straw and blow as hard as you can you will only get a little bit of air out..doesn't matter how much pressure you apply. Your air-side and fluid-side flow in the radiator will max out. You can get the largest fan and the largest pump, but it will still only pump so much liquid thru and so much air thru the fins..your lines will burst and the fan-blown air that can't get thru the fins due to the pressure drop will go around or not move at all..that means Mdot and h are maxed. So, we have A, mdot, Cp, h, and Tambient as constants and possibly T1r if the computer is at steady state

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Originally Posted by relttem

That is exactly true, but not all radiators can provide that necessary delta, therfore they are limited. All radiators have a specified heat load that they max out at..

it all goes back to

mdot*Cp*(T2r-T1r) = h*A*(T2-T1)

again, h, is related to the air speed, but it has a limit. So, when that has peaked out you only have one option left and that is to lower T1. T1 is going to be your ambient temp, so it can only go so low in practical cases. Sure you can run cooled air thru the radiator, or Nitrogen gas etc, but in practical systems it will be ambient. So, the equation part on the right hand side is maxed out and will equal X Watts.

Now, we move to the left hand side. Cp is constant. T1r, mdot and T2r can/will vary. Mdot will max out due to pressure drop thru the radiator and pump limitations. Now, if we keep pumping heat into the system, T1 goes up and T2 goes up, thus the run away system

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I don't need a lesson in the math. I was referring to your statement that the heat load doesn't matter. I was pointing out that it does have a factor in the time it takes for the system to reach equilibrium. Something you ignored and instead decided to launch into yet another pointless lecture.

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Originally Posted by Martinm210

I just wish there was a way we could automatically log and plot the temps on the crystalfontz. I'm sure it could be done if someone had the programming expertise.

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Doesn't the Crystalfontz unit update the .csv file in realtime? It should be possible pull the file with another program in progress, throw it into Excel, and throw the data into the graphing wizard to see how the curve is forming. If Excel won't accept a file that another program owns, you could use a program like Textpad to do that, and save it as a different file first.

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Originally Posted by Erasmus354

I don't need a lesson in the math. I was referring to your statement that the heat load doesn't matter. I was pointing out that it does have a factor in the time it takes for the system to reach equilibrium. Something you ignored and instead decided to launch into yet another pointless lecture.

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sorry, I don't mean them to be pointless. Its hard to determine what everyone is referring to on here, so I just try to generalize. But, if your heat load is too high it will never reach equilibrium..that is all I was trying to point out.

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Originally Posted by Martinm210

That was a 590 watt run.

The wierd thing I'm noticing is the time to equilibrium is not really heat based as much as it is the Delta and the volume of water in the test system.

I also ran an 200 watt load that took just as long because the delta was high from running very low 400RPM fans.

My reservoir for the heaters is a 4" x 24" piece of PVC, so when I add up the volume from my heater reservoir AND the large radiator, it's well over a full gallon of water in my test loop.

I have found that you can speed up the heatup process by estimating the delta from previous runs and heating the loop without fans first....just don't forget about it:eek:

I think ideally for this delta T testing you want the bare minimum of water volume. That and simply alot of time and leave the room and come back in a couple of hours for a really good run. My data points are falling on the trendlines much more accurately with this run. Now instead of stopping the logging run, I simply copy the file over to another and open it, plot the run and check to see if it's good enough to stop before I stop it.

I just wish there was a way we could automatically log and plot the temps on the crystalfontz. I'm sure it could be done if someone had the programming expertise.

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My res holds about a half gallon, I'm using 3" PVC.

I've got the templates down for testing, just have to copy paste from 635_log.csv into the excel template. About as minimal as you can make it...I'll send you a workbook. :up:

Apologies for a bit of thread necromancy.

The reason why more flow doesn't necessarily mean better is because ine can only remove so much heat from one surface given so much fluid flow. This is why radiators designed for low airflow can only do so much when given higher flow rate fans.

Similarly, this goes the same for coolant flow, but with a twist, you would want maximum turbulence and maximum amount of dwell time for your coolant inside the tubes, however this doesn't bode very well for your water blocks.

There's always a sweet spot between pressure drop and heat dissipation radiator designers want to achieve. More often than not, its very counter intuitive and requires a lot of testing and validation.

You can increase your FPI only so much that it prevents you from using audibly acceptable fans, but you can only bring it down so much that your heat transfer potential, relative to the radiator footprint, is optimally utilized.

Sorry Martinm, did not see this until today as I only stop in here occasionally. The question you are asking is studied in chemical engineering under reactor well dynamics. Without the math to confuse, consider the simple steam locomotive. As I add in more coal, I can drive the speed up to perhaps 60 mph. Quickly though, there comes a point, where no matter how much more coal I add in, I cannot drive the locomotive any faster (in fact, due to thermal convection resonance, I will actually slow down slightly). Same effect here, increasing flow rate (under fixed fan speed) increases turbulence as flow changes from laminar to turbulent (up to Reynolds # 4000 or so) and dissipates more heat for any given radiator until you pass the turbulent boundary and reach chaotic flow (this is the result of the partial derivatives to zero in Navier Stokes), at which point cavitation sets in and less reward is seen for ever increasing pressure, until the flow is so chaotic that initial boundary conditions consume more energy in transition than in radiation. In laymen's terms, the fluid is now moving under such increased pressure that more energy is exchanged within the fluid than can be radiated thru the rad's surface. That is why the delta T drops slightly.

@ hwlabs "Apologies for a bit of thread necromancy."

Thanks, hadn't read this one.

Ya, that last one sounds really good:up: