I just want to say that I'm a complete noob to water cooling, yet I have taken a hydraulics class and I wanted to make sure I'm getting what is being said here..
Something for the Flowrate sticky. The old one has tons of great info, some applies, some is way out and does not apply to our needs. This will kinda sum it up.
Flow rate is important to the effective operation of a CPU block. Modern blocks need 1 to 1.5 GPM of flow rate to effectly remove heat. below 1 GMP you can impact the CPU. Above 1.5 the benefits drop off and flatten out on curves. Look inside any modern block. And if you can find some of the simulations posted how the water acts in the block, I have seen them in the past.
Fluid needs to be of high enough flow to get very turbulent inside the block. This causes more molecules to contact the warmer copper surfaces, thus collecting more heat to be dissipated in the rad.
I don't want to get too technical here, but the amount of head these pumps produce isn't enough to create a high pressure scenario in regards to having more molecules make contact. You're talking about compression and water, in liquid form, is as dense as it will get. I think what you're saying is you want the flow rate to be just right so that the water has time to absorb the heat. Not too fast to not absorb the heat, yet not too slow for the water to stay and act as an insulator. What I think you mean is having a good velocity profile through the whole system.
We want linear laminar? flow in the hoses, but not in the blocks. You reduce flow and you can severly impact the cooling capability of the CPU block.
Technically, the water will be turbulent through the whole loop. Pumps, blocks, and radiators will cause a turbulent flow because everything except the pump will cause a drop in energy head. So, the type of flow is not really an issue because it is not a high water pressure scenario. It boils back down to good
volumetric flow rate, which is dependent on the pump. Keep in mind that flow rate never changes in a Pipes in Series, which is what water cooling is.
Another thing. Slowing the flow in the rad also reduces flow in the blocks. So as it enters the block it's cool. When it exits it's warmer than it could be, because it's flowing slower. So besides the impingment problems metioned above, you can look at it this way.
Unless you mean volumetric flow, I think you're talking about velocity. The pump controls the volumetric flow rate, not the radiator and block.
Lastly, we are dealing with a closed loop. Lets set the flow rate at 2 GPM. For easy demonstration purposes this will cause a water molecule to make a loop every 5 seconds, and stay in the rad for 1 second. So, every minute the molecule will be in the rad for 12 seconds.
Lets reduce the flow to 1 GPM. Every 10 seconds. 2 seconds. The molecule will still be in the rad for 12 seconds. Reducing flow rate does not keep the water in the rad for any longer...........................
I don't know where you're getting that data as it's nearly impossible to 'track' a molecule in that closed loop.
In summary, the purchase of a good pump and not getting crazy with two many blocks and 90 degree fittings your flow rate will be just fine. All modern rads and recommended pumps are just fine even with two GPU blocks and a CPU block. Add in a full set of Mosfet blocks etc and you might be hurting for flow rate. How do you know? Your temps are poor. It can be tested with a 10 gallon bucket. Fill use the 10 gallons as your res. Bleed the loop. Then put the pump out hose into the sink and turn it on. How long does it take the 10 gallons to get to 2 gallons? There is your systems flow rate.
All of this can be supported by tests at Martins and Skinnees and other info on the web. Someone is welcome to add all the goodies and links if they wish to make it into a true sticky.
I really want to see a few graphs from Martin & Skinnee. I really would like to see a test done on one PC with a water cooling setup of just having the CPU cooled, and see how the temperature of the CPU changes when you change the volumetric flow rate of the pump. Then another graph of another water cooling setup but this time, the GPU is also water cooled and the same comparison is done...
Otherwise, all I have to say is that by reading this and applying my knowledge I've learned in hydraulics, the key factor that determines good cooling is volumetric flow rate from a pump with a good pump pressure head.
You want it so that when the water reaches the pump on the intake side, the drop in pressure head due to ALL of the blocks, radiator, and elbows is still above atmospheric. It also prevents water from 'lingering' inside a block and start acting as an insulator rather than dissipate it.
high pump head, and the right volumetric flow will create a very efficient cooling set up.