Quote:
Originally posted by Myrd
This is the direct heat gain profile of most water cooled systems
[list=1][*]System pump imparts energy to water in the forms of Flow, Heat, entropy[*]Water passes to tubing which will remove heat from the enclosure to due Entropy. Heat always migrates to cold.[*]Water now slightly warmer 'Calories in gain from the tubing more dependent of Flow rate' moves to cooling block of component.[*]Based on temp. difference between the water and the cooling block and again the flow rate. Heat migrates to the liquid medium.[*]Water enters tubing where gains from the enclosure are now almost nonexistant.[*]Water enters radiator where in an ideal system it should reduce in velocity to enable a greater thermal transfer. [*]Water returns to the pump where velocity increases again.[/list=1]
The heat gain from the pump is not worth the reduced capacity caused by it preceeding the radiator. You want to have the least possible loss to entropy at the cooling block. Entropy caused by the tubing and fittings is from 'Laminar Flow'.
 For once my job actually is fun!
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Myrd,
I appreciate your enthusiasm, but a couple of your statements are either misleading or altogether incorrect. The two that particularly stick out regard heat moving from the enclosure through the tubing to the fluid and the second regards slowing flow in the radiator.
For starters, our plastic tubing is a pretty good insulator. Even if the fluid was 10°C cooler than the air within the case (extreme delta T) there would be little energy exchange across the tubing. As to the radiator, one only needs to consider heat flux between the tubing within it and the fluid. Heat flux integrated over the total surface area equals the total heat transfer. Heat flux depends on many factors, one of which is velocity. Higher velocity equals higher heat transfer coefficient. Higher heat transfer coefficient equals greater heat transfer for a given delta T or lower required delta T for a given amount of heat transfer. The differences are pretty minor and overall results are impacted a lot more by air flow, but nonetheless it is a fact that heat dissipation in a radiator will be more efficient at higher velocity. Only when the energy required to drive the higher flow rate exceeds the incremental gain in convection coeffient will reducing flow improve heat exchanger efficiency.
If you want the coolest fluid to strike the block, there can be no doubt that the radiator should preceed the block. Again, for "typical" flow rates, the differences are all but immeasurable, but it does not change the fact about which location is "best". On a more practical note, what really ought to dictate radiator position is which option yields the coolest air to the fans. A 2°C drop in air temperature will have a larger effect than moving the radiator ahead of vs after the pump.