[Sirpent]

Thanks, myv65!
I think I agree with most of what you wrote. But, still, my question was slightly different. Probably I just didn't use the right words. I'll try to be more precise. I'm building a simple mathematical model of the situation (trying to keep worms in their can).

So assume that we have an ideal situation, where the cpu generates a given wattage, all the heat goes to water, the air temperature in the case always stays the same, and we can vary the water flow in the system without generating extra heat. Suppose that the heat flux F1 from cpu to water equals C1(Tcpu - Twater-wb), where Twater-wb is the "averaged" temperature of water in the waterblock, and the heat flux F2 from water to air equals C2(Tair - Twater-rad), where Twater-rad is the "averaged" temperature of water in the radiator and Tair is the "averaged" temperature of air (the arithmetic mean of the in-case and exaust air temperatures). For simplicity, we can even assume that Tair is the (supposedly constant) in-case temerature. In a stabilized system, we have F1 = F2 = (heat generated by cpu); and we can also think that Twater-wb = Twater-rad. Two biggest difference between this simple model and the real life situation are, imho, the following: (1) in reality, C1 and C2 are not constants, they depend on the flow (or, really, local velocity, as you wrote, but the flow is our only input parameter) - higher flows mean higher numbers; (2) higher flows generate more heat even if the pump is 100% effective (and it's not). The two correction terms work one against another and it seems that the first one usually wins in the typical range of applications (higher flow -> lower cpu temperature). But my question is: is it true that in the simplified model (I think it's really the simplest possible one) the cpu temperature does not depend on the flow?

Originally posted by gmat:
I'm not sure of what you mean, but if you mean "heat tranfer" the Fourier formula Q=UAdeltaT answers it. Q=heat tranfer, A = contact area, U = heat coeffficient, which depends on such things as turbulence and flow. U will get higher if flow gets higher...

I don't know how big or how small are changes of U in a typical system (any data?). So I am asking what are the answers for the "first approximation" model where U=constant.