forced convective heat transfer is related to turbulence and surface area. Turbulence is directly related to water velocity, not flow rate, by the appearance of Reynolds number in calculation.
I think the term I am digging for is called the h coefficient (encompasses the S.A. and Re and some more terms) but I dont have any of that in front of me at the moment. I am betting that Dave or Bill know it much better.
Anyway flow rate is useful because it can be measured anywhere in the loop (constant) much more easily than velocity at a lot of points. Plus I assume water velocity can be estimated at any point with cross sectional area.
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