Great Post, Daggit!

Water (or any fluid) flowing through tubes very slowly will have a laminar flow pattern, which is to say that the water along the walls stays along the walls and the water in the center stays in the center of the tube. There is no cross mixing. This is a very in-efficient way to transfer heat since only the water near the wall heats up much in the tube...the heat doesn't have time to get to the center.

At very high flow rates the flow is fully turbulent, meaning that the is nearly full mixing within the tube and water near the wall at one point gets moved to the center and vice versa. This allows more heat transfer since more water contacts the tube wall and can get to near wall-temperature.
I JUST saw something exactly like this in the latest Cycle World I got this week. Kevin Cameron in his "Top Dead Center" column, talking about engine cooling says nearly the same thing, but not as clearly. I've been taking the position that the greatest flow rate your system can tolerate will give you the most bang for the buck. I think the real limiting factors are the pressure the panels can tolerate without leaking, and the amount of additional flow the pump can sustain before you lose effective water motion from your regular returns. You seem to reach these limits long before others.

In my Fanta-Sea solar deck panels, leaking is a MAJOR issue--I've just purchased a plastic welder so I can fix a panel with a split seam. As it's a special panel cut to fit around the skimmer, I can't use my other spare panels instead--I must fix it.

BTW, I surprised about wind-chill. I didn't think it included evaporating water--just the flow over the surface. I have ridden a motorcycle at 10 to 20 degrees F for hours on end without a windshield when I was young and foolish and it was one of the most unpleasant painful experiences that did NOT require a trip to a doctor! I still remember spending over an hour and a half in a Burger King in Cheraw, SC in 1980, just thawing out...

My point? I certainly wasn't sweating, but that wind DEFINITELY made it feel like 20 below and it was just sucking the heat out of me, despite multiple layers and wind-proofing.