Originally posted by gaara
Simple energy fundamentals. Does it take you more work to run around a track that is a mile long, or 2 miles long? You want to pump under as little stress as possible. The only time the water is cooled is when the heat has a chance to dissipate, and naturally this is only done when it's going through the radiator. Therefore the only way for it to cool more efficently is to add a radiator with a bigger surface area, hence why you see dual and even quad fan rads.
As for the configuration of inlet/outlet, yes you do want the inlet in the middle as in that situation the water will be hitting the direct center of your core and idealling right on top of your core, and then displacing outwards. When it's off to the side, the actual contact of the water is not as strong relative to the location of the core, and it does not displace in a natural radius
It all traces back to the zero slip theory.. this says that the particle of water touching the edge of your none moving surface (your tube) does not move. so when the pump is pumping, the rest of the coolant has to sheer away from this non-moving particle. the water in the middle of your tube moves faster than the outer water at a logrhythmic rate. this is why you want large diameter tubing, the water in the middle of a 1/2" tube can move faster than the fluid in the middle of a 3/8" tube because there is less fluid friction to overcome.
along the lines of fluid friction, the longer your tubes are, the more friction needs to be overcome along with the given of gravity. this is what your 'pump head' is refering to. how high your pump can drive water straight up against gravity. if you have a significant amount of fluid friction, it takes away from how high your pump can push against gravity.
Your radiator is the largest source of fluid friction. this is desired, however, because convective heat transfer is directly proprotional to the amount of fluid element (a small piece of fluid) shear stress, which is also directly proportional to fluid friction. AKA the more shear you have, the move heat transfer and the more fluid friction.
how you apply this theory:
the more viscosity you have (aka the more your fluid resists shear stress) the more friction you will have and therefore the more heat transfer you can get out of your radiator... but you will also need a pump with more head! This is why people say antifreeze is better than water, it has a higher viscosity... but it also holds less heat by volume than water so i think its a wash.
If you get a bigger radiator, you may need a bigger pump to overcome the additional fluid friction.