Higher flow rates increase turbulence in the water tubes (many tubes are scored  or raised inside to increase mixing of water during flow) which means more mixing and more water that 'sees' the tube surface, so more opportunity for heat conductance ..... all at the cost of increased pressure required.  This should work until the centrifugal pump begins to stall (pressure increases causing flow to decrease to the point that pump head pressure begins to fall, at which point the pump begins to add heat to the system.   All radiators have 'ideal' flow rates for resultant 'ideal' pressure drops for best heat rejection. 

Summary:  Turbulent flow in radiators provides much more heat rejection potential than laminar flow, whether it's the water inside the tubes or the air going around the tubes.  And the key on both sides is to allow for a good low pressure area on the downstream side so all the pressure drop is located where you want it in the radiator.

Phil Stitzer
ex plumber

Lehanover@aol.com wrote:

In a message dated 9/30/2007 10:32:24 A.M. Eastern Daylight Time, eanderson@carolina.rr.com writes:

 Mark, if you really had excess air flowing through your radiators the coolant would drop more than 4 Deg F.  In fact, the more air flow the more coolant Delta T you would drop through the radiator. 

<snip>

 

That's exactly what I HAD thought, until I was told that the air could pass through too fast and not pick up as much heat.  This didn't make sense to me.  Maybe I wasn't listening closely and missed the point altogether (wouldn't be the first time). 

 

 

This is one of the oldest myths around - that air or coolant will flow too fast to pick up the heat.  It just IS NOT factual.  The more mass flow you have, the more heat you will carry away.  It appears that some early experimenters noted that if you slowed the flow of coolant through a radiator that there was a greater temperature drop of the fluid than if it flowed through faster.  better myth.  I once had an debate

Ed, Could you comment on this?:

 

On race cars I modify the radiators to be double or triple pass. By inserting baffles in the tanks, to force the coolant through a fewer number of tubes, and therefore at a higher velocity. The effect is that the radiator has 1/3 or 1/2 the tube count but the tubes are twice or three times as long and have exactly the same area exposed the airflow. It has never failed to work for me.

 

My thinking is that the flow rate remains very energetic and is scrubbing the inside of the tubes with gusto, dumping more heat than a slower flowing coolant.

 

What do you think?

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