Reply to "double water pumps."

Coolant speed needs to be fast enough to have a
turbulent boundary layer while staying under the cavitation
speed of the water pump. At slower speeds, flow is laminar. In laminar flow,
Newtonian fluids that come in contact with a surface exhibit a characteristic
velocity profile. The fluid layer that is in contact with surface has zero
velocity. The velocity increases from zero to freestream as you move away from
the surface. Laminar flowing or standing water is a poor thermal conductor
(actually it's a pretty decent insulator). Refer to:

http://www.engineeringtoolbox....ductivity-d_429.html

The thermal conductivity ( in units of k - W/(m.K) ) of a cast iron 351C block
is 55. The conductivity of carbon steel coolant pipes is 43 while an aluminum
radiator is 205 and a copper one is 401. Water is several orders of magnitude
lower at 0.58. Thermodynamics are driven by temperature differentials.
The greater the temperature differential, the more effective the cooling.
Increasing the speed of the coolant flow so that it has a turbulent boundary
layer introduces mixing which carries higher temperature coolant from the
center of the tube to contact the lower temperature surface, providing better
thermal transfer between the coolant and the tube surface.

Dual pass radiators double the path length so velocity is
also doubled. Smaller diameter pipes also increase velocity. This is done to
promote a turbulent boundary layer for increased cooling. Also, once a coolant
reaches a certain temperature (it's pressure corrected vapor point), it no
longer absorbs additional heat and, as it approaches that temperature, it
picks up heat at an increasingly slower rate. Slow coolant speed leads to
localized hot spots and boiling in places like cylinder head passages near
exhaust ports.

Dan Jones