Hey all,
I swore to myself I wasn't gonna throw in my two cents worth about
this whole fast/slow coolant flow thing, cuz I figured someone like
Mr. Fisher would post yet another eloquent answer and everyone would
leave with a smile on his or her face. Well, he did, but people still
don't have smiles on their faces. Maybe this will work....
One good way to analyze a problem like this is to break it down into
two extreme situations. If we are dealing with changing the speed of
the coolant flow, our two extremes are (1) coolant flows super-duper
fast (approaching infinity) and (2) coolant flows really-really slow
(approaching mass flow=0 ).
Ok, suppose the coolant went really really fast. In this case, a
small unit of fluid would not spend much time in the radiator in a
single pass. Thus it would not lose much heat. On the other hand, it
would not spend much time in the block -- so it won't gain much heat.
This is called the steady-state condition. The fluid works its way
through the cooling system at approximately the same temperature. A
good thing about this is that the entire radiator is heated (from the
inside) to the same temperature. And as we all intuitively know (or
learned in our physics classes) the greater the heat differential
between two bodies, the greater the amount of heat transfer. This
means we can optimally use our radiator! Assuming that we can get
just enough air to rush by the radiator to remove the heat generated
in the block, there will be no net increase in the temperature of the
coolant.
Now then, assume that the fluid is going really slow. Then a small
unit of fluid would lose all of its energy in the first portion of its
pass through the radiator (because it sits there forever). For the
remainder of its pass through the radiator, it is the same temperature
as the ambient air. Which is nice and cool but in effect this makes
the majority of the radiator useless! Unfortunately, the fluid now
has to enter the block. The heat from combustion is passed to this
fluid which, since it sits in the block forever, now is hotter than
its boiling point. Solid-to-liquid heat transfer is much more
efficient than solid-to-gas heat transfer, so the engine block
overheats, siezes up and solves the cooling problem since the engine
no longer produces heat. It just radiates like a brown-body.
It is easy to get sucked into the "if it stays in the radiator longer
it get cooler" arguement but not if one looks at the entire system as
a whole.
One thing this arguement doesn't include is the effects from turbulent
flow. Laminar flow through the coolant passages would not transfer
heat as effectively as would turbulant flow. I would expect in this
case that turbulence introduced at the thermostat would have a
negligable effect upon heat transfer.
My credentials: BS in Physics from HMC
Scientist in Air Force Rocket Propulsion Lab
Burned myself with scalding liquids several times
I hope all this made sense and yet wasn't too simplistic.
Timothy Radsick
'72 Triumph GT6
'74 Porsche 914 (NO RADIATOR REQUIRED!!!!)
'91 Honda CBR600F2 (my only reliable transportation)
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