----- Original Message -----
From Peter C. <nosimport at mailbag.com>
> The coolant needs to go slow enough through the engine
to pick up
> the heat, but not too slow or the temperature difference between
iron and
> coolant wont be great enough.
I barely remember my 4th year thermodynamics - but here goes. The
RATE of heat transfer between a FIXED surface area of engine metal
and coolant linearly depends on the temp difference (delta T). So
cooler coolant equals more transfer. The metal don't care if the
coolant is moving or at speed.
Therefore slower flow means the coolant next to the metal warms up
more, the delta T is less, and heat transfer is less (assuming the
metal temp remains constant, i.e. let's call the metal to be at
momentary equilibrium). The other thing you cannot avoid with slow
flow in the engine is that you will wind up with a cool end and a
hot end of the motor. The potential for design hot spots is
increased.
Also --- flow rates introduce the topic of turbulence and boundary
layers (Reynolds numbers) next to the metal. Boundary layers
(slower flow) are effectively insulators. Turbulence (high speeds)
is better for heat transfer. I don't know if internal engine water
passages are experiencing turbulence, except for sure at the water
pump impellers and probably not at all in the rad cores. Rads are
more complex in that you have fluid flow on one side and varying
temperature/speed air flow on the other. Unlike motors rads are
not so restricted to an "operating temperature" or a "mechanical
configuration" and therefore can have long passages for increased
time exposure, can have space shuttle fin technology, can have a
hot end and a cool end etc. Has anyone noticed that the water
passages in a motor block have not changed much in 100 years, but
look at what has happened to rad technology in terms of size,
weight and efficiency.
There is a long engineering equation that tries to simulate
reality for practically everything, up to and including rising
bubbles in a boiling pot. A lot of car stuff such as balancing the
heat output of the engine with the heat dissipation at the rad and
in the engine compartment is derived from empirical test data and
component suppliers specs. This thread is struggling with the end
result of a 30's Brit low production volume engineering mentality,
that designed everything with a minimum theoretical calculation
contingency safety factor. No allowance for loss of strength due
to corrosion, for Arizona temperatures, for poor owner maintenance
and lastly they designed RHD cars and converted (with compromises)
80% of them to LHD.
Bottom line, with marginal cooling, you have to guess or figure
out where your weakest and or cheapest link is - and say goodbye.
My take - speed it up in the engine (without cavitation), slow it
down in the rad (bigger rad), max air movement (shrouds and fans).
When you have excess cooling capacity, then the thermostat can
once again become the Controller.
Mike L
60A,67E,59Bug
|