stutzman wrote:
>
> in the TR4 comp manuel Kas said to break the bellows and re-install the
> thermostat. "You must have a restriction in the system or the water pump
> will force the coolant through the radiator too fast for proper cooling to
> take place, and the engine can overheat very quickly." His advice in the
> TR6 comp manuel is similar.
With all due respect to Kas, I've weighed this one back and forth for
some time. Like others here, I've had some training in thermodynamics,
and think that there are some minor considerations that may interfere
with any pure thermodynamics theory, but have come to the opinion that
their effects are small to negligible. The question is of materials, and
as Bill Babcock says, of boundary layer effects.
Two things regarding materials are of interest--coefficient of heat
transfer through the radiator material, and coefficient of heat transfer
off the material to air. Both of these, effectively, are rates, and by
definition, are time-dependent. Therefore, it would seem that residence
time of the hot coolant in contact with the radiator material would be
of consideration.
As for boundary layer effects, if non-compressible fluids behave
similarly to compressible fluids, the greater the flow, the thinner the
boundary layer, but, as well, the greater stagnation of the boundary
layer. In the case of heat transfer, this creates two temperature
gradients--one from the hot fluid through the stagnant boundary layer
(which, being stagnant, is closer in temperature to the radiator
material), and the second is through the radiator material itself. Since
heat transfer is also linearly dependent upon the temperature gradient,
better heat transfer through the radiator would occur if there were no
boundary layer of coolant, which is a heat moderator, rather than a good
heat conductor as is the metal of the radiator.
All that said, the gross thermodynamic theory still applies. In a closed
system, temperature equilibrium depends on only two things--the amount
of heat produced by the engine in a fixed period of time, and the amount
of heat rejected by the cooling system in that same time. If the heat
rejection capacity of the radiator exceeds the heat production of the
engine, under optimum conditions, total heat in the system (and,
therefore, indicated temperature) can be regulated by thermostat.
If the heat production of the engine exceeds the heat rejection capacity
of the cooling system, that's where the rate-dependent materials
considerations come into play in any overheating equation. The materials
and design of the radiator will determine _how quickly_ the engine will
overheat if heat production exceeds heat rejection.
The effect of a restriction in the thermostat housing is build pressure
in the block and the head to minimize nucleate boiling, purely and
simply. The lower the pressure in the cooling jacketing of the engine,
the greater the likelihood of nucleate boiling (this phenomenon hasn't
been adequately explained, I think--it has specifically to do with the
ability of water vapor to transfer much less heat as compared to liquid
water--the higher the pressure, the more difficult it is for the coolant
to boil, and if liquid coolant can't boil, more of the liquid is in
contact with the surface area transmitting combustion heat--transferring
much more heat to the coolant than can steam vapor). It has to do with
how many molecules of coolant are in contact with the cooling jacket,
and the space between those molecules. The farther apart the molecules,
the more difficult the heat transfer. As well, with more molecules of
coolant in contact with hot areas of the jacket, the greater the heat
transfer.
The intent of increasing water pump pulley diameter, thus reducing speed
of rotation of the pump at high operating rpm, is to minimize the
_cavitation_ of a highly inefficient stock pump--cavitation creates
vacuum bubbles which act much like nucleate boiling--they are a point of
compressibility in an otherwise non-compressible fluid, which creates an
effective pressure loss in the system between the pump outlet and the
outlet to the radiator, and effectively increases nucleate boiling. The
original stock pumps have simple straight vanes cast into the impeller
plate, and those create turbulence at high speed which creates those
vacuum bubbles.
Cheers.
--
Michael D. Porter
Roswell, NM (yes, _that_ Roswell)
[mailto:mporter@zianet.com]
Don't let people drive you crazy when you know it's within walking
distance.
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