mike
thanks for taking the time to impart some of your knowledge. I appreciate
it and actually understood much of what you said.
bruce
----- Original Message -----
From: "Michael D. Porter" <mporter@zianet.com>
To: "stutzman" <stutzman@adelphia.net>
Cc: "MARK J WEATHERS" <markjwea@email.msn.com>; <fot@autox.team.net>
Sent: Wednesday, March 05, 2003 1:36 AM
Subject: Re: Head Cooling Problems
>
>
> 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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