Subject:  Re: Heat flow
   Date:  Wed, 29 Apr 1998 14:48:28 -0700

Jeff Spitzer wrote:

At 06:34 AM 4/29/98 -0700, you wrote:

My question is; what about the coolant with a boiling temp of 369
degrees F at sold by Evans  Cooling systems 446 Lancaster Ave. Frazer,
Penn. 19355 (610) 889 9933? What will it do at high temps and pressures
long term?

Paul

As usual I have very little time to repond here but I wanted to respond to
these posts about a high boiling point fluid being some kind of advantage.
It's not!  In fact, most heat carried away from the high heat flux areas of
ANY engine is only possible through nucleate boiling.  For those of you who
don't know what nucleate boiling is it is a microscopic phase change
(boiling) that occurs in a the fluid boundary layer which creates intense
microscopic turbulence.  Also the vapor bubbles are carried away from the
surface by the process and are immediately recondensed by the cooler bulk
fluid.  If one calculates the heat flux that can occur with pure convection
it immediately becomes obvious that nucleate boiling is required for high
flux.  So, if the boiling point of the coolant is increased the wall temp
must increase proportionally to acheive nucleate boiling and remove the
heat.  Higher boiling temp coolant will always make the engine run hotter
(for the same coolant bulk temp).  One upside of nucleate boiling is that
the heat flux is exponential with temperature difference between the wall
and the bulk fluid for wall temps above the boiling point.  In other words,
for typical heat flux values, the wall adjacent to the coolant will never
run more than tens of degrees hotter than the bulk fluid (as long as
reasonable flow is provided) and even if the heat flux doubles the wall
temps will only go up by tens of degrees.  Engines which experience
localized cooling problems are always to be blamed on poor flow
distribution or too thick walls (note that the coolant side wall is
maintained not the combustion side) and not on coolant properties (unless
the BP is too high)  The real issues with coolant properties and temps show
up in the radiator where temp and convection coefficients are all that is
available to transfer heat.  Here is where either extremely hot fluid or
one with great thermal/boundary properties (e.g. pure water) can make an
improvement.

Anyone who runs anything but water in an engine designed for water deserves
what he gets.
Jeff Spitzer

I always remember this old message from Jeff on the CD. :-)
Jeff did not hold back :-) Jeff should know about this as
he tried PG in the Moller engines.

BTW Rolls Royce tried pure ethylene glycol back in the 30's in the Merlin
and gave up. They found that 70/30 EG/H2O mix pressurized  was better by 70 degrees
F on the cyl head temp than atmospheric pure EG. There was a "creep" problem with
pure EG
whatever that was. This according to Major Piston Engines of WWII by Victor Bingham 
Page 116. There is that reference to cyl head metal temperature again. 

Paul Lamar
 
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