Bill Freeman wrote:

Paul,
Were you disagreeing with my comments on boiling and a
higher boiling temp coolant?  Not sure what to make of your
comment.   I *did* look at the piece you sent to me privately
before you posted it.   I generally agree with your source
(Heinz), I just now saw that the heat transfer increases by
TWO orders of magnitude as nucleat boiling comes on
line rather than "just" one order of magnitude.  I think we are
in general agreement on this, except I contest your comment
on steam creating an "air pocket".   :-)   Russell agrees with
me, but as he says - a nit, not worth much picking.  The only
part of Heinz that I question is the continued existence of the
steam bubbles for large distances away from the hot spot.
I would expect the cooler (sub boiling temp) turbulant gross coolant
would quickly condense the bubbles unless the liquid coolant
was extremely close to the boiling temp itself.  Typical auto coolant
temps are 200-220F and under pressure the EG/water
has something like a 260F boiling temp.  Steam bubbles should
quickly condense when washed into the turbulent stream of
liquid coolant.  Heinz says they stay intact until they reach the
surface in a coolant tank.  The conditions that form the bubble
are only controlled by temp and press, not by any free surface
of trapped air that might exist somewhere in the engine.  Bubbles
would rise, of course, but would not necessarily seek out some
air pocket, whether in the block or in a coolant expansion tank-
just a high point.  And remember I stipulated adequate coolant
flow rates and radiator size plus airflow being adequate.

re: "part of Heinz that I question"
The difference may lie in the temperatures cited.  You say that "typical
auto coolant temps are 200-220F".  Is that average system temperature, or
is that a "typical" temperature at a particular point in the system?  Is it
possible that the local coolant temperature near the nucleation site
("near" in this case means within the water jacket region, i.e. a dozen
millimeters or so) has reached the coolant's boiling temperature?  Under
those circumstances, the vapor bubbles from the nucleation site would
remain bubbles until the coolant had been cooled slightly, which might be
the radiator tank or possibly the hose to the radiator.

My problem with Heinz Heisler's treatise is that it doesn't discriminate
between the initial nucleate boiling, where the bubbles depart the surface
and quickly re-condense ("quickly" in this case being on the order of a few
millimeters distance), and later nucleate boiling where the bubbles do not
re-condense until the bubbles (and the surrounding coolant) enter the
radiator.

In the general scheme of things, sure, the later nucleate boiling is even
better since the heat of condensing isn't dumped into the coolant until the
coolant is in the radiator.  But for *our* purposes, this later nucleate
boiling has a huge potential problem: if the vapor bubbles get hung on some
structure, they can coalesce and form a "steam pocket".  The steam pocket
then promotes a "pseudo-film" boiling local condition, which can rapidly
result in localized overheating.

And just to make things even more difficult, there are not good names for
the initial and later phases of nucleate boiling, making conversation about
it a hassle.

Russell Kent

 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/            http://www.linux.org