<Snip!>

Tom Kendall wrote

Uh oh... I see this graph is making the rounds again.  I know a lot of the
people on this site know about it's limitations, but some of the newer folks
may not.  So, at the risk of sounding off on an old subject...

The graph is reasonably accurate.  But it is also deceiving.  If you are
heating something up that is being cooled on the "back side" by boiling, you
will get up to the top of the "peak".  But then watch out.  The heat
transfer does *not* continue smoothly down the peak and then back up again!
If you can increase the heat rate enough to leave the "peak" (no small
feat... that is a LOT of heat transfer), then you "jump" all the way to the
right into the "stable film" regime.

The temperature difference goes through the roof and things break FAST.
This is what Paul has alluded to when he talks of "departure from nucleate
boiling" (aka DNB).

The devil of it is, even if you recognize the condition and immediately
reduce the heat input (by pulling the throttle for instance), the heat
transfer now takes it's sweet time crawling down the curve until it gets to
the bottom, then it leisurely climbs back up to the peak from the right.
You don't get a "jump" back to the peak.  The temperatures stay very high
while the engine cools down more or less slowly.  Plenty of time for things
to break and not a thing you can do about it.

Except of course, to make sure you always operate to the left of the peak.

Oh yeah... one other little thing to note: both axes on the graph are
logarithmic.  You go from ~50 degrees F between the wall temp and the
cooling fluid to several thousand degrees F difference.  Instant melted
rotor housing... if the scenario is really achievable.  We're talking about
forced convection which is a much better heat removal mechanism.  As best as
I can recall, there's over 3 decades (1000 times) improvement in heat
removal from subcooled boiling to DNB when talking forced convection.

You might mention that there is no water flow here.
This was done in a pot on top of the stove :-)
It is the only one I have on this subject. If
you have some others I would love to have copies.

It is ironic that you chimed in Tom. I was going to use
a nuclear reactor as an example of a big generator of heat that
can indeed be cooled :-)
 
Paul Lamar
 
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