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I did my doctoral research from 1986-1988 measuring the heat transfer to the
walls of a diesel engine combustion chamber. This work was sponsored in part
by the "Consortium for Use of Ceramics in Internal Combustion Engines". No,
I don't know much about ceramic coatings or their effect on heat transfer.
This research was sponsored when ceramic coatings were not so durable and
they thought it might be due to high cyclic thermal stresses due to local
differences in heat transfer rate. Not the thermal stress due to the
temperature gradient into the wall, but along its surface. I didn't work
with the coatings but measured the local heat transfer rates, and passed the
data on to the next poor chump for him to explain. And I've done nothing in
this field since then (who cares about diesels, anyway? Not me) so my
information is 12 years old.
Round about then, one of the recognized world leaders in the field, Gerhard
Woschni (some German guy working for some German company), ran experiments
with ceramic coated cylinders and found that the heat losses were not reduced
as expected. I don't recall whether this meant "less reduced than expected"
or "not reduced at all". He speculated that the higher wall temperatures
resulted in a smaller "quenching distance", which is the thin layer adjacent
to the wall where combustion cannot be supported due to the interaction of
the wall surface with the molecule fragments ("radicals") that are essential
in the flame propagation process. Rather like a "chemical boundary layer" if
you will. (As an aside, this is how spark arresting mufflers work. Have a
screen whose mesh size is smaller than the quenching distance and the flame
can't get through.) This meant that the hot gas was closer to the wall, thus
the temperature gradient was stronger, the gas temperature adjacent to the
wall hotter, and the total heat transfer rate through the wall not reduced
proportionately with the reduced thermal conductivity of the ceramic coating.
Kinda like doubling the insulation in your house and then finding out that
the outside temperature is now 300 degrees. It's still gonna get hot inside.
This was just speculation on Woschni's part. There was no controversy on the
part of his experimental findings, but the explanation was merely an unproven
hypothesis in 1988, which is when they gave me my degree and I bugged out of
the diesel field, never to return.
I have no intention to offend in this posting, but if there's someone I've
omitted offending (I've gotten Germans and diesel enthusiasts and automotive
engineers, I think) I apologize. Send me an e-mail and I'll get you next
time....
- Rob Wolf
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