Posted for "Fred Moreno" <fredmoreno@optusnet.com.au>:


 
 Rod:
 
 
 
 Correct, I did not consider coating inside and outside.  So let’s consider
 it for a moment.
 
 
 
 Wall heating from the of the exhaust manifold is primarily by means of
 convective heat transfer (heat transfer through the thin boundary layer
 along the wall of the tube from the hotter gases to the cooler tube wall),
 not via thermal radiation from the hot gasses (which are “optically thin”
 and unable to radiate much thermal energy over the short distance of the
 tube diameter).  There is likely a small temperature drop from interior
 surface of the ceramic to the tube wall (a modest thermal insulating effect)
 which would be beneficial.
 
 
 
 But I would expect the benefit to be small, and offset by the insulating
 effect of the ceramic on the outside which would raise the wall temperature
 in the same way.  That is, dual insulating ceramic coating (inside and out)
 would seem to cancel the benefits out.  Theoretically, one would put ceramic
 on the interior wall only, and let the outside lose heat as rapidly as
 possible to keep the wall temperature lower and gain some benefit.  As noted
 earlier, do not do this on your turbo airplane if you like your turbine
 wheels.
 
 
 
 Technically, it would be nice to get some real, verifiable DATA.  But it
 would be hard to get, requiring a before and after coating test with a
 calibrated optical pyrometer to detect wall temperature while an engine is
 running on a dynamometer.  And calibrating the pyrometer requires detailed
 knowledge of thermal emissivity of the raw exhaust manifold compared to the
 ceramic coated manifold.  If the ceramic has a lower thermal emissivity than
 the metal, the heat transfer to the surroundings will be smaller, but the
 tube wall temperature higher because of the reduced radiation heat loss.
 Manifold lifetime would suffer as a result, although how much is an open
 question.  I merely point out that small temperature increases at these
 operating temperatures can have large impact on strength, corrosion rate,
 and increasing brittleness.
 
 
 
 My view: if you like the appearance of ceramic coating on your exhaust
 manifold in favour of the grey/black of the oxidized stainless, then coat
 away.  At least you have a chance to detect cracks and deterioration.  Coat
 and then inspect regularly.
 
 
 
 At the end of the day, I think it is an issue of aesthetics and has to be
 justified on that basis.  As a thermal management tool, ceramic coatings on
 the exhaust manifold seem to be a double edged tool.  If you want thermal
 management under the cowl, do it another way, or show the benefits via
 verifiable data.
 
 
 
Fred Moreno