From: "Marvin Kaye" <marv@lancaironline.net>
Subject: Re: [LML] Coating and covering exhaust manifolds
To: lml
Date: Thu, 13 Jul 2006 02:14:41 -0400
Message-ID: <web-1234899@logan.com>
In-Reply-To: <008901c6a62c$9366af70$c211140a@fred>
References: <008901c6a62c$9366af70$c211140a@fred>
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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