Colyn wrote: 

 

"That data makes it pretty clear.

I had a mechanical engineer/metallurgist friend who was a pilot at one point.   His comment on the equipment was "Airplanes barely work.  Helicopters don't work"   I think this exhaust situation is an example of "barely work".   Those margins are really thin.

So does this also mean that it is better to paint stainless black than leave it shiny?"

 

No.

 

Above 1000-1200F stainless steel turns gray/black on its own due to oxidation of the chromium.  It has an emissivity (ability to dump heat via radiation) of nearly 1.0 making it approach a theoretically perfect infrared emitter.  Leaving it alone and letting nature take its course works just fine. 

 

And just to warn folks: what is "non- black" to your eye may be "black" in the infrared red.  Things that look  white in visible light are frequently varying shades of black (emitters or absorbers) in the infra red. 

 

Since sunlight radiation peaks in the visible (not a coincidence),  your eyes work well for evaluating what is "black"  in sunlight.  That is why the upward-facing surfaces of composite airplanes should be white, to stay cool in the sun. 

 

But the white paint re-radiates in the infrared efficiently.  Thermal analysis in sunlight is a different kettle of fish from hot engine parts.   For exhaust pipes and engine compartments, naturally black pipes are good, and silvery surfaces to reflect the heat dumped from the pipes are also good.  Hence the recommendation for shiny aluminumized fabrics to line the inside of the cowl.

 

Fred