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This is coming from Question 1 and further down Question 2.

I think the nacelle and exhaust cone do affect the contrail formation in some way, what is the relationship between them? Do the famous 787 Chevrons also affect the contrails? And How does it compare with engines with exhaust mixer?

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Ultimately the engine is dumping water into the atmosphere and this will quickly cool to ambient temperature. Depending on the relative humidity at altitude it may or may not condense. These factors are irrespective of the engine design provided that combustion is more or less complete, so the only factor that will significantly affect contrail formation is the extent to which the exhaust gases are distributed. This will certainly be affected by the design but is hard to predict in general how; it’s mostly down to fluid dynamics well downstream of the engine itself.

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    $\begingroup$ Interestingly, piston engines can also make contrails. Fans could be mentioned as a factor "to which exhaust gasses are distributed". A contrail is essentially a cloud, and their (longer lasting) ice crystals must form before the water in the exhaust stream evaporates. Very cold temperatures with higher relative humidity favor contrail formation. $\endgroup$ May 22 '21 at 11:50
  • $\begingroup$ For whatever it's worth, B-2 bombers often leave very distinctive contrails that can be recognized at a glance. I can't quite remember exactly what it is about them that catches the eye, but they are distinctive. Of course, in 4-engine airliners like 747 the contrails become entrained in the wingtip vortices in a way that never happens with other aircraft-- $\endgroup$ May 23 '21 at 14:09
  • $\begingroup$ @quiet flyer given the variety of different engine designs in use, I’d say that since only a handful of aircraft have visibly different contrails that’s empirical evidence that engine design such as chevrons doesn’t make a significant difference. $\endgroup$
    – Frog
    May 23 '21 at 20:21

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