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When it comes to fully subsonic propulsion, all outlets are converging nozzles (I did say fully subsonic so no converging diverging). From what I understand, they are flow regulating devices (changing the exit diameter allows to control the mass flow through the propulsion system, be it a turbofan or ducted electric fan).

Why don't we ever use diverging outlets, i.e. diffusers? Is it due to the back pressure? Wouldn't such a shape increase the pressure recovery, i.e. more thrust.

diverging outlet propulsion system

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  • $\begingroup$ "Wouldn't such a shape increase the pressure recovery" - The pressure recovery is the pressure immediately forward of the fan, divided by the free stream total pressure, isn't it? How would a diverging outlet increase the pressure immediately forward of the fan? Maybe I'm totally misunderstanding something. $\endgroup$ – Terran Swett Sep 6 '19 at 20:41
  • $\begingroup$ sorry, i meant the after the fan. by turning the speed into pressure $\endgroup$ – toshi ba Sep 6 '19 at 23:13
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The basic answer is that pressure differential at the exit isn't what you want for thrust. You want the exhaust stream to have velocity. If it exits the nozzle with excess pressure, that extra pressure essentially gets converted into plume size (see the underexpanded streams on rocket first stages at high altitudes as an example).

This is why subsonic nozzles converge, and supersonic nozzles converge to a Mach 1 throat and then diverge.

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  • $\begingroup$ "If it exits the nozzle with excess pressure, that extra pressure essentially gets converted into plume size" yeah but that pressure creates forward thrust on the diffuser walls, unlike a nozzle which is a dragful components. wouldnt that increase the overall thrust, despite the plume size? $\endgroup$ – toshi ba Sep 25 '19 at 13:16
  • $\begingroup$ I think that maybe the disconnect is that yes, having higher-pressure gas on the exit side of the engine than the inlet DOES generate some thrust. But The Environment around the aircraft is big, and a lot of that pressure/potential energy goes into just pushing around air in the environment. It's more efficient to convert the pressure into momentum, which is a nozzle's job. You can only trade momentum for other momentum, but you can trade the potential energy of high-pressure gas for heat and noise and other not-thrusty things. $\endgroup$ – Erin Anne Sep 25 '19 at 19:42

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