I've read in many books that afterburners are designed not to affect the upstream engine components, so if there's a large amount of combustion going on after the addition of afterburner fuel, how does the reaction of the air to the sudden acceleration not force air back into the turbine stages?
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$\begingroup$ How would this affect upstream components? There’s more combustion occurring, so there’s more air needed. This should pull air AFT instead of forcing it forward. $\endgroup$– MD88FanCommented Jun 12, 2021 at 22:47
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2$\begingroup$ @MD88Fan Please don't answer in the comments, as this bypasses quality control (the infamous downvote). More combustion does not equal "pulling" air - a jet engine is not comparable to natural convection. $\endgroup$– SanchisesCommented Jun 13, 2021 at 9:20
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2 Answers
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The afterburner will always affect the upstream flow, as it is subsonic throughout the entire engine (remember, hot air has a higher speed of sound). A properly designed afterburner should therefore not significantly increase the pressure upstream, because this may result in a compressor stall.
Typically this is solved by increasing the nozzle opening area, so that the nozzle back pressure is reduced. Variable geometry nozzles are the norm for reheat capabilities. This in turn keeps the pressure at the turbine outlet roughly the same. Still, most engines require to be at some minimal power level before igniting the afterburner.
answered Jun 13, 2021 at 13:47
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It's pretty simple. When in reheat, the core engine is already running at maximum thrust, so the inertia of the mass flow out the turbine section is such that the reheated air has nowhere else to go but out the back.
The afterburner is more or less a ramjet, except with the main engine's exhaust doing the ramming in place of high airspeed.
