15.7 Exhaust — nozzles, noise reduction and thrust reversers

Scope

A level 1, B1 level 2: construction and operation; convergent, divergent and variable-area nozzles; engine noise reduction; thrust reversers.

Exhaust system layout

Downstream of the last turbine stage:

• Exhaust cone (tail cone / inner cone) and struts — the cone forms a diffusing (divergent) annular passage that slows the gas leaving the turbine and straightens it (struts remove residual swirl). Slowing the gas reduces friction losses in the jet pipe.
• Jet pipe (tailpipe) — carries gas to the nozzle; insulated or shrouded to protect structure; expansion joints allow for heating.
• Propelling nozzle — accelerates the gas to produce thrust; its area sets the back-pressure on the turbine and therefore the engine's operating point.
• Materials: nickel or titanium alloys, stainless steel; heat shields and insulating blankets.

Turbofans may have separate hot and cold nozzles (short cowl) or a mixed exhaust (long cowl with a forced mixer — lobed mixer that blends hot core and cold bypass flow, reducing noise and slightly improving efficiency).

Nozzle types

Nozzle	Principle	Use
Convergent	Accelerates subsonic gas; maximum exit velocity is Mach 1 at the exit; above the critical pressure ratio the nozzle is choked and the excess pressure gives pressure thrust	Almost all subsonic civil engines
Divergent	Subsonic flow slows and pressure rises — used as the diffuser section behind the turbine, not as a propelling nozzle for subsonic flow	Exhaust cone region
Convergent–divergent (C-D)	Gas reaches Mach 1 at the throat, then accelerates further to supersonic in the divergent part — converts the pressure that would be "pressure thrust" into extra velocity (full expansion)	Supersonic aircraft, afterburning engines
Variable-area	Nozzle area changed by petals/flaps or an "eyelid"; opened when afterburning (reheat) to keep turbine back-pressure correct; also used on some high-bypass engines (variable fan nozzle) to optimise fan operating line	Military engines; some turbofans

Key fact: In a convergent duct subsonic flow speeds up; in a divergent duct subsonic flow slows down. Supersonic flow does the opposite — it speeds up in a divergent duct.