Exhaust
Propelling nozzles, the exhaust cone, convergent-divergent nozzles and thrust reversers.
Exhaust, summary notes
- The exhaust system collects the turbine-exit gas and accelerates it through a propelling nozzle to produce (or complete) thrust. In a convergent nozzle the gas speeds up as the area reduces; most subsonic engines use a fixed convergent nozzle.
- The exhaust cone (bullet) and struts straighten the swirling gas leaving the turbine and prevent the flow from expanding back into a low-pressure, turbulent wake behind the turbine disc.
- A convergent-divergent nozzle is used where the gas is supersonic (afterburning/military engines) to expand it efficiently; nozzle area may be variable on such engines.
- Thrust reversers redirect the exhaust (and/or bypass air) forward to help decelerate the aircraft on landing, cascade/blocker-door types for fan air, clamshell/bucket types for the core; they must stow positively and are interlocked to ground/idle logic.
- Exhaust-gas temperature is measured here or upstream of the turbine as a key engine limit; noise-suppressing mixers and chevrons reduce jet noise.
- ⚠ Exam trap: a convergent nozzle ACCELERATES the subsonic gas (area down, velocity up), the opposite of the divergent intake; thrust reversers act mainly on the high-mass bypass/fan air on a high-bypass engine.
On a high-bypass turbofan, which airstream does the thrust reverser mainly act on during landing, and why?
Mostly the bypass (fan) air, because that is where most of the engine's mass flow and thrust comes from. Cascade-type reversers with blocker doors redirect the fan air forward to produce reverse thrust; acting on the small, hot core flow alone would give little deceleration.
Exhaust concept map
Exhaust
Exhaust quiz
Exhaust, quiz
1. A convergent exhaust propelling nozzle:
Accelerates the gas to increase its velocity and produce thrustSlows the gas down, raising its static pressure so that more work can be taken from itHas no effect on thrust, since all of the useful work has already been taken by the turbine2. A thrust reverser produces reverse thrust by:
Redirecting the gas stream forwardsStopping the engineIncreasing the exhaust velocity rearwards3. A propelling nozzle passes 55 kg/s at a jet velocity of 600 m/s, with the aircraft static. The gross thrust is:
33 kN9.2 kN55 kN4. A convergent propelling nozzle:
Accelerates the subsonic gas as area reducesSlows the gas downAdds fuel5. The exhaust cone (bullet) behind the turbine:
Straightens the flow and prevents a turbulent wakeAdds thrust reverseCools the oil6. A convergent-divergent nozzle is used where the gas is:
Supersonic (e.g. afterburning engines)Always subsonicStationary7. On a high-bypass turbofan, thrust reversers act mainly on the:
Bypass (fan) airSmall hot core flow onlyOil system8. A cascade-type thrust reverser uses:
Blocker doors and cascade vanes to turn fan air forwardA clamshell over the core onlyWater injection