Jet Engine Nozzle Flashcards
What is the purpose of the jet engine nozzle?
Increase exhaust gas velocity before discharge
Collecting and straightening the gas flow from the turbine
Explain the principles of a propelling nozzle
Turbine exit - diffuser - jet pipe (- afterburner) - nozzle.
- Jet pipe length decided by the engine position
- Area increase between turbine and jet nozzle reduces velocity and friction loss
- Afterburner may be installed in jet pipe if thrust booster is repuired
- Depending on location and if an afterburner is installed, some or all components may be compromised
Explain the principles of a convergent nozzle
The nozzle is >-shaped (wider inlet, smaller exit).
- Used if the nozzle pressure ratio is less than 2
- Common in low thrust engines for subsonic aircraft
Explain the principle of a convergent-divergent nozzle
Shaped ><. The smallest cross section area is called throat.
- Used if nozzle pressure ratio is high
- In supersonic aircraft, it is common to also have variable geometry and other aerodynamic features
Explain flow principles for nozzles
- Back pressure need to be lower than the pressure inside the nozzle (otherwise no flow)
- If back pressure is too low, nozzle becomes choked (exit speed is Mach 1, mass flow cannot increase anymore)
- In convergent-divergent nozzle: normal shock waves are formed downstream of the throat, if the speed at the throat is Mach 1
Explain the principles of a flexible nozzle exit area
- Nozzle are can change and automatically adapt to changes in pressure, temp, density and mass flow
- Typically used in fighters with afterburners and large flight envelope
- Mechanically complex and heavy
Explain the principles of thrust vectoring
- The nozzle can be moved up and down, left and right
- Enables shorter TO and landing distances
- Higher manoeuverability
- Helps stabilize aerodynamically unstable aircraft designs
