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
