Lecture Notes on Gas Turbine Engine Exhaust System

Importance of Exhaust System

• Often underrated but significantly influences engine performance.
• Ensures gases exit in the correct direction and at optimum velocity for thrust.

Key Characteristics

• Turbojet Engines: Thrust provided by exhaust gases.
• Turboprop Engines: Thrust provided by propeller; outlet nozzle design critical.

Exhaust System Temperature

• Normal gas temperature: 550-850°C.
• With afterburner: can rise to 1,500°C.
• Aircraft fuselage protection: clearance and insulation needed.

Gas Velocity

• Range: 750-1250 feet per second (around 0.5 Mach).
• Long jet pipes can cause turbulence, decreasing efficiency and thrust.

Jet Pipe Design

• Divergent volume within convergent casing.
• Exhaust Cone: Reduces gas velocity and minimizes turbulence.
• Supports rear turbine bearing and straightens residual whirl.

Convergent Propelling Nozzle

• Increases gas velocity to Mach 1.
• Nozzle 'choked' at sonic speed.

Safety Considerations

• Dangerous zones around intake and exhaust.
• High gas velocity and temperature.
• Risk of foreign object ingestion.

Exhaust System in Bypass Engines

• More complex with two gas streams.
• Mixer Unit: Combines bypass air and hot exhaust gases.
• Integrated exhaust nozzle for efficiency.

Noise Considerations

• Major noise source: engine, particularly exhaust.
• Regulations necessitating noise reduction.
• Noise generated by compressor, fan, turbine, and exhaust.
• Noise reduction by decreasing exhaust jet velocity.

Noise Reduction Techniques

• Use of noise-absorbing materials in engine and bypass ducts.
• Weight increase and higher skin friction as disadvantages.

Legacy Noise Reduction Systems

• Older aircraft: corrugated internal mixers and lobe-type nozzles.
• Corrugated mixers reduce noise but may impact performance.

Conclusion

• Exhaust system design is critical to engine performance and noise management.