15.12 - Air Systems

Question 1

How is the internal air system of a gas turbine engine defined?

Answer 1

The engine internal air system is defined as those airflows which do not directly contribute to the engine thrust.

Question 2

List four functions of the internal air system:

Answer 2

• Internal cooling
• Blade-tip clearance control
• Nacelle cooling
• Customer Air supply (cabin pressurization, cabin heat, airframe anti-icing)
• Engine Anti-ice

Question 3

In relation to turbine blades, what is the difference between ‘convection’ and ‘film’ cooling?

Answer 3

Convection cooling involves the compressor discharge air making two or three passes within the cooling passages of the blade before being discharged through the blade tip or holes in the trailing edge.

Film or Impingement cooling allows the internal air to escape at the leading edge of the blade through a row of small holes forming a cool heat shield boundary layer across the faces of the blade.

Question 4

State the purpose of compressor and turbine blade tip clearance control.

Answer 4

To minimize tip losses and increase engine fuel efficiency. This is done by an ACC (Active Clearance Control) system which blows either cold fan discharge air or mixed air coming from the fan and compressor to the turbine case to either expand or contract the case material to keep minimal blade tip clearance through all phases of engine RPM.

Question 5

What are internal air systems used for?

Answer 5

The engine internal air system is defined as those airflows which do not directly contribute to the engine thrust.

• Used for engine cooling

Question 6

What air is normally used to cool nozzle guide vanes?

Answer 6

Compressor air

(Internal cooling system)

Question 7

Where is film cooling used?

Answer 7

Turbine Blades

Question 8

What method is normally used to cool engine accessories?

Answer 8

Fan air (bypass air)

Question 9

What air is normally used to cool turbine discs?

Answer 9

Compressor air

Question 10

Where is bleed air tapped from?

Answer 10

Compressor

Question 11

How is turbine tip clearance maintained?

Answer 11

AAC = Active Clearance Control

This is done by either blowing a stream of either cold fan discharge air or mixed air coming from the fan and compressor to the turbine case.

Cold air contracts casing, mixed hot air expands it minimising tip clearance