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Engines 7815 > Engine Turbines > Flashcards

Engine Turbines Flashcards

1
Q

How does a turbine differ from a compressor?

A

The expanding gas is moving in a direction of decreasing pressure.

As pressure increases in the compressor, the direction of gas flow wants to take the easiest route out which is back on itself unlike the turbine stage where it’s energy is so high, it wants to propel itself out its natural gas flow direction.

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2
Q

What does a turbine stage consist of?

A

Nozzle Guide Vanes followed by a row of rotating turbine blades

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3
Q

What are the turbine blades attached to?

A

A turbine disk which is then attached to a drive shaft

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4
Q

As the gas flow moves rearwards from the HP turbine to the LP turbine in a multi-spool, explain what is happening to the NGVs

A

The NGV stages gradually get bigger to account for the correct amount of energy to be extracted due to the loss of energy in the gas stream

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5
Q

What are the number of turbine stages dependent on?

A
  1. Amount of power the turbine has to produce
  2. Number of compressors driven
  3. Largest turbine diameter the engine design allows
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6
Q

What are the sub assemblies of a single stage turbine?

A
  1. NGVs
  2. Turbine blades
  3. Turbine disk
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7
Q

Describe the basic construction and operation of NGVs

A

Convergent duct to allow an increase in kinetic energy in the gas stream by converting thermal and pressure energy

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8
Q

State the function of a shroud on a NGV

A

To ensure rigidity, create a smooth passage for the gas flow and prevent tip leakage

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9
Q

What is the purpose of a turbine blade?

A

To extract energy from the gas flow to drive the compressor, accessories and/or high speed output shaft

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10
Q

What shape is a turbine blade?

A

Aerofoil shaped

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11
Q

What are the 3 types of turbine blades?

A

Impulse
Reaction
Impulse-Reaction

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12
Q

Where are pure impulse blades found?

A

Used in the air starter and APUs

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13
Q

Why are reaction blades not used?

A

They are inefficient

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14
Q

Because the impulse-reaction blades are 50-50, what shape does it take?

A

A twisted aerofoil shape from root to tip

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15
Q

What is the twisted profile known as on a turbine blade?

A

Stagger angle

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16
Q

What does a stagger angle ensure?

A

Ensures that the gas flow does equal work at all positions along the length of the blade

Ensures it enters the exhaust system with a uniform axial velocity

17
Q

How are turbine blades attached to the turbine disk?

A

Fir tree root method

18
Q

What must the turbine disk be capable of withstanding?

A

High centrifugal loads

High thermal loads

19
Q

How does the turbine disk cope with the high loads?

A

Wider at the root and tapers towards the rim to carry the bade root fixings and air seal

20
Q

What type of turbine blades are used for smaller GTE to reduce the centrifugal loading imposed on the disk?

A

A BLISK

21
Q

What is required in design to account for a temporary increase in length during operation?

A

A clearance gap between the turbine blade tip and the casing

22
Q

What is permanent increase in length of the turbine blade known as?

A

Blade creep

23
Q

What is creep defined as?

A

The continuous extension of a component under loads, even though the load is less than that which would cause fracture

24
Q

What is the degree of creep dependent on?

A

Operating temperature
Higher the temp, higher the creep

25
Q

What are the 2 ways to cool NGVs and turbine blades using compressor bled air?

A

Convection Cooling
Film Cooling

26
Q

How is Convection cooling achieved?

A

Passing compressor bled air through holes or hollow sections within the blades or NGVs

27
Q

How is Film cooling achieved?

A

Passing compressor bled air over the surface of the blades and NGVs which protects them from the heat of the gas flow.

28
Q

State a disadvantage of film cooling and how designers rectify it

A

Expensive to develop and produce

Designers concentrate on providing cooling to the portions of NGVs and blades that most require them

29
Q

How is the rear face of the turbine disk protected?

A

By an exhaust cone or sheet metal cover.
COmpressor bled air is directed onto the exhaust cone or beneath the sheet metal cover for cooling purposes

Engines 7815 (5 decks)

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