15.6 turbines

Question 1

What is the purpose of the gas generator turbine?

Answer 1

To absorb 60-70% of the total pressure energy from the exhaust gasses

Question 2

What does the turbine transform a portion of the exhaust gases kinetic energy into?

Answer 2

Mechanical energy for gas gen compressor and accessories

Question 3

Where is the turbine section in a gas turbine engine?

Answer 3

Behind the combustion chamber outlet

Question 4

What are the two types of turbines?

Answer 4

Radial
Axial

Question 5

What are the advantages of radial flow turbines?

Answer 5

Simple design
Easy to manufacture

Question 6

Disadvantages of radial flow turbines:

Answer 6

Only allow small airflows
Less efficient bc Hugh aerodynamic losses bc airflow must pass through the turbine opposing centrifugal forces

Question 7

Where are radial flow turbines used?

Answer 7

Small gas turbine engines e.g APU

Question 8

How many stages do radial turbines have?

Answer 8

One

Question 9

How many stages in an axial flow turbine?

Answer 9

Any amount necessary o

Question 10

Advantage of axial turbines:

Answer 10

Allow the very high airflow required to create high thrust

Question 11

What are the three types of turbine blades?

Answer 11

Impulse
Reaction
Reaction impulse

Question 12

What is the relationship between the area between the inlet and exit of impulse blades?

Answer 12

Area between inlet and exit is equal

Question 13

How does the impulse force act on a turbine wheel?

Answer 13

Not directly in the plane or rotation but resolved into two components

Question 14

What are the key concepts for impulse turbines?

Answer 14

Stator/ nozzle guide vanes: pressure drop
Rotor blades: velocity direction changes bc of its shape, reaction force of the pressure drop that makes the blades spin

Question 15

What are the key concepts of pure reaction turbines?

Answer 15

• stator/ fixed nozzle guide vanes: only change the direction
• converging blade accelerates the gas

Question 16

In a reaction type turbine, what type is the blade at the roots and tip?

Answer 16

Root: impulse
Tip: reaction type

Question 17

How are turbine blades cooled?

Answer 17

Heat barrier coatings e.g ceramic coating
Airflow cooling
Cast as a single crystak giving the blades better strength and heat properties

Question 18

How are turbine blades constructed?

Answer 18

Forged or cast
Precision cast and finish ground to desired shape

Question 19

What are the parts of a turbine rotor?

Answer 19

Shaft
Disc
Blades

Question 20

What does the turbine disc (wheel) do?

Answer 20

Transmits torque from blades to shaft

Question 21

How is the turbine disc connected to the shaft?

Answer 21

Bolted or welded

Question 22

What is the shaft made of?

Answer 22

High strength alloy stell

Question 23

What does the shaft do?

Answer 23

Transfers power to the compressor

Question 24

What are the stresses on a rotor?

Answer 24

Centrifugal loads due to high RPM
Thermal loads due to hot exhaust gases

Question 25

Is the discs rim or Center higher temp?

Answer 25

- rim bc of gas exposure and heat from the blades - Cooling air bleed often used to cool the disc face

Question 26

How is the blade root attached to the disc?

Answer 26

Disc rim has slots or groves for the blades Disc expands when hot for a tighter blade fit

Question 27

What is the most common and second most common disc root types?

Answer 27

1st Fir tree root Dovetail root (less common, more in compressors)

Question 28

What are the four types of blade retention methods?

Answer 28

Peening: notch in the blade root filled by punching in disc metal Welding Lock tabs bent or golfed in after installation Riveting Tang so the blade can only be inserted in one direction

Question 29

What’s the most common shaft to disc attachment?

Answer 29

Bolting

Question 30

How is the shaft to compressor coupled?

Answer 30

Centrifugal compressor: splined coupling Axial: splines or welded/ bolted

Question 31

What’s the three purposes of turbine inlet guide vanes?

Answer 31

1) Accelerate the hot gases from the combustion chamber by turning pressure + heat energy into high-velocity airflow. 2) Direct that flow at the correct angle onto the turbine blades to maximize energy extraction. 3) Withstand extreme temperatures – it’s the hottest metal-contact point in the engine.

Question 32

How do inlet guide vanes allow thermal expansion?

Answer 32

Loose fit in shrouds Sometimes the shrouds are segmented Extra support via inner and outer rings

Question 33

What is the d ensign of inlet guide vanes?

Answer 33

- inner and outer shrouds - fixed vanes between them that are angled and shaped like an aero foil to from multiple small nozzles

Question 34

What type of damage is allowable and not allowable on turbine blades?

Answer 34

Cracks are not acceptable Minor pitting from FOD may be blended with stoning/ polishing

Question 35

How are turbine discs and blades inspected?

Answer 35

Visually Dye penetrant and other NDT methods

Question 36

Where do you find stress rupture cracks?

Answer 36

- at right angles to blade edges - typically on leading or trailing edges - hairline cracks but may grow in size

Question 37

What does deformation from over temperature on turbine discs on blades look like?

Answer 37

- waviness on the leading edge in the first stage blades

Question 38

What is creep?

Answer 38

- long term molecular deformation due to high stress + high temp - causes blades to stretch and change shape which can lead to scoring on the turbine casing over time