15.6 Turbine Section

Question 1

what is the purpose of the turbines

Answer 1

to turn kinetic energy of the exhaust gasses into mechanical energy. also turns the fans, compressor and shafts

Question 2

what percentage of total pressure is absorbed by the turbine ?

Answer 2

60-70%

Question 3

what are the two different types of turbines used on gas turbine engines ?

Answer 3

• radial flow
• axial flow

Question 4

what are the vanes called at the entrance of the turbine

Answer 4

Nozzle guide vanes

Question 5

where are radial flow turbines used?

Answer 5

small gas turbine engines e.g APU

Question 6

what are some disadvantages of a radial flow turbine?

Answer 6

• only allows small airflows
• less efficient due to high aerodynamic losses

Question 7

where are axial flow turbines commonly used?

Answer 7

• modern gas turbine engines

Question 8

what is the benefit of using axial flow turbines ?

Answer 8

• allows very high airflow required to create high thrust for modern engines

Question 9

what does the turbine convert gas energy into ?

Answer 9

mechanical energy in the form of torque

Question 10

what happens to airflow velocity as it goes through the nozzle guide vanes ?

Answer 10

velocity increases

Question 11

what are the three clasifications of turbine blades ?

Answer 11

• impulse configuration
• reaction configuration
• reaction-impulse configuration

Question 12

what happens to total pressure and velocity in an impulse configuration?

Answer 12

total pressure decreases rapidly and velocity increases

Question 13

what happens to total pressure and velocity of airflow in a reaction configuration?

Answer 13

doesn’t change the pressure and changes direction of airflow

Question 14

why might a reaction impulse configuration be used ?

Answer 14

mixture of both impulse and reaction most efficient way of absobing energy in the turbine

Question 15

what does reaction impulse configuration require to obtain the maximum rate of momentum change?

Answer 15

high velocity gas

Question 16

What are most turbine blades made of?

Answer 16

precision cast or made of composite alloys

Question 17

what is the advantage of casting a single crystal?

Answer 17

better strength and heat properties

Question 18

what are some ways turbine blades can be protected from heat?

Answer 18

• ceramic coating
• single crystal casting
• airflow cooling

Question 19

what does the turbine assembly consist of ?

Answer 19

• stator ( nozzle and inlet guide vanes )
• turbine blades ( rotot )
• both connected via the turbine shaft

Question 20

where are inlet nozzle vanes located?

Answer 20

• directed aft of the combustion chambers and immediately forward of the turbine wheel

Question 21

why are inlet nozzle vanes contoured ?

Answer 21

to discharge gas at extremely high speed by converting heat energy into velocity

Question 22

other than increasing speed, what is the other job of the inlet nozzle vanes?

Answer 22

to deflect airflow to a specific angle

Question 23

how many shrouds are used in an inlet nozzle vane assembly ?

Answer 23

• inner and outer shrouds

Question 24

inlet nozzle vanes are exposed to extreme heat, what should be taken into consideration when designing the assembly?

Answer 24

• thermal expansion

Question 25

how is thermal expansion accounted for with inlet nozzle vanes ?

Answer 25

thermal expansion is absorbed by the loose assembly of the shrouds

Question 26

what does the support ring facilitate ?

Answer 26

-keeps the vanes in place, otherwise they would fall out
- increases support and rigidity

Question 27

how else could thermal expansion be accounted for in the shrouds?

Answer 27

vanes are welded/ riveted and the shrouds will have seperations in them to allow sufficient space for thermal expansion

Question 28

what sort of cracks or damage are premitted on a blade?

Answer 28

none

Question 29

how is crack detection carried out?

Answer 29

usually visual, however other methods such as dye penetrants can be used

Question 30

how can slight pitting due to FOD be corrected ?

Answer 30

blended by stoning or polishing

Question 31

what tools are used to inspect turbine blades for stress rupture cracks and deformation?

Answer 31

strong light and a magnifying glass

Question 32

how do stress rupture cracks appear on a turbine blade ?

Answer 32

minute hairline cracks

Question 33

how may deformation due to heat damage appear?

Answer 33

waviness along the leading edge

Question 34

what else must be suspected if a stress crack or deformation is found ?

Answer 34

an over temperature condition

Question 35

what is creep?

Answer 35

a slow structural deformation caused by prolonged exposure to high stress

Question 36

what promotes creep?

Answer 36

extreme temperatures in the turbine area

Question 37

what must be inspected once creep is identified ?

Answer 37

turbine disc for hardness and stretch

Question 38

Why are there operational limits for turbines

Answer 38

Due to the centrifugal force and temperature

Question 39

What is the difference between a turbine disc and a turbine wheel

Answer 39

A turbine wheel has blades installed onto the disc

Question 40

How do you relieve temperature stresses on the disc

Answer 40

Been cooling air back onto the face of the disc

Question 41

How are blades installed onto discs

Answer 41

In grooves or notches

Question 42

What are the two types of disc retention

Answer 42

Bulb and fir tree

Question 43

What are the two ways of holding discs in place

Answer 43

Bolting and welding

Question 44

How are blades held in place in centrifugal compressors

Answer 44

With splined couplings