15.5 - Turbines

Question 1

What are the two types of turbine?

Answer 1

Radial flow

Axial flow

Question 2

How many turbines can you have in a radial jet engine?

Answer 2

Only one single stage turbine.

Question 3

What are the advantages of a radial turbine?

Answer 3

Simple in design

Easy to manufacture

Question 4

What are the disadvantages of a radial turbine?

Answer 4

Only allows small air flows or low axial air velocity
Less efficient due to high aerodynamic losses
Short service life under high temperature loads.

Question 5

Where would you find a radial turbine used?

Answer 5

APU

Question 6

What are the advantages of an axial flow turbine?

Answer 6

Can be many stages, allowing drive of accessories and large fan.
Allows very high airflow (needed to create high thrust in modern engines)

Question 7

How can the power output of an axial turbine be increased?

Answer 7

Increasing its diameter - however this increases drag since the engine is bigger and also greater stresses imposed due to higher centrifugal forces created.

Therefore, multiple stages are used with a smaller turbine diameter.

Question 8

How does the efficiency of a turbine increase with the blade speed?

Answer 8

Increases proportionally with the square of the blade speed.

Question 9

What is the purpose of the turbine?

Answer 9

Converts the gas energy from combustion into torque.

Question 10

What are the three types of turbine blade shape?

Answer 10

Impulse type
Reaction type
Impulse/reaction type

Question 11

What is an impulse type turbine blade?

Answer 11

They make their rotation only by the impact of gas flow. (Like a water wheel).
Rarely seen, used on older jet engines but may be found in air starters.
F=ma relationship

Question 12

What is a reaction type turbine blade?

Answer 12

They rotate due to the lift they create as the gas stream flows over them.

Question 13

Which law does a reaction turbine blade use?

Answer 13

Newton’s 3rd law.

For every action, is a equal and opposite reaction.

Question 14

What type of turbine blade is most commonly used in a modern jet engine?

Answer 14

Impulse/Reaction type

Question 15

In an impulse/reaction blade, which part has the greatest angle of incidence?

Answer 15

Greater at its tip than its base

Question 16

What is the advantage of using an impulse/reaction turbine blade?

Answer 16

The gas flow enters the exhaust system with a uniform axial velocity.

Question 17

What is the most common turbine blade fixing?

Answer 17

The fir tree fixing

Question 18

How are the turbine blades retained in their grooves?

Answer 18

Rivets, lock tabs, lock wires and roll pins

Question 19

Blades that have a normal metal crystallised form are called…?

Answer 19

Equiaxed crystal mixture

Question 20

What is the problem with equiaxed crystal blades?

Answer 20

The boundaries of the grain or crystal in the material is usually the starting point of any failure. It requires the blades to be internally cooled

Question 21

Which type of blade is very creep resistant (but not extremely creep resistant)?

Answer 21

Directional solidified structure blades

Question 22

What is the advantage of a directional solidified structure over an equiaxed crystal mixture blade?

Answer 22

The blades have longer, slender crystals aligned in columns, which means they have fewer grain boundaries and are very creep-resistant.

Question 23

What is the advantage of using a single crystal blade?

Answer 23

No boundaries, eliminating corrosion and extremely creep resistant blade.

Question 24

What is creep?

Answer 24

A function of centrifugal force, material temperature and time. The blade stretches beyond its ability to reform its original length.

Question 25

When would you find primary creep?

Answer 25

During engines first run

Question 26

When would you find tertiary creep?

Answer 26

During operating overloads

Question 27

What is a turbine’s service life calculated using?

Answer 27

Secondary creep. A strain/time graph over the life of the turbine.

Question 28

What are nozzle guide vanes made from?

Answer 28

Nickel alloys

Question 29

What does anodising a blade do?

Answer 29

It’s an electrolytic process which increases corrosion resistance and wear resistance on the turbine blades

Question 30

What is the advantages of using turbine cooling?

Answer 30

Increases service life
Increases efficiency (active clearance control)

Question 31

What are the 3 types of turbine cooling methods?

Answer 31

Convection cooling
Impingement cooling
Film cooling

Question 32

What is the most effective method of cooling turbines?

Answer 32

Film cooling
Maximum cooling effect with minimum cooling air.

Disadvantage, is precision made; therefore costly.

Question 33

Where is it only necessary to cool in a turbine?

Answer 33

Nozzle guide vanes and HPT

Question 34

What are the purpose of the nozzle guide vanes?

Answer 34

Gas flow is accelerated but static pressure decreases

Question 35

What is 1 turbine stage classed as?

Answer 35

NGV and rotor

Question 36

What is turbine creep caused by?

Answer 36

High RPM and high temperatures

Question 37

Why are nozzle guide vanes fitted?

Answer 37

They increase the velocity of the gas

Question 38

What are blue and golden deposits evidence of on a turbine blade?

Answer 38

Titanium

Question 39

Which parts of an Impulse-Reaction blade are tip/root?

Answer 39

Tip - Reaction

Root - Impulse