15.5 - Turbines Flashcards
What are the two types of turbine?
Radial flow
Axial flow
How many turbines can you have in a radial jet engine?
Only one single stage turbine.
What are the advantages of a radial turbine?
Simple in design
Easy to manufacture
What are the disadvantages of a radial turbine?
Only allows small air flows or low axial air velocity
Less efficient due to high aerodynamic losses
Short service life under high temperature loads.
Where would you find a radial turbine used?
APU
What are the advantages of an axial flow turbine?
Can be many stages, allowing drive of accessories and large fan.
Allows very high airflow (needed to create high thrust in modern engines)
How can the power output of an axial turbine be increased?
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.
How does the efficiency of a turbine increase with the blade speed?
Increases proportionally with the square of the blade speed.
What is the purpose of the turbine?
Converts the gas energy from combustion into torque.
What are the three types of turbine blade shape?
Impulse type
Reaction type
Impulse/reaction type
What is an impulse type turbine blade?
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
What is a reaction type turbine blade?
They rotate due to the lift they create as the gas stream flows over them.
Which law does a reaction turbine blade use?
Newton’s 3rd law.
For every action, is a equal and opposite reaction.
What type of turbine blade is most commonly used in a modern jet engine?
Impulse/Reaction type
In an impulse/reaction blade, which part has the greatest angle of incidence?
Greater at its tip than its base
What is the advantage of using an impulse/reaction turbine blade?
The gas flow enters the exhaust system with a uniform axial velocity.
What is the most common turbine blade fixing?
The fir tree fixing
How are the turbine blades retained in their grooves?
Rivets, lock tabs, lock wires and roll pins
Blades that have a normal metal crystallised form are called…?
Equiaxed crystal mixture
What is the problem with equiaxed crystal blades?
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
Which type of blade is very creep resistant (but not extremely creep resistant)?
Directional solidified structure blades
What is the advantage of a directional solidified structure over an equiaxed crystal mixture blade?
The blades have longer, slender crystals aligned in columns, which means they have fewer grain boundaries and are very creep-resistant.
What is the advantage of using a single crystal blade?
No boundaries, eliminating corrosion and extremely creep resistant blade.
What is creep?
A function of centrifugal force, material temperature and time. The blade stretches beyond its ability to reform its original length.
When would you find primary creep?
During engines first run
When would you find tertiary creep?
During operating overloads
What is a turbine’s service life calculated using?
Secondary creep. A strain/time graph over the life of the turbine.
What are nozzle guide vanes made from?
Nickel alloys
What does anodising a blade do?
It’s an electrolytic process which increases corrosion resistance and wear resistance on the turbine blades
What is the advantages of using turbine cooling?
Increases service life Increases efficiency (active clearance control)
What are the 3 types of turbine cooling methods?
Convection cooling
Impingement cooling
Film cooling
What is the most effective method of cooling turbines?
Film cooling
Maximum cooling effect with minimum cooling air.
Disadvantage, is precision made; therefore costly.
Where is it only necessary to cool in a turbine?
Nozzle guide vanes and HPT
What are the purpose of the nozzle guide vanes?
Gas flow is accelerated but static pressure decreases
What is 1 turbine stage classed as?
NGV and rotor
What is turbine creep caused by?
High RPM and high temperatures
Why are nozzle guide vanes fitted?
They increase the velocity of the gas
What are blue and golden deposits evidence of on a turbine blade?
Titanium
Which parts of an Impulse-Reaction blade are tip/root?
Tip - Reaction
Root - Impulse
