Construction (turbine) Flashcards
What is the purpose of a turbine?
The purpose of the turbine is to extract sufficient energy from the gas stream to drive the compressor and accessories, and to provide shaft power for a propeller or rotor in certain engines.
How does a turbine produce necessary driving torque?
A turbine produces necessary driving torque by extracting energy from hot gases released from the combustion chamber, expanding them to a lower pressure and temperature, and converting gas energy into mechanical energy.
What is the maximum rotation speed of turbine blade tips?
The turbine blade tips may rotate up to 1500 ft/sec.
What is the range of Turbine Entry Temperature (TET)?
The Turbine Entry Temperature (TET) could be between 700 and 1700°C.
What are the three main parts of an engine turbine stage?
The three main parts of an engine turbine stage are:
1. Nozzle guide vanes (NGVs)
2. Rotating turbine disc
3. Turbine blades
What is the shape and function of nozzle guide vanes?
The nozzle guide vanes are of an aerofoil shape, forming a convergent duct between adjacent vanes, and are usually hollow for cooling by compressor delivery pressure.
What materials are turbine discs typically made from?
Turbine discs are usually manufactured from nickel-based alloys and are machine forged with an integral shaft or a flange for shaft attachment.
How is heat conduction limited between turbine blades and the disc?
A flow of cooling air is passed across both faces of the turbine disc to limit the effect of heat conduction from the turbine blades to the disc.
What sections do turbine blades consist of?
Turbine blades consist of a combination of impulse (50%) and reaction (50%) sections, blended together with the impulse section at the root and reaction at the tip.
What drives the turbine in a gas turbine engine?
The initial impact of the gas on the impulse section, coupled with the resulting acceleration of the gas through convergent passages providing a reaction force.
What are the two types of turbines based on gas flow?
- Turbine driven by the impulse of the gas flow only.
- Turbine driven by the impulse of the gas flow and its subsequent reaction as it accelerates through the converging blade passage.
What is the purpose of the stagger angle in turbine blades?
To ensure that the gases flowing from the combustion system do equal work at all positions along the length of the blade and to ensure that the flow enters the exhaust system with a uniform axial velocity.
Where is the stagger angle greater in turbine blades?
The stagger angle is greater at the tip than at the root.
What is the importance of attaching blades to the turbine disc?
The method of attaching the blades to the turbine disc is important because the stress in the disc around the fixing or in the blade root affects the limiting rim speed.
What is a Fir Tree Root in turbine blade attachment?
The Fir Tree Root involves very accurate machining to ensure that the loading is shared by all the serrations. The blade is free in the serrations when the turbine is stationary and is stiffened in the root by centrifugal force when the turbine is rotating.
What is a BLISK?
A BLISK is a method of fixing where the blades are permanently attached to the disk, either by machining the whole component from a solid forging or by friction welding the blade segments to the disk.
What is the purpose of a shroud in turbine blades?
A shroud is fitted to reduce the loss of efficiency through gas leakage across the blade tips, which is made up of a small segment at the tip of each blade.
What must turbine blades be resistant to?
Turbine blades must be designed to be resistant to fatigue,
thermal shock,
corrosion,
oxidization,
high temperature, centrifugal force,
high rotational speeds,
high frequency gas flows.
What is turbine blade creep?
Creep is the phenomenon where turbine blades slowly grow in length over time due to operational stress, especially under high temperatures and centrifugal loads.
A small turbine blade weighing only 2oz may exert a load of over two tons at maximum speed.
What factors contribute to the elongation of turbine blades?
The elongation of turbine blades in normal service use is due to three factors: centrifugal force, high temperature, and running hours.
During which time periods is creep likely to occur?
Creep is likely to occur during three time periods: primary (new engine during initial runs), secondary (normal service use), and tertiary (excess temperature and running hours).
What advancements have been made in turbine blade development?
Developments have been made to prolong turbine blade life and protect them from heat damage, such as moving from plain castings to directionally solidified blades.
What is the benefit of directionally solidified blades?
Directionally solidified blades are cooled during casting to form metal grains in one direction, reducing grain boundaries and potential weaknesses.
What technique is used in current designs to improve temperature and creep resistance?
A technique designed by Rolls Royce encourages the metal to grow into one single crystal during casting.
What materials are metal blades made from?
Metal blades are made from Nickel based alloys.
What is the current leader in heat resistance for engine components?
The single crystal design is the current leader in heat resistance.
What materials are being developed for high temperature areas of engines?
Ceramics are being developed for use in high temperature areas of engines.
Why must the flow characteristics of a turbine be matched with those of the compressor?
To avoid conditions that cause compressor surge or choke, which lead to a loss of efficiency.
What happens if the NGV’s allow too low a gas flow?
A back pressure would build up causing the compressor to surge.
What happens if the gas flow is too high?
The compressor would choke, leading to a loss of efficiency.
What changes through the turbine due to duct shape and rotational force?
Pressure, temperature, and velocity all change through the turbine.
What is the effect on P.T.V through convergent ducts?
P decreases, T decreases, and V increases.
What is the effect on P.T.V through the turbine blades?
P decreases, T decreases, and V decreases.
Why does velocity decrease through the turbine blades?
Velocity decreases due to the convergent shape opposing gas laws and energy being absorbed by the engine.
What is the first maintenance activity typically carried out on turbines?
Turbine Tip Clearance check (Creep)
What is the second maintenance activity typically carried out on turbines?
Bearing Wear/Failure
What is the third maintenance activity typically carried out on turbines?
Run Down Check. Run Down time must be within limits (Timed by the pilot)
What is the fourth maintenance activity typically carried out on turbines?
Overheating Check. Look for signs of burnt blades
What components are checked for FOD Damage?
FOD Damage Check on:
(1) Compressor
(2) Turbine
(3) Exhaust
What is the sixth maintenance activity typically carried out on turbines?
Corrosion/Erosion. Prevented by compressor washing.
(KLP 37.02.04)
