Turbine Engine Turbine Section

Question 1

State the purpose and operation of the turbine section

Answer 1

When gas leaves combustion chamber they have a high pressure and temp. And the turbine’s job is to efficiently extract that energy from the gasses.

Uses this energy to turn the compressor and the engine driven accessories.

Question 2

Describe how a turbine blade extracts energy from the gas stream and drives the wheel/disc.

Answer 2

The turbine uses the expansion of the gas flow as it moves in the direction of decreasing pressure. As the turbine is connected to the compressor when turbine moves it moves too.

Question 3

What are the two types of turbines.

Answer 3

Axial flow turbine - turbine that’s turned by hot gasses from the combustion chamber and which is physically connected to a compressor stage.

Free power - type of axial flow turbine where the turbine connected to the drive shaft is not connected to a compressor stage. ( turboprop aircraft have engine that can start but prop wont turn known as a free power turbine engine)

Question 4

Describe the casing component

Answer 4

Part of the turbine that enclose the turbine stators and rotor.

Front end connects the combustion chamber discharge area, while rear connects to the exhaust nozzle.

Question 5

Describe nozzle guide vanes. (3)

Answer 5

Vanes designed to present the air to the first turbine wheel at the most efficient AoA for max engine efficiency, they also allow expansion and acceleration of the hot gasses.

Must withstand higher gas temps than turbine blade and are hollow so that they can be cooled by passing HP air.

Manufactured with shrouds fitted top and Botton of vane which form a smooth passage for gas flow and avoid leakage of air around the tips of the vane.

Question 6

Describe a turbine wheel/disc/rotor

Answer 6

Rotating part of the turbine. Most engine typically need more then one wheel to absorb all of the energy from hot gasses leaving combustion.

So most engine have multiple wheels which each increase in diameter and blade size as they near the exhaust nozzle.

Question 7

Describe a nozzle

Answer 7

Primary functions to generate thrust and to act as a control valve for the engine.

Question 8

Describe turbine blade manafacturing

Answer 8

Must have high fatigue strength, good creep resistance qualities (creep is tendency of blade to elongate), resistance to thermal shock and resistance to corrosion.

Question 9

What is the connection to the turbine wheel (fir tree root)

Answer 9

A fir tree shaped connection at the root of the blade. This type of fixing holds the blade loosely when the turbine is stationary and provides a firm fitting by centrifugal force when turbine is rotating.

Question 10

What is BLISK

Answer 10

Blade and disk, another method of securing the blade to the disk, by bonding the blade to disk to produce a single unit.

This reduced number of components while eliminating a source for cracks due to the fir tree root attachment. 8% more efficient.

Question 11

Describe impulse blades and when they’re used

Answer 11

Like a waterwheel.

A pressure drop that occurs in the convergent NGV (nozzle guide vanes) passages which means corresponding increase in velocity.

This high velocity gas is directed at the rotor blades where the passages are constant in area and so theres no further pressure drop, Only the direction of airflow is changed, producing an impulse (sudden force) on the blade causing it to rotate.

Used for starter turbines and APU’s.

Question 12

Describe reaction blades

Answer 12

The entire pressure drop takes place between the rotor blades which have convergent passages in the direction of flow. NGV only guide the airflow onto the rotors.

Turbine is driven round by reaction force resulting from the acceleration of the gas through the converging passage between the rotors.

Both direction and magnitude of the gas velocity are changed.

Question 13

Describe impulse-reaction blades.

Answer 13

Comprise between both blades. Where blade root is largely impulse balding and blade tip is mainly reaction blading.

Question 14

What is the most popular blade and why

Answer 14

Impulse-reaction

As the blade tip speed is faster than the root speed, the most efficient type of blade type is used along the length of each blade. This is important so that the work done by the blade (and the forces on it) are equal along the entire length of the blade.

Question 15

Identify factors which limit the power available from the turbine section. (4)

Answer 15

Turbine entry temperature - gasses leaving the combustion can be extremely hot and on leaving the combustion they immediately meet the NGV followed by the first stage of HP turbine. NGV are cooled by air from compressor

Metal fatigue - of the blades due to heat and successive heating-cooling cycle that an engine goes through on every start and stop. Meaning they’re limited by number of cycles.

Corrosion and oxidation - caused by combustion gasses containing corrosive elements, such as lead, sodium and potassium. These coat the turbine blades and eat away at the blade and NGV’s.

Abrasion - occurs due to particles striking the blade and wearing parts of it away.

Question 16

What is turbine blade creep and its 3 stages.

Answer 16

Where the individual turbine blades become longer over time due to centrifugal forces and heat involved. Eventually blades will begin to damage the casing.

Primary creep, when creep rises rapidly over a brief period and then slowly decreases.

Secondary creep, where blades creep at a relatively small and steady rate over a longer period of time.

Tertiary creep, where the rate is accelerated until the material breaks. This is towards the end of the blade’s life.

Question 17

State reasons for compressor-turbine matching why and how its achieved.

Answer 17

to obtain the maximum efficiency and performance of the engine. The turbine must be able to absorb all of the power given to it.

If there was too low a maximum gas flow, then a backpressure would build up causing the compressor to surge. If too high a gas flow, this would cause the compressor to choke. Both conditions would cause a loss of efficiency and a reduction in thrust.

Careful design of the turbine blades, selecting the correct number of turbine stages and the use of multi-spool engines, allow manufacturers to match turbine gas flow to compressor flow.

Question 18

Describe multi stage turbines.

Answer 18

Turbine blades have to extract sufficient work from the gas flow to drive compressor in as few stages as possible.

HP turbine driving HP compressor spool can be single stage as it receives gas of high energy but by the time the gases reach the IP or LP, more blade area is needed if a proper work balance is to be maintained. Done by a multi-stage turbine with increasingly larger blades.

Question 19

State why turbine assemblies increase in diameter towards the rear of the engine.

Answer 19

To extract the maximum amount of energy from the gas flow