Turbine Engine Section

Question 1

Purpose and operation of turbine

Answer 1

• Turn the compressor
• Powers generators, hydraulic pumps etc

Question 2

Function of casing and associated structures

Answer 2

• Constrains and directs the airflow.

Question 3

Function of turbine wheel/disc

Answer 3

• Absorbs the energy from the airstream and convert it into useful rotational motion

Question 4

Function of the shaft

Answer 4

• Connects the turbine wheel through the centre of the engine to the corresponding compressor stages

Question 5

Function of nozzle guide vanes

Answer 5

• PLaced after the combustion chamber before the first set of turbine blades
• Present airflow

Question 6

Function of the blades

Answer 6

• Connected tot he wheels/discs and come in several different types.
• Absorb energy

Question 7

Impulse

Answer 7

• Gas passes through a convergent area in the NVG, increasing in velocity and reducing pressure
• gas enters the constant cross sectional area of the turbine blades
• gas changes direction without icy change to velocity and pressure
• Change in direction is what causes the impulse force

Question 8

Reaction

Answer 8

• Act in the opposite way
• Cross sectional area between the NGVs remain constant - results to only small changes in pressure and velocity
• Results in a. Change in direction to the gas only
• Reduction in pressure and increase in velocity from the convergent nature of the rotating turbines

Question 9

Impulse reaction

Answer 9

• Typically a combination of the two types of blades are used
• Blade root - mostly impulse due to the low rotational velocity
• Blade tip - Mostly reaction due to the high rotational velocity
• AKA blade twist
• Equal amounts of work done along the blade

Question 10

Most common type of turbine blade design

Answer 10

• Impulse- reaction = best efficiency

Question 11

Factors that limit the power available from the turbine section

Answer 11

• ## Thermodynamic and mechanical losses result in 90% efficiency

Question 12

Multi-stage turbines

Answer 12

• Use multiple stages of turbine blades to power each separate compressor stage
• Allows them to operate at their own discreet speeds

Question 13

Why do turbine assemblies increase in diameter towards the rear of the engine

Answer 13

• As the gas stream will have energy extracted by each successive turbine stages the turbine wheel diameters and turbine blade length ewill have to increase in order to extract roughly the same amount of energy from the weakened gas stream

Question 14

Turbine blade creep and state the causal factors for this condition

Answer 14

• Overtime turbine will creep.
• Primary creep during manufacturing
• Need to avoid tertiary creep
• Defined engine hours
• Within defined temperatures and rotational limits