Axial Flow Turbines Questions and Answers

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the cause. In a turbine the flow accelerates through both rows, so the pressure gradient is favourable, the boundary layers stay thin and separation is unlikely. In a compressor the flow is decelerating against an adverse gradient, which thickens the boundary layers and risks separation. Turbine efficiencies of 90 to 92 per cent against compressor efficiencies of 85 to 90 follow directly, and so does the fact that one turbine stage can drive several compressor stages.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 describe the division of labour in a turbine stage. Statement 3 is impossible: the vanes are stationary, and no work can be transferred across a stationary surface. They convert pressure to velocity and set the direction, and every joule of shaft work comes out of the rotor.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the cooling air is bled from the compressor and routed around the combustor, and turbine entry temperatures now exceed the melting point of the blade alloy, which is only possible because the metal never sees the gas temperature. Statement 3 is wrong - the bleed air has been compressed but not heated in the combustor, and it dilutes and disturbs the gas path when it rejoins, so it carries a real cycle penalty of several per cent.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 give the two accumulation rules. Statement 3 is wrong and is precisely the point: creep depends on hours at temperature, so it scales with sector length, while thermal fatigue depends on the number of starts and is almost indifferent to it. The two therefore respond quite differently to an engine’s duty pattern, and that is why maintenance is tracked in both hours and cycles.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are the definitions: an impulse stage takes its whole static pressure drop in the vanes and none in the rotor, while a fifty per cent reaction stage splits it equally. Statement 3 is the reverse of the truth - the impulse stage extracts about twice the work but at lower efficiency, because the entire drop occurring in the vanes gives a very high rotor entry velocity and correspondingly large friction and secondary losses.

Answer: B

Only the second does work on the shaft, and the last two protect the blade rather than contributing to the thermodynamics. Each is present because something in the environment would otherwise limit what the stage can achieve.

Answer: C

The first and third accumulate per hour at temperature while the second accumulates per cycle. Which one binds first depends on how the engine is flown, which is why maintenance intervals are set on both hours and cycles.

Answer: D

The first three all spend compressor air while the fourth spends none, which is why coatings were such a valuable addition. In practice the methods are combined, since none alone gives the margin a modern entry temperature demands.

Answer: A

The first three describe the stage at a single radius, while the fourth describes how the design varies along the span. Together they account for both the aerodynamic character of the stage and the pronounced twist of its blades.

Answer: B

Correct answer: The nozzle guide vanes

Work is force times velocity at the surface doing it, and the vanes do not move, so they can extract nothing. Their function is to accelerate the gas and turn it into the plane of rotation. The high pressure, low pressure and free power turbine rotors all extract work, each driving a different shaft.