Axial Flow Turbines Questions and Answers

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.

Answer: C

Correct answer: False, the stationary vanes come first in a turbine

The two machines are ordered oppositely. A compressor stage begins with the rotor, which adds energy, and follows it with a stator that recovers pressure from the swirl. A turbine stage begins with the stationary vanes, which accelerate the gas and set its direction, and follows them with the rotor, which extracts the work. The reason is the direction of the pressure gradient in each machine.

Answer: D

Correct answer: It imposes no penalty because it is reheated in the combustor

This is the false statement. The cooling air is bled from the compressor and routed around the combustor, so it has been compressed at full cost but never receives the heat addition that would make it useful work. It then dilutes and disturbs the main gas path when it rejoins. The penalty is real and amounts to several per cent of cycle efficiency, which is why cooling air is used as sparingly as the metal temperature allows.

Answer: A

Correct answer: No, it extracts more work but at lower efficiency

The impulse stage takes its whole pressure drop in the vanes, giving a high rotor entry velocity and roughly twice the work of a fifty per cent reaction stage at the same blade speed - but that high velocity brings larger friction and secondary losses, so the efficiency is lower. Fewer, harder-working, less efficient stages against more, gentler, more efficient ones is the trade, and modern practice generally favours reaction.

Answer: B

Correct answer: Reducing the blade speed

Work per stage is \( U\Delta C_w \), so reducing the blade speed reduces it directly. The other three all increase it: a higher blade speed raises \( U \), greater tangential turning raises \( \Delta C_w \), and moving towards impulse staging raises \( \Delta C_w \) for a given blade speed by taking the whole pressure drop in the vanes.

Answer: C

A choked throat fixes the corrected mass flow independently of downstream conditions, which gives the engine a stable and predictable flow capacity. Adjusting that throat area is a standard way of rematching an engine during development.

Answer: D

Correct answer: Nozzle throat choking

Choking of the first stage throat is a designed operating condition, not damage - it is what fixes the engine’s corrected mass flow. Creep, thermal fatigue and hot corrosion are the genuine life limiters, and blades are removed on whichever of them accumulates first, tracked in hours at temperature for the first and third and in cycles for the second.

Answer: A

Correct answer: No, it is usually higher

Turbine efficiencies are typically 90 to 92 per cent against 85 to 90 for a compressor, because the flow accelerates through a turbine and the favourable pressure gradient keeps the boundary layers thin. The compressor must decelerate the flow against an adverse gradient, which risks separation and limits how much turning each stage can take. Cooling air does cost the turbine something, but not enough to reverse the comparison.

Answer: B

Correct answer: An improvement in stage efficiency

Greater tip clearance always costs efficiency. Gas escapes over the tip doing no work, and the leakage jet emerging on the suction side disturbs the passage flow and thickens the casing boundary layer, so the penalty exceeds the leaked mass alone. The other three all follow from opening the gap, and closing it in cruise is exactly what active clearance control is for.

Answer: C

Removing grain boundaries removes the sites where creep damage accumulates, raising the usable metal temperature substantially. The casting process is slow and costly, and the temperature gain is what repays that expense.