Axial Flow Turbines Questions and Answers

Answer: A

Blade speed rises with radius while the work is held uniform along the span, so the reaction must climb outward. The large change in section shape from root to tip is the visible consequence of exactly that requirement.

Answer: B

The ceramic layer is worth a hundred degrees or more of metal temperature, and creep life halves for every fifteen. Losing it converts a comfortable design margin into an immediate life limitation even with cooling working perfectly.

Answer: C

Every kilogram of cooling air has absorbed compressor work and then bypassed the combustor, so it returns little. Beyond a point the cycle loss from the extra bleed cancels the gain the higher entry temperature was meant to deliver.

Answer: D

Correct answer: nozzle guide vanes

A turbine stage begins with the stationary row, in contrast with a compressor stage, which begins with the rotor. The vanes accelerate the gas, converting pressure into velocity, and turn it into the plane of rotation so that it meets the rotor at the correct relative angle. The rotor then extracts the work by turning the flow back towards axial.

Answer: A

With no pressure change across the rotor, the relative velocity is unchanged in magnitude and the blades merely turn the flow. Zero reaction gives the most work per stage at a noticeable cost in efficiency compared with a reaction design.

Answer: B

Correct answer: choked

The first stage nozzle guide vanes are sized so that their throat is sonic over almost the whole operating range. A choked throat passes a fixed corrected mass flow regardless of downstream conditions, so the vanes act as the engine’s flow-metering orifice - and altering their throat area is the standard way of rematching an engine that runs too close to surge or too far from it.

Answer: C

The rate depends steeply on both temperature and stress, halving the life for roughly every fifteen kelvin of metal temperature. That sensitivity is why cooling effectiveness and thermal barrier coatings are worth so much.

Answer: D

Grain boundaries are where creep damage accumulates at high temperature, so eliminating them raises the usable metal temperature substantially. The casting process is slow and costly, which the resulting performance gain repays.

Answer: A

An accelerating flow keeps its boundary layer attached through turning that would separate a diffusing one. Everything else in the table follows from that single difference, including why a few turbine stages drive many compressor stages.

Answer: B

A margin of several hundred kelvin lets the gas exceed the alloy's melting point while the metal stays safely below it. The whole modern gas turbine cycle rests on that margin, which is why losing a coating is immediately serious.