Axial Flow Turbines Questions and Answers

Answer: C

Correct answer: 1.0 per cent

Both must be in the same units:

A rule of thumb is that each one per cent of clearance to span costs one to two per cent of stage efficiency, so this small gap matters - and it is why active clearance control, cooling the casing in cruise to shrink it onto the blades, is fitted to large engines.

Answer: D

Correct answer: 545 kJ/kg

Specific work is power over mass flow:

Which at a \( c_p \) of 1150 corresponds to a total temperature drop of about 475 K across the turbine - a figure typical of a high pressure turbine driving a large compressor.

Answer: A

Correct answer: Twice that of the reaction stage

At the same blade speed and flow coefficient, an impulse stage turns the flow through roughly twice the tangential angle of a fifty per cent reaction stage, and work is \( U\Delta C_w \), so it extracts about twice as much. The price is efficiency: the whole pressure drop occurs in the vanes, giving a very high velocity into the rotor and greater friction and secondary losses. Fewer stages, each less efficient, is the trade.

Answer: B

The metal must sit below its melting point while the gas around it is hotter still, which cooling and a thermal barrier coating together achieve. Running gas hotter than the alloy melts is normal practice and depends entirely on that margin.

Answer: C

Both statements are true and the second is the reason. An accelerating flow keeps its boundary layer attached through turning that would separate a diffusing one immediately, so each turbine stage does several times the work of a compressor stage.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. Once the throat is sonic, information cannot travel upstream, so the corrected flow is set by the throat area and the upstream conditions alone and is independent of everything downstream. The vanes therefore act as a fixed orifice for the whole engine, which is why the engine’s corrected flow is essentially constant over the operating range and why the throat area is the rematching parameter.

Answer: A

Both are true and the second is what makes the first possible. The margin between gas and metal is created entirely by the cooling scheme, which is why losing a coating or blocking a cooling passage is so immediately serious.

Answer: B

Both statements are true and the second is the reason. The work spent compressing that fifteen to twenty per cent of core flow is largely wasted, which is why reducing cooling flow is a standing objective in turbine development.

Answer: C

The first is correct and the second is not. Zero reaction gives the largest turning and therefore the most work, but the efficiency is noticeably lower, which is exactly the trade that decides between the two arrangements.

Answer: D

Both hold and the second is the mechanism. A blade at fifty per cent reaction at the mean radius may be close to impulse at the root and strongly reactive at the tip, which is why the section shape changes so much along its length.