Axial Flow Turbines Questions and Answers

Answer: C

Correct answer: Fatigue damage accumulates per cycle

Reading the third column, creep, oxidation and hot corrosion all accumulate with time at temperature, while thermal fatigue accumulates with each start and shutdown, since it is the temperature gradient during those transients that strains the blade. An engine making many short sectors therefore accrues fatigue damage far faster relative to its flying hours, and a cycle-based maintenance interval tracks the damage that actually governs.

Answer: D

A quarter of the stage loss from a gap of well under a millimetre is a large return for cooling the casing in cruise. The measure attacks the loss directly rather than working around it, which is why it became standard.

Answer: A

A root running near impulse and a tip running strongly reactive need quite different profiles and stagger angles. The pronounced twist and taper of a turbine blade is the direct physical expression of that requirement.

Answer: B

Both statements are true and the second is the reason. Each turbine stage extracts several times what a compressor stage absorbs, which is why three or four turbine stages routinely drive a dozen or more compressor stages.

Answer: C

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

Both are true and the reason gives the mechanism. Because the throat is choked, its area alone fixes the corrected mass flow the engine will pass, and nothing downstream can alter it. Opening the throat lets the engine swallow more, which lowers the compressor working line and increases the surge margin; closing it does the reverse. A small change of area is therefore a powerful and precise rematching tool.

Answer: D

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

Both are true and the reason supplies the urgency. A thermal barrier coating may hold the metal 100 kelvin or more below the gas, and creep life roughly halves for every 15 kelvin the metal temperature rises. Losing the coating over part of the aerofoil therefore does not shorten the life a little - it can reduce it by an order of magnitude, which is why coating loss found at inspection is acted on at once.

Answer: A

Both statements are true and the second is the reason. At fifteen to twenty per cent of core flow the penalty is substantial, which is why beyond a point extra cooling cancels the gain the higher entry temperature was meant to give.

Answer: B

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

Both are true and the reason explains the twist. In a free vortex design the tangential velocity varies as \( 1/r \), so the degree of reaction rises from root to tip - a blade may be nearly impulse at the root and highly reactive at the tip. The velocity triangles therefore differ at every radius, and the section must be twisted and reshaped along the span to suit each one.

Answer: C

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

Both are true and the reason gives the justification. Since thermal fatigue damage is done by the start-up and shutdown transients rather than by the cruise, a thirty minute sector damages the blade almost as much as a ten hour one. Counting hours would therefore badly understate the damage on a short-haul engine, and the maintenance interval is set in cycles instead.

Answer: D

Correct answer: A is false but R is true

The reason is true - a shroud does reduce tip leakage and damp vibration by tying the blades together at the tip - but the assertion is false. The shroud is extra mass at the largest radius, so it adds substantially to the centrifugal load, and on the hot, highly stressed high pressure stage that penalty is unacceptable. Shrouds are therefore common on low pressure turbine blades and rare on high pressure ones.