Axial Flow Turbines Questions and Answers

Answer: C

The ejected air hugs the surface and insulates the metal from the mainstream gas. It is far more effective than internal convection alone but disturbs the external flow, so each row of holes costs aerodynamic efficiency as well as air.

Answer: D

The ceramic has a very low conductivity, so a layer a fraction of a millimetre thick can drop the metal temperature by a hundred degrees or more. It supplements cooling rather than replacing it, and its adhesion is often what limits the blade's life.

Answer: A

Grain boundaries are where creep damage accumulates at high temperature, so eliminating them raises the usable temperature substantially. The casting process is slow and expensive, which is accepted because the temperature gain translates directly into engine performance.

Answer: B

The blade lengthens gradually until it either rubs the casing or ruptures, and the rate depends steeply on both temperature and stress. A modest rise in metal temperature can halve the life, which is why cooling effectiveness is worth so much.

Answer: C

Each start and shutdown imposes a temperature gradient that the blade must accommodate, and the resulting strain accumulates over many cycles. Engines used on short sectors therefore see far more damage per flying hour than those on long ones.

Answer: D

The metal reacts with oxygen and with contaminants such as sulphur and salt, losing material steadily. Protective coatings are applied specifically against this, and their loss often ends a blade's life before creep or fatigue would have.

Answer: A

Leakage over the tip bypasses the blade passage entirely and mixes destructively with the main flow. Active clearance control, which cools the casing to shrink it onto the tips in cruise, exists to recover exactly this loss.

Answer: B

A shroud seals the tip and damps vibration, at the cost of mass at the largest radius where centrifugal stress is highest. That trade is why shrouds are common on low pressure turbines and rare on the hottest high pressure stages.

Answer: C

That air has been compressed and then never heated, so the work spent on it is largely wasted. Cooling flows of fifteen to twenty per cent of core airflow are typical, and reducing them is a standing objective in turbine development.

Answer: D

Because a turbine accelerates the flow, its efficiency is typically higher than a compressor's, commonly above ninety per cent. That relative ease is one reason a gas turbine's development effort concentrates on the compressor and the combustor.