Axial Flow Turbines Questions and Answers

Answer: A

The stationary vanes accelerate the gas and turn it into the plane of rotation, and the rotor then extracts work from it. The order is the reverse of a compressor stage, where the moving row comes first and the stationary row follows to diffuse the flow.

Answer: B

An accelerating flow keeps its boundary layer attached through turning that would separate a diffusing one immediately. That single difference is why a handful of turbine stages can drive a compressor of a dozen or more.

Answer: C

The same relation governs turbines and compressors alike, differing only in the sign of the tangential velocity change. Work depends solely on blade speed and how far the rotor turns the flow tangentially, which is why both quantities are pushed as high as materials allow.

Answer: D

With no pressure change across the rotor, the relative velocity is unchanged in magnitude and the blades merely turn the flow. Zero degree of reaction gives the highest work per stage but at noticeably lower efficiency than a reaction design.

Answer: A

The equal division makes the velocity triangles mirror images and lets a similar profile serve both rows. It is aerodynamically the most efficient arrangement, which is why most modern turbines are designed close to it at the mean radius.

Answer: B

Blade speed rises with radius while the tangential velocity distribution is chosen to keep the work uniform, so the reaction climbs outward. A blade designed for fifty per cent at the mean radius may be close to impulse at the root and highly reactive at the tip.

Answer: C

Once choked, the corrected mass flow through the turbine is fixed by the throat area regardless of what happens downstream. That fixed capacity is what makes the first vane row the effective flow control of the whole engine.

Answer: D

Correct answer: Corrected mass flow is fixed

A choked throat passes the maximum flow the upstream conditions allow, and nothing downstream can change it. The first stage nozzle guide vanes therefore fix the engine’s corrected mass flow \( \dot{m}\sqrt{T_0}/p_0 \) at a single value over almost the whole operating range, which is why the throat area is the designer’s principal handle for rematching an engine. Pressure ratio, turbine entry temperature and fuel flow all still vary with throttle.

Answer: A

The gas can be made far hotter than any alloy tolerates, so the ceiling is metallurgical rather than thermodynamic. Modern engines run entry temperatures well above the melting point of the blade alloy, which is possible only because of cooling.

Answer: B

The air picks up heat from the internal walls and is then discharged, usually at the trailing edge or the tip. It is the simplest of the cooling schemes and is normally combined with film cooling on the most heavily loaded blades.