Axial and Centrifugal Compressors Questions and Answers

Answer: A

The rotor adds energy to the air and raises both its velocity and its pressure, and the stator then converts part of that velocity into further pressure while removing swirl. Both rows diffuse the flow, which is what makes a compressor so much harder to design than a turbine.

Answer: B

Work is done on the air by the moving blades, so both total pressure and total temperature rise across the rotor. The stator that follows changes neither total temperature nor the energy of the flow, converting velocity into static pressure instead.

Answer: C

Because it does no work, the total temperature is unchanged across it and only static pressure rises as the velocity falls. Removing the swirl left by the rotor is equally important, since the next rotor expects the flow to arrive at a particular angle.

Answer: D

The relation \( w = U \Delta c_\theta \) says that work depends only on how much the blade turns the flow in the tangential direction. It applies to axial and centrifugal machines alike, which is why it is the natural starting point for any stage analysis.

Answer: A

A fifty per cent reaction stage divides the static pressure rise equally between rotor and stator, which makes their velocity triangles mirror images. That symmetry is efficient and lets one blade profile serve both rows, which is why it is so common in practice.

Answer: B

The absolute velocity seen from the casing and the relative velocity seen from the rotating blade differ by the blade speed, and drawing that vector relation is the whole of stage kinematics. Every quantity in the Euler equation is read directly off those triangles.

Answer: C

Diffusion in a decelerating flow is inherently prone to separation, so each row can turn and slow the air only so far. That is why a compressor needs many stages where a turbine achieves a similar pressure change in very few, its flow being accelerated rather than diffused.

Answer: D

Stage pressure ratios of about 1.2 to 1.4 multiply through ten or more stages to give overall ratios above forty in the largest engines. Multiplying rather than adding is what makes a long compressor so effective despite each stage's modest contribution.

Answer: A

The radius increase does much of the work, which is why a single stage can achieve a pressure ratio of four or more where an axial stage manages perhaps 1.3. The penalty is a large frontal area and the difficulty of staging such machines in series.

Answer: B

The impeller adds the energy, the diffuser converts the high velocity leaving it into static pressure, and the volute collects the flow and delivers it onward. Roughly half the pressure rise typically occurs in the diffuser rather than in the impeller itself.