Compressor

Question 1

Explain some fundamental things about the axial compressor

Answer 1

• Can have very high overall pressure ratios = reduces specific fuel consumption
• High efficiency and large flow rate for a given frontal area
• High stage pressure ratios reduces needed number of stages
• Not justifible for industrial gas turbines as high Mach numbers and large gas deflection is implied by high stage pressure ratios
• Generally more conservative design than aero-engines

Question 2

Explain some fundamental things about the centrifugal compressor

Answer 2

• In engines where the flow is too small to be efficiently handled by an axial compressor
• Shorter, wider
• High pressure ratio per stage = fewer stages
• Flow is turned 90 degrees in compressor, which is why the diameter is larger

Question 3

Explain diffusion

Answer 3

In the stator, the airflow is decelerated by both the passage through blades and the curvature of the blade which causes a deflection
High deflection can give a higher pressure rise in a single stage, but it also causes risk of stall

Question 4

How is allowed diffusion decided?

Answer 4

At early design stage: de Haller number.
At final design: diffusion factor, derived based on velocity gradient form on the suction surface of the blade

Question 5

What causes losses in a blade row?

Answer 5

Boundary layer growth on the suction and pressure side of the blade surfaces.
They cause a wake after the blades, which drops the stagnation pressure

Question 6

Explain compressor surge

Answer 6

• Flow is too slow, starts to separate over blade
• High pressure at back of compressor
• High enough pressure = entire compressor stall
• Reverse flow = pressure is high in combustor and turbine
• Air leaves through inlet until the pressure in the compressor is low enough
• Pressure starts to build again
• If blockage is not relieved, the compressor starts to surge (process happens over and over again very quickly)