Explain some fundamental things about the axial compressor
- 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
Explain some fundamental things about the centrifugal compressor
- 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
Explain diffusion
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
How is allowed diffusion decided?
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
What causes losses in a blade row?
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
Explain compressor surge
- 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)
