Inlet and Compressor (construction, operation, stall and surge)
What this domain tests
This domain covers how the inlet delivers uniform, high-pressure air to the compressor and how the compressor raises pressure without stalling. The exam tests inlet pressure recovery and distortion, axial versus centrifugal construction, stage matching, and the difference between rotating stall and surge.
Core rules
- Inlet total-pressure recovery is pt2/pt0; losses reduce thrust and can trigger compressor instability. Distortion is non-uniform total pressure or swirl at the fan/compressor face.
- An axial compressor has alternating rotor and stator rows; each stage gives a modest pressure ratio. A centrifugal compressor gives a higher pressure ratio per stage but is heavier and has a narrower flow range.
- Diffusion converts velocity into pressure. If incidence is too high, the boundary layer separates: rotating stall appears as a cell of stalled flow; surge is an axisymmetric, often violent, reversal of flow.
- Stall/surge controls include bleed valves, variable inlet guide vanes and stators, and active clearance control.
Worked example
A ten-stage axial compressor has an average stage pressure ratio of 1.30. Overall pressure ratio is about 1.30^10 = 13.8. If one stage stalls and its pressure ratio falls to 1.20 while the rest hold 1.30, the overall ratio becomes 1.20 x 1.30^9 = about 12.7, a loss of about 8%. That illustrates why stage matching matters: one weak stage degrades the whole compressor.
Traps
Treating stall and surge as synonyms; assuming pressure ratios add linearly; ignoring corrected speed; forgetting that bleed valves open near idle to protect the low-pressure compressor.