GATE 2022 AE – Question 30
A high-pressure-ratio multistage axial compressor encounters an extreme loading mismatch during starting. Which of the following technique(s) can be used to alleviate this problem?
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Correct answer: (B) Variable angle stator vanes; (C) Blow-off valves; (D) Multi-spool shaft
Explanation
In a high-pressure-ratio multistage axial compressor, at low speed (during starting) the density ratio is much lower than at design. The **front stages** then pass too little flow compared with the rear stages' demand, so the front stages stall, while the rear stages choke. This is the starting-loading mismatch.
Remedies:
- **Variable-angle stator vanes (B):** adjust the incidence on the blades at off-design speeds so that the front stages do not stall. ✓
- **Blow-off (bleed) valves (C):** release some air from the middle of the compressor, which raises the flow through the front stages. ✓
- **Multi-spool shaft (D):** the low-pressure and high-pressure parts run at different speeds, which matches the stages better to the flow. ✓
- **Blade cooling (A):** protects the turbine blades against the heat. It has no effect on compressor starting. ✗
Answer **B, C and D**.