Question

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?

• A. Blade cooling
• B. Variable angle stator vanes
• C. Blow-off valves
• D. Multi-spool shaft

Hint:

Compressor starting problems are usually solved by improving flow matching between stages using variable stator vanes, bleed valves, or independent spool speeds.

Answer 1

The Correct Option is B, C, D

Step 1: Understand the problem of loading mismatch in compressors.
A high-pressure-ratio axial compressor often faces flow instability and mismatching of air mass flow during starting conditions. This leads to issues such as compressor surge or stall because the early stages cannot handle the large mass flow while the later stages demand high pressure rise. To avoid such mismatches, different control techniques are used during startup.
Step 2: Evaluate each option based on compressor starting behaviour.
(B) Variable angle stator vanes help control the airflow angle entering rotor blades, improving stability and reducing mismatching at low speeds.
(C) Blow-off valves temporarily discharge excess air from intermediate stages to avoid back-pressure buildup during starting, preventing surge.
(D) Multi-spool shafts allow different compressor sections to rotate at different speeds, improving matching between stages during startup.
(A) Blade cooling is used mainly for turbine thermal management and does not address compressor matching problems, so it is not useful here.
Step 3: Conclusion.
Techniques that effectively alleviate loading mismatch during starting are variable stator vanes, blow-off valves, and multi-spool shafts. Therefore, the correct options are (B), (C), and (D).