In an axial-flow compressor stage, rotating stall is fundamentally caused by:

(A) Flow separation on compressor blade suction surfaces due to excessive positive incidence angles at reduced mass flow rates

(B) Supersonic shock wave detachment occurring exclusively at maximum design mass flow rate

(C) Choking in the turbine nozzle guide vanes at idle throttle

(D) Thermal expansion mismatch between the turbine disk and casing

Answer Key:

A

### 5-Stage Derivation & Analytical Solution: 1. **Stage 1: Velocity Triangle:** Relative flow angle $\beta_1 = \arctan(U / C_a)$. Decreasing mass flow rate decreases axial velocity $C_a$. 2. **Stage 2: Incidence Angle:** Incidence $i = \beta_1 - \beta_{1,blade}$ increases. 3. **Stage 3: Aerodynamic Stall:** Excessive positive incidence exceeds the stall limit, triggering boundary layer separation and rotating stall cells. 4. **Stage 4: Selection:** Option A is correct.

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