QB_AE_PROP_NEW_025
Section Question Bank
MSQ · 2 Marks
Propulsion • Centrifugal and Axial Compressors - Rotating Stall and Surge
Which of the following statements regarding aerodynamic instability phenomena (stall and surge) in turbomachinery compressors are **CORRECT**?
(A) Rotating stall is a localized flow instability that propagates circumferentially around the compressor annulus at a speed typically between $20\%$ and $70\%$ of rotor speed
(B) Surge is a complete system-level axisymmetric instability characterized by violent, low-frequency longitudinal oscillations of mass flow rate and total pressure throughout the entire engine duct
(C) Operating a compressor beyond the surge line on its characteristic map leads to flow reversal through the compressor stages
(D) Increasing the mass flow rate through a compressor at constant rotational speed moves its operating point closer to the surge boundary
Answer Key:
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Step-by-Step Verified Solution
### 1. Strategy & Theoretical Principles 1. **Rotating Stall:** - A local aerodynamic disturbance on one blade causes flow separation on its suction side, throttling the flow passage. - Diverted air increases the incidence angle on the adjacent blade in the direction opposite to blade rotation, causing it to stall, while decreasing incidence on the opposite blade. - Consequently, the stall patch rotates circumferentially in the direction of rotor rotation at a speed lower than the rotor ($20-70\%$ of $N$). 2. **Surge:** - Occurs when the slope of the compressor characteristic curve becomes sufficiently positive ($dp/d\dot{m} > 0$). - It is a global 1D limit-cycle oscillation of the entire compression system where mass flow can reverse violently, causing severe structural vibrations and flameout. 3. **Compressor Map Dynamics:** - The surge line is located at low mass flow rates for a given rotational speed. - *Increasing* mass flow rate moves the operating point to the right, toward the **choke line**, moving it *further away* from the surge boundary. ### 2. Statement Evaluation - **Statement A:** **CORRECT.** Accurate description of rotating stall. - **Statement B:** **CORRECT.** Accurate description of surge. - **Statement C:** **CORRECT.** Deep surge features catastrophic flow reversal. - **Statement D:** **INCORRECT.** Increasing mass flow moves away from surge toward choke. ### 3. Exam Trap & Standard Reference - **Trap Alert:** Surge happens when mass flow is reduced (throttling), not when mass flow is increased. - **Standard Reference:** Saravanamuttoo, H.I.H., *Gas Turbine Theory*, Chapter 5.