What occurs when surge happens in a compressor?

• A. An unstable flow arising when mass flow is too high for the pressure ratio, causing forward flow.
• B. An unstable flow arising when mass flow is too low for the pressure ratio, causing reverse flow and possible compressor damage.
• C. A smooth, stable increase in flow with temperature rise.
• D. A condition only seen at idle with no risk to the engine.

Answer: (B)

Surge happens when the flow through the compressor is too small to support the pressure rise the rotor is trying to produce. In this situation, the flow can detach and reverse in parts of the blade passages, causing large, unstable pressure pulsations and oscillations. Those reverse flows and rapid changes in speed and pressure put abnormal loads on the blades and seals, creating a real risk of damage to the compressor components. This unstable, backflow behavior occurs for a given pressure ratio only when mass flow falls below the stable operating line, which is why surge is a low-flow, high-pressure-ratio instability. It’s not a smooth increase in flow, nor something that only happens at idle.