h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Gas turbine engines take relatively quite a bit of time to spool up to max speed. If you try to accelerate them up too fast you get something called surge which is where the flow of air through the engine breaks down.

Incidently, some of the air guides or stators in the compressors can be turned to help with starting the engine.

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Can you elaborate on "breaks down"? This (to me) implies that the flow through the engine must be laminar, at least at startup, and that if you try to start it up too fast the Reynolds number goes above 2100 (transitional flow) or above 4000 (turbulent flow) and this causes problems. Am I barking up the wrong tree, and what are the negative effects of "surge"?

Thanks...
H.

I suppose that during a surge the blades of the compressor stall and the air flow becomes turbulent. The air flow can through the enine can reverse completely. There are various measurements done to cause the fuel system to reduce the fuel flow which will hopefully clear the surge and allow the engine to recover, though it is possible to have a locked in surge where about the only thing to do is to shut down the engine completely. A surge can cause damage to the turbine blades which can become overheated, and mechanical shock can damage other parts of the engine.

Accelerating the engine too fast from idle to max or intake distortion can cause a surge.

(I'm sorry if this is not totally clear. It is sometime since I dealt with gas turbine engines!)