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  • Compressor stall

    I can not understand why a compressor stall is more likely to occour at a low RPM (Q210706 for example).
    The compressor blade is an aerofoil. The faster it spins (higher RPM) the higher its angle of attack, and therefore - the higher its chance to stall.
    In a low rotational speer (low RPM), the angle of attack gets shallower - less chance of stall.
    What am I missing?

  • #2
    The compressor is designed to run efficiently at optimum RPM for the majority of its time-on-wing working life. This, needless to say is high RPM. Therefore the compressor blade angles are set for high rotational speed. Each stage possesses its own aerodynamic performance and handling characteristics subtly different from adjacent stages.

    Looking at the compressor map (fig.14.24) one can clearly see the reduction of the safety margin at lower RPM. Even with the use of VIGVs, VSVs and inter-stage bleed valves, the unstable area is breathing down our necks!

    The point you are missing Random is Axial Flow.

    Axial air flow through the compressor must be maintained, and at lower RPM and during accelerations the risk of 'choking' the rear stages is more of a risk.

    Rgds

    Fraser.

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    • #3
      I understand.
      Would it be correct to say that a compressor can stall at a speed higher or lower that the optimal one?
      Would it be correct to say that the likelyhood of a surge is higher at a lower axial speed?

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      • #4
        From the Rolls Royce book:

        "At higher operating speeds, if the conditions imposed upon the compressor force operation beyond the limits of the stability line, the rear stages will become overloaded, and an instantaneous breakdown of the airflow through the compressor occurs, leading to surge. During the surge event, the inlet mass flow varies with time,as the compressor flow oscillates between stalled and unstalled. Due to loss of pressure rise capability across the compressor stages,the high pressure air in the combustion system may be expelled forward ( negative flow direction) resulting in loss of thrust. This 'deep' surge produces a loud bang and possibly combustion gases exiting the intake"

        (We covered lower RPMs yesterday)

        So stall may occur across the range of all operating speeds as a result of reducing axial flow incompatible for that condition, causing breakdown of axial flow (so lower speed?, yes, if you like).

        Fraser.

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        • #5
          Understood. Thank you very much.

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