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ACTIVELY CONTROLLED COMPONENTS
Table 2.1: Typical Surge Margin Requirements at ISA, SLS [2.22]
Fan LPC/IPC HPC
Military fighter 15 – 20% 20 – 25% 25 – 30%
Commercial aircraft 10 – 15% 15 – 20% 20 – 25%
As the worst case has to be considered here, not all of the accounted effects are necessarily to occur during the
entire mission of the engine. The surge margin requirement limits the exploitable performance of a compressor
in terms of pressure ratio, efficiency, and engine dynamics.
The mechanical integrity and airworthiness of the compressor parts are very important requirements of the
components. The harsh operating conditions together with the strict reliability requirements make it difficult
to design compressor parts with enough life. The parts have to operate at high temperatures and at extensive
rotational speeds. However, the component is optimized for a minimum mass which results in thin structures
with high stress levels. These structures are susceptible to vibrations at different engine orders of the shaft
speed. An additional requirement is given by loss-minimized aerodynamic shapes, which often contradict life
demands.
2.3.2.2 Active Surge Control
Active surge control offers two major advantages for a gas turbine. A direct benefit is given by the enhanced
stability of the compressor. This additional margin may be used to close existing lacks of stability or to tighten
operability requirements through quicker engine acceleration and deceleration or additional allowances for
inlet distortion. An additional benefit can be gained addressing the compressor design. If parts of the surge
margin stack up could be provided by active systems only if required, the basic stack up would allow for
lower incremental surge margin. This would release new design space which can be used to optimize overall
performance, component efficiency and parts count (airfoils or stages) and thus production and maintenance
costs.
Figure 2.4: Enhanced Compressor Operating Point by Active Surge Control [2.23].
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