Zero-stage

Jet engines are often uprated by adding a zero-stage to the front of a compressor.[1]

At a given core size, adding a stage to the front of the compressor not only increases the cycle overall pressure ratio, but increases the core mass flow.

This is demonstrated by the following relationship:

w_2 = (w_2 \sqrt{T_3}/P_3) * (P_3/P_2) *  (\sqrt{T_2/T_3}) * (P_2/\sqrt{T_2}) \,

where:

core mass flow = w_2 \,

core size =(w_2 \sqrt{T_3}/P_3) \,

core total head pressure ratio = (P_3/P_2) \,

inverse of core total head temperature ratio = T_2/T_3 \, i.e. (P_3/P_2 \,)

core entry total pressure = P_2 \,

core entry total temperature = T_2 \,

So basically, increasing (P_3/P_2) \, increases w_2 \,.

On the other hand, adding a stage to the rear of the compressor increases overall pressure ratio, decreases core size, but has no effect on core flow. This option also needs a Turbine with a significantly smaller flow capacity to drive the compressor.

References

  1. Hooker, Sir Stanley (1984). Not much of an Engineer, P. 153. Airlife Publishing Ltd, Shewsbury, England ISBN 0906393353.


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