Zero-stage
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Jet engines are often uprated by adding a zero-stage to the front of a compressor. 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:
<math>w_2 = (w_2 \sqrt{T_3}/P_3) * (P_3/P_2) * (\sqrt{T_2/T_3}) * (P_2/\sqrt{T_2}) \,</math>
where:
core mass flow = <math>w_2 \,</math>
core size =<math>(w_2 \sqrt{T_3}/P_3) \,</math>
core total head pressure ratio = <math>(P_3/P_2) \,</math>
inverse of core total head temperature ratio = <math>T_2/T_3 \,</math> i.e. f(<math>P_3/P_2 \,</math>)
core entry total pressure = <math>P_2 \,</math>
core entry total temperature = <math>T_2 \,</math>
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.
