Compressor and turbine performance characteristics

Compressor characteristic

The performance map plots the pressure ratio against the corrected mass flow $\dot m\sqrt{T_{01}}/p_{01}$ for lines of constant corrected speed $N/\sqrt{T_{01}}$, with islands of constant isentropic efficiency.

 p02/p01
    |           surge line
    |          /
    |         *-----.  N3
    |        /       \
    |       *-----.   |
    |      /       \  |  N2
    |     *-----.   | |
    |    /       \  | |  N1
    |   /         | | |
    +---------------------------- m*sqrt(T01)/p01
      each speed line starts at the surge line (*) and
      drops almost vertically at its right end (choke)

• Surge: at low flow the blades stall, the delivery pressure collapses and the flow reverses; the cycle repeats with violent pulsations. The surge line bounds the left side of the map.
• Choking: at high flow the Mach number reaches 1 at some section (rotor inlet or a throat), the mass flow cannot increase further, and the speed lines become vertical.
• The operating line must lie between the surge and choke limits, in the region of high efficiency.

Turbine characteristic

The map of expansion ratio against $\dot m\sqrt{T_{01}}/p_{01}$ for different values of $N/\sqrt{T_{01}}$ shows that once the nozzle (or stage) is choked, the flow parameter becomes constant and independent of speed. The curves for different speeds therefore merge at high expansion ratio. The efficiency is flat over a wide range, so turbines are less sensitive than compressors to off-design operation.

Centrifugal and axial compressors