A radial inlet pressure distortion index is derived using a mathematical compressor model, the two-dimensional continuity equation and the expression for radial velocity upstream of a low hub-tip ratio, multi-stage compressor. These expressions are combined with two conditions required for inlet flow similarity at compressor stall; namely, constant rotor angle of attack and constant ratio of mixing time to air residence time in the blade passage. The theory was tested on a twin-spool turbojet engine at sea level static conditions.

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