An experimental investigation is conducted to study the details of instability inception in a transonic axial flow compressor. The experimental results indicate that the compressor instability is initiated through the development of a low-frequency axisymmetric disturbance. The frequency of this disturbance is approximately the Helmholtz frequency of the test facility. The low-frequency disturbance can be detected over 3000 rotor revolutions before the compressor becomes unstable. Further experimental investigations illustrate that this low-frequency axisymmetric disturbance is initiated at the hub region of the compressor. This new kind of instability inception is termed “partial surge.” Examination of the design parameters of the compressor indicates that a high diffusion factor in the rotor root region might be the cause of the partial surge type instability inception.

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