Unsteady flow and the rotor rotational behavior during surge in a single stage centrifugal compressor have been investigated to study the physical mechanism of this special operating period. The experiments were performed for the compressor with vaneless and vaned diffuser at different rotational speed. The centrifugal compressor was driven by a 1350 kw dc-motor with Leonard-control. Unsteady pressure was measured with fast response dynamic transducers while rotor speed was simultaneously measured with a capacitive impulser. At the beginning and the end of the surge, the pressure signals measured in the meridional direction from the impeller inlet to the diffuser exit show obvious pressure oscillation. During the whole surge period, the pressure signal at the impeller exit shows periodic fluctuations with very large amplitudes at a high frequency. The further pressure analyses indicate that these pressure fluctuations is produced by the pressure difference across blade pitch. The pressure side and the suction side of the blade are inversed compared with that in the normal compressor operating condition. This reverse pressure difference produces an additional torque on the impeller blades in the impeller rotation direction. The rotating speed of the compressor rotor during surge was also studied. It is shown that during the surge the rotor speed increases slowly until the surge stops. Since the compressor-motor system is controlled by the Leonard-set, the increase in rotor speed indicates the torque reduction of the compressor.

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