The unsteady flow at small flow rates is always the most important of typical unsteady phenomena in centrifugal compressors, since it is closely related to the operating safety and efficiency. To study the mechanism of stall and surge generation, an experimental research on an industrial centrifugal compressor with variable vaned diffuser is carried out to study the unsteady flow structure from design point to surge. A multi-phase dynamic pressure measurement is conducted, based on 23 dynamic pressure sensors mounted on the shroud side casing surface of the compressor. The sensors are circumferentially distributed in a non-uniform manner at seven different radial positions, including the impeller region, the vaneless region and the diffuser throat region. Real-time data is recorded during the whole valve-adjusting process. The characteristics of pressure fields at some specific operating conditions are focused on, especially the pre-stall, stall precursor, stall and surge conditions. According to the multiphase data association, the originating position of the stall precursor can be determined. The features of the unsteady flow structure are also obtained, such as the surge pattern and the propagation direction of stall cells. In addition, when the diffuser vane setting angle (OGV) is turned up, the core factors to trigger total instability will change. In order to visually show how the tip leakage and separation vortex in the impeller gradually affect the flow structure in the vaned diffuser region and even the whole machine, numerical simulation and dynamic mode decomposition (DMD) method are used to study the flow mechanisms. The numerical simulation result is well matched with the experimental result. With the help of the DMD method, a few low-frequency tip leakage vortex structures are extracted from the unsteady numerical result over a period of time, which correlate with the experimental result. Meanwhile, on this issue, the feasibility of dynamic experimental analysis combined with multi-channel numerical simulation analysis is verified and discussed. Through the two analytic methods, a detailed understanding of the unsteady flow structure in the centrifugal compressor with variable vaned diffuser is obtained.

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