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Unsteady flow phenomena are common in turbomachinery and have serious impacts on its stable operation. This paper reports a self-induced unsteady phenomenon inside a centrifugal impeller due to strong secondary flow that has not been previously reported. A high-loading centrifugal impeller is selected as the research object, an accurate three-dimensional model is established, and exhaustive numerical simulations are performed based on unsteady compressible Navier–Stokes equations. The unsteady performance of the compressor is measured in detail during the experiment, and the experimental results are compared with the numerical simulation results to verify the reliability of the numerical simulation method. The results of the unsteady pressure signal show the existence of unsteady phenomenon when the operating point is far away from the choke boundary. The power spectrum results of the unsteady pressure distribution at the impeller blade tip are analyzed, and two different modes of this unsteady phenomenon are confirmed. Higher-order frequency characteristics are not observed near the stall boundary. The detailed analysis of the flow field inside the impeller clarifies that the instability of the interface formed between the secondary flow and the main flow induces this unsteady phenomenon. This unsteady pressure disturbance propagates inside the blade passage through pressure waves, which contributes to the instability of the interface and the existence of this self-induced unsteady phenomenon. [ABSTRACT FROM AUTHOR]

A comprehensive quantitative analysis of a high-loading transonic centrifugal compressor is performed under the design point. The reason for secondary flow within the impeller is examined under the approximation of boundary layer theory. Comparing the amplitudes of different terms of the Navier–Stokes (N–S) equations reveals that the acceleration that creates secondary flow within the shear layer is caused primarily by the Coriolis force difference, which has often been neglected in previous research. Migration flow on the wall surface further leads to the formation of three-dimensional secondary flow structures that occupy the entire blade passage. The vortex structure inside the impeller is visualized, and the contribution of the different vortices to the wake is clarified. The wake is ultimately composed of high-loss fluid that includes migration flow from the hub to the tip on the blade suction surface, leakage flow, and recirculating flow at the impeller outlet. By decomposing the N–S equations, it is clarified that the pressure distribution in the blade passage is mostly determined by the acceleration caused by the three-dimensional secondary flow structure, especially the momentum convection term. Therefore, the relationship between the secondary flow and the impeller loading is established. Furthermore, the natural frequency of the unsteady fluctuation of the secondary flow inside the impeller is discovered by unsteady numerical simulation, revealing the evolution of vortex structures. [ABSTRACT FROM AUTHOR]