Quantitative Analysis on Influence of Secondary Flow for Centrifugal Impeller Internal Flow

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.