The three-dimensional flow in centrifugal impellers is investigated on the basis of a detailed analysis of the results of numerical simulations. In order to gain confidence in this process, an in-depth validation is performed, based on computations of Krain’s centrifugal compressor and of a radial pump impeller, both with vaneless diffusers. Detailed comparisons with available experimental data provide high confidence in the numerical tools and results. The appearance of a high loss ‘wake’ region results from the transport of boundary layer material from the blade surfaces to the shroud region and its location depends on the balance between secondary and tip leakage flows and is not necessarily connected to 3D boundary layer separation. Although the low momentum spots near the shroud can interfere with 3D separated regions, the main outcome of the present analysis is that these are two distinct phenomena. Part I of this paper focuses on the validation base of the numerical approach, based on fine mesh simulations, while Part II presents an analysis of the different contributions to the secondary flows and attempts to estimate their effect on the overall flow pattern.
The three-dimensional flow in centrifugal impellers is investigated on the basis of a detailed analysis of the results of numerical simulations. An in-depth validation has been performed, based on the computations of Krain’s centrifugal compressor and a radial pump impeller, both with vaneless diffusers and detailed comparisons with available experimental data, discussed in Part I, provide high confidence in the numerical tools and results. The low energy, high loss ‘wake’ region results from a balance between various contributions to the secondary flows influenced by tip leakage flows and is not necessarily connected to 3D boundary layer separation. A quantitative evaluation of the different contributions to the streamwise vorticity is performed, identifying the main features influencing their intensity. The main contributions are: the passage vortices along the end walls due to the flow turning; a passage vortex generated by the Coriolis forces proportional to the local loading and mainly active in the radial parts of the impeller; blade surface vortices due to the meridional curvature. The analysis provides an explanation for the differences in wake position under different geometries and flow conditions. A secondary flow representation is derived from the calculated 3D flow field for the two geometries validated in Part I, and the identified flow features largely confirm the theoretical analysis.
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