Numerical modeling of the dynamic unsteadiness with global mode decomposition approach in a high-speed compressor with abnormal diffuser vane

Adjustable vane diffuser is addressed as one of the main techniques in centrifugal compressor in seeking of wider stability margin. Flow instabilities, however, may be driven by the three-dimensional flow separation induced by the abnormal vane in the diffuser. In the present work, a numerical investigation of the radial vaned diffuser with abnormal vane was applied to identify the correlation between stall inception and asymmetric flow distribution. Unsteady numerical simulations of the centrifugal compressor with vaned diffuser was carried out and validated against experimental data. Then, the aerodynamic characteristics and evolution process of the flow instabilities in vaned diffuser was analyzed with the compressed dynamic mode decomposition in term of perturbation frequency and dominating flow mode. Finally, concerning the relative position of the volute tongue and abnormal vane, the influence of the abnormal vane on the matching between the diffuser and volute regarding the stall mechanism was summarized. The results indicate that the abnormal vane needs to be altered large enough, which is above 10°, to trigger the flow instability of vaned diffuser before the spike stall in impeller. With the increase of stagger angle alternation, the stall mechanism turns from a global flow separation in multiple passages into a regional and abrupt phenomenon in limited passages. With the mode composition approach, the flow separation near diffuser outlet and the tornado-type vortices at the leading edge of the abnormal vane are responsible for the flow instability of the case with 15° vane alternation. When combined with the volute, the case with abnormal vane close to the volute tongue tends to be more unstable than that 180° away from volute tongue. The tornado-type vortices are also moved to the trailing edge rather than the leading edge under the influence of the asymmetric volute.