Investigation of rotating stall in radial vaneless diffusers with asymmetric inflow

Despite its simple geometry, the turbulent flow in vaneless diffusers is asymmetric and highly skewed. In the present work, a frozen eddy viscosity approach was employed to investigate the instability in a vaneless diffuser with different axial widths. A turbulent stability analysis was performed around the numerically computed mean flows with a non-uniform inflow for both isolated and full-annular vaneless diffusers. The predictions of the flow instability frequency and coherent structure were validated against experimental data. By performing a structural-sensitivity analysis corresponding to the leading eigenvalue, the instability mechanisms for the isolated and full-annular vaneless diffusers were revealed. The sensitivity analysis indicated that the interaction between the boundary layer and the main flow may have been the primary cause of the self-excited instabilities in a narrow diffuser under both axisymmetric and asymmetric inflow. The contribution of the reverse flow near the walls was relatively small. However, the influence of the separation flow near the wall on the instabilities of a wide diffuser was significant, particularly under high-skew inflow conditions. The wavemaker regions were located on the shroud side near the inlet and the hub side near the outlet. When connected to the impeller in the upstream direction, the diffuser outlet backflow was responsible for instability in the diffuser with a radius ratio of 1.53. The jet-wake flow in the diffuser inlet had little impact on the flow instability.