Formation and suppression of the instability in radial vaneless diffusers

The change of width radius ratio may lead to different physical mechanisms for the instabilities in vaneless diffusers. Reynolds-averaged Navier–Stokes numerical simulations were performed and the results were used to present the mean flow. A quasi-laminar mixed approach based was applied on the vaneless diffusers with narrow and moderate width. The frequency of the perturbations in the vaneless diffuser flow with different axial widths was validated against the experimental data. Moreover, in order to reveal the wavemaker region, the structural sensitivity analysis of the vaneless diffuser flow was carried out. The sensitivity of the eigenvalue to the mean flow modifications were used to guide the flow control. The results indicate that the viscosity tends to stabilize the narrow diffusers but destabilize the moderate width diffusers. For narrow diffusers, the wavemaker regions lie at the centerline near the intersection region of boundary layers. However, the wavemaker regions lie beside the centerline for moderate width diffusers. The occurrence of instabilities may be related with the inflection point of the velocity distribution in axial direction. Given the distribution of the sensitivity to mean flow modifications, the flow control effect may be associated with the amplitude of viscosity stress at the local flow field.