Global Stability and Sensitivity Investigation of Nonlinear Dynamics of Compressor Diffuser with Numerical Volute Approach

Although the increase of the width ratio of a radial diffuser generally leads to a more unstable condition, the effect of volute was seldom considered in previous experimental and numerical studies because of the mismatching between the changed diffuser and the original volute. In this work, vaneless diffusers with different width ratios were theoretically and numerically investigated. In particular, a turbulent stability analysis based on different flow planes was performed using a spectral collocation method. To consider the combined impact of width ratio and volute, the base flow of a diffuser with a numerical volute imposed at the outlet was computed and validated against experimental data. The sensitivity of the eigenvalue was further obtained to localize the wave-maker region of the diffuser by means of a continuous adjoint procedure. The results indicated that the asymmetric flow field could be well recovered by the numerical volute. With a small width ratio of 0.06, the stability margin of the compressor was not significantly widened with a stall first occurring in the impeller, although the diffuser becomes more stable. The asymmetric flow condition led to the regional enhanced global modes in the circumferential direction for the diffuser with middle and large width ratios. The high-sensitivity region of the diffuser with a middle width ratio lay at the 90°-160° position in a scattered form, indicating that the combined effect of the inflow wake and outlet reverse flow near the shroud wall were crucial for flow instability. However, for a diffuser with a large width ratio, the dominant factor in instability is the outlet reverse flow due to the delay of the separation point.