Sensitivity Analysis of the Vaneless Diffuser Flow under Circumferential Asymmetric Inflow/Outflow Condition

Unsteady flow is often observed in high-speed centrifugal compressor. The physical mechanism for it is often the combining effect of critical geometric parameters and boundary conditions. The vaneless diffuser flow in a centrifugal compressor is studied in the present work. Based on the global linear stability theory, a 2D stability analysis method is established at the r-θ plane considering both the turbulent viscosity and molecular viscosity. The direct modes are obtained. Then, the adjoint modes are also obtained based on adjoint method. And the structural sensitivity is derived based on both the direct and adjoint modes. Finally, the influence of jet-wake flow and volute are studied. The results indicates that the reverse flow at the flow at the shroud side is crucial for the stability of vaneless diffuser. The influence of jet-wake flow mainly lies at the stall cell number rather than the physical explanation of stall. The asymmetric structure of volute results into redistribution of flow field. The interaction between the reverse flow and inflow wake flow at the circumferential position 90◦∼135◦ away from the volute tongue is the main cause for the occurrence of stall.