Investigation of the Aerodynamic Losses Caused by the Impeller/Volute Interaction in a Transonic Centrifugal Compressor

Abstract The aerodynamic losses caused by the impeller/volute interaction in a transonic centrifugal compressor stage with a vaneless diffuser for turbocharger applications are numerically analyzed. In the operating point investigated a high volute mismatching induces an inhomogeneous pressure profile in circumferential direction with a pressure difference of up to 1 bar at the diffuser outlet, which corresponds to about 20% of the average pressure in this plane. This inhomogeneity propagates upstream and causes unsteady losses in the highly loaded impeller, which are investigated in detail. URANS simulations are conducted to enable a detailed analysis of the highly unsteady flow, evidenced by a high acoustic Strouhal number Sr = 0.18. To model the influence of the volute, an experimentally measured inhomogeneous static pressure profile is used as a boundary condition at the diffuser outlet. At the operating point investigated, the inhomogeneity reduces the time-averaged total-to-static stage efficiency Δηis,ts,4 by up to −1.2%, about two-thirds of which are caused by the impeller, as measured by the entropy increase. The analysis points out a large positive pressure gradient in the direction of rotation at the impeller outlet as the origin of these losses. An interaction region at the impeller outlet is identified using a velocity decomposition in the absolute and the relative frame of reference. The analysis shows that the interaction region impacts the flow inside the impeller significantly causing additional losses and that the identified loss mechanisms are of unsteady origin. Therefore, time accurate unsteady simulations are shown to be necessary for accurately predicting the losses in a centrifugal compressor with a high volute mismatching.