Effect of Vaned Diffuser on a Small Centrifugal Impeller Performance

A small transonic centrifugal compressor with 3D vaned diffuser has been developed for a turbine engine by Nanjing University of Aeronautics and Astronautics. The centrifugal impeller’s diameter is approximately 80 mm. The impeller was improved by reducing the static pressure gradient in spanwise direction. But the improvement is reduced by the interaction between the impeller and the diffuser, when it works with the diffuser. So, the effect of the vaned diffuser on the performance and flow structure of the impeller was studied by steady and unsteady numerical simulations. The total pressure ratio and efficiency of the impeller with a vaned diffuser are lower than those with a vaneless diffuser. Further, the difference decreases as the operation point moves from the surge to the choke. Analysis of steady flow proves that the meridional bend and the vanes in the 3D vaned diffuser are the main causes. When the impeller operates with the vaned diffuser, the vortex at the shroud of the impeller exit is pushed toward the trailing edge of the impeller blade, which leads to an altered flow structure in the impeller passages. As a result, the loss of the impeller increases, and consequently the efficiency as well as the pressure ratio drops. On the other hand, the unsteady analysis shows that the flow pattern at the impeller exit is circularly affected by features of the stagnation, the local acceleration, and the shock wave in the semi-vaneless space of the diffuser. The flow at the different spanwise location in the trailing edge of the impeller blade can be influenced by the different flow feature, therefore, the spanwise distributions of the flow parameters is disturbed.