STEADY/UNSTEADY DIFFUSER FLOW IN A TRANSONIC CENTRIFUGAL COMPRESSOR

The results of steady and unsteady RANS simulations performed in a transonic centrifugal compressor with a radial vaned diffuser are compared. The paper gives the detailed analysis of the internal structures which develop in the diffuser. Although the overall performances of the compressor are well described by steady RANS, large discrepancies have been observed in the physics of the diffuser flow, both types of simulation predicting different flow separations. These results are examined in order to choose a flow control strategy, relying on boundary layer aspiration. The comparison between RANS and URANS results shows that no valid physical analysis can be expected in this case from steady simulations. Consequently, any flow control device should be designed by taking into account only unsteady results.