Analyses of impeller-vaneless-diffuser-scroll interactions in a radial compressor

Centrifugal compressors have reached advanced stages of their development, and it is only through a detailed understanding of their complex airflows that improvements in the overall performance will be achieved. This article presents a numerical investigation of unsteady flows through the different components of a radial compressor, emphasizing on the impeller-vaneless-diffuser-scroll interactions. A transient rotor-stator simulation model was used for the calculations carried out by means of the code CFX-TASC flow, at design point and near stall and choke operating conditions. Spectral analyses of the pressure fluctuation-related different component interactions have resulted in the amplitudes and speeds of rotation of the main energetic modes and have revealed that the flow presents a space-time periodic behaviour that may be described by the double Fourier decomposition. The spatial mode analysis was therefore introduced to describe the interaction mechanisms, whereas the time mode analysis during the working time of the machine has permitted to determine different frequencies, and hence the most prevailing modes and their originating sources. These spectral analyses have permitted a good understanding of the flow interaction mechanisms and revealed the computations limits, depending on whether one-passage or a full-rotor simulation is considered. Principally, these limitations are related to an underestimation of pressure fluctuation amplitudes and a failure in detecting all harmonics lower than the number of passages per component. Finally, this study may help in envisaging the local treatments to reduce noise levels and increase the compressor stability.