Analysis of the Rotordynamic Response of a Centrifugal Compressor Subject to Aerodynamic Loads due to Rotating Stall

In the current industrial research on centrifugal compressors, manufacturers are showing increasing interest in the extension of the minimum stable flow limit in order to improve the operability of each unit. The aerodynamic performance of a compressor stage is indeed often limited before surge by the occurrence of diffuser rotating stall. This phenomenon generally causes an increase of the radial vibrations, which, however, is not always connected with a remarkable performance detriment. In case the operating curve has been limited by a mechanical criterion, i.e. based on the onset of induced vibrations, an investigation on the evolution of the aerodynamic phenomenon when the flow rate is further reduced can provide some useful information. In particular, the identification of the real thermodynamic limit of the system could allow one to verify if the new load condition could be tolerated by the rotordynamic system in terms of radial vibrations. Within this context, recent works showed that the aerodynamic loads due to a vaneless diffuser rotating stall can be estimated by means of test-rig experimental data of the most critical stage. Moreover, by including these data into a rotordynamic model of the whole machine, the expected vibration levels in real operating conditions can be satisfactorily predicted. To this purpose, a wide-range analysis was carried out on a large industrial database of impellers operating in presence of diffuser rotating stall; the analysis highlighted specific ranges for the resultant rotating force in terms of intensity and excitation frequency. Moving from these results, rotordynamic analyses have been performed on a specific case study to assess the final impact of these aerodynamic excitations.