A systematic approach to estimate the impact of the aerodynamic force induced by rotating stall in a vaneless diffuser on the rotordynamic behavior of centrifugal compressors

One of the main challenges of the present industrial research on centrifugal compressors is the need of extending the left margin of the operating range of the machines. As a result, interest is being paid in accurately evaluating the amplitude of the pressure fluctuations caused by rotating stall, which usually occurs prior to surge. The related aerodynamic force acting on the rotor can produce subsynchronous vibrations, which can prevent the machine to further operate, in case their amplitude is too high. These vibrations are often contained thanks to the stiffness of the oil journals. Centrifugal compressors design is, however, going towards alternative journal solutions having lower stiffness levels (e.g. Active Magnetic Bearings or Squeeze Film Dampers), which hence will be more sensitive to this kind of excitation: consequently, a more accurate estimation of the expected forces in presence of dynamic external forces like those connected to an aerodynamically unstable condition is needed to predict the vibration level and the compressor operability in similar conditions. Within this scenario, experimental tests were carried out on an industrial impeller operating at high peripheral Mach number. The dedicated test rig was equipped with several dynamic pressure probes that were inserted in the gas flow path; moreover, the rotor vibrations were constantly monitored with typical vibration probes located near the journal bearings. The pressure field induced by the rotating stall in the vaneless diffuser was reconstructed by means of an ensemble average approach, defining the amplitude and frequency of the external force acting on the impeller. The calculated force value was then included in the rotordynamic model of the test rig: the predicted vibrations on the bearings were compared with the measurements, showing satisfactory agreement. Finally, the prospects of the proposed approach are discussed by investigating the response of a real machine in high-pressure functioning when different choices of journal bearings are made.