Rotordynamic Force Coefficients for Open and Shrouded Impellers

The present work deals with experimental measurements of rotordynamic force coefficients for two types of impellers: open (unshrouded) and closed (front shrouded). The impellers were designed to work in a turbopump with a cryogenic fluid, but experiments were performed in a test rig where water is used as working fluid. All the tests were conducted with a vaned diffuser. For both configurations, the supply pressure is 0.6 MPa, knowing that the whole test rig is pressurized at 0.5 MPa. The rotor is centered and the operating rotating speeds are 2000, 3000, and 5000 rpm. For each rotor speed, according to a double conical hydrostatic thrust bearing and eight piezo electric actuators, dynamic displacements in two different directions are applied to the rotating impeller, within a frequency range from 20 to 110 Hz. The resulting displacements and forces are measured in order to identify rotordynamic force coefficients using complex impedances. Stiffness and damping coefficients are compared and discussed in order to highlight the influence of the shroud together with the eye-packing seal and front casing on the impellers performance.