The rotordynamic forces on an open-type centrifugal compressor impeller inwhirling motion

In recent years, increasing interest has been given to the rotordynamic for ces on impellers, from the view point of the shaft vibration analysis. Prev ious experimental and analytical results have shown that the fluid-induced forces on closed-type (with shroud) centrifugal impellers in whirling motio n contribute substantially to the potential destabilization of subsynchrono us shaft vibrations. However, to date nothing is known of the rotordynamic forces on open-type (without shroud) centrifugal impellers. This paper exam ines the rotordynamic fluid forces on an open-type centrifugal compressor i mpeller in whirling motion. For an open-type impeller, the variation of the tip clearance due to the whirling motion is the main contribute to the rot ordynamic forces. Experiments were performed to investigate the rotordynami c forces by direct measurement using a force balance device, and indirectly from the unsteady pressure on the casing wall over a range of whirl speed ratio (Omega/omega) for several flow rates. In this paper, the following re sults were obtained: (1) Destabilizing forces occur at small positive whirl speed ratio (0 < Omega/omega < 0.3) throughout the flow range of normal op eration; (2) At smaller flow rate with inlet backflow, the magnitude of the fluid force changes dramatically at a whirl speed ratio close to Omega/ome ga = 0.8, resulting in destabilizing rotordynamic forces. From the measurem ent of unsteady inlet pressure, it was shown that the drastic changes in th e fluid force are related to the coupling of the whirling motion with a rot ating pow instability, similar to "rotating stall"; (3) The forces estimate d from the unsteady pressure distribution an the casing wall and those esti mated from the pressure difference across the impeller blades were compared with the results from the direct fluid force measurements The direct fluid forces correlate better with the forces due to the pressure distribution o n the casing wall.