The Rotordynamic Forces on an Open-Type Centrifugal Compressor Impeller in Whirling Motion

Abstract

In recent years, increasing interest has been given to the rotordynamic forces on impellers, from the view point of the shaft vibration analysis. Previous experimental and analytical results have shown that the fluid-induced forces on closed-type (with shroud) centrifugal impellers in whirling motion contribute substantially to the potential destabilization of subsynchronous shaft vibrations. However, to date nothing is known of the rotordynamic forces on open-type (without shroud) centrifugal impellers. This paper examines the rotordynamic fluid forces on an open-type centrifugal compressor impeller 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 rotordynamic forces. Experiments were performed to investigate the rotordynamic forces by direct measurements using a force balance device, and indirectly from the unsteady pressure on the casing wall over a range of whirl speed ratio (Ω/ω) for several flow rates. In this paper, the following results were obtained: (1) Destabilizing forces occur at small positive whirl speed ratio (0 ≤ Ω/ω ≤ 0.3) throughout the flow range of normal operation; (2) At smaller flow rate with inlet backflow, the magnitude of the fluid force changes dramatically at a whirl speed ratio close to Ω/ω = 0.8, resulting in destabilizing rotordynamic forces. From the measurement of unsteady inlet pressure, it was shown that the drastic changes in the fluid force are related to the coupling of the whirling motion with a rotating flow instability, similar to “rotating stall”; (3) The forces estimated from the unsteady pressure distribution on the casing wall and those estimated 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 on the casing wall.

The Effect of Casing Geometry on Rotordynamic Fluid Forces on a Closed Type Centrifugal Impeller in Whirling Motion

“…The experimental facility was already described in previous papers by Yoshida et al [1] or Suzuki et al [2] so only a quick overview will be presented here. Figure 1 shows both an overall view of the facility and a detailed view of the mechanism used to impose whirling motion to the shaft.…”

Rotordynamic forces acting on a centrifugal open impeller in whirling motion by using active magnetic bearing

“…On the other hand, there have been few reports on the rotordynamic forces acting in ¤open¥ impellers. In industrial open centrifugal impellers, it has been reported that rotordynamic forces encourage instability [3,4]. The present study focuses on the rotordynamic forces acting on the ¤open¥ impeller in a rocket engine turbopump.…”