The Rotordynamic Fluid Forces on an Artificial Heart Pump Impeller in Whirling Motion

In centrifugal pumps for artificial hearts, a magnetic drive and lightly loaded journal bearing system are often used to avoid leakage and to minimize the damage on blood cells. In such system, the rigidity of the bearing is small and the impeller usually rotates over the critical speed. For such cases, the rotordynamic fluid forces play an important role for the stability of operation. In the present study, the characteristics of the rotordynamic fluid forces on the impeller were examined. The destabilizing fluid force which encourages the whirling motion of the impeller occurred in the range of 0<ω/Ω?0.75, where ω and Ω are the angular velocities of the whirling motion and the rotation of the impeller, respectively. The steady fluid forces in the case without the whirling motion were also investigated. The magnitude of the steady fluid force was consistent with the unsteady fluid force in ω/Ω≅0. The forces estimated from the unsteady pressure distribution on the casing were in agreement with the force measured directly by 4-axis sensor. The effects of leakage flow were also examined. It was found that the leakage flow has a stabilizing effect on the whirling motion of the impeller.