Analyses of the Unsteady Fluid Force on the Impeller of a Centrifugal Blood Pump

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The present study deals with the analyses of the rotor-dynamic fluid force acting on the impeller of a miniature centrifugal pump developed as a left ventricular assist device. The final goals are to clarify the causes of the unstable problems on the impeller motion of a magnetically-coupled centrifugal pump and to present the design standards for vibration problems. The in-house blood pump originally designed as a left ventricular assist device was selected as the object pump. The displacement of the impeller was measured using two laser displacement meters to obtain the lateral loci of the impeller under the wide range of working conditions. The average position of the impeller shifted in the direction of the outlet port as the flow rates increased at the all investigated rotational speeds (1500 - 3000 rpm). The frequencies of the whirling motion of the impeller were between 0.7 and 0.5, and the rotational velocity components around the impeller bush was influential on the frequency of the whirling motion. This flow works also as the leakage flow, which is expected to wash the bottom area of the pump casing to avoid thrombus formation inside the pump. The practical operational conditions of the pump include flow fluctuation of a large-scale, which was caused by the beatings of the native heart of the patients. The impeller displacements were also measured under the pulsatile conditions by the use of pulsatile blood pump placed at the upstream of the test pump. The results showed no significant difference from the loci of the steady flow rate conditions.For the measurements of the rotordynamic fluid forces, a 7.5 time scale-up model of -the impeller was manufactured and tested under whirling motion. The results are that the unsteady forces are generated on the impeller with the open-blades without the shroud, and that unsteady force becomes steady at the frequency ratio of 0.7 to 0.8.