Improvement of washout flow in a centrifugal blood pump by a semi-open impeller

The purpose of this study is to evaluate the modification of the impeller configuration of a centrifugal blood pump by means of flow visualization. Design of the pump geometries to obtain sufficient washout flow throughout the pump, especially in the backside of the impeller, is one of the crucial factors for a long-term use of the pump. We have compared the effect of the impeller shroud on the washout condition through the pump by flow visualization technique of oil film method. The oil film patterns on the impeller surface and on the rear casing were between an open impeller and a semi-open impeller of the centrifugal blood pump developed at our institute. The running condition of the pump was the rotational speed of 3000 rpm for several different flow rates. The summarized features of flow patterns were as follows. A) open type: The flow in the center area of the suction side of the impeller tended to be stagnant. Separated flow at the impeller inlet was observed, but no separation on the rest area on the pressure side. The flow entering the backside of the hall was circumferentially uniform and entered the central washout hall without large tangential component. B) semi-open type: Stagnant area on the suction side is much smaller than that of the open type due to the corner vortices developed at the corner edge of the passage. Reattachment point was observed on the outer edge of the pressure side. The flow patterns appeared on the casing indicated that the washout flow in the semi-open type seemed to have higher angular velocity and strong swirling flow at the bottom of the rear casing although it was not circumferentially uniform but entered mainly from the location of the outlet port. In summary, attachment of the impeller shroud contributed to the reduction of stagnant area, and have altered the flow patterns in the backflow to increase washout.