Effect of Washout Hole Geometry on a Centrifugal Blood Pump

Quantitative flow visualization was performed to obtain suitable design criteria of a centrifugal blood pump. Particular attention was paid to the effect of washout hole geometry to prevent thrombus formation which may be caused by stagnation behind the impeller. Experiments were conducted with 2.5 times scale-up models based on Nikkiso HPM-15 with 64 wt% sodium iodide solution as working fluid and 150 μm silicon dioxide particles as tracer. A 7 W Ar-ion laser light sheet and a 4,500 frames/sec, 256x256 pixel high speed video camera were used for lighting and for photographing respectively, and image analyses were made by particle tracking velocimetry. All driving conditions were set for a flow rate of 5 1/min and a head of 300 mmHg representing extracorporeal condition. Results were as follows: flow through the washout holes caused by the pressure difference induces a secondary flow in the gap behind the impeller that prevents stagnation of the flow. Comparing various diameters and radial positions of washout holes, it was found that the optimal washout hole minimizes the stagnation behind the impeller, whereas it hardly affects the flow behavior in the outlet or pressure-head curves. Not only small holes but also too large holes decrease washout effect because of the appearance of reversal flow. The best washout was achieved by the standard hole model, and the flow rate through the washout holes was found to be almost half the external flow rate.