Quantitative Visualization of Flow Through a Centrifugal Blood Pump: Effect of Washout Holes

Abstract

To clarify the effect of washout holes on the flow in a centrifugal blood pump to prevent blood stagnation, a quantitative flow visualization technique was applied to compare flows in models with and without washout holes. A scaled-up model of a prototype pump and a high speed video camera were used for the flow visualization, and images were processed by particle tracking velocimetry. Particular attention was paid to the flow through the gaps behind and in front of the impeller. The results showed that in the gap behind the impeller, washout holes caused not only an inward flow, but also an increase in the tangential velocities. In the gap in front of the impeller, washout holes caused an outward flow and a decrease in the tangential velocities. Head flow characteristics were little affected by the washout holes in this initial design for which the flow through the washout holes was set to be approximately 10% of the flow in the external circuit. These results suggest that the flow through washout holes is significant in the prevention of blood stagnation in 2 ways. First, the inward radial velocity behind the impeller and outward velocity in front of the impeller result in fluid exchange, and second, a tangential velocity increase reduces fluid stagnation behind the impeller.

Washout Hole Flow Measurement for the Development of a Centrifugal Blood Pump

“…The radial pressure gradient is proportional to the centrifugal force, which is proportional to the square of the tangential velocity component. Therefore, the pressure gradient is reduced by the washout holes as low flow momentum is conveyed to the front gap by the washout hole flow (7). Figure 4b shows the radial distribution of the normalized pressure in the back gap of the impeller.…”

“…Images are then digitized by a film scanner into a personal computer, and water levels are read and converted to static pressures. Figure 2b shows the experimental apparatus for flow visualization (7), for which the working fluid is a 64% NaI solution with 0.1% Na 2 S 2 O 3 as stabilizer (specific gravity 1.9). The fluid has the same refractive index (1.49) as acrylic resin.…”

Critical review of current left ventricular assist devices

“…77 Washout holes are effective in reducing blood stagnation and hence in preventing thrombus formation in the gap behind the impeller. 78 When the external circuit resistance increases (i.e. the specific speed increases) the pressure difference increases between the entrance and exit of the washout holes and causes a higher flow rate via the washout holes.…”

Flow Visualization as a Complementary Tool to Hemolysis Testing in the Development of Centrifugal Blood Pumps

“…The visualization apparatus consisted of a 250% scaled‐up pump model with a 6 vaned semiopen impeller (Fig. 1, diameter: 125 mm, similar to HPM‐15, (Nikkiso Co., Ltd., Tokyo, Japan), high speed video camera (4,500 frame/s, Photron, Tokyo, Japan), Argon ion laser light sheet (7 W, LEXEL, Fremont, CA, USA), and 4 frame particle tracking software (Current, Kanomax, Osaka, Japan) (3). The pump model was made of square transparent acrylic block for flow visualization.…”