Flow investigation in a centrifugal pump

Abstract

From multiple medical studies on ventricular assist devices, thrombosis and haemolysis within centrifugal blood pumps have shown to be greatly affected by leakage flow characteristics. Leakage flow occurs within the clearance gap between impeller wall and volute wall of a centrifugal pump. Stagnation of blood flow can cause blood to clot while high blood flow rate accompanied by shearing can cause blood to rupture. Leakage flow can also reduce the volumetric efficiency of a centrifugal pump. These effects are undesirable and hence form the study motivation for this project.

According to many researchers investigating flow patterns within ventricular assist devices, it is difficult to measure the leakage flow rates in the clearance gap experimentally. One of the many experimental methods to obtain leakage flow rates is to drive back flow of fluid through a pump with dummy impeller using an auxiliary pump. Then, a coupling method that uses differential pressure can extrapolate total leakage flow rates for pump with real impeller. Thus, Computational Fluid Dynamics (CFD) simulation was utilised in this study to validate results obtained from the experimental dummy impeller method and investigate leakage flow characteristics using a numerical approach.

CFD simulations were done for a miniature pump with 50mm diameter impeller. 80 cases were simulated in total and the variables include operating speed (1500rpm, 2000rpm), gap width (0.2mm, 0.3mm, 0.4mm, 0.5mm), and impeller type (real, dummy). It was found that pump performance is better when gap width is decreased. Total leakage flow rates increased with higher differential pressure across gap and larger gap width. Total leakage flow rates obtained from experiments and CFD simulation for both operating speeds have less than 10% difference on average, except for gap width of 0.5mm. While within experimental and numerical errors, pump with dummy impeller and CFD simulations were able to estimate total leakage flow rates fairly well.