Computational fluid dynamics analysis of blade tip clearances on hemodynamic performance and blood damage in a centrifugal ventricular assist device

Abstract

An important challenge facing the design of turbodynamic ventricular assist devices (VADs) intended for long-term support is the optimization of the flow path geometry to maximize hydraulic performance while minimizing shear-stress-induced hemolysis and thrombosis. For unshrouded centrifugal, mixed-flow and axial-flow blood pumps, the complex flow patterns within the blade tip clearance between the lengthwise upper surface of the rotating impeller blades and the stationary pump housing have a dramatic effect on both the hydrodynamic performance and the blood damage production. Detailed computational fluid dynamics (CFD) analyses were performed in this study to investigate such flow behavior in blade tip clearance region for a centrifugal blood pump representing a scaled-up version of a prototype pediatric VAD. Nominal flow conditions were analyzed at a flow rate of 2.5 L/min and rotor speed of 3000 rpm with three blade tip clearances of 50, 100, and 200 microm. CFD simulations predicted a decrease in the averaged tip leakage flow rate and an increase in pump head and axial thrust with decreasing blade tip clearances from 200 to 50 microm. The predicted hemolysis, however, exhibited a unimodal relationship, having a minimum at 100 microm compared to 50 microm and 200 microm. Experimental data corroborate these predictions. Detailed flow patterns observed in this study revealed interesting fluid dynamic features associated with the blade tip clearances, such as the generation and dissipation of tip leakage vortex and its interaction with the primary flow in the blade-blade passages. Quantitative calculations suggested the existence of an optimal blade tip clearance by which hydraulic efficiency can be maximized and hemolysis minimized.

FIG. 1

(a) Meridional sectional view of shrouded (closed) centrifugal pump illustrating typical retrograde flow in the clearance between the shroud and housing. (b) Meridional section of unshrouded (semi-open) impeller. (c) Typical tip leakage flow for unshrouded impeller from pressure-side to suction-side of blade, illustrating the formation of tip leakage vortex.

FIG. 2

Centrifugal pediatric VAD prototype including the 12.5 mm impeller, outlet, and inlet volutes.

FIG. 3

CFD meshes for scaled-up experimental pump with 46 mm diameter impeller. (a) Entire flow domain, (b) detail of C-grid around the leading edge of blades and the grid at the blade tip region (top of blade tip), and (c) detail of C-grid near the trailing edge of blades and the partial grid of volute and hub regions.

FIG. 4

Comparison of the computed pressure head versus measured H-Q curves for three different blade tip clearances of the scaled-up model at 3000 rpm.

FIG. 5

(a) Tip leakage flow through each of six blade tip gaps for different tip clearances (flow rate = 2.5 L/min, and pump speed = 3000 rpm); blades numbered clockwise, starting with blade closest to cutwater. (b) The mean leakage flow through six blade tips versus blade tip clearances.

FIG. 6

Velocity field illustrating tip leakage for three clearances studied. Left column: relative velocity vectors projected onto four radial sections of r = 7.5, 10, 15, and 20 mm, colored according to velocity magnitude. Middle column: particle pathlines. Right column: Scalar shear stress distribution on cut-plane through mean height of blade tip clearance. (Rotational direction is clockwise.)

FIG. 7

Maximal shear stress on the middle plane of blade tip clearances.

FIG. 8

Pathlines from tens of representative particles (out of a total of 6000 particles) released to compute the shear history and estimate hemolysis.

FIG. 9

Mean estimated blood damage through the pump model for three different blade tip clearances (based on 6000 particles).

Fig. 10

Proportion of total blood damage contributed by different pump components for three blade tip clearances.