Experimental investigation of unsteady flow behaviour within a sac-type ventricular assist device (VAD)

The flow behaviour of human blood in a pneumatically driven sac-type ventricular assist device (VAD) has been simulated by a non-Newtonian polymer (Separan) solution, and investigated by both a one-component laser Doppler anemometer (LDA) in back-scattering mode and flow visualisation. The device was driven by a pneumatic driver with a squarewave output pressure, and was operated at a pumping rate of 70 bpm, and a systolic duration of 35% of the pumping cycle. The preload and the average afterload to the device were set at 10 mmHg and 100 mmHg, respectively. The output flowrate was 4.05 l min-1. The wall of the peripheral and the central parts of the blood chamber of the VAD were well 'washed' by a main vortex developed during filling and the flow components perpendicular to the base of the VAD produced by the diaphragm motions, respectively. Flow recirculation regions were found at several locations. Around the junction between the tube sections and the blood chamber, the recirculation with an associated fluid particle deposition indicates an underlying risk of thrombus formation. This risk will be reduced by modifying the device design. The maximum turbulent shear stress estimated from the maximum turbulent normal stress (74 N m-2), is unlikely to cause significant damage to blood for the exposure times found. The maximum particle residence time was found to be closely related to the heart rate and the degree of completion of ejection/filling. At a given flow rate, the operating conditions which produce the highest stroke volume give the shortest residence times.