Downloaded from http://journals.lww.com/asaiojournal by BhDMf5ePHKbH4TTImqenVAHxkFJp/XpPk1L/H3vMGwqMxG9jwOd8eHu+WdxxmltmG+Y2f0aqIM4= on 07/30/2018 Particle Image Velocimetry Measurements of Blood Velocity in a Continuous Flow Ventricular Assist Device STEVEN W. DAY,JAMES C. MCDANIEL,HOUSTON G. WOOD,PAUL E. ALLAIRE,NICOLAS LANDROT, AND ANTHONY CURTAS The third prototype of a continuous flow ventricular assist device (CFVAD3) is being developed and tested for implan- tation in humans. The blood in the pump flows through a fully shrouded four-bladed impeller (supported by magnetic bear- ings) and through small clearance regions on either side of the impeller. Measurements of velocities using particle image velocimetry of a fluid with the same viscosity as blood have been made in one of these clearance regions. Particle image velocimetry is a technique that measures the instantaneous velocity field within an illuminated plane of the fluid field by scattering light from particles added to the fluid. These mea- surements have been used to improve understanding of the fluid dynamics within these critical regions, which are possi- ble locations of both high shear and stagnation, both of which are to be avoided in a blood pump. Computational models of the pump exist and these models are currently being used to aid in the design of future prototypes. Among other things, these models are used to predict the potential for hemolysis and thrombosis. Measurements of steady flow at two operat- ing speeds and flow rates are presented. The measurements are compared with the computed solutions to validate and refine, where necessary, the existing computational models. ASAIO Journal 2001; 47:406 –411. T he CFVAD3 is the third prototype in a design evolution of a continuous flow, magnetically suspended centrifugal pump intended for use as a left ventricular assist device. This pump is part of the HeartQuest™ series being developed by MedQuest Products, Inc., the Utah Artificial Heart Institute, and the Uni- versity of Virginia. The pump is designed to deliver 6 L/min at 100 mm Hg pressure rise while running at 2,000 rpm. The blood in the pump flows through a fully shrouded four-bladed impeller (supported by magnetic bearings) and through small clearance regions on either side of the impeller. The absence of seals in the clearance regions between the impeller and pump housing is one of the principle advantages of magnetically suspended pumps in terms of durability and blood damage (Figure 1). The flow within the small clearance region between the impeller and the housing is complicated. The fluid field is determined by the interaction of a pressure force acting radially inward (toward the pump inlet) and an outward radial force due to viscous interaction with the rotat- ing impeller. The fluid dynamics within this region are also of great significance, because there is the potential for both he- molysis due to high shear stress and thrombosis resulting from low velocity regions near the surfaces of the housing and impeller. It is desirable in terms of blood damage to have continual movement of the blood in one direction through the clearance regions, known as “washing.” This design avoids regions of stagnation and prolonged exposure to this poten- tially high shear. However, inward radial flow contributes to the inefficiency of the pump. The design goal is to minimize the net flow through the clearance gaps, while still avoiding blood damage. This paper describes measurements intended to characterize the flow in one of these gap regions within a CFVAD3 prototype. The measurements are intended to lend insight into the nature of this flow and to validate and refine computational fluid dynamics (CFD) models of this flow. Com- putational results for this geometry were first reported by Anderson et al. 1 The clinical version (CFVAD4) of the pump is currently being designed relying heavily on CFD. Validation of the CFD models is critical. Materials and Methods Optical Measurement Technique: Particle Image Velocimetry Optical measurement techniques offer several advantages over physical probes. Optical techniques do not interfere with the fluid, whereas traditional mechanical measurement probes may distort the flow that they are measuring. Furthermore, optical techniques offer the advantage that light may be used to probe regions in which one could not easily locate a phys- ical probe, such as the small clearance regions inside the heart pump. Using a laser as a light source, it is possible to very accurately locate and shape this illuminated plane so that one can make measurements within a very thin measurement volume. Particle image velocimetry (PIV) is an optical technique that measures the instantaneous velocity field within an illumi- nated plane of the fluid field using light scattered from particles seeded into the fluid. It differs from flow visualization, because it is quantitative, whereas flow visualization is qualitative. PIV also has the advantage of making a simultaneous measurement at many points within an illuminated plane, as opposed to a single point measurement using laser Doppler velocimetry. PIV has very recently matured and proven to be a very useful and practical tool for studying a wide range of fluid flows. 6 A schematic of a basic PIV system is shown in Figure 2. Successive image pairs within the laser illuminated plane are captured using a high-speed digital camera. Using a pulsed laser, it can be ensured that each image pair may be obtained at an effectively instantaneous time (very small relative to the From the Mechanical and Aerospace Engineering Department, Uni- versity of Virginia, Charlottesville, Virginia. Submitted for consideration July 2000; accepted for publication in revised form December 2000. Reprint requests: Steven W. Day, Mechanical and Aerospace Engi- neering, University of Virginia, Charlottesville, VA 22903. ASAIO Journal 2001 406