Impeller behavior and displacement of the VentrAssist implantable rotary blood pump

The VentrAssist implantable rotary blood pump, intended for long-term ventricular assist, is under development and is currently being tested for its rotor-dynamic stability. The pump is of the centrifugal type and consists of a shaftless impeller, also acting as the rotor of the brushless DC motor. The impeller remains passively suspended in the pump cavity by hydrodynamic forces, resulting from the small clearances between the impeller outside surfaces and the pump cavity. In the older version of the pump tested, these small clearances range from approximately 50 microm to 230 microm; the displacement of the impeller relative to the pump cavity is unknown in use. This article presents two experiments: the first measured displacement of the impeller using eddy-current proximity sensors and laser proximity sensors. The second experiment used Hall-effect proximity sensors to measure the displacement of the impeller relative to the pump cavity. All transducers were calibrated prior to commencement of the experiments. Voltage output from the transducers was converted into impeller movement in five degrees of freedom (x, y, z, theta(x), and theta(y)). The sixth degree of freedom, the rotation about the impeller axis (theta(z)), was determined by the commutation performed by the motor controller. The impeller displacement was found to be within the acceptable range of 8 micro m to 222 microm, avoiding blood damage and contact between the impeller and cavity walls. Thus the impeller was hydrodynamically suspended within the pump cavity and results were typical of centrifugal pump behavior. This research will be the basis for further investigation into the stiffness and damping coefficient of the pump's hydrodynamic bearing.     

Dynamic characteristics of a magnetically levitated impeller in a centrifugal blood pump

Centrifugal blood pumps that employ hybrid active/passive magnetic bearings to support noncontact impellers have been developed in order to reduce bearing wear, pump size, the power consumption of the active magnetic bearing, and blood trauma. However, estimates made at the design stage of the vibration of the impeller in the direction of passive suspension during pump operation were inaccurate, because the influence of both the pumping fluid and the rotation of the impeller on the dynamic characteristics was not fully recognized. The purpose of this study is to investigate the dynamic characteristics in a fluid of a magnetically levitated rotating impeller by measuring both the frequency response to sinusoidal excitation of the housing over a wide frequency range and the displacement due to input of a pulsatile flow during left ventricular (LV) assist. The excitation tests were conducted under conditions in which the impeller was levitated in either air or water, and with or without rotation. The experimental and analytical results indicate that vibration of the impeller due to the external force in water was decreased, compared with that in air due to the hydraulic force of water. The axial resonant frequency rose quadratically with rotational speed, and the tilt mode had two resonant frequencies while rotating due to the gyroscopic effect. With the pump inserted into a mock systemic circulatory loop, the dynamic stability of the impeller when pulsatile pressure was applied during LV assist was verified experimentally. The amplitudes of vibration in response to the pulsatile flow in the passively constrained directions were considerably smaller in size than the dimensions of initial gaps between the impeller and the pump housing.     

Test Controller Design, Implementation, and Performance for a Magnetic Suspension Continuous Flow Ventricular Assist Device

A new continuous flow ventricular assist device using full magnetic suspension has been designed, constructed, and tested. The magnetic suspension centers the centrifugal pump impeller within the clearance passages in the pump, thus avoiding any form of contact. The noncontact operation is designed to give very high expected mechanical reliability, large clearances, low hemolysis, and a relatively small size compared to current pulsatile devices. A unique configuration of magnetic actuators on the inlet side and exit sides of the impeller provides full 5 axis control and suspension of the impeller. The bearing system is divided into segments which allow for 3 displacement axes and 2 angular control axes. The controller chosen for the first suspension tests consists of a decentralized set of 5 proportional integral derivative (PID) controllers. This document describes both the controller and an overview of some results pertaining to the magnetic bearing performance. The pump has been successfully operated in both water and blood under design conditions suitable for use as a ventricular assist device.     

The Effects of Ambulatory Accelerations on the Stability of a Magnetically Suspended Impeller for an Implantable Blood Pump

This article describes the effects of ambulatory accelerations on the stability of a magnetically suspended impeller for use in implantable blood pumps. A magnetic suspension system is developed to control the radial position of a magnetic impeller using coils in the pump casing. The magnitude and periodicity of ambulatory accelerations at the torso are measured. A test rig is then designed to apply appropriate accelerations to the suspension system. Accelerations from 0 to 1 g are applied to the suspended impeller with ambulatory periodicity while the radial position of the impeller and power consumption of the suspension system are monitored. The test is carried out with the impeller suspended in air, water, and a glycerol solution to simulate the viscosity of blood. A model is developed to investigate the effects of the radial magnetic suspension system and fluid damping during ambulatory accelerations. The suspension system reduces the average displacement of the impeller suspended in aqueous solutions within its casing to 100 µm with a power consumption of below 2 W during higher magnitude ambulatory accelerations (RMS magnitude 0.3 g). The damping effect of the fluid is also examined and it is shown that buoyancy, rather than drag, is the primary cause of the damping at the low displacement oscillations that occur during the application of ambulatory accelerations to such a suspension system.     

Parameter Estimation and Actuator Characteristics of Hybrid Magnetic Bearings for Axial Flow Blood Pump Applications

Axial flow blood pumps are generally smaller as compared to centrifugal pumps. This is very beneficial because they can provide better anatomical fit in the chest cavity, as well as lower the risk of infection. This article discusses the design, levitated responses, and parameter estimation of the dynamic characteristics of a compact hybrid magnetic bearing (HMB) system for axial flow blood pump applications. The rotor/impeller of the pump is driven by a three-phase permanent magnet brushless and sensorless motor. It is levitated by two HMBs at both ends in five degree of freedom with proportional–integral–derivative controllers, among which four radial directions are actively controlled and one axial direction is passively controlled. The frequency domain parameter estimation technique with statistical analysis is adopted to validate the stiffness and damping coefficients of the HMB system. A specially designed test rig facilitated the estimation of the bearing's coefficients in air—in both the radial and axial directions. Experimental estimation showed that the dynamic characteristics of the HMB system are dominated by the frequency-dependent stiffness coefficients. By injecting a multifrequency excitation force signal onto the rotor through the HMBs, it is noticed in the experimental results the maximum displacement linear operating range is 20% of the static eccentricity with respect to the rotor and stator gap clearance. The actuator gain was also successfully calibrated and may potentially extend the parameter estimation technique developed in the study of identification and monitoring of the pump's dynamic properties under normal operating conditions with fluid.     

Fluid dynamic characteristics of the VentrAssist rotary blood pump

The VentrAssist pump has no shaft or seal, and the device is unique in design because the rotor is suspended passively by hydrodynamic forces, and urging is accomplished by an integrated direct current motor rotor that also acts as the pump impeller. This device has led to many challenges in its fluidic design, namely large flow-blockage from impeller blades, low stiffness of bearings with concomitant impeller displacement under pulsatile load conditions, and very small running clearances. Low specific speed and radial blade off-flow were selected in order to minimize the hemolysis. Pulsatile and steady-flow tests show the impeller is stable under normal operating conditions. Computational fluid dynamics (CFD) has been used to optimize flow paths and reduce net axial force imbalance to acceptably small values. The latest design of the pump achieved a system efficiency of 18% (in 30% hematocrit of red blood cells suspended in phosphate-buffered saline), and efficiency was optimized over the range of operating conditions. Parameters critical to improving pump efficiency were investigated.     

A hydrodynamically suspended, magnetically sealed mechanically noncontact axial flow blood pump: design of a hydrodynamic bearing

To overcome the drive shaft seal and bearing problem in rotary blood pumps, a hydrodynamic bearing, a magnetic fluid seal, and a brushless direct current (DC) motor were employed in an axial flow pump. This enabled contact-free rotation of the impeller without material wear. The axial flow pump consisted of a brushless DC motor, an impeller, and a guide vane. The motor rotor was directly connected to the impeller by a motor shaft. A hydrodynamic bearing was installed on the motor shaft. The motor and the hydrodynamic bearing were housed in a cylindrical casing and were waterproofed by a magnetic fluid seal, a mechanically noncontact seal. Impeller shaft displacement was measured using a laser sensor. Axial and radial displacements of the shaft were only a few micrometers for motor speed up to 8500 rpm. The shaft did not make contact with the bearing housing. A flow of 5 L/min was obtained at 8000 rpm at a pressure difference of 100 mm Hg. In conclusion, the axial flow blood pump consisting of a hydrodynamic bearing, a magnetic fluid seal, and a brushless DC motor provided contact-free rotation of the impeller without material wear.     

Evaluation of hemolysis in the VentrAssist implantable rotary blood pump

The VentrAssist implantable rotary blood pump (IRBP) is an implantable centrifugal blood pump with a hydrodynamically suspended impeller; optimal efficiency requires small running clearances (70-300 microm). The effect of running clearance and polish on hemolysis was evaluated in vitro. Three different human blood suspensions were compared: phosphate buffered saline (PBS), plasma volume expander (Hemaccel), and whole blood. The test conditions were: blood hematocrit 30%, flow rate 5 L/min, pressure across pump 100 mm Hg, 6 h flow period, and 37 degrees C. Normalized Index of Hemolysis (NIH) for the Biomedicus BP-80, used as a control, was: 0.0040 +/- 0.0023 (n = 9; x +/- SD) and 0.00014 +/- 0.00009 (n = 5) for pooled blood suspensions in PBS and Hemaccel respectively, and 0.00053 +/- 0.0002 (n = 3) in whole blood. Hemolysis was reduced by improved surface finish and unaffected by running clearance. NIH for the VentrAssist IRBP with 0.2 microm Ra surface finish was 0.000167 +/- 0.00007 (n = 4) g/100 L in whole human blood, demonstrating minimal hemolysis.     

Detection of left ventricle function from a magnetically levitated impeller behavior

The magnetically levitated (Mag-Lev) centrifugal rotary blood pump (CRBP) with two-degrees-of-freedom active control is promising for safe and long-term support of circulation. In this study, Mag-Lev CRBP controllability and impeller behavior were studied in the simulated heart failure circulatory model. A pneumatically driven pulsatile blood pump (Medos VAD [ventricular assist device]-54 mL) was used to simulate the left ventricle (LV). The Mag-Lev CRBP was placed between the LV apex and aortic compliance tank simulating LV assistance. The impeller behavior in five axes (x, y, z, theta, and phi) was continuously monitored using five eddy current sensors. The signals of the x- and y-axes were used for feedback active control, while the behaviors of the other three axes were passively controlled by the permanent magnets. In the static mock circuit, the impeller movement was controlled to within +/-10-+/-20 microm in the x- and y-axes, while in the pulsatile circuit, LV pulsation was modulated in the impeller movement with the amplitude being 2-22 microm. The amplitude of impeller movement measured at 1800 rpm with the simulated failing heart (peak LV pressure [LVP] = 70 mm Hg, mean aortic pressure [AoP(mean)] = 55 +/- 20 mm Hg, aortic flow = 2.7 L/min) was 12.6 microm, while it increased to 19.2 microm with the recovered heart (peak LVP = 122 mm Hg, AoP(mean) = 100 +/- 20 mm Hg, aortic flow = 3.9 L/min). The impeller repeated the reciprocating movement from the center of the pump toward the outlet port with LV pulsation. Angular rotation (theta, phi) was around +/-0.002 rad without z-axis displacement. Power requirements ranged from 0.6 to 0.9 W. Five-axis impeller behavior and Mag-Lev controller stability were demonstrated in the pulsatile mock circuit. Noncontact drive and low power requirements were shown despite the effects of LV pulsation. The impeller position signals in the x- and y-axes reflected LV function. The Mag-Lev CRBP is effective not only for noncontact low power control of the impeller, but also for diagnosis of cardiac function noninvasively.     

Antithrombogenicity of the Gyro permanently implantable pump with the RPM dynamic suspension system for the impeller

In 1995, a group at Baylor College of Medicine started to develop the NEDO biventricular assist device (BVAD) using two Gyro permanently implantable (PI) centrifugal pumps. This pump consists of a sealless pump housing and an impeller supported with a double pivot bearing. In May 2001, an RPM dynamic suspension system (RPM-DS) for the impeller was developed to improve durability and antithrombogenicity without a complex magnetic suspension system. From March 2000 to March 2002, eight BVAD bovine experimental studies were performed for more than 1 month. Two pumps were implanted in two cases without the RPM-DS (group A) and in six cases with the RPM-DS (group B). In group A, the survival period was 45 and 50 days. The primary reason for termination was an increase in the requiring power, which was related to deposition of white thrombus on the bottom bearing. In group B, the survival period was 37, 48, 51, 60, 80, and 90 days. The reasons for termination were not related to thrombus formation. No thrombus was observed in the pumps except for one right pump. In that experiment, the thrombus formation may have occurred when that pump had a low flow rate at a level of 1 L/min for 6 hr. These studies demonstrate the apparent antithrombogenic effect of RPM-DS. The NEDO BVAD is ready to move into a 3-month preclinical system evaluation.     

Magnetically suspended centrifugal blood pump with an axially levitated motor

The longevity of a rotary blood pump is mainly determined by the durability of its wearing mechanical parts such as bearings and seals. Magnetic suspension techniques can be used to eliminate these mechanical parts altogether. This article describes a magnetically suspended centrifugal blood pump using an axially levitated motor. The motor comprises an upper stator, a bottom stator, and a levitated rotor-impeller between the stators. The upper stator has permanent magnets to generate an attractive axial bias force on the rotor and electric magnets to control the inclination of the rotor. The bottom stator has electric magnets to generate attractive forces and rotating torque to control the axial displacement and rotation of the rotor. The radial displacement of the rotor is restricted by passive stability. A shrouded impeller is integrated within the rotor. The performance of the magnetic suspension and pump were evaluated in a closed mock loop circuit filled with water. The maximum amplitude of the rotor displacement in the axial direction was only 0.06 mm. The maximum possible rotational speed during levitation was 1,600 rpm. The maximum pressure head and flow rate were 120 mm Hg and 7 L/min, respectively. The pump shows promise as a ventricular assist device.     

Evaluation of hydraulic radial forces on the impeller by the volute in a centrifugal rotary blood pump

In many state-of-the-art rotary blood pumps for long-term ventricular assistance, the impeller is suspended within the casing by magnetic or hydrodynamic means. For the design of such suspension systems, profound knowledge of the acting forces on the impeller is crucial. Hydrodynamic bearings running at low clearance gaps can yield increased blood damage and magnetic bearings counteracting high forces consume excessive power. Most current rotary blood pump devices with contactless bearings are centrifugal pumps that incorporate a radial diffuser volute where hydraulic forces on the impeller develop. The yielding radial forces are highly dependent on impeller design, operating point and volute design. There are three basic types of volute design--singular, circular, and double volute. In this study, the hydraulic radial forces on the impeller created by the volute in an investigational centrifugal blood pump are evaluated and discussed with regard to the choice of contactless suspension systems. Each volute type was tested experimentally in a centrifugal pump test setup at various rotational speeds and flow rates. For the pump's design point at 5 L/min and 2500 rpm, the single volute had the lowest radial force (～0 N), the circular volute yielded the highest force (～2 N), and the double volute possessed a force of approx. 0.5 N. Results of radial force magnitude and direction were obtained and compared with a previously performed computational fluid dynamics (CFD) study.     

Numerical analysis of blood flow in the clearance regions of a continuous flow artificial heart pump

The CFVAD3 is the third prototype of a continuous flow ventricular assist device being developed for implantation in humans. The pump consists of a fully shrouded 4-blade impeller supported by magnetic bearings. On either side of this suspended rotating impeller is a small clearance region through which the blood flows. The spacing and geometry of these clearance regions are very important to the successful operation of this blood pump. Computational fluid dynamics (CFD) solutions for this flow were obtained using TascFlow, a software package available from AEA Technology, U.K. Flow in these clearance regions was studied parametrically by varying the size of the clearance, the blood flow rate into the pump, and the rotational speed of the pump. The numerical solutions yield the direction and magnitude of the flow and the dynamic pressure. Experimentally measured pump flow rates are compared to the numerical study. The results of the study provide guidance for improving pump efficiency. It is determined that current clearances can be significantly reduced to improve pump efficiency without negative impacts.     

Disposable MagLev Centrifugal Blood Pump Utilizing a Cone‐Shaped Impeller

To enhance the durability and reduce the blood trauma of a conventional blood pump with a cone‐shaped impeller, a magnetically levitated (MagLev) technology has been applied to the BioPump BPX‐80 (Medtronic Biomedicus, Inc., Minneapolis, MN, USA), whose impeller is supported by a mechanical bearing. The MagLev BioPump (MagLev BP), which we have developed, has a cone‐shaped impeller, the same as that used in the BPX‐80. The suspension and driving system, which is comprised of two degrees of freedom, radial‐controlled magnetic bearing, and a simply structured magnetic coupling, eliminates any physical contact between the impeller and the housing. To reduce both oscillation of the impeller and current in the coils, the magnetic bearing system utilizes repetitive and zero‐power compensators. In this article, we present the design of the MagLev mechanism, measure the levitational accuracy of the impeller and pressure‐flow curves (head‐quantity [HQ] characteristics), and describe in vitro experiments designed to measure hemolysis. For the flow‐induced hemolysis of the initial design to be reduced, the blood damage index was estimated by using computational fluid dynamics (CFD) analysis. Stable rotation of the impeller in a prototype MagLev BP from 0 to 2750 rpm was obtained, yielding a flow rate of 5 L/min against a head pressure in excess of 250 mm Hg. Because the impeller of the prototype MagLev BP is levitated without contact, the normalized index of hemolysis was 10% less than the equivalent value with the BPX‐80. The results of the CFD analysis showed that the shape of the outlet and the width of the fluid clearances have a large effect on blood damage. The prototype MagLev BP satisfied the required HQ characteristics (5 L/min, 250 mm Hg) for extracorporeal circulation support with stable levitation of the impeller and showed an acceptable level of hemolysis. The simulation results of the CFD analysis indicated the possibility of further reducing the blood damage of the prototype MagLev BP.     

Computational Fluid Dynamics Analysis of Blade Tip Clearances on Hemodynamic Performance and Blood Damage in a Centrifugal Ventricular Assist Device

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 µm. 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 µm. The predicted hemolysis, however, exhibited a unimodal relationship, having a minimum at 100 µm compared to 50 µm and 200 µm. 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.     

Real‐Time Observation of Thrombus Growth Process in an Impeller of a Hydrodynamically Levitated Centrifugal Blood Pump by Near‐Infrared Hyperspectral Imaging

Understanding the thrombus formation in cardiovascular devices such as rotary blood pumps is the most important issue in developing more hemocompatible devices. The objective of this study was to develop a hyperspectral imaging (HSI) method to visualize the thrombus growth process within a rotary blood pump and investigate the optical properties of the thrombus. An in vitro thrombogenic test was conducted using fresh porcine blood and a specially designed hydrodynamically levitated centrifugal blood pump with a transparent bottom. The pump rotating at 3000 rpm circulated the blood at 1.0 L/min. The bottom surface of the pump was illuminated with white light pulsed at the same frequency as the pump rotation, and the backward‐scattered light was imaged using the HSI system. Using stroboscopic HSI and an image construction algorithm, dynamic spectral imaging at wavelengths ranging from 608 to 752 nm within the rotating pump was achieved. After completing the experiment, we collected the red thrombus formed in the pump impeller and quantified the thrombus hemoglobin concentration (Hb_thrombus). The spectrum changed around the center of the impeller, and the area of change expanded toward the impeller flow path. The shape corresponded approximately to the shape of the thrombus. The spectrum change indicated that the light scattering derived from red blood cells decreased. The Hb_thrombus was 4.7 ± 1.3 g/dL versus a total hemoglobin of 13 ± 0.87 g/dL. The study revealed that Hb_thrombus was reduced by the surrounding blood flow.     

Parametric Study of Blade Tip Clearance, Flow Rate, and Impeller Speed on Blood Damage in Rotary Blood Pump

Phenomenological studies on mechanical hemolysis in rotary blood pumps have provided empirical relationships that predict hemoglobin release as an exponential function of shear rate and time. However, these relations are not universally valid in all flow circumstances, particularly in small gap clearances. The experiments in this study were conducted at multiple operating points based on flow rate, impeller speed, and tip gap clearance. Fresh bovine red blood cells were resuspended in phosphate-buffered saline at about 30% hematocrit, and circulated for 30 min in a centrifugal blood pump with a variable tip gap, designed specifically for these studies. Blood damage indices were found to increase with increased impeller speed or decreased flow rate. The hemolysis index for 50-µm tip gap was found to be less than 200-µm gap, despite increased shear rate. This is explained by a cell screening effect that prevents cells from entering the smaller gap. It is suggested that these parameters should be reflected in the hemolysis model not only for the design, but for the practical use of rotary blood pumps, and that further investigation is needed to explore other possible factors contributing to hemolysis.     

Development of the Baylor/NASA Axial Flow Ventricular Assist Device: In Vitro Performance and Systematic Hemolysis Test Results

Abstract: Our newly developed axial flow pump consists of a flow tube, an internal rotating impeller, and a fixed flow stator (we call the stator) behind the impeller. This pump produces a flow of 3 to 8 L/min against 50 to 150 mm Hg pressure difference, respectively, in the range of 10,000 to 16,000 rpm. An axial flow pump that will be used as a ventricular assist device (VAD) has to have low hemolytic and good antithrombogenic characteristics. This paper will show how to decrease the hemolytic properties of this axial flow pump systematically using a test matrix. The test variables evaluated were impeller blade tip geometry, impeller flow tube clearance (radial clearance), impeller stator clearance (axial clearance), impeller blade number, stator blade number, and impeller length. All in vitro hemolysis tests were performed at 5.0 L/min against 100 mm Hg pressure difference using a total of 83 bags of fresh bovine blood. The results were as follows: the impeller blade tip geometry did not significantly effect hemolysis, a 0.005-inch and a 0.009-inch radial clearance were significantly (p < 0.01 or 0.001) less hemolytic than the other clearances, a 0.075-inch axial clearance was significantly (p < 0.05) more hemolytic than the other clearances, two-and six-bladed impellers were significantly (p < 0.01 and 0.02, respectively) less hemolytic than a four-bladed impeller, a five-bladed stator was significantly (p < 0.05 or 0.01) less hemolytic than the other stators, and the impeller length did not make a significant difference. Currently, the best index of hemolysis is 0.031 ± 0.018 g/100 L, and using parameters from these results, implantable devices are being fabricated.     

Experimental and Numerical Investigation of an Axial Rotary Blood Pump

Left ventricular assist devices (LVADs) have become a standard therapy for patients with severe heart failure. As low blood trauma in LVADs is important for a good clinical outcome, the assessment of the fluid loads inside the pump is critical. More specifically, the flow features on the surfaces where the interaction between blood and artificial material happens is of great importance. Therefore, experimental data for the near‐wall flows in an axial rotary blood pump were collected and directly compared to computational fluid dynamic results. For this, the flow fields based on unsteady Reynolds‐averaged Navier–Stokes simulations‐computational fluid dynamics (URANS‐CFD) of an axial rotary blood pump were calculated and compared with experimental flow data at one typical state of operation in an enlarged model of the pump. The focus was set on the assessment of wall shear stresses (WSS) at the housing wall and rotor gap region by means of the wall‐particle image velocimetry technique, and the visualization of near‐wall flow structures on the inner pump surfaces by a paint erosion method. Additionally, maximum WSS and tip leakage volume flows were measured for 13 different states of operation. Good agreement between CFD and experimental data was found, which includes the location, magnitude, and direction of the maximum and minimum WSS and the presence of recirculation zones on the pump stators. The maximum WSS increased linearly with pressure head. They occurred at the upstream third of the impeller blades and exceeded the critical values with respect to hemolysis. Regions of very high shear stresses and recirculation zones could be identified and were in good agreement with simulations. URANS‐CFD, which is often used for pump performance and blood damage prediction, seems to be, therefore, a valid tool for the assessment of flow fields in axial rotary blood pumps. The magnitude of maximum WSS could be confirmed and were in the order of several hundred Pascal.     

Computational flow study of the continuous flow ventricular assist device, prototype number 3 blood pump

A computational fluid dynamics study of blood flow in the continuous flow ventricular assist device, Prototype No. 3 (CFVAD3), which consists of a 4 blade shrouded impeller fully supported in magnetic bearings, was performed. This study focused on the regions within the pump where return flow occurs to the pump inlet, and where potentially damaging shear stresses and flow stagnation might occur: the impeller blade passages and the narrow gap clearance regions between the impeller-rotor and pump housing. Two separate geometry models define the spacing between the pump housing and the impeller's hub and shroud, and a third geometry model defines the pump's impeller and curved blades. The flow fields in these regions were calculated for various operating conditions of the pump. Pump performance curves were calculated, which compare well with experimentally obtained data. For all pump operating conditions, the flow rates within the gap regions were predicted to be toward the inlet of the pump, thus recirculating a portion of the impeller flow. Two smaller gap clearance regions were numerically examined to reduce the recirculation and to improve pump efficiency. The computational and geometry models will be used in future studies of a smaller pump to determine increased pump efficiency and the risk of hemolysis due to shear stress, and to insure the washing of blood through the clearance regions to prevent thrombosis.