An estimation method of hemolysis within an axial flow blood pump by computational fluid dynamics analysis

Evaluation of hemolysis within a blood pump on a computer is useful for developing rotary blood pumps. The flow fields in the axial flow blood pump were analyzed using computational fluid dynamics (CFD). A blood damage index was calculated based on the changes in shear stress with time along 937 streamlines. Hemolysis of the pumps was measured using bovine blood. A good correlation between the computed and measured hemolysis results was observed. CFD analysis is useful for estimating hemolysis of rotary blood pumps on a computer.     

A comparative study between flow visualization and computational fluid dynamic analysis for the sun medical centrifugal blood pump

Flow visualization experiments and computational fluid dynamic (CFD) analyses were performed and the results were compared to clarify the detailed fluid dynamic characteristics for the prototype design of a centrifugal pump, namely, an implantable ventricular assist system from Sun Medical, whose hemocompatibility was previously demonstrated in a series of animal experiments. The flow visualization was conducted with particle tracking velocimetry, and the CFD analysis was performed with STAR-CD software. The findings were as follows: (1). There were no flow separations around the curved open impeller. (2). Antithrombogenic design concepts for the inducer and the vane-shaft clearance were effective in producing axial velocity along the shaft surface and generat-ing suitable shear rates against the stationary fluid. (3). Unsteady vortex shedding in the outlet, which adversely affected the fluid dynamic efficiency, was observed clearly by flow visualization. Comparison of velocity distribution measured by flow visualization and CFD analysis showed reasonably good correlation. Our findings indicate that the impeller is suitable for an implantable artificial heart. The techniques of flow visualization and CFD analysis are complementary evaluation tools in research and development efforts.     

Validation of an axial flow blood pump: computational fluid dynamics results using particle image velocimetry

A magnetically suspended axial flow blood pump is studied experimentally in this article. The pump casing enclosed a three-blade straightener, a two-blade impeller shrouded by a permanent magnet-embedded cylinder, and a three-blade diffuser. The internal flow fields were simulated earlier using computational fluid dynamics (CFD), and the pump characteristic curves were determined. The simulation results showed that the internal flow field was basically streamlined, except the diffuser region. Particle image velocimetry (PIV) measurement of the 1:1 pump model was conducted to validate the CFD result. In order to ensure the optical access, an acrylic prototype was fabricated with the impeller driven by a servomotor instead, as the magnet is opaque. In addition to the transparent model, the blood analog fluid with the refractive index close to that of acrylic was used to avoid refraction. According to the CFD results, the axial flow blood pump could generate adequate pressure head at the rotating speed of 9500rpm and flow rate of 5L/min, and the same flow condition was applied during the PIV measurement. Through the comparisons, it was found that the experimental results were close to those obtained by CFD and had thus validated the CFD model, which could complement the limitation of the measurement in assessing the more detailed flow fields of the axial flow pump.     

A novel integrated rotor of axial blood flow pump designed with computational fluid dynamics

Due to the smaller size, smaller artificial surface, and higher efficiency, axial blood pumps have been widely applied in clinic in recent years. However, because of its high rotor speed, axial flow pump always has a high risk for hemolysis, which the red blood cells devastated by the shearing of tip clearance flow. We reported a novel design with the integrated blade-shroud structure that was expected to solve this problem by abolishing the radial clearance between blade and casing designed with the techniques of computational fluid dynamics (CFD). However, the numerical simulation result of the newly designed structure showed an unexpected backflow (where flow velocity is reverse of the main flow direction) at the blade tip. In order to eliminate this backflow, four flow passes were attempted, and the expansion angles (which reflect the radial amplification of the flow pass, on the meridional section, and should be defined as the angle between the center line of the flow pass and the axial direction) of the blades of the integrated rotor are 0 degrees, 8 degrees, 15 degrees, and 20 degrees, respectively. In the CFD result, it could be easily found as the expansion angles increased, the backflow was restrained gradually, and was eliminated at last. After numerous "cut and try" circles, the pump model was finally optimized. The numerical simulation of this model also showed a stable hydraulic characteristic.     

Studies of turbulence models in a computational fluid dynamics model of a blood pump

Computational fluid dynamics (CFD) is used widely in design of rotary blood pumps. The choice of turbulence model is not obvious and plays an important role on the accuracy of CFD predictions. TASCflow (ANSYS Inc., Canonsburg, PA, U.S.A.) has been used to perform CFD simulations of blood flow in a centrifugal left ventricular assist device; a k-epsilon model with near-wall functions was used in the initial numerical calculation. To improve the simulation, local grids with special distribution to ensure the k-omega model were used. Iterations have been performed to optimize the grid distribution and turbulence modeling and to predict flow performance more accurately comparing to experimental data. A comparison of k-omega model and experimental measurements of the flow field obtained by particle image velocimetry shows better agreement than k-epsilon model does, especially in the near-wall regions.     

A validated computational fluid dynamics model to estimate hemolysis in a rotary blood pump

A major part of developing rotary blood pumps requires the optimization of hemolytic properties of the entire pump. Application of a suited computational fluid dynamics (CFD)-based hemolysis model allows approximation of blood damage in an early phase of the design process. Thus, a drastic reduction of time- and cost- intensive hemolysis experiments can be achieved. For the MicroDiagonal Pump (MDP), still under development at Helmholtz-Institute in Aachen, Germany, different pump configurations have been analyzed, both numerically and experimentally. The CFD model of the pump has been successfully validated based on the comparison of the pressure head curves (H-Q curves), as discussed in a prior publication. In the present study, the authors focus on the development of a semiempiric blood damage model using the CFD and in vitro hemolysis data. On the one hand, mean key characteristics (shear stress and exposure time) and other characteristics affecting blood damage have been calculated based on numerical data. On the other hand, in vitro hemolysis tests have been accomplished in order to determine the hemolytic curves of two different pump configurations (with the same impeller but different tip clearances). Finally, a new function based on a general power law has been defined by means of the mean key characteristics. The unknown constants of the function have been determined by multidimensional regression analysis using the hemolytic curves. For the final validation of this new blood damage model, the calculated and the in vitro obtained hemolysis indices at the specific VAD operating point have been compared for all pump configurations. The comparison showed an excellent agreement, both qualitatively and quantitatively.     

Computational fluid dynamics analysis of an intra-cardiac axial flow pump

A low rate of hemolysis is an important factor for the development of a rotary blood pump. It is, however, difficult to identify the areas where hemolysis occurs. Computational fluid dynamics (CFD) analysis enables the engineer to predict hemolysis on a computer. In this study, fluid dynamics throughout intracardiac axial flow pumps with different designs were analyzed three-dimensionally using CFD software. The computed pressure-flow characteristics of the pump were in good agreement with the measurements. The Reynolds shear stress was computed along particle trace lines. Hemolysis was estimated on the basis of shear stress (tau) and its exposure time (Deltat): dHb/Hb = 3.62 x 10(-7)(tau)(i)(2.416) x Delta(t)(i)(0.785). Particle damage increased with time along the particle trace lines. Hemolysis of each of the pumps was measured in vitro. The computed hemolysis values were in good agreement with the experimental results. CFD is a useful tool for developing a rotary blood pump.     

Assessment of hemolysis related quantities in a microaxial blood pump by computational fluid dynamics

A computational assessment or even quantification of shear induced hemolysis in the predesign phase of artificial organs (e.g., cardiac assist devices) would largely decrease efforts and costs of design and development. In this article, a general approach of hemolysis analysis by means of computational fluid dynamics (CFD) is discussed. A validated computational model of a microaxial blood pump is used for detailed analysis of shear stress distribution. Several methods are presented that allow for a qualitative assessment of shear stress distribution and related exposure times using a Lagrangian approach and mass distribution in combination with shear stress analysis. The results show that CFD offers a convenient tool for the general assessment of shear-induced hemolysis. The determination of critical regions and an estimation of the amount of blood subject to potential damage in relation to the total mass flow are shown to be feasible. However, awareness of limitations and potential flaws in CFD based hemolysis assessments is crucial.     

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.     

Design optimization of blood shearing instrument by computational fluid dynamics

Rational design of blood-wetted devices requires a careful consideration of shear-induced trauma and activation of blood elements. Critical levels of shear exposure may be established in vitro through the use of devices specifically designed to prescribe both the magnitude and duration of shear exposure. However, it is exceptionally difficult to create a homogeneous shear-exposure history by conventional means. This study was undertaken to develop a Blood Shearing Instrument (BSI) with an optimized flow path which localized shear exposure within a rotating outer ring and a stationary conical spindle. By adjustment of the rotational speed and the gap dimension, the BSI is designed to generate shear stress magnitudes up to 1500 Pa for exposure time between 0.0015 and 0.20 s with a pressure drop of 100 mm Hg. Computational fluid dynamics (CFD) revealed that a flow path designed by first-order analysis and intuition exhibited unfavorable pressure gradient, vortices, and undesirable regions of reverse flow. An optimized design was evolved utilizing a parameterized geometric model and automatic mesh generation to eliminate vortices and reversal flow and to avoid unfavorable pressure gradients. Analysis of the flow and shear fields for the extreme limits of the shear gap demonstrated an improvement in homogeneity due to shape optimization and the limitations of an annular shear device for achieving completely uniform shear exposure.     

Computational fluid dynamics analysis of the pediatric tiny centrifugal blood pump (TinyPump)

We have developed a tiny rotary centrifugal blood pump for the purpose of supporting circulation of children and infants. The pump is designed to provide a flow of 0.1-4.0 L/min against a head pressure of 50-120 mm Hg. The diameter of the impeller is 30 mm with six straight vanes. The impeller is supported by a hydrodynamic bearing at its center and rotated with a radial coupled magnetic driver. The bearing that supports rotation of the impeller of the tiny centrifugal blood pump is very critical to achieve durability, and clot-free and antihemolytic performance. In this study, computational fluid dynamics (CFD) analysis was performed to quantify the secondary flow through the hydrodynamic bearing at the center of the impeller and investigated the effects of bearing clearance on shear stress to optimize hemolytic performance of the pump. Two types of bearing clearance (0.1 and 0.2 mm) were studied. The wall shear stress of the 0.1-mm bearing clearance was lower than that of 0.2-mm bearing clearance at 2 L/min and 3000 rpm. This was because the axial component of the shear rate significantly decreased due to the narrower clearance even though the circumferential component of the shear rate increased. Hemolysis tests showed that the normalized index of hemolysis was reduced to 0.0076 g/100 L when the bearing clearance was reduced to 0.1 mm. It was found that the CFD prediction supported the experimental trend. The CFD is a useful tool for optimization of the hydrodynamic bearing design of the centrifugal rotary blood pump to optimize the performance of the pump in terms of mechanical effect on blood cell elements, durability of the bearing, and antithrombogenic performance.     

Numerical analysis of the inner flow field of a biocentrifugal blood pump

Implantable ventricular assist devices have been regarded as a promising instrument in the clinical treatment of patients with severe heart failures. In this article, a three-dimensional model of the Kyoto-NTN magnetically suspended centrifugal blood pump was generated and a computational fluid dynamics solution of the inner flow field of the pump including the static pressure distributions, velocity profiles, and the shear stress distributions of the blood was presented. The results revealed that reverse flow generally occurred in the impeller blade channels during the operation of the pump, due to the imbalance of the flow and the pressure gradient generated in the blade channels. The flow pattern at the exit of the blade channels was varying with its angular positions in the pump. The reverse flow at the exit of the impeller blade channels was found to be closely related with the static pressure distribution in the volute passage. Higher pressure in the volute caused severe backflow from the volute into the blade channels. To clarify the effects of a moving impeller on the blood, shear stresses of the blood in the pump were investigated according to the simulation results. The studies indicated that at the beginning of the splitter plate and the cutwater, the highest shear stress exceeded 700 Pa. At other regions such as the inlet and outlet of the impeller blade channels and some regions in the volute passage, shear stresses were found to be about 200 Pa. These areas are believed to have a high possibility of rendering blood trauma.     

Computational fluid dynamics investigation of a centrifugal blood pump

Abstract:  In the development of a ventricular assist device, computational fluid dynamics (CFD) analysis is an efficient tool to obtain the best design before making the final prototype. In this study, different designs of a centrifugal blood pump were developed to investigate flow characteristics and performance. This study assumed the blood flow as being an incompressible homogeneous Newtonian fluid. A constant velocity was applied at the inlet; no slip boundary conditions were applied at device wall; and pressure boundary conditions were applied at the outlet. The CFD code used in this work was based on the finite volume method. In the future, the results of CFD analysis can be compared with flow visualization and hemolysis tests.     

Comparison of hydraulic and hemolytic properties of different impeller designs of an implantable rotary blood pump by computational fluid dynamics

A mixed-flow blood pump for long-term applications has been developed at the Helmholtz-Institute in Aachen, Germany. Central features of this implantable pump are a centrally integrated motor, a blood-immersed mechanical bearing, magnetic coupling of the impeller, and a shrouded impeller, which allows a relatively wide clearance. The aim of the study was a numerical analysis of hydraulic and hemolytic properties of different impeller design configurations. In vitro testing and numerical simulation techniques (computational fluid dynamics [CFD]) were applied to achieve a comprehensive overview. Pressure-flow charts were experimentally measured in a mock loop in order to validate the CFD data. In vitro hemolysis tests were performed at the main operating point of each impeller design. General flow patterns, pressure-flow charts, secondary flow rates, torque, and axial forces on the impeller were calculated by means of CFD. Furthermore, based on streak line techniques, shear stress (stress loading), exposure times, and volume percentage with critical stress loading have been determined. Comparison of CFD data with pressure head measurements showed excel-lent agreement. Also, impressive trend conformity was observed between in-vitro hemolysis results and numerical data. Comparison of design variations yielded clear trends and results. Design C revealed the best hydraulic and hemolytic properties and was chosen as the final design for the mixed-flow rotary blood pump.     

Numerical investigation of the effect of blade geometry on blood trauma in a centrifugal blood pump

Fluid dynamic forces in centrifugal blood pump impellers are of key importance in destruction of red blood cells (RBCs) because high rotational speed leads to strong interaction between the impeller and the RBCs. In this paper, three-dimensional models of five different blade geometries are investigated numerically using the commercial software CFX-TASCflow, and the streaklines of RBCs are obtained using the Lagrangian particle tracking method. In reality, RBCs pass through the pump along complicated paths resulting in a highly irregular loading condition for each RBC. In order to enable the prediction of blood damage under the action of these complex-loading conditions, a cumulative damage model for RBCs was adopted in this paper. The numerically simulated percent hemoglobin (%HB) released as RBCs traversed the impeller and volute was examined. It was observed that the residence time of particles in the blade passage is a critical factor in determining hemolytic effects. This, in turn, is a function of the blade geometry. In addition, it was observed that the volute profile is an important influence on the computed HB% released.     

Computational fluid dynamics of gap flow in a biocentrifugal blood pump

The centrifugal blood pump with a magnetically suspended impeller has shown its superiority as compared to other artificial heart pumps. However, there is still insufficient understanding of fluid mechanics related issues in the clearance gap. The design nature of the pump requires sufficient washout in the clearance between the impeller and the stationary pump housing inner surface. In this study, numerical simulations were carried out to investigate the flow fields in the gap of the Kyoto-NTN centrifugal blood pump. The flow patterns in the gap region of the blood pump were presented and regions of high and low velocity were identified. It was found that the radial velocity of the blood in the gap was closely related to the pressure distribution at the exit of the impeller, both the highest pressure gradient and the highest radial velocity in the gap occurred at an angular position of 170 degrees . The mass flow rate in the gap was estimated to be 25.2% of the pump outflow, which is close to the measurement results of a five times enlarged test pump. The wall shear stresses on the gap surface were found to be over 21 Pa and below 300 Pa, which is correspondingly higher than the threshold of thrombi formation and is lower than the shearing threshold of red blood cells. Comparison of the 1 : 1 simulation model with the measurement results on a five times enlarged test pump indicates that there are some differences in the resulting radial velocity distributions in the gap and thus the washout mechanism. Two symmetrical high washout regions at both the cutwater and splitter plate were observed in the simulation instead of a single washout region at the splitter plate found in the experimental study. This may be due to the scaling effect of the enlarged test pump; also the medium used in the experiment is different from the simulation.     

Computational fluid dynamics prediction of blood damage in a centrifugal pump

This study explores a quantitative evaluation of blood damage that occurs in a continuous flow left ventricular assist device due to fluid stress. Computational fluid dynamics (CFD) analysis is used to track the shear stress history of 388 particle streaklines. The accumulation of shear and exposure time is integrated along the streaklines to evaluate the levels of blood trauma. This analysis, which includes viscous and turbulent stresses, provides a statistical estimate of possible damage to cells flowing through the pump. In vitro normalized index of hemolysis values for clinically available ventricular assist devices were compared to our damage indices. This allowed for an order of magnitude comparison between our estimations and experimentally measured hemolysis levels, which resulted in a reasonable correlation. This work ultimately demonstrates that CFD is a convenient and effective approach to analyze the Lagranian behavior of blood in a heart assist device.     

Computational fluid dynamics performance prediction for the hydrodynamic bearings of the ventrassist rotary blood pump

Finite-volume computations are described for laminar flow in the hydrodynamic bearings supporting 2 different versions of the impeller of the VentrAssist rotary pump. Pressure boundary conditions are taken from prior computations of turbulent flow in the whole pump with frictionless sliding of the impeller on the inside of the pump body. By investigating various impeller positions, the true ride height is determined. Net lift and combined drag from all 8 bearings of the 4-bladed impeller are compared with predictions based on 2-D theory. The computations also reveal the extent of net force and moment acting to move the impeller away from its nominal axis of rotation.     

Computational fluid dynamics as a development tool for rotary blood pumps

Computational fluid dynamics (CFD) is beginning to significantly impact the development of biomedical devices, in particular rotary cardiac assist devices. The University of Pittsburgh's McGowan Center for Artificial Organ Development has extensively used CFD as the primary tool to analyze and design a novel axial flow blood pump having a magnetically suspended rotor. The blood-contacting surfaces of the pump were developed using a design strategy based on CFD that involved closely coupling a Navier-Stokes solver to a parameterized geometry modeler and advanced mesh movement techniques. CFD-based blood damage models for shear-induced hemolysis as well as surrogate functions describing thrombosis potential were employed to help guide design improvements. This CFD-based design approach resulted in the timely development of a pump subjected to multiple geometric refinements without building expensive physical prototypes for each design iteration. A physical prototype of the final improved pump was fabricated and experimentally analyzed using particle imaging flow visualization. The CFD predicted results correlated well with the experimental data including pressure-flow (H-Q) performance and specific flow field features. It is estimated that the present CFD-based design approach shortened the overall design time frame from an order of years to months.     

Computational fluid dynamics analysis of hydrodynamic bearings of the VentrAssist rotary blood pump

The computational fluid dynamics (CFD) package CFX-TASCflow was applied to simulate the flows through the blood pump hydrodynamic bearings. The three-dimensional flow patterns through the bearings were predicted and the hydraulic performance analyzed. The computations were carried out at 3 axial positions of the pump impeller. Net lift force away from the nearer part of the housing increased when the impeller moved closer to this part. Radial force and drag force were also found. Separated flows were observed at the leading and trailing edge of the bearing gap. To test the CFD package, a series of two-dimensional computations were also carried out for various bearing geometries. The results were compared with published experimental data.