PIV experimental study on the flow field characteristics of axial flow blood pump under three operating conditions

: In order to investigate the internal flow field of an axial blood pump, two-dimensional (2D) particle image velocimetry (PIV) was applied to test the blood pump under three different conditions. Acquiring the images of inlet, front guide vane, impeller, back guide vane and outlet area, the distribution of internal flow field was then analysed. The rotation speed of the blood pump was set to 6000, 7000 and 8000 r/min, respectively. The results show that the flow field of blood pump is stable relatively at the inlet area. Some vortices and reflux existed due to the block of guide vane in the import and impeller area. With the increase of rotating speed, the overall disturbance degree of flow field increases. Under the condition of low rotation speed, the flow field acceleration is insufficient. Under the medium rotation speed condition, the flow field is stable and the velocity distribution is even. Under the condition of high rotating speed, the number of vortices in the flow field increased significantly, the flow separation phenomenon at the impeller was obvious and more unstable flow appeared at the back guide vanes and outlet area.


Introduction
Nowadays, there are more than 20 million patients with heart failure in the worldwide [1]. It is also that the incidence rate of heart failure is rising continuously. Left ventricular assist device helps patients to carry out blood circulation. It has become an effective solution in the absence of a heart transplant donor. As a second-generation blood pump, the axial flow blood pump generates continuous flow output through high-speed rotation of the impeller. In the flow field of blood pump, a velocity gradient is formed due to changes in flow rate and flow direction [2], vortex, reflux and separation flow [3] and other reasons, causing mechanical damage to red blood cells, which results in haemolysis [4]. Therefore, the study on internal flow field of blood pump can provide a key reference for improving haemolysis performance and optimising blood pump structure.
Particle image velocimetry (PIV), as a kind of effective flow field test method, can reflect the action of missing particle motion [5,6]. It has been applied to study the blood pump flow field gradually in recent years [7][8][9]. Liu et al. [10,11] used PIV to photograph the inlet and outlet areas of the axial flow blood pump, and obtained that the inlet flow field was stable and the outlet flow field had spiral flow. Day and Mcdaniel [12] et al. used PIV to measure the steady and unsteady flow fields of centrifugal blood pump. Wang [13] et al. used PIV to verify the performance of the blood pump impeller design. Shi et al. [14] compared the flow field simulation results of the axial bleeding pump with the PIV results and concluded that the three turbulence models and PIV results had the same trend, proving the effectiveness of numerical simulation.
This paper puts forward a kind of analysis methods to study the axial flow blood pump impeller and its import and outlet areas with PIV experiment. Using PIV technique to obtain the image of internal flow field provided evidence for the design of blood pump.

Experimental model
The experiment model was independently designed by the Central South University (CSU). As shown in Fig. 1, it consists of a front vane, an impeller and a back vane. A permanent magnet is set in the inside of the impeller so that external magnet could drive its rotation. The design flow rate is 5 L/min, the rated pressure differential is 13.3 kPa and the rotation speed of pump depends on external drivers, which are made of a titanium alloy. In order to reduce the light reflection during the PIV experiment, the components of the blood pump were painted to matte black.

Experimental device
The experimental device adopts the LaVision PIV system. The main components are described as follows. The laser adopts the British Litron LDY300, and the maximum working frequency is 90 Hz. A LaVision ImagerProSX 5M dual shutter camera is used, with a resolution of 2448 × 2050 and each single pixel size of 3.45 × 3.45 μm. The synchronous shooting system includes a LaVision 1108090 synchroniser, a rotary encoder, a signal transmission cable and Davis processing software.
The arrangement of the experimental device is shown in Fig. 2.
The blood pump is installed in a transparent acrylic round pipe. A tank full of water is set in the outer edge of the pipe to adjust the refractive index caused by pipe deformation. The laser is irradiated directly from the upper position of the axis of the blood pump. A motor device with a strong magnet is set behind the blood pump. The motor drives the strong magnet to rotate to generate alternating magnetic field, which rotates the blood pump. The motor rotates once a week, and the blood pump rotates synchronously for two weeks. The rotary encoder is installed on the motor to output the shooting trigger signal. Pressure gauges are installed at the inlet and outlet of the blood pump. An ultrasonic flow sensor is installed outside the pipeline. The model of flow sensor is SNOWFLOW CO.55/100 V2.0. In order to maintain the viscosity stability during fluid circulation, a temperature water bath is added in the loop, and the temperature is sent in the range of 40-80 °C.

Fluid and tracer particle selection
For the simulation of blood properties in pump fluid, 27.3% glycerine aqueous solution of density 1.07 kg/m 3 is selected; it is approximation to the indicators of blood. In order to guarantee the trace properties and light scattering of particles, we selected hollow glass beads of size 10 μm. The size of the tracer particle is selected to be similar to the size of a blood cell for better experimental results.

Photography solution
According to the structural composition of the axial flow blood pump, the inlet area and front vanes, the radial area of the impeller and the back vanes, and the outlet area were photographed. The rotary encoder mounted on the drive motor will output signals at the same position, to control the camera shooting at every revolution.

Soft set-up of PIV
The principle of PIV was calculating the time interval between two images to obtain the arithmetic cross-correlations. Then, the information such as velocity field was obtained; the time interval will directly influence the accuracy of the result. The best choice is the particles moving 5-8 pixels in each interval. Therefore, the time intervals of the import, export and impeller were set as 150, 150 and 50 μs, respectively. In Davis software, the external trigger mode was selected as a camera model; the shooting frequency was set the same as the motor encoder output signal frequency. Shooting occurs once, while the blood pump rotates for two rounds. Inquiry window size is set to be 32 × 32, and it is calculated with the adaptive PIV algorithm.

Results
The experimental results are shown as absolute speed; the direction of the arrow represents the speed direction, and colour represents the speed magnitude. A total of 100 pieces were shot under each rotating speed; the results were calculated by average processing. Due to the vertical laser, the shunt field where below the blood pump directly is blocked, but does not affect the validity of the experimental results.

Flow field distribution of inlet and front vane
As can be seen from Fig. 3, the velocity field at the inlet is relatively stable, with the maximum velocity of 0.5 m/s. The velocity direction of particles in the flow field is basically the same as the axis of blood pump. The particles accelerated into the impeller area at the leading vanes, and the flow direction was basically consistent with the contour of the front guide vanes. With the increase of rotating speed, the flow field velocity at the inlet increased significantly and more unstable flows were generated. Fig. 3a shows the speed of 7000 r/min; the inlet velocity is smooth and the flow is gentle, but the impeller generates a larger vortex at the upper right acceleration area. Fig. 3b shows the speed of 8000 r/min. The flow field at inlet and the leading zone is stable, and the acceleration is even. With the further increase of speed, the flow field at the entrance is seriously disturbed. Fig. 3c shows that under the condition of 9000 r/min, large areas of vortex, reflux and flow separation occur near the wall of the pipeline and front vane area.

Flow field distribution of impeller
As shown in Fig. 4, the maximum velocity at the impeller is 2.5 m/s. The photographed part at the upper part of the impeller is divided into A, B, C and D areas. The velocity of the four areas increases successively. In region A, flow is stable and the flow direction basically follows the impeller rotation direction, but small eddy currents occur in the left under the condition of 9000 r/min. In region B, the flow field is stable under all three working conditions. However, eddy currents and severe turbulence occur on the right side near the blade in regions C and D. Especially, the backflow is obvious and large vortexes occur in region D. There was a small range of reflux near the wall of blood pump body.

Flow field distribution of outlet and back vane
As shown in Fig. 5, the maximum flow velocity at the back guide vane and the outlet is 2.0 m/s. The eddy currents and the reflow

Conclusion
(i). The flow field of the blood pump at the inlet is relatively stable. In the impeller area, the flow field presents an unstable flow state due to the rapid increase of Reynolds coefficient. In the leading vanes and region D of the impeller area, there is partial reflux due to the baffle of guide vane. (ii). With the increase of rotating speed, the overall disturbance degree of flow field increases. Under the condition of 7000 r/min, the flow field acceleration is insufficient, leading to the reflux at the upper part of the leading blade, and the reflux in the D area of the impeller is relatively small. Under the condition of 8000 r/min, the flow field is relatively stable and the acceleration is even. Under the condition of 9000 r/min, the number of vortices in the flow field increased significantly, the flow separation at the impeller was obvious, and more unstable areas appeared at the trailing guide vanes and exits. Thus, 8000 r/min is the most suitable speed for this pump. (iii). As the velocity is perpendicular to the axis in the flow after the back vane, the fluid is sprayed to the upper wall. Thus, the jet area is formed at the outlet area. Therefore, it is necessary to modify the streamline of the back guide vane, which provides a clear idea for the structural optimisation of the blood pump. (iv). There is a small range of backflow near the wall of the blood pump, so it is necessary to consider the selection of wall function in flow field simulation.

Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 51475477 and 31670999).