# Influence of Impeller Speed Patterns on Hemodynamic Characteristics and Hemolysis of the Blood Pump
## 3 Results
### CFD Results of the Hemodynamic Characteristics and Hemolysis  
The hemolysis index (HI) of the impeller with three speed patterns was compared and analyzed. According to Figure 12a, the HI corresponding to the three speed patterns met the requirements of clinical use. The average HI of the constant speed pattern was \(1.75\times 10^{-5}\) in a cardiac cycle. The sinusoidal and pulsatile speed patterns were \(2.11\times 10^{-5}\) and \(2.05\times 10^{-5}\), respectively. The latter two kinds of speed patterns led to an increase in HI, but the changing trend of HI was more consistent and closer. The change in the HI of the blood pump was small under the condition of continuous flow with a constant speed pattern. The HI amplitude of the other two patterns changed to a certain extent, indicating that the impeller speed had a direct effect on the degree of hemolysis, but the hemolysis value did not increase by an order of magnitude. Therefore, it might be related to the small range of impeller speed fluctuation. The blood velocity in the groove of the hydraulic bearing of the blood pump was monitored. The blood velocity in the groove was basically stable at 0.34 m/s when the blood pump worked at a constant speed, as shown in Figure 12b. Under the action of the impeller with sinusoidal and pulsatile speed patterns, the blood velocity fluctuation range of the blood pump was about 0.1 m/s. Therefore, with a periodic change in the impeller speed, the flow field of the blood pump also changed at any time. The impeller speed directly affected the distribution characteristics of the flow field in the spiral groove of hydrodynamic bearing because of the small clearance between the impeller and the upper and lower pump shells and the effect of fluid viscosity. With a periodic change in the impeller speed, the flow field of the blood pump also changed at any time, and the changing trend of the flow velocity was close to that of the impeller speed. The HI and shear stress distribution in the flow field of the blood pump driven by the sinusoidal speed pattern at 0.9 s are shown in Figure 13a,b. The distribution of HI was more uniform, and its value was larger at the outlet of the impeller and in the solute runner. Most areas had low shear stress, but the shear stress in the spiral groove area was larger, which reached 155 Pa. Therefore, the spiral groove part was the main area causing blood damage, as shown in Figure 13b. When blood flowed through these areas, the high shear stress affected the blood and led to the destruction of blood cells. Under the action of the pulsating flow field, the flow path of the flow field of the blood pump is shown in Figure 14a. No obvious eddy and flow stagnation zone were found. The high shear stress in the flow process was mainly in the secondary flow passage of the impeller and the solute. The flow fields in some areas of the spiral groove were analyzed, as shown in Figure 14b. The blood flow path in the spiral groove was more uniform and regular. The blood flowed from the outer end of the spiral groove to the inner end in a spiral form, and was not disturbed by the changing impeller speed. Figure 12: (**a**) Changes in hemolysis index (HI) under the three speed patterns. (**b**) Blood velocity in the spiral groove of the hydraulic bearing of the blood pump varied with time. Figure 13: (**a**) HI distribution of the sinusoidal speed pattern at 0.9 s. (**b**) High shear stress region of the spiral groove in the sinusoidal speed pattern at 0.9 s.  
The blood flow trajectory in the spiral groove showed that the hemolysis index fluctuated to different degrees in 0.9, 1.2, and 1.4 s, according to the HI change trend of the pulsating speed pattern. Therefore, the three moments were used for analysis, as shown in Figure 15a-c. The blood velocities corresponding to the ascending, peak, and descending phases of AOP at different moments were different. Moreover, the flow velocity was the highest during the peak pressure period.