W.K. Chan

The flow patterns within the impeller passages of a centrifugal blood pump model

The effects of impeller geometry on the performance of a centrifugal blood pump model [the MSCBP design of Akamatsu and Tsukiya (The Seventh Asian Congress of Fluid Mechanics (1997), 7-10) at a 1:1 scale] have been investigated both experimentally and computationally. Four impeller designs were tested for pump hydraulic performance at the operating point (i.e. 2000 rpm), using blood analog as the working fluid. Each impeller has seven blades with different configurations including the radial straight blade and backward swept blade designs. The results show that both designs can achieve a stable head of about 100 mm Hg at the operating point. Subsequent investigations involved the visualization of the relative flow field within the impeller passages via the image de-rotation system coupled with a 2.5 W argon ion laser. Flow structures in all sectors of each impeller were examined and discussed. To further quantify the possible effects of blade geometry to thrombus formation and hemolysis, computational fluid dynamics (CFD) was used to simulate a simplified two-dimensional blade-to-blade flow analysis so as to estimate the shear stress levels. The results indicate that the stress levels found within the blade passages are generally below the threshold level of 150 N/m(2) for extensive erythrocyte damage to occur. There are some localized regions near the leading edge of the blades where the stress levels are 60% above the threshold level. However, given such a short residence time for the fluid particles to go through these high shear stress regions, their effects appear to be insignificant.

Measurements of a step cylinder

Abstract This paper presents the measurements of surface pressure distributions and velocity in the far wake of a step cylinder. It was found that the big cylinder generated higher drag force than the small cylinder. This difference in the drag force affected the wake generated behind the step cylinder. A perfect wake profile can be found behind the small cylinder and twin peak velocity profiles were found behind the big cylinder. As the velocity profiles are expressed as the defect with respect to the free stream velocity, the twin peak velocity profiles behind the big cylinder should be due to the ‘downwash’ effect from the small cylinder with relatively small obstruction to the flow.

Prediction of leakage flow in a shrouded centrifugal blood pump.

This article proposes a phenomenological model to predict the leakage flow in the clearance gap of shrouded centrifugal blood pumps. A good washout in the gap clearance between the rotating impeller surfaces and volute casing is essential to avoid thrombosis. However, excessive leakage flow will result in higher fluid shear stress that may lead to hemolysis. Computational fluid dynamics (CFD) analysis was performed to investigate the leakage flow in a miniaturized shrouded centrifugal blood pump operating at a speed of 2000 rpm. Based on an analytical model derived earlier, a phenomenological model is proposed to predict the leakage flow. The leakage flow rate is found to be proportional to h(α) , where h is the gap size and the exponent α ranges from 2.955 to 3.15 for corresponding gap sizes of 0.2-0.5 mm. In addition, it is observed that α is a linear function of the gap size h. The exponent α compensates for the variation of pressure difference along the circumferential direction as well as inertia effects that are dominant for larger gap clearances. The proposed model displays good agreement with computational results. The CFD analysis also showed that for larger gap sizes, the total leakage flow rate is of the same order of magnitude as the operating flow rate, thus suggesting low volumetric efficiency.

Preliminary measurements of an enlarged blood pump model

Abstract The centrifugal blood pump with a magnetically suspended impeller has shown its superiority over other artificial hearts. However, there is still insufficient understanding of fluid mechanics related issues in the clearance gap. The design requires sufficient washout in the clearance between the impeller and stationary surfaces. As the gap is only 0.2 mm in width, it is very difficult to conduct measurements with present instrumentation. A 5:1 enlarged pump model was thus designed and constructed according to specifications. Flow characteristics and gap pressure measurements of the model are very close to the prototype.

Preliminary measurements of wall shear stress

Abstract This paper presents the investigation of wall shear stress of the fluid flow in the clearance gap between the impeller face and inlet casing of a blood pump model. This was accomplished through hot-wire anemometry and rotating disk apparatus. The pump assigned was a 5:1 scaled-up model of the Kyoto-NTN magnetically suspended impeller centrifugal blood pump. Regions of high and low shear stresses were identified. These correspond to spots where likelihood of hemolysis and thrombus formation are high. The hot-wire probe orientation was shown to have a significant influence on the wall shear stress readings.

Comparison of flow characteristics of enlarged blood pump models with different impeller design

In earlier studies, a 5:1 enlarged pump model of the Kyoto-NTN Magnetically Suspended Centrifugal Blood Pump has been constructed and the gap pressure and wall shear stress have been measured. With the satisfactory results obtained and also as air was the medium used in the previous investigations, another 5:1 enlarged pump model using water as the medium was thus designed and constructed. Five different impeller blade profile designs are used in the present study. By varying (1) the design of the blade profiles i.e. forward, radial and backward, (2) the number of blades used and (3) the rotating speeds, the flow characteristics of the pump are investigated. It is found that the impeller with the higher number of blades used with the forward and straight blade profiles have the best performance.

Characteristics of thin film probe with temperature compensation

Abstract Analytical and experimental studies were carried out using an electrolytic technique to measure the thickness of a thin liquid film. It was found that the relationship between the thickness and the output voltage depends strongly on the area and the configuration of the probe. For a specific configuration and area of the probe, the output voltage depends on both the thickness and the temperature of the liquid. In order to achieve the output voltage as a unique function of the film thickness, the automatic temperature compensation method was applied. Comparisons were also made with analytical solutions. Good agreement was obtained.