Experimental and Analytical Performance Evaluation of Short Circular Hydrodynamic Journal Bearings Used in Rotary Blood Pumps

Abstract

Rotary blood pumps (RBPs) have demonstrated considerable promise while treating heart failure patients, such that they are being placed at an earlier stage of the disease. These devices may therefore be required to operate for prolonged durations which yields the need for RBPs exhibiting high durability, reliability, and blood compatibility. Noncontacting bearings, utilizing magnetic and/or hydrodynamic suspension techniques, appear to provide a suitable solution to these challenges. Hydrodynamic suspension has the advantage that it does not need feedback control systems. Among various hydrodynamic bearing types, the circular journal bearing has the particular benefit of easy manufacturing. This study presents methods to evaluate the performance of short (length to diameter ratio <1) circular hydrodynamic journal bearings (HJBs) for RBPs. Analytical calculations with specific boundary conditions are presented to predict the rotor's eccentricity under equilibrium states and thus the related performance parameters such as load capacity, power loss, and shear rates. These results and boundary conditions were confirmed experimentally in a specially designed test set-up. The bearing performance was found to correlate to analytical solutions using the full Sommerfeld boundary condition instead of the half Sommerfeld condition conventionally used for such applications. Geometrical and operational parameter variations showed that HJB designs with a short Sommerfeld Number SS >0.02 can provide sufficient fluid film thicknesses and low shear rates. The measurements were further used to evaluate the bearings' stability. The estimation of the stability threshold drawn in relation to a modified stability index and the equilibrium eccentricity of the rotor allows the prediction of stability for short circular HJB designs under full Sommerfeld condition.

Multivariate Assessment of the Effect of Pump Design and Pump Gap Design Parameters on Blood Trauma

“…Boehning et al. provide further details on the calculation of the eccentric rotor position based on these parameters . …”

“…The eccentricity is caused by a radial force F and is a result of the choice of the magnitude of the design parameters force F, rotational speed n, gap length L, diameter D, and fluid viscosity (η = 3.6 mPa·s, assumed constant). Boehning et al provide further details on the calculation of the eccentric rotor position based on these parameters (7).…”

“…The CorAide as an example is suspended in radial direction with a plain radial hydrodynamic bearing. The use of such a bearing in rotary blood pumps was also investigated by Boehning et al Their study gave insight into rotor stability, eccentricity of the rotor, and minimal film thickness values (7). Using analytical equations, Boehning et al calculated averaged and peak shear stress values in the bearing gap and thereby achieved a first hemolytic characterization of a gap design.…”

Slip and turbulence phenomena in journal bearings with application to implantable rotary blood pumps

“…The second limitation is that the materials are different to those used in blood pumps. A water-glycerol blood analog solution is used instead of real blood following Boehning et al [24]. Further, the journal and sleeve are fabricated from acrylic rather than the titanium typically used in blood pumps.…”

“…Several researchers have described evidence of slip [13,14] and turbulence [15] in journal bearing systems. Since analytical solutions are used for the prediction of blood pump behaviour [24], work is required to verify assumptions in classical theory and allow behaviour prediction for journal bearings in blood pumps, with different geometries and low viscosity working fluid. This paper uses analytical, computational fluid dynamics (CFD) and experimental methods to investigate and describe the behaviour of journal bearings with various geometries and working fluid similar to that found in centrifugal rotary blood pumps.…”