Session: 03-02-01 Bio-Inspired and Biomedical Fluid Mechanics

Paper Number: 86732

86732 - Methodology for Experimental Investigation of Hydraulic And Mechanical Characteristics of Ventricular Assist Devices 

Measurement of hydraulic and mechanical characteristics of a pump becomes increasingly complex as pump size decreases. Characteristic parameters to be measured impose significant challenges on measurement equipment, precision of manufactured components, and materials used. This paper uses the example of a mechanically suspended ventricular assist device (impeller outer diameter D₂=18.7mm) to demonstrate a method for determining bearing losses of an arbitrary pump test section and for deducing hydraulic parameters (in particular hydraulic power loss) with high measurement precision. The main challenge with these sizes is to achieve reliable reproducibility of measured data. Isolation of bearing losses is best achieved by using ceramic materials, as these have lowest friction values and highest reproducibility among tested components. Furthermore, standard seals such as rotary shaft seals and mechanical seals cannot be used due to their lack of reproducibility. Therefore, a custom developed ceramic gap seal is presented that meets such specific requirements.

A speed range from 3600 rpm to 7000 rpm has been investigated. Measured impeller torques are in range of 1.0 mNm to 5.4 mNm and friction torques in the mechanical suspension are in range of 0.23 mNm to 0.31 mNm.

Measured internal power of the pump ranges from 0.40 W to 3.97 W. For the design point at 5600 rpm and a flow rate of Q=4.5 L/min, P_hyd=0.94 W relates to useful hydraulic power (48.6%) and P_L=0.99 W to hydraulic power loss (51.4%). For the quantification of measurement precision, a Gaussian Error Propagation was performed for all parameters over the entire operating range.

Determined relative measurement uncertainty amounts for pressure head–flow-curve at 3600 rpm with an average value of 3.15% (min. 1.53%, max. 16.45%, median 2.06%). With increasing speed, relative measurement uncertainty reduces to a mean value of 0.52% at 7000 rpm (min. 0.41%, max. 0.90%, median 0.50%).

Relative measurement uncertainty for hydraulic power loss averages 2.40% at 3600 rpm (min. 1.46%, max. 2.76%, median 2.56%). It likewise decreases with increasing speed to a mean value of 1.69% at 7000 rpm (min. 0.58%, max. 2.26%, median 2.10%).

Hydraulic efficiency at a speed of 3600 rpm amounts to a maximum of 42.8% and increases steadily as speed increases to a maximum of 51.7% at 7000 rpm. Relative measurement uncertainty for hydraulic efficiency at 3600 rpm averages 3.21% (min. 1.93%, max. 13.77%, median 2.23%). Relative measurement uncertainty for hydraulic efficiency reduces to 1.89% on average at 7000rpm (min. 1.30%, max. 3.70%, median 1.82%).

Investigation of dimensionless curves indicates a Reynolds dependence in hydraulic power loss. Reynolds dependence leads to an improvement in hydraulic efficiency with increasing speed.

The presented methodology is applicable to any pump type of similar size and thus provides the possibility to investigate especially ventricular assist devices with respect to their hydraulic parameters (particularly hydraulic power loss) with high precision and accuracy.

Presenting Author: Julija Peter Technische Universitaet Berlin