Investigation of the near-wall flow physics of blood in narrow gaps at technically relevant Reynolds numbers

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Ventricular assist devices (VADs) are implanted in people with severe heart failure to maintain the blood circulation. The most commonly implanted VADs are turbopumps. A major aim of a VAD optimization is to minimize flow-induced blood damage that occurs due to non-physiologically high stresses. This blood damage is often computed using flow simulations and empirical blood damage models. However, it is currently not possible to calculate quantitatively correct damage values in VAD simulations, e.g., for hemolysis (damage to erythrocytes).A possible reason for the miscalculation of hemolysis could be found in the treatment of blood in the VAD simulation as single-phase fluid. However, multiphase flow effects could occur. For example, erythrocyte migration could happen in the blood flow in narrow VAD gaps, where the highest stresses act, with erythrocytes migrating to the center of the gap and a cell-free plasma layer (CFL) forming near the walls. This CFL will affect the acting stresses and hemolysis in the VAD gaps. However, the CFL has not yet been investigated experimentally in VAD gaps, nor has it been considered in VAD simulations.Therefore, the main objectives of the project are the experimental flow field investigation in blood-flowing, narrow gaps; the characterization of the CFL in the gap; as well as the numerical modeling of the gap flow.Since an accessibility for measurements in a VAD gap is limited, the geometric parameters and flow conditions of the gaps are mimicked in microchannels, through which animal blood flows. On the one hand, optical flow investigations will be carried out using micro-particle image velocimetry (μ-PIV) and astigmatism particle tracking velocimetry (APTV), in order to describe the blood flow and to characterize the CFL. On the other side, wall shear stress measurements will be performed in the microchannels by using direct wall shear stress sensors. For this purpose, single-phase blood analogues fluids (BEF) will be also used. By comparing the wall shear stresses between BEF and animal blood, the difference in the acting stresses due to the plasma layer in the animal blood can be quantified.Subsequently, the blood flow in the microchannels will be numerically modeled by means of flow simulations with an extended fluid model, which takes the plasma layer into account. After that, a validation of the model and a comparison with current fluid models will be performed. In the last project objective, the influence of different inlet geometries on the plasma layer formation in the gap will be also investigated and recommendations for the design of the inlet area will be derived.The achievement of the project goals forms the basis for an improved understanding of blood flow in the narrow gaps of VADs and their correct numerical modelling. Based on this, blood damage in VADs can be calculated in an improved way in the future.