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

心脏辅助装置(VAD)被植入严重心力衰竭患者体内以维持血液循环。最常见的植入式VAD是涡轮泵。VAD优化的一个主要目标是最大限度地减少由于非生理性高应激而导致的血液损伤。这种血液损伤通常使用流动模拟和经验血液损伤模型来计算。然而，目前还不可能在VAD模拟中计算定量正确的损伤值，例如，对于溶血(对红细胞的损害)。在VAD模拟中将血液作为单相流体处理可能是导致溶血计算错误的原因。然而，可能会出现多相流效应。例如，红细胞在狭窄的VAD间隙中的血流中可能发生迁移，在那里应力最高，红细胞迁移到间隙的中心，并在壁上形成无细胞的血浆层(CFL)。这种CFL会影响VAD间隙中的作用应力和溶血。然而，VAD间隙中的CFL尚未被实验研究，也没有在VAD模拟中被考虑。因此，本项目的主要目标是在狭窄的间隙中进行血液流动的实验流场研究、间隙中CFL的表征以及间隙流动的数值模拟。由于在VAD间隙中测量的可达性有限，因此在动物血液流动的微通道中模拟间隙的几何参数和流动条件。一方面，将利用微粒子图像测速仪(μ-PIV)和散光粒子跟踪测速仪(APTV)进行光流研究，以描述血液流动和表征CFL。另一方面，将使用直接壁面剪应力传感器在微通道中进行壁面剪应力测量。为此，还将使用单相血液类似物液体(BEF)。通过比较BEF和动物血液的壁面剪应力，可以量化动物血液中由于血浆层引起的作用应力的差异，从而利用扩展的流体模型对微通道中的血液流动进行数值模拟，其中考虑了血浆层。之后，将对模型进行验证，并与当前的流体模型进行比较。在最后的项目目标中，还将研究不同的进口几何形状对缝隙内等离子体层形成的影响，并对进口区域的设计提出建议。项目目标的实现为更好地了解VAD狭窄缝隙内的血液流动及其正确的数值模拟奠定了基础。在此基础上，今后可以用改进的方法计算VADS的血液损伤。

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.