Effect of Particle Migration on the Stress Field in Microfluidic Flows of Blood Analog Fluids at High Reynolds Numbers

Abstract

In the present paper, we investigate how the reductions in shear stresses and pressure losses in microfluidic gaps are directly linked to the local characteristics of cell-free layers (CFLs) at channel Reynolds numbers relevant to ventricular assist device (VAD) applications. For this, detailed studies of local particle distributions of a particulate blood analog fluid are combined with wall shear stress and pressure loss measurements in two complementary set-ups with identical flow geometry, bulk Reynolds numbers and particle Reynolds numbers. For all investigated particle volume fractions of up to 5%, reductions in the stress and pressure loss were measured in comparison to a flow of an equivalent homogeneous fluid (without particles). We could explain this due to the formation of a CFL ranging from 10 to 20 μm. Variations in the channel Reynolds number between Re = 50 and 150 did not lead to measurable changes in CFL heights or stress reductions for all investigated particle volume fractions. These measurements were used to describe the complete chain of how CFL formation leads to a stress reduction, which reduces the apparent viscosity of the suspension and results in the Fåhræus–Lindqvist effect. This chain of causes was investigated for the first time for flows with high Reynolds numbers (Re∼100), representing a flow regime which can be found in the narrow gaps of a VAD.

Viscosity Modeling for Blood and Blood Analog Fluids in Narrow Gap and High Reynolds Numbers Flows

“…In this Reynolds number definition, c represents bulk velocity, and denote density and viscosity (measured in a rheometer), and L is the characteristic length. Similarly to Fåhræus and Lindqvist, the study of Knüppel et al [ 15 ] revealed a significant reduction in stresses and pressure losses across the gap at high . Based on this experimental basic research, it was anticipated that a particle migration will also occur within real VAD gap flows, significantly influencing the flow dynamics in the gap.…”

“…The foundation of the model lies in the experimental analysis of particle distribution within a particle-laden gap flow operating at high Reynolds numbers. For a comprehensive understanding of the experimental setup, readers are directed to our earlier publication in Knüppel et al [ 15 ].…”

“…We performed direct numerical simulations (DNSs) across Reynolds numbers ranging from 50 to 150. The computational domain of the microchannel closely resembles those used in the experiments conducted by Knüppel et al [ 15 ], and it is illustrated in Figure 7 . The computational grid comprises 1.25 million nodes and features an aspect ratio of 65, grid angles of 45°, and a volume change of 1.…”

“…To test and validate the viscosity model, the simulations were compared with data from two different measurement campaigns. The first campaign involved BAF, and these measurements are comprehensively reported in Knüppel et al [ 15 ]. In this campaign, wall shear stresses (WSSs) and pressure losses in a particle-laden flow through a microchannel at different Reynolds numbers were conducted for .…”

“…Cell migration in these gaps has not yet been researched deeply, but it is hypothesized that migration effects occur here [ 5 , 7 , 9 ]. For initial fundamental studies on CFL formation in VAD gaps, Knüppel et al replicated the pressure-driven component of the gap flow in an experiment with geometric parameters and flow conditions similar to those in the VAD gaps [ 15 ], see Figure 2 . They utilized optically accessible blood analog fluids (BAFs) with volume fractions up to to investigate stresses, pressure losses, and viscosity reductions in the gap flow.…”