An investigation of multiphase flow in complex microchannel geometries for hemodialysis systems using residence time distribution analysis Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9019s732b

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  • There are approximately 2.2 million cases of End-stage Renal Disease (ESRD) worldwide and current hemodialysis treatment is costly and ultimately insufficient compared to kidney transplantation. There is a strong need to arrive at a better solution for treating ESRD. Microchannel-based hemodialysis is one proposed option, already shown to have improved filtration performance characteristics over conventional hemodialysis. It has the potential to greatly improve the quality of life for patients but before microchannel hemodialysis is utilized in patient homes, there are still important challenges that must be overcome. A test loop was developed for measuring the pulse response of a dye tracer injected into a dialyzer device for the purpose of detecting defects, developing residence time distributions and characterizing lamina design. Four lamina designs were investigated: 1) a 60 microchannel array with slim asymmetric triangular headers, 2) a 60 microchannel array with wide asymmetric triangular headers, 3) a 60 microchannel array with wide asymmetric triangular headers filled with micro-post features, and 4) a micro-post grid pattern throughout the entire lamina. As a diagnostic tool for detecting defects, the pulse response test loop performed close to expectations, able to detect a difference in mean residence time for design 1 when only 4 of 60 channels were obstructed or 4.1% difference in total system volume. Residence time distributions (RTD) of the dialyzer devices were obtained by deconvoluting the pulse response measurements. RTD variance tended to lower for designs that are more dominated, volume-wise, by the microchannel array versus the headers. These results also pointed out a discrepancy between the idealized conceptual device and the real fabricated device by emphasizing how issues such as sagging or bulging, fabrication tolerances, and miniscule misalignments can significantly impact a microchannel device. A multi-segmented CFD model developed for pairing with the pulse response test loop and dialyzer, showed good agreement between visual observation of the tracer in simulations and experiments, and the shape and peak height of the output profiles. In addition to this work, a Polyethylene oxide (PEO) – polybutadiene (PB) – polyethylene oxide triblock polymer coating, originally intended to improve device biocompatibility and therapeutic applications, was investigated for its effect on reducing bubble stagnation in microchannel lamina for dialysis use, thereby improving filtration performance. Experimental results show a statistically significant reduction in blocked channels by stationary bubbles for a 40 microchannel PEO-coated lamina versus a similar uncoated lamina between nominal average channel velocities of 1.8 to 3.6 cm/s. A numerical simulation based on the Lattice Boltzmann modeling approach indicate that beneficial effects may be ascribed to the maintenance of a lubricating, thin liquid film around the bubble. Experimental and numerical simulation results serve to validate the additional bubble lubricating utility of the PEO-PB-PEO coating.
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