Design of passive micromixers in microfluidic systems: a computational study
Okuducu, Mahmut Burak
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Passive micromixers are miniaturized mixing devices that are employed in microfluidic systems to mix two or more fluids utilizing the energy in the flow system in microchannels. In passive micromixers, fluid mixing arises as a challenging task since strictly laminar fluid flow and extremely low molecular diffusion constants inherently create a though mixing environment. In microchannels, high advection dominance that is Peclet (Pe) number is in the range of 104–106 and small interfacial area between fluid bodies prevent improving the degree of mixing over a short distance. In such circumstances, mixing length increases substantially to obtain an acceptable mixing efficiency which is not desirable due to the fact that elongated mixing domains are against the micromixer design. In passive micromixers, developing special geometric designs is essential to increase mixing performance and reduce mixing length. Besides the tough mixing conditions in passive micromixers, the numerical simulations of high Pe transport systems is problematic in terms of controlling false diffusion errors in numerical solutions. Evaluation of the degree of mixing without appropriate analysis of the contribution of false diffusion errors cause overestimation of mixing performance. As reported in this thesis, most technical literature on this subject shows signs of reported mixing conditions which are not reliable. In the current passive micromixer literature, several passive micromixer designs have been proposed to improve fluid mixing at microscales. In most of these efforts, the enhancement of mixing efficiency usually takes place as a trade-off between mixing length, energy required and design complexity. Furthermore, in several numerical studies, the magnitude of false diffusion errors is usually disregarded or underestimated. In this dissertation, a comprehensive research is conducted on the extent of false diffusion errors in numerical simulations of microscale mixing systems and two different three-dimensional passive micromixer designs are developed which show promising results. Computational Fluid Dynamics (CFD) tool is employed to investigate fluid flow and transport of a passive scalar in microchannels. In the first part of the research, false diffusion effects are examined in both unidirectional and multidirectional fluid flow conditions employing different passive micromixer designs. The outcomes indicate that the numerical investigations of advection-dominant systems require extreme caution because the actual performance of a micromixer may be masked partially or entirely by false diffusion errors depending on the several factors in numerical simulations. In the second part of the study, the convex semi-circular-ridge (CSCR) passive micromixer design is developed. It is demonstrated that the convex alignment of semi-circular elements in the streamwise direction yields a specific, helicoidal fluid motion along the mixing channel which in turn enhances fluid mixing. The CSCR design reduces inhomogeneity between fluids by offering a two-way mixing mode depending on the flowrate imposed. In the third part of the dissertation, the circular-shaped fluid overlapping (CSFO) passive micromixer is designed particularly for extremely low flowrate conditions. It is shown that the CSFO design developed allows forming a large interfacial area between fluid bodies without requiring a complex flow development in the flow domain. Therefore, a rapid inter-diffusion between fluids is enabled and mixing distance is reduced substantially. Overall, the contribution of this dissertation to the current passive micromixer literature is two-fold. First, the extent of false diffusion effects in numerical simulations of advection-dominant transport systems are disclosed in detail. The results are of crucial importance to the accurate evaluation of the degree of mixing and reporting physical mixing outcomes in numerical passive micromixer studies. Second, two novel passive mixing approaches are introduced for microfluidic systems. The CSCR and CSFO passive micromixers both diminish mixing distance under low pressure drop conditions and present high integrability with microfluidic systems due to their simple design structures.