Design of passive micromixers in microfluidicsystems: A Computational stud
Başlık çevirisi mevcut değil.
- Tez No: 720963
- Danışmanlar: DR. MUSTAFA M. ARAL
- Tez Türü: Doktora
- Konular: İnşaat Mühendisliği, Çevre Mühendisliği, Civil Engineering, Environmental Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2020
- Dil: İngilizce
- Üniversite: Georgıa Instıtute Of Technology
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
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Özet (Çeviri)
In microfluidic systems, passive micromixers are employed to mix at least two fluids homogeneously exploiting the fluid flow energy in microchannels. In these microscale mixing units, strictly laminar fluid flow that is Reynolds (Re) << 100 and very low molecular diffusion constants—typically in the range of 10-9 –10-11 m2 /s— fundamentally create tough conditions in terms of both yielding a high mixing efficiency over a short distance and controlling false diffusion errors in numerical simulations. In passive micromixers, developing special geometric designs are essential to increase mixing performance and reduce mixing length. In the current literature, although various passive micromixer configurations are proposed, the improvement of mixing in these designs is usually possible compromising at least one of the following criteria: short mixing length, low energy requirement, and design simplicity in terms of fabrication. Besides, in numerical passive micromixer investigations, the magnitude of false diffusion errors is usually disregarded or underestimated. Evaluation of the degree of mixing in these devices without appropriate analysis of the contribution of false diffusion cause over estimation of mixing performance. The objective of this research is to characterize the extent of false diffusion errors in numerical simulations of microscale mixing systems and develop efficient three-dimensional micromixer designs in which all the above standards are ensured in tandem. In the first part of the study, a comprehensive research is performed on false diffusion errors in numerical simulations of passive micromixers. The effect of false diffusion is investigated in both simple unidirectional and complex three-dimensional fluid xviii flow conditions. Several test scenarios are established to monitor and quantify the extent of false diffusion generation in numerical solutions. It is shown that the scalar transport simulations produce considerably high numerical errors compared to the numerical solution of the flow field. Thus, the use of flow parameters in grid studies should be avoided. Instead, mixing efficiency needs to be employed as the parameter to show the actual discrepancy between different grid levels and to determine a feasible grid size that is computationally inexpensive and produces insignificant amount of false diffusion. Moreover, the difference between mesh densities should be high enough to be able to expose false diffusion errors accurately. The simulation results of different numerical algorithms show that while Finite Element Method (FEM) and Finite Volume Method (FVM) resolve the flow domain almost identically, both numerical techniques exhibit different false diffusion generation inclination in scalar transport simulations. In FVM, false diffusion errors are reduced substantially when the flow vectors are orthogonal to the cell faces in the computational domain. Hence, the lowest numerical diffusion errors are observed in cases where the flow is unidirectional in the micromixer and hexahedron mesh elements are used. When prism and tetrahedral mesh structures are applied, the physical effect of the molecular diffusion constant tested is overshadowed by false diffusion errors that are generated during the numerical solution of advection diffusion (AD) equation. In complex fluid flow conditions, even though computational domain is discretized with hexahedron elements, contribution of additional dimensions inherently prevent retaining a good mesh flow alignment in micromixers. Thus, the continuous violation of orthogonality in such flow regimes causes generating high amount of false diffusion in numerical solutions which in turn masks the physical effects of molecular diffusion and increase the xix performance of micromixer unphysically. In FEM, the selection of an appropriate stabilization type, which is used to suppress the oscillations in the simulations, is crucial to control the false diffusion in numerical solutions. It is found that consistent stabilization option manages false diffusion generation successfully and provide almost identical outcomes with FVM whereas artificial stabilization approach offers a stable solution at a cost of excess unphysical diffusion in numerical solutions. In the second part of the study, fluid mixing in 3-D classical T–shaped (CT) passive micromixers is investigated in a wide range of flow, scalar transport and fluid injection conditions. It is shown that the improvement of mixing performance in CT micromixer configurations is quite low due to inefficient manipulation of fluid bodies in the mixing channel. Although vortex and engulfment flow profiles are fed by alternative split type inlets, the degree of mixing is not raised beyond 40%. In addition, the above complex flows are developed at a cost of a high pressure drop in the CT micromixer. The pressure drops measured are around 8.5 and 15.1 kPa for the two highest flow conditions tested, i.e., Re = 160 and 240, respectively. Considering the limitations in the CT geometry in terms of creating an effective complex flow profile, a novel convex semi-circular-ridge (CSCR) passive micromixer design is developed. It is demonstrated that the convex alignment of semi-circular elements 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. While a rapid interdiffusion between fluid bodies is ensured in low flow conditions, the increasing deformation rate of fluid bodies with rising flowrates improves fluid mixing due to chaotic advection. In test cases examined, homogeneous scalar concentration distributions with a xx mixing efficiency over 80% and a pressure drop less than 5 kPa are obtained in a mixing length less than 2000 µm. When it is compared to the CT micromixer, the novel design developed increases mixing efficiency and mixing quality values by the factors of 8.7 and 3.3, respectively. It is also shown that different orientations of mixing elements in the mixing channel adversely affect the mixing performance by disturbing the formation of helicoidal-shaped flow profile. In the third part of the dissertation, a novel fluid overlapping mixing method is introduced along with nested type inlets. By this diffusion–based mixing approach, the improvement of mixing efficiency over a short distance is aimed particularly in very low flow conditions, i.e., Re < 10, where the development of a complex flow profile is difficult. The behavior of fluid flow in rectangular and circular designs is analyzed based on the simulation outcomes of a single–mixing–box fluid overlapping micromixer configuration. It is shown that the circular geometry presents optimum conditions in terms of the uniform distribution of fluid flow over an impermeable surface in the mixing box. The mixing of fluids is investigated in a circular–shaped fluid overlapping (CSFO) passive micromixer in the Re number range of 0.1–10. In the CSFO design, different scalar transport scenarios and inlet types are tested for both constant and sequential fluid injection modes. The outcomes show that the CSFO micromixer design provides considerably high mixing efficiencies over a very short distance in the main streamwise direction. When Re = 0.1, 0.5 and 1 flow conditions are simulated with the constant scalar injection mode, almost complete mixing is observed in a mixing distance between 260 µm and 470 µm for the most difficult mixing condition tested. In higher molecular diffusion scenarios of Re = 0.1, 0.5 and 1 flow conditions, fluid mixing is completed in a mixing distance less than 260 xxi µm. When the sequential scalar injection mode is applied at Re = 1, it is found that even the lowest injection frequency tested provides more than 90% mixing efficiency for the smallest molecular diffusion constant simulated. The effect of injection frequency is more visible in higher flow cases that are Re = 5 and Re = 10. Operating the CSFO design with sequential injection mode reduces the complete mixing distance noticeably. In the highest flow condition examined, the maximum pressure drop is measured to be less than 1.4 kPa between inlet and outlet of the CSFO micromixer. Based on the studies reported in this thesis three technical papers are published in SCI indexed journals on the subject which already received numerous citations over a very short period.
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