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Chaotic Advection And Its Role In The Water Quality Of The Lower Hudson River Estuary
Published in Nada Assaf-Anid, Hazardous and Industrial Wastes Proceedings of the Thirty-Third Mid-Atlantic Industrial and Hazardous Waste Conference, 2001
Amvrossios C. Bagtzoglou, Andrei Novikov, Brandon Gallagher, Rene Chevray
It has long been recognized that particle paths in a smooth laminar flow may be chaotic. This phenomenon where a flow is laminar and some particle paths are chaotic is called Lagrangian turbulence but we rather prefer the name chaotic advection established by Aref (1984) because the flow is in fact not turbulent. When discrete particles are introduced into a chaotically advected medium, they are often distributed rapidly over the accessible space. Under certain conditions it is also possible that the particles entrained in this flow become segregated to a specific region, causing that way local inhomogeneity or changes in the boundary geometry. This kind of processes could be observed in many natural phenomena such as the action of tidal waves and tidal currents in particle and sediment transport. There, the incoming and outgoing tides generate periodic streamline patterns, which could induce chaotic Lagrangian trajectories. Particle transport in such conditions could be simulated with the process of industrial mixing or of dispersion of pollutants in natural media.
Characterization of Chaotic Stirring and Mixing Using Numerical Tools
Published in Sushanta K. Mitra, Suman Chakraborty, Fabrication, Implementation, and Applications, 2016
Shizhi Qian, Bayram Celik, Ali Beskok
Hereafter, bold letters represent vectors. In such velocity fields, fluid elements that are originally close to one another trace paths that diverge rapidly (exponentially fast in the ideal case), so that the material is dispersed throughout the volume very efficiently. This typically leads to significantly fast mixing. Therefore, chaotic advection in LOC devices can provide the best possibility of achieving efficient and thorough mixing of fluids.
Introduction to Microfuidics
Published in Simona Badilescu, Muthukumaran Packirisamy, BioMEMS, 2016
Simona Badilescu, Muthukumaran Packirisamy
It is known that turbulence in flows facilitates rapid mixing at values of Re > 2,000, which can be reached in microchannels only at very high, impractical flow velocities (~10 msec−1). Such high flow velocities require high sample consumption and high pump pressures that are difficult to be achieved in microchips. Unfortunately, for slow microfluidic flows, diffusion alone is usually not sufficient to mix fluids, as it would require very large diffusion rates. It does not happen fast enough, especially in the case of assays that involve large particles such as cells. Various mixing principles have been applied to passive micromixers in order to achieve higher mixing efficiencies. Interdigital multilamination, split and recombination, reduced diffusion length design, vortex generation, and chaotic mixing are just a few of them. An efficient passive micromixer with complex three-dimensional geometries is utilized to enhance fluid lamination, stretching, and folding for optimal mixing. In a microfluidic device, there are two ways of mixing fluid streams, namely, passive mixing and chaotic advection or active mixing, as shown in Figure 8.4. Passive mixers use appropriate channel geometries to fold fluid streams in order to increase the area over which diffusion occurs. Examples of passive mixing include distributive mixer, static mixer, T-type mixer, and vortex mixer. The Coanda effect is used to make an in-plane micromixer that splits the fluid streams and recombines them to induce mixing. Rapid mixing with low reagent consumption can be achieved using chaotic advection. Chaotic advection increases the area over which diffusion occurs by continuously stretching and refolding the two fluids to achieve layers of fluids (striations) that become thinner and thinner until mixing becomes rapid. Some of the designs employed for passive mixing are bifurcation flow distribution structures, focusing structures for flow compression, flow obstacles within microchannels, multihole plates, tiny nozzles, and T- and Y-flow configurations.
Effectiveness of MPCMS on Straight, Dimple-cavity & Rib-groove microchannel heat sinks – a comparative study
Published in Experimental Heat Transfer, 2022
John Peter R, K R Balasubramanian, Divakar s, Jinshah B s
With the higher hydraulic diameter in the dimple-cavity channel, the residence time of the MPCM increases, leading to enhanced heat transfer compared to a straight channel. These enhancements are due to the flow transition, curvature breaking of the boundary layer through the intermittent secondary passage through the corrugated channel. This chaotic advection in the expansion-compression section by dimple-cavity enhances the fluid mixing and heat transfer rates. In the rib-groove channel, the same characteristics are found due to its structure, leading to low residence time, and inducing the phase change process. The fluid velocity in this region of the groove is less than that of the narrow region due to the suddenly widened cross-sectional area. Due to this flow, stagnation occurs, leading to a fall in heat dissipation from the bottom surface of the groove. Further, the rectangular ribs will separate the fluid flow into two small vortices in the corner of the rib, leading to a high flow velocity, which flushes at the bottom surface of the groove to enhance the fluid transport.
A numerical parametric study to enhance thermal hydraulic performance of a novel alternating offset oblique microchannel
Published in Numerical Heat Transfer, Part A: Applications, 2021
Sanskar S. Panse, Srinath V. Ekkad
Chai et al. [20–22] published a three-part research investigating heat transfer, pressure, and THP of microchannels featuring fan-shaped ribs on microchannel side walls arranged in aligned and staggered manner. Effect of parameters like the rib height, width, and spacing was analyzed on their performance. Results showed lower height, large spacing of fan-shaped ribs in aligned arrangement provided optimal performance. The Nusselt number increased by about 5–100% with very high frictional losses about 1–8.3 times compared to the baseline case. Highest THP of 1.33 was reported. As opposed to ribs extending into fluid domain and blocking the flow passages, cavities promise enhancement in heat transfer exceeding penalty in pressure drop by increasing the flow area. Xia et al. [23, 24] investigated the performance of microchannels with triangular and semicircular cavities. The cavities induced fluid recirculations which reduced the boundary layer thickness. The chaotic advection brought about by cavities enhanced fluid mixing and resulted in a uniform wall temperature. Triangular cavities provided 40–60% increase in heat transfer with ∼20–30% penalty in frictional losses, while semicircular reentrant cavities reduced fan power by 20% for constant thermal resistance.
Numerical simulation of geometry effect on mixing performance in L-shaped micromixers
Published in Chemical Engineering Communications, 2020
Figure 12 represents the distribution of the mixing index along the flow path in four microchannels at different Reynolds numbers. At Re = 20, the mixing index along serpentine microchannels increases with a smaller gradient as compared to high Reynolds numbers due to weaker secondary flows. Additionally, mixing in the L-shaped micromixer improves steadily along the flow path. An increase in flow velocity in the serpentine microchannels leads to a significant increase in the effect of centrifugal force and a stronger chaotic advection, and therefore, better mixing. Mixing index in the straight microchannel increases slowly along the mixing channel with almost the same gradient at all Reynolds numbers. Mixing index values in serpentine microchannels are considerably more than those in the straight microchannel due to recurring turns, especially at high Reynolds numbers. As mentioned before, a microchannel with a shorter total streamwise length is suitable. Mixing index reaches the maximum value of about 88% at Lt=5000 μm in the L-shaped micromixer, and Lt=7000 μm in the 90° and 60° V-shaped micromixers at Re = 200. Hence, total length of the microchannels can be reduced to 5000 μm in the L-shaped micromixer, and 7000 μm in the 90° and 60° V-shaped micromixers. Mixing index does not increase steadily along the flow path in the serpentine microchannels to achieve complete mixing at high Reynolds numbers.