摘要 :
With significant centrifugal effects and Dean vortices, the spiral microchannels are being widely used in microfluidic systems, especially for high-efficiency mixing. However, the spirals are with various shapes and their radii co...
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With significant centrifugal effects and Dean vortices, the spiral microchannels are being widely used in microfluidic systems, especially for high-efficiency mixing. However, the spirals are with various shapes and their radii continuously vary along the spiral, which causes the flow fields and mixing process complicated. This research focused on the flow behaviors and mixing performance of spiral micromixers with various spiral structures. Five classical spirals of Archimedean, Logarithmic, Hyperbolic, Golden and Fibonacci spirals, and two structures of centrosymmetric spiral structure and single spiral structure were investigated. Numerical simulation and experimental validation were conducted with Re from 0.1 to 100. The results showed that the Archimedean spiral presented the best mixing performance among the five spirals, since its radius of curvature kept a small value in the whole flow path which brought about stronger Dean vortices. Moreover, it was found that Dean vortices was inapplicable to characterize chaotic advection in the centrosymmetric spiral structure due to vortices degradation. Vortices degradation resulted from the counter-rotating Dean vortices, which suppressed the chaotic advection and the mixing process. The results also revealed that vortices varied periodically along the flow path in single spiral structure. Increasing the Reynolds number accelerated the periodical variation of vortices, and therefore mixing developed more rapidly. Moreover, a novel parameter named vortices advection intensity (Vai) was proposed to characterize chaotic advection. The calculated results of Vai agreed well with the results of mixing performance under the condition of chaotic advection dominated mixing. The parameter Vai could reliably characterize chaotic advection.
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摘要 :
This paper designs three quasi-geostrophic barotropic models with a radial/horizontal grid length being 2 km, one in the polar coordinates, one on a stationary typhoon circulation condition and another on a non-stationary typhoon ...
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This paper designs three quasi-geostrophic barotropic models with a radial/horizontal grid length being 2 km, one in the polar coordinates, one on a stationary typhoon circulation condition and another on a non-stationary typhoon circulation condition in the Cartesian coordinates, to investigate the effects of azimuthal and radial linear advections, and nonlinear advection on the inward propagation of mesoscale vorticity and the changes of typhoon intensity. Results show that the azimuthal linear advection may result in the formation of spiral vorticity bands; the radial linear advection in a certain parameter set is able to transfer vorticity inwards, leading to a slight enhancement of typhoon; the nonlinear advection of perturbation vorticity on a stationary typhoon circulation condition may transfer more vorticities inwards, thus resulting in a distinct enhancement of typhoon; and the nonlinear advection on a non-stationary typhoon circulation condition possesses duality, i. e. on the one hand, the advection increases the vorticity of inward propagation, thus favorable to the intensification of typhoon, and on the other hand, in the inward propagation process of vorticity the originally concentric and axisymmetric structure of typhoon basic flow is damaged, and a complex flow pattern forms, which in turn tends to weaken the circulation of typhoon. At last the paper discusses the possible applications of those results in typhoon intensity prediction.
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