Technical Note
Micromixing enhanced by pulsating flows

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Abstract

We consider the micromixing enhancement by pulsating flows. Dimensionless governing equations and boundary conditions were developed for T-type micromixers with two inlet pulsating flows. The problem involves a set of parameters. Three key dimensionless parameters are identified: the Reynolds number, the Strouhal number, and the disturbance amplitude. Suitable Strouhal number or disturbance amplitude causes symmetrical meniscus-shape mixing interfaces, separating the whole mixing channel into a set of segments. Thus uniform exit species concentration can be reached. Too large or too small Strouhal number or disturbance amplitude yields the meniscus-shape mixing interfaces deviating from the centerline of the mixing channel, deteriorating the mixing performance. The optimized disturbance amplitude is increased with increases in Strouhal numbers. Low Reynolds number needs larger disturbance amplitude.

Introduction

Micromixing becomes difficult due to the lack of turbulence for low Reynolds number flows in microscale [1]. It can only be fulfilled by molecular diffusion, behaving very slowly. Inlet pulsating flows can enhance the micromixing process. This concept of mixing was first proposed by Volpert et al. [2]. Bottausci et al. [3] designed a micromixer with three cross-stream secondary channels, providing time-dependent actuation of the flow stream perpendicular to the main stream. It is shown that mixing was substantially improved with oscillatory flows in multiple side channels. Cross-channel micromixers were reported in [4], [5], [6], in which the main stream was perturbed by an external oscillatory-flow excitation in side channel. Adjusting amplitude and frequency of the oscillatory flow, the wavy and chaotic regimes were obtained. A spatiotemporal resonance phenomenon was observed in their experiments. It is found that the resonant states could be used to separate particles of different sizes. Niu and Lee [7] studied chaotic behaviors in a micromixer with multiple side channels stirred by oscillatory flows. They found that Lyapunov exponent is closely related to the amplitude and frequency of stirring and can be used to optimize design and operation of micromixers.

Nguyen and Huang [8], [9] reported a micromixer with pulsating flows at the entrances using two piezoelectric valves. Truesdell et al. [10] proposed the pulsation action to mix two streams entering a tube from two separated branches of a bifurcation at low Reynolds numbers. The two pinch valves used in their experimental setup deform the flexible tubing immediately upstream of the connection. The mixing quality was evaluated by measuring the fluorescence resulting from the chemical reaction of species transported in the two streams. The results showed that the mixing process was almost complete. Glasgow and Aubry [11] studied the enhanced micromixing by the computational fluid dynamics (CFD). All channels were 200 μm wide by 120 μm deep. They found that the best results occurred when both inlets were pulsed out of phase. The interface was shown to stretch, retain one fold, and sweep through the confluence zone, leading to good mixing within 2 mm downstream of the confluence, i.e., about 1 s of contact. A simple time-dependent analytical solution for the pulsating flow enhanced micromixing was proposed by Nguyen and Huang [12].

In this paper, we study the micromixing enhancement by pulsating flows. The dimensionless governing equations and boundary conditions are developed. Three key parameters were identified. The optimal parameters for the micromixing enhancement were discussed.

Section snippets

Numerical simulation

T-type micromixers are used, whose dimensions and coordinates are shown in Fig. 1. The original point (0, 0, 0) is at the confluence center between the two inlet channels. The x-coordinate is at the mixing channel centerline. The y-coordinate is the chip width direction perpendicular to the x-coordinate. The z-coordinate refers to the channel depth direction. The mixing channel has the length of L, with the channel width of Wc and depth of H. The two inlet channels have the same cross section as

Results and discussion

The disturbance amplitude significantly influences the mixing performance. During the pulsating flow, the whole mixing channel is separated by a set of meniscus-shape mixing interfaces. The segmentation number is the number of sections separated by meniscus-shape interfaces. At Re = 10, f = 10 and 100 Hz, the disturbance amplitude of 5 yields five segments separated by four meniscus-shape mixing interfaces. The meniscus-shape mixing interfaces deviate from the mixing channel centerline. The tip of

Conclusions

We consider the micromixing enhancement by pulsating flows. Dimensionless governing equations and boundary conditions were developed for T-type micromixers with two inlet pulsating flows. The problem involves a set of parameters. Three key dimensionless parameters are identified: the Reynolds number, the Strouhal number, and the disturbance amplitude. Suitable Strouhal number or disturbance amplitude causes symmetrical meniscus-shape mixing interfaces, separating the whole mixing channel into a

Acknowledgment

This work is supported by the Natural Science Foundation of China with the Contract Nos. 50776089 and 50825603.

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