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Research Papers: Flows in Complex Systems

Flow Pulsation and Geometry Effects on Mixing of Two Miscible Fluids in Microchannels

[+] Author and Article Information
Houssein Ammar, Ahmed Ould el Moctar, Bertrand Garnier

Laboratoire de Thermocinétique
Lunam, Polytech Nantes
UMR CNRS 6607,
Nantes 44306, France

Hassan Peerhossaini

Laboratoire Interdisciplinaire des Energies de Demain (LIED)
Université Paris-Diderot,
Sorbonne Paris Cité UMR CNRS 8236,
Paris 75013, France
email: hassan.peerhossaini@univ-paris-diderot.fr

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received July 31, 2013; final manuscript received April 6, 2014; published online September 10, 2014. Assoc. Editor: Daniel Maynes.

J. Fluids Eng 136(12), 121101 (Sep 10, 2014) (9 pages) Paper No: FE-13-1463; doi: 10.1115/1.4027550 History: Received July 31, 2013; Revised April 06, 2014

Many microfluidic applications involve chemical reactions. Most often, the flow is predominantly laminar, and without active or passive mixing enhancement the reaction time can be extremely long compared to the residence time. In this work we demonstrate the merits of the combination of flow pulsation and geometrical characteristics in enhancing mixing efficiency in microchannels. Mixing was studied by introducing a mixing index based on the gray level observed in a heterogeneous flow of pure water and water colored by rhodamine B. The effects of the injection geometry at the microchannel inlet and the use of pulsed flows with average Reynolds numbers between 0.8 and 2 were studied experimentally and numerically. It appeared that the mixing index increases with the nondimensional residence time (τ), which is inversely proportional to the Reynolds number. In addition, we show that the mixing efficiency depends strongly on the geometry of the intersection between the two fluids. Better mixing was achieved with sharp corners (arrowhead and T intersections) in all cases investigated. In pulsed flow, the mixing efficiency is shown to depend strongly on the ratio (β) between the peak amplitude and the mean flow rate. Optimal conditions for mixing in the microchannels are summarized as a function of Reynolds number Re, the ratio β, and the geometries.

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Figures

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Fig. 1

The four intersection geometries studied, consisting of two inlet channels and one micromixer outlet channel: (a) right angle intersection, (b) “Y” intersection, (c) “T” intersection, and (d) arrowhead intersection. All channels are 500 μm wide, 50 μm deep, and 38 mm long.

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Fig. 2

Schematic diagram of flow visualization in the microchannel by fluorescence technique

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Fig. 3

The excitation and emission spectra (λmax(ex) = 554 nm, λmax(em) = 579 nm) of rhodamine B in water [50]

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Fig. 4

Flow cross section at the entrance of Y intersection channel for a continuous flow with V0 = 0.022 m s−1, Re = 2: (a) experiment and (b) numerical simulation

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Fig. 5

Experimental and numerical profiles of Mi in the flow cross sections (Fig. 4) in a Y intersection with continuous flow rate for average velocity V0 = 0.022 m s−1 and Re = 2: (a) cross section CD in Figs. 4(a) and 4(b) cross section AB in Fig. 4(a)

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Fig. 6

Experimental mixing index as a function of the nondimensional residence time in the microchannel for the confluence geometry with continuous flow rate

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Fig. 7

Numerical mixing index as a function of the nondimensional residence time in the microchannel for the confluence geometry with continuous flow rate

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Fig. 8

Experimental mixing index at the micromixer channel outlet as a function of Reynolds number for all geometries with a continuous flow rate

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Fig. 9

Experimental mixing index as a function of position in micromixer channel for different values of β for all geometries (Re = 0.8)

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Fig. 10

Experimental mixing index as a function of the position in the microchannel for β = 2 (pulsed) and β = 0 (continuous) for the right angle intersection (Re = 0.8)

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Fig. 11

Numerical mixing index as a function of position in microchannel for different β values for all geometries (Re = 0.8)

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Fig. 12

Experimental mixing index at microchannel outlet versus β ratio for all geometries (Re = 0.8)

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