Online proceedings - EDA Publishing Association

The mixing length is a distance that a fluid will keep its
original characteristics before dispersing them into the
surrounding fluid. By means of the mixing length, the
required channel length of the micromixer can be
demonstrated. Fig. 8 shows a series of images captured at
specific segments of the channels at specific flow velocity,
and the fabricated microchannels which consist of twenty
two identical mixing elements are studied. The flow rates are
0.15 ml/min corresponding to the Re equal to 25 and K equal
to 11.18. As shown in Fig. 8(a), the flow proceeds forward
near the inlet, the mixing of two fluids is only through
molecular diffusion across the interface of the two liquids. So
two parallel streams meet at the exact center of the channel
and, thus, the interface is clearly observed in the channel.
Then more reacted solution stream passes through the inner
half of the channel and the variation of the concentration
distribution along the radical direction can be seen clearly.
Furthermore, the reacted solution is spread across the
channel and multiple streams become visible. For the
staggered curved channels with constant-width structures
shown in Fig. 8(b), the similar results can be observed. With
smaller impact effect near the recombined portions in the
channels than that of staggered curved channels with tapered
structures, it shows the mixing is not very well. The mixing
performance of the continuous curved channels is
demonstrated in Fig. 8(c). Lack of SAR structures the mixing
is poorer than that of staggered curved channels with tapered
structures. However, the path length per segment is the
longest compared with that of staggered curved channels.
The Dean Vortices inside the continuous curved channels
induce strong secondary flows. The mixing is comparable to
that of staggered curved channels with constant-width
structures. The mixing index at twenty two specific locations
is measured and calculated at different micromixers. The
dashed line represents a mixing index equal to 0.9, and the
mixing length is the channel length required for achieving
the mixing index of 0.9. Mixing index of 0.9 denotes that
mixing fluids are in a well mixed status. The resulting mixing
lengths are at the seventh, eighteenth and over twenty-second
segments corresponding to the downstream distances of 24.5
mm, 63 mm and 77 mm, respectively. Due to the large vortex
flow combined with the SAR effect and the impact effect,
two fluids are folded into each other. Notably, due to the
increases of the interfaces of the two fluids, mixing is
improved.
Staggered Curved Channels with Tapered structures
1 5
10 15
(a)
11-13
May 2011, Aix-en-Provence, France
Staggered Curved Channels with Constant-width structures
1 5
10 15
(b)
Continuous Curved Channels
1 5
10 15
(c)
(d)
Fig. 8. Top view images of the specific segments of the staggered channel
with tapered structure at specific Reynolds number. Mixing index changes
along the downchannel direction of the micromixer for different
micromixers.
The increased interface area of two fluids can promote a
mass transfer based on diffusion. The configurations of
interfacial lines between two different fluids play an
important role in the microchannels. The numerical results of
the interfacial line length at four specific locations are
calculated. Initially, the fluid interface is described by a
vertical straight line across the inlet. The shapes of the
interfaces of the cross-sectional planes for staggered curved
channels with tapered structures at different Re are shown in
Fig. 9(a). The flow near the central axis is the most strongly
affected by the inertia. In the case of Re of 10, the interfacial
distortion is negligible. For Re equal to 50, the interface is
much more distorted. The interface stretching factors are
poltted in Fig. 9(b) as a function of the number of mixing
segments. This factor is defined as the interface length at a
certain position divided by the initial interface length. From
the figure, it is obvious that at Re=10 nearly no stretching
occurs, while for Re=50, the stretching can be seen obviously.
The plots of the interfacial line length as a function of
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