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Research Papers

Three-Dimensional CFD Model of Pressure Drop in µTAS Devices in a Microchannel

[+] Author and Article Information
Damena D. Agonafer

Department of Mechanical Science and Engineering,  University of Illinois at Urbana-Champaign, Urbana, IL 61801agonafer@illinois.edu

J. Yeom, M. A. Shannon

Department of Mechanical Science and Engineering,  University of Illinois at Urbana-Champaign, Urbana, IL 61801

J. Electron. Packag 133(3), 031011 (Sep 26, 2011) (6 pages) doi:10.1115/1.4004217 History: Received October 06, 2010; Revised March 06, 2011; Published September 26, 2011; Online September 26, 2011

Microposts are utilized to enhance heat transfer, adsorption/desorption, and surface chemical reactions. In a previous study [Yeom , J. Micromech. Microeng., 19, p. 065025 (2009)], based in part on an experimental study, an analytical expression was developed to predict the pressure drop across a microchannel filled with arrays of posts with the goal of fabricating more efficient micro-total analysis systems (µTAS) devices for a given pumping power. In particular, a key figure of merit for the design of micropost-filled reactors, based on the flow resistance models was reported thus providing engineers with a design rule to develop efficient µTAS devices. The study did not include the effects of the walls bounding the microposts. In this paper, a three-dimensional computational fluid dynamics model is used to include the effects of three-dimensionality brought about by the walls of the µTAS devices that bound the microposted structures. In addition, posts of smaller size that could not be fabricated for the experiments were also included. It is found that the two- and three-dimensional effects depend on values of the aspect ratio and the blockage ratios. The Reynolds number considered in the experiment that ranged from 1 to 10 was extended to 300 to help determine the range of Re for which the FOM model is applicable.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Microchannel filled with an N by M array of microposts. Channel dimensions are also defined. (b) The SEM image of an exemplary device used to measure the pressure drop across an array of Si microposts. (c) The optical microscope images of various β and N. Scale: width of the channel = 2 mm.

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Figure 2

(a) A SEM image of an array of micropost structures in the µPC and (b) a schematic of a unit cell for modeling

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Figure 3

(a) A SEM image of an array of micropost structures, (b) a geometry of the half-channel model, and (c) the geometry of the periodic model

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Figure 4

The vector plot shown above is the half-channel model. The inlet is shown on the left of the domain where the axis is located. As can be observed, the velocity field is repetitive which led to analyzing the system as a periodic model.

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Figure 5

The pressure drop across the first four post of the half-channel model for β = 0.65. The pressure drop across each post remains the same after the first post.

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Figure 6

The figure shows a mesh for a micropost for β = 0.65. The number of elements used is scaled up depending on the aspect ratio that is used.

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Figure 7

Maximum absolute scalar velocities (u, v, w) for the x, y, and z directions for AR = 0.1. The plot shows for an aspect ratio of 0.1, the velocity component in the normal direction is negligible at any β values.

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Figure 8

Parametric plot (z parameter) of the periodic part of the static pressure, p'(x,y,z), across each post for N = 40, y = 100 μm: (a) β = 0.56, (b) β = 0.73, and (c) β = 0.90. The three plots show that the flow field goes from three- to two-dimensional as β approaches one.

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Figure 9

Pressure drop comparison of numerical and experimental data [3]. The experimental and numerical data agree within experimental uncertainty for all ranges of β above 0.1. Experimental data are not available below 0.2 of β due to the fabrication challenges.

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Figure 10

The normalized drag per unit length as a function of flow rate for different β. The flow in this range is of Stokes type.

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Figure 11

FOM for N = 40 and N = 200 showing that above a certain N, there is an optimal range of β that results in a maximum FOM

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Figure 12

The effect of aspect ratios have over the normalized drag per unit length β = 0.562. As the aspect ratio decreased, the normalized drag increased by over a factor of 5. This trend shows that as the aspect drops below 0.1, the effect of the walls need to be considered.

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Figure 13

The pressure drop per unit cell versus Reynolds number for a fixed β. The flow stops behaving as Stokes’ type for Re > 100.

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