Computational study of laminar flow in microchannels with abrupt expansion or contraction at a 90° miter bend.

摘要

Rapid prototyping technologies have radically reduced the time required for the development and field disposal of microfluidic devices. Xurography is one such technology, in which the microstructures are fabricated in double-sided adhesive tape using a cutting process. This cutting process is highly imprecise, particularly when cutting features such as bends and junctions where the blade is required to change cutting direction. As a result of this imprecision, expanding or contracting microchannels can be produced while fabricating serpentine microchannels. In order to efficiently use xurography to fabricate microfluidic systems for applications such as heat exchangers, it is necessary to characterize the effects of these abrupt expansions and contractions at 90° miter bends.;A steady state incompressible flow simulation of water in microchannels containing a 90° miter bend is conducted. Microchannels with miter bends, including abrupt expansions and contractions after the miter bend in some cases, are used in these simulations. The aspect ratio of these channels ranges from 0.2 to 1.0 and the area ratio of contraction and expansion ranges from 0.33 to 0.91 and 1.1 to 3, respectively. Commercially available software packages, Fluent and Gambit, are used for this purpose. A pressure-based solver using a fully-coupled implicit algorithm with algebraic multigrid method is used for the simulation. Third-order MUSCL scheme is used for the momentum discretization and the PRESTO scheme for the pressure.;Excess loss coefficients, flow development lengths downstream of the miter bend, and the length and width of the recirculation zones, both upstream and downstream of the miter bend, are evaluated for the microchannels with different aspect ratios and area ratios for Reynolds numbers ranging from 5 to 600. It is observed that the critical Reynolds number Re cr of the microchannel is a function of the aspect ratio. An axial flow vortex that develops in the outlet channel results in the independence of the primary recirculation zone on Reynolds number and decreasing secondary recirculation zone's penetration into the outlet with Reynolds number. The development length downstream of the miter bend is better predicted by the conventional model than the more advanced models based on aspect ratio. Bend excess loss coefficients Kb are in reasonable agreement with the microscale experimental loss coefficients reported in the literature. The bend excess loss coefficients decrease with increasing area ratio Ar in both the expansion and contraction channels, when the losses are normalized with the inlet kinetic head. Bend excess loss coefficient Kbe for the expansion channel remains nearly constant for high expansion ratios. Bend excess loss coefficients for expansions and contractions in the 90° miter bend microchannels are far less than the similar experimental data reported in the literature, due to the surface roughness, inconsistent channel dimensions, and the damage to the channel walls caused by the cutting process.