Microchannel Flow Measurement Using Micro Particle Image Velocimetry

Since microfabrication techniques are typically planar processes, microchannel flows typically have significant predevelopment due to the upstream reservoir having the same height as the microchannel. The main concerns of the current study are categorized into finding the effects of typical microchannel geometry on the velocity entrance length in the laminar flow regime and providing the turbulence transitional Reynolds number range using the details of the velocity profile rather than global measurements of pressure drop. A rectangular micro-channel of aspect ratio ~2.65 and the hydraulic diameter 380μm was used in this study. Micro particle image velocimetry measurement was performed to measure the velocity profiles. The entrance length is reduced about 45% and the transitional velocity profile is measured at Re=2900. The velocity profiles do not show deviation from the fully developed laminar flow profiles up to Re=2100. Related to the flow transition, the close resemblance between the correlation function peak broadening and the turbulence intensity is observed. INTRODUCTION In studies of duct flow, entrance effects and laminar to turbulent transition are two of the most fundamental and important subjects. These features are important design parameters in practical applications because properties, such as pressure gradient, wall shear stress and heat transfer coefficient, show the different behaviors according to both the flow regime, i.e. laminar or turbulent, and the flow region, i.e. the hydrodynamic developing region(the entrance region) or the developed region. The pressure drop in the developing region exceeds its value in the fully developed region because it must overcome the wall shear and increase the flow momentum. As channel flow applications have been extended to the small scale, the microchannels for various purposes have been constructed using microfabrication techniques. Microchannels for electronics cooling or that take advantage of the high surface to volume ratio are typical applications. Tuckermann and Pease[1] fabricated a microchannel array on the backside of VLSI(Very Large Scale Integrated) circuit for cooling purposes and showed that cooling performance was significantly increased. 1 Copyright © 2002 by ASME Analytical and experimental work to understand the flow physics in rectangular macroscale channels were performed extensively in 60’s and 70’s.[2-12] Most works were devoted to finding both the pressure drop and the velocity profiles in the entrance region, along with the entrance length. On the other hand, specific analyses of the microchannel flow have been focused on the size effects on the friction factor and the flow transition, since the Wu and Little[13] observed the abnormal behavior in the microchannel flow. Later, deviations from conventional hydrodynamic observations were found by several experimenters but the results are not conclusive yet.[13-16] The velocity profiles in rectangular channels are somewhat more complicated than those of circular channels. In a rectangular channel, four boundary layers begin at the each wall like the boundary layer growth on the flat plate and merge at some distance downstream. The hydrodynamic entrance region is where the velocity boundary layer, the pressure gradient, and wall shear stress are developing. The entrance length is the distance to attain their constant conditions (i.e. fully developed conditions). The definition of the entrance length can be made based on the pressure gradient as well as the velocity profile. The pressure entrance length is usually somewhat shorter than the velocity entrance length.[5] To decide the precise entrance length is difficult because of the asymptotic approaches of the three variables to their constant conditions.[25] For engineering purposes, the entrance length is defined, in general, as the axial distance required for the centerline velocity to reach 99% of the fully developed velocity. There exist several correlations for entrance length estimates.[24] The uniform velocity profile, typically assumed at the entrance when viscous fluid is entering a duct, is seldom achieved in practice because the flow development, taking place in the contraction section, is unavoidable.[5] Further, the velocity overshoot caused by the abrupt change of the velocity at the wall and the sharp turn of the velocity at the entrance make it also difficult to have the uniform inlet velocity profiles. [3,6,8,10,11,23]