Entrance length, turbulence intensity, and local pressure measurement in microchannel flows

Abstract

Fundamental aspects of the channel flows were experimentally investigated in a microchannel with the planar upstream geometry, i. e. the entrance length and the critical Reynolds number. The effects of the upstream reservoir geometry resulting from typical microfabrication processes on the entrance length of a rectangular microchannel has been studied for a large range of Reynolds numbers (i.e. 1, 10, 20, 30, 50, 70, 100, 210, 1030, 1810, and 2100). The microchannel dimension was 252.29 μm × 694.15 μm × 120 mm. Micro particle image velocimetry was used to measure the velocity profiles in the microchannel. An almost constant entrance length is found for low Reynolds number flows. The entrance lengths for ReD = 210 and 1030 show ∼50% reduction while the entrance lengths are increased for Re = 1810 and 2100 by ∼80%. There exists a nonlinear relation between the entrance length and Reynolds number in the range of 1030 ≤ ReD ≤ 1810. Since the channel has far larger scale compared to where the continuum theory breaks down, the effect is solely from the entrance geometry change. Critical Reynolds number in a rectangular microchannel has been investigated. Laminar flow behavior is observed up to ReD = 2100 while transitional flow is generated at ReD = 2940 in agreement with well established macroscopic observations. Turbulence intensity is calculated and confirms that transition occurs between Reynolds numbers 2100 and 2940. In addition, a new correlation analysis for turbulence intensity measurement has been developed. The results are compared with the turbulence intensity calculated from the velocity field. The turbulence intensity derived from the correlation analysis showed the very close trend compared to the turbulence intensity measured from the velocity field. An inexpensive and simple pressure measurement technique which could be used in a microchannel using air compression is demonstrated. The ideal gas law and surface tension are used to measure the local static pressure in the microchannel. The prototype device was built with SU-8. For the interfacial pressure jump calculation, the contact angle of a water drop on an SU-8 surface was measured and the contact angle was found as 76.0° ± 6°. Because of the complicated boundary condition of the pressure tap, an accurate interfacial pressure jump was not acquired. Long pressure taps showed better sensitivity. The measured pressure shows lower values than the theoretically calculated values. However, the sensor can still be calibrated and used for pressure measurement