Mean flow structure in high aspect ratio microchannel flows

An experimental investigation of water flow through a high aspect ratio rectangular microchannel was conducted to further understand fluid dynamic characteristics in microchannels and to test the validity of macroscale theories that are commonly utilized at the microscale. A rectangular microchannel with nominal dimensions of 500 μm in height, 6 mm in width, and 32.8 cm in length was CNC machined into an aluminum blank. The test-section was completed by attaching a cap blank to the microchannel blank. Pressure and velocity data were obtained over a Reynolds number range from 173 to 4830, where the Reynolds number is based upon hydraulic diameter and channel average velocity. Velocity data were obtained using molecular tagging velocimetry (MTV). Laminar dimensionless velocity and coefficient of friction data are in agreement with macroscale theory. Transition from laminar flow, based upon a change in dimensionless velocity profile shape, occurs at a Reynolds number of 2800. This transitional Reynolds number is in excellent agreement with integral results and macroscale experimental results. Fully developed turbulent flow is found to exist at a Reynolds number of 4800. Inner normalized mean velocity profiles scale in the near-wall region, whereas the profiles of Reynolds stress and production of kinetic energy do not scale on inner variables. The inner normalized mean velocity profiles exhibit increasingly logarithmic-like behavior through the transitional regime. The experimental trends for the inner normalized mean velocity, Reynolds stress, and turbulence production are consistent with macroscale experimental and direct numerical simulation data. At this scale, influences reliably attributable to microscale effects were not detected in either the laminar or turbulent measurements of the present study.