Characterization of the Hydrodynamically Developing Flow in a Microtube Using Molecular Tagging Velocimetry

There is a need for increased understanding of the momentum transport phenomena in micro-fluidic geometries to aid in the design and optimization of such devices. Micro-molecular tagging velocimetry (μMTV) has been used to characterize the hydrodynamic developing flow in a microtube with an inner diameter of 180 μm. μMTV is a non-intrusive laser-based technique for obtaining detailed measurements of velocity profiles in flows dominated by a single velocity component. μMTV measurements are made by directing an ultra-violet laser beam into a flow containing phosphorescent tracer molecules. The laser beam excites a line of phosphorescence in the flow. Subsequently, two digital images, separated by a short time delay, of the line are captured by a CCD camera. The displacement of the tracer molecules between the images can be determined from the two images and the velocity of the flow is thus calculated. Velocity profile data at ten axial locations within the hydrodynamic developing region of a 180 μm diameter tube were acquired using the μMTV approach. The uncertainty for these measurements ranged from 1.5% to 5.5% of the center line velocity. Data were taken at Reynolds numbers, Re, of 60, 140, 290, and 340. It was observed that a vena-contracta existed in the first few tube diameters for all Re. The velocity profiles obtained very close to the tube entrance exhibited a uniform velocity core flow surrounded by regions of relatively stagnant fluid in the near wall regions. The profiles evolved in the downstream direction until the classical parabolic distribution was observed. The total hydrodynamic entry length agrees well with values published in the literature for macroscale flows, obtained from numerical simulation.