Characterization of the Hydrodynamically Developing Flow in a Microtube Using MTV

Micro-molecular tagging velocimetry $(μMTV)$ has been used to characterize the hydrodynamic developing flow in a microtube inlet with a nominal inner diameter of $180μm$⁠. Velocity profile data at 11 axial locations within the hydrodynamic developing region were acquired using the $μMTV$ approach and the results represent the first characterization of hydrodynamically developing pipe flow at the microscale. The uncertainty in measurements of time-averaged velocity profiles ranged from 6% to 7% of the centerline velocity. The uncertainty in instantaneous measurements is in the range 8%–16% of the peak maximum velocity. Data were taken at Reynolds numbers of 60, 100, 140, 290, and 350. The data suggest the formation of a vena contracta with either locally turbulent flow or unsteady laminar flow separation early in the tube for the larger Reynolds (Re) numbers, which is quite different from macroscale experiment or numerical simulation where a vena-contracta is not observed for $Re<500$⁠. The velocity profiles obtained very near the tube entrance exhibited a uniform velocity core flow surrounded by regions of relatively stagnant fluid in the near wall regions. The size of the inferred recirculation zones, measured velocity rms, and maximum shear rates all exhibit increasing magnitude with increasing Reynolds number. The velocity profiles were observed to evolve in the downstream direction until the classical parabolic distribution existed. The total hydrodynamic entry length agrees well with values published in the literature for laminar flow with a uniform inlet velocity, despite the existence of the observed vena contracta.