Sudden expansions in circular microchannels: flow dynamics and pressure drop

Micro particle shadow velocimetry is used to study the flow of water through microcircular sudden expansions of ratios e = 1.51 and e = 1.96 for inlet Reynolds numbers Re (d) < 120. Such flows give rise to annular vortices, trapped downstream of the expansions. The dependency of the vortex length on the Reynolds number Re (d) and the expansion ratio e is experimentally investigated in this study. Additionally, the shape of the axisymmetric annular vortex is quantified based on the visualization results. These measurements favorably follow the trends reported for larger scales in the literature. Redevelopment of the confined jet to the fully developed Poiseuille flow downstream of the expansion is also studied quantitatively. Furthermore, the experimentally resolved velocities are used to calculate high resolution static pressure gradient distributions along the channel walls. These measurements are then integrated into the axisymmetric momentum and energy balance equations, for the flow downstream of the expansion, to obtain the irreversible pressure drop in this geometry. As expected, the measured pressure drop coefficients for the range of Reynolds numbers studied here do not match the predictions of the available empirical correlations, which are commonly based turbulent flow studies. However, these results are in excellent agreement with previous numerical calculations. The pressure drop coefficient is found to strongly depend on the inlet Reynolds number for Re (d) < 50. Although no length-scale effect is observed for the range of channel diameters studied here, for Reynolds numbers Re (d) < 50, which are typical in microchannel applications, complex nonlinear trends in the flow dynamics and pressure drop measurements are discovered and discussed in this work.