Anomalous Reduction in Pressure Drops of the Water Flow through Micro-Orifices in High Velocity Ranges

Micro‐fluid mechanics and its application is one of the most attracting subjects in recent fluid mechanics and fluid engineering. We measured the pressure drop in water flowing through micro‐orifices in high velocities approaching two hundreds meters per second at the maximum. It was found that the measured pressure drop agrees with the prediction of Navier‐Stokes equation for the orifice of 100 μm diameter, but it is lower than the prediction for the orifices less than 50 μm diameter, especially almost two orders of magnitude lower for 10 μm and 5 μm diameters. This huge reduction was confirmed to have no relation to the orifice shape whether the corner of the orifice hole is round or sharp. Also, deformation of orifice foils by pressurization was not a factor of this phenomenon.